Cylinder misfire troubleshooting!

[centaurus]

Ok, this is the deal - the bottom cylinder is misfiring. Usually it misses every second or every third beat, mainly at idle and when going with a constant speed. Also I noticed that not all the explosions as equally powerful. When accelerating hard, usually sounds and feels like it fires properly (but I cant' be 100% sure). However, recently (in the last two days) in two occasions, it completely stopped firing after about 1/2h or riding and it wouldn't start even when going all the way to 10-11k rpm. Because it misfires, it smokes more than the other one. The other cylinder never missed a bit and runs very smoothly.
The spark plug from the faulty cylinder looks wet with petrol, but not oil fouled.

Thinks I've checked:
-checked the compression - the same on both cylinders
-checked that there is no oil leak from the gearbox
-switched spark plugs between cylinders, the bottom one keeps misfiring, the top one runs smoothly
-replaced spark plugs (twice) with new ones and with the correct specs
-checked the continuity of the wires from the ECU to the coil
-check that the coils have the same electrical resistance
-tried to swap coils - the misfire stayed with the same cylinder
-visually checked the spark while the engine was running - it seemed to fire -every time and of blue color, so I'm assuming it's not a spark problem, but I -can't be 100% sure (it could be the wrong voltage or something else that can't be seen with the naked eye)
got the carbs cleaned at the shop (at least that's what I've been billed for :D)
-tried playing with the air screw on the carb. I made it smoke less, but the misfiring remained.

Anybody has any idea as to what should I check next? What could be causing it?

At this point I can only think of the carb (unballanced or poorly set up) or a faulty ECU.
Before opening the carb (I don't know what I'm doing but the shop guys look like they don't either, so there's no point paying them to tell me that they don't know how to fix it) or taking the ECU to diagnostics, I was wondering if anybody knows what could cause this or what else could I test/check.

Thanks in advance.

[Kickaha]

-tried to swap coils - the misfire stayed with the same cylinder
.

did you swap the leads and plug caps with the coils?

[vifferman]

did you swap the leads and plug caps with the coils?
That's what I was going to ask. I had a misfire problem on the FahrtSturm, and it turned out to be due to a very small tear in the rubber boot on the front cylinder's sparkplug lead. I wouldn't have even thought to check it, except I saw on another forum that someone else had similar problems and that's what the cause was. I'm pharkt if I know how that could make a difference, but replacing it made the problem disappear. :confused:

[centaurus]

the plug caps and the coils are one piece. The plug caps can't be detached from the coils.

[vifferman]

Well, that kinda proves it's not electrical, because you swapped all the electrical bits to no avail. There's only electrical, fuel, and mechanical.
Electrical: you've done all you can apart from swapping the ECU with another known good one.
Mechanical: check for air leaks between the carb and cylinder. Check the zorst too, that it's seated properly, not coked up on that header, etc.
Fuel: the shop supposedly cleaned the carb, but there's still a possibility of wear, crap still being in there, or something being incorrectly adjusted. Crud under a needle, water in the fuel bowl, overfuelling due to a holed float, etc.

If you're not up to doing the work yourself (have you got a good workshop manual?), find someone with a good reputation for working on this kind of bike.

[centaurus]

If you're not up to doing the work yourself (have you got a good workshop manual?), find someone with a good reputation for working on this kind of bike.

I would give anything for a workshop manual for this bike, but looks like there is no workshop manual in English. Haynes don't make one and Honda made only in Japanese.

After the fiasco with the shop (he kept the bike two weeks and charged me a lot of money and didn't even notice that the cylinder was misfiring) I am apprehensive about taking it to a mechanic. I don't have money to throw away, and when it comes to carburettors, they can easily spend a lot of hours and still not fix it.

I have found some bits and pieces of info on the net about nsr250 carburettors and I think I will have to take my chances and do it miself.

[rogson]

Choke cable hung-up or plunger not seating properly?

[centaurus]

Choke cable is ok. the lever that it actioned by the cable moves properly. I have no idea how to check the plunger

[centaurus]

Mechanical: check for air leaks between the carb and cylinder. Check the zorst too, that it's seated properly, not coked up on that header, etc.

What's a zorst? I haven't encountered the term before.

[xwhatsit]

What's a zorst? I haven't encountered the term before.

An exhaust... just say the word `exhaust' fast in a kiwi accent and you'll understand lol.

[centaurus]

An exhaust... just say the word `exhaust' fast in a kiwi accent and you'll understand lol.

Thanx xerxesdaphat. Now I understand. LOL!



Check the zorst too, that it's seated properly, not coked up on that header, etc.

Are you refering at the exhaust like in exit from the cylinder? So I should take the expansion chamber off the cylinder and check the connection between the two of them?

What about the exhaust valves? I checked that they are in the right position, but a worn valve could cause misfire or something like that?

[xwhatsit]

Thanx xerxesdaphat. Now I understand. LOL!




Are you refering at the exhaust like in exit from the cylinder? So I should take the expansion chamber off the cylinder and check the connection between the two of them?

What about the exhaust valves? I checked that they are in the right position, but a worn valve could cause misfire or something like that?

Lol his sentences aren't so easy... but I think he means the actual silencer. So he's talking about where the silencer joins onto the headers, and that the headers aren't carboned up. But it all applies; make sure that there is no leaks anywhere in the exhaust system, at any of the joins, and decarboning the entire exhaust system is probably a good idea anyway (removes another variable at the very least).

[centaurus]

Lol his sentences aren't so easy... but I think he means the actual silencer. So he's talking about where the silencer joins onto the headers, and that the headers aren't carboned up. But it all applies; make sure that there is no leaks anywhere in the exhaust system, at any of the joins, and decarboning the entire exhaust system is probably a good idea anyway (removes another variable at the very least).

Recently I had to take off both silencers - one of them had a missing screw and the on the other one, the plate welded to the rest of the exhaust that helps securing the silencer was broken so I had to weld it back. Neither of the silencers are clogged (you can see through the inner pipe).

[centaurus]

I took off the expansion chamber from the "bad" cylinder today and cleaned the carbon deposits on the cylinder head around the valve and on the edge of the expansion chamber. Didn't seem too much, just a thin layer. This didn't make any difference.

While testing, I realized that if I pull the choke, the bad cylinder misfires less, so I started to assume fuel starvation.

I took three deep breaths... :gob: and started dismantling the carburettors. Inside I found everything clean and in order:
- I checked that the choke plungers were not stuck
- I extracted the slow jet and blew it with compressed air
- I switched the slow jets between carbs (in case the slow jet is bad, the problem would move to the other cylinder)
- I checked that the float was still air tight
- Seemed that the float arm wasn't bent enough so I tried to adjust it so the needle would open as fas as the one on the other carb.
- checked that the rubber boots between the carbs and the engine were still air tight.

After I put everything back, the engine seemed to run better. The "bad" cylinder fires pretty much every time, however the explosions don't seem totally equal in force like on the other cylinder. The good part is that from what I could see, this happens only at idle. As soon as I turn the throttle both cylinders seem to fire properly. Had to close the air screw further away from the manual position though.

I am gonna put it back and try to ride it for a few days to see if it dies again or not. The only thing that worries me is that sometimes after I release the throttle, the revs remain around 3k. After a while (~5-20 sec.) they come down to 1.25k. If I open the choke, the revs seem to drop instantly to the normal idle (1.25k). Anybody knows what could be causing this?

[xwhatsit]

I took off the expansion chamber from the "bad" cylinder today and cleaned the carbon deposits on the cylinder head around the valve and on the edge of the expansion chamber. Didn't seem too much, just a thin layer. This didn't make any difference.

While testing, I realized that if I pull the choke, the bad cylinder misfires less, so I started to assume fuel starvation.

I took three deep breaths... :gob: and started dismantling the carburettors. Inside I found everything clean and in order:
- I checked that the choke plungers were not stuck
- I extracted the slow jet and blew it with compressed air
- I switched the slow jets between carbs (in case the slow jet is bad, the problem would move to the other cylinder)
- I checked that the float was still air tight
- Seemed that the float arm wasn't bent enough so I tried to adjust it so the needle would open as fas as the one on the other carb.
- checked that the rubber boots between the carbs and the engine were still air tight.

After I put everything back, the engine seemed to run better. The "bad" cylinder fires pretty much every time, however the explosions don't seem totally equal in force like on the other cylinder. The good part is that from what I could see, this happens only at idle. As soon as I turn the throttle both cylinders seem to fire properly. Had to close the air screw further away from the manual position though.

I am gonna put it back and try to ride it for a few days to see if it dies again or not. The only thing that worries me is that sometimes after I release the throttle, the revs remain around 3k. After a while (~5-20 sec.) they come down to 1.25k. If I open the choke, the revs seem to drop instantly to the normal idle (1.25k). Anybody knows what could be causing this?

Hey! Progress! Awesome :D

As for your idle that doesn't settle back down, I'm pretty sure I know what this is. Had the same thing on my bike, and this is what it turned out to be. When you pulled apart the carbs, you probably disturbed the seal between the float bowl and the carb body. So whatever reason (screws not tight enough, gasket leaking, whatever) there is now an air leak in the float bowl. This will cause strange idling.

The other option, perhaps, is if you accidentally changed the float level, but it usually seems to be a leak in the float bowl.

Good luck! Sounds like you're getting there.

[Ixion]

Your problem is almost certainly (on the basis of the information you provide) , an air leak into the induction system (which includes the crankcases), causing a lean mixture. This will cause miss firing bad running and a very fast idle.

Likely culprits are a worn or misassembled carb (probable), a leaking crankcase seal (possible), other crankcase or barrel air leaks.

[centaurus]

After further testing, it turns out the job on the carbs didn't do anything.

Yesterday I rechecked the lot, including the exhaust valves position.

Today, after additional further testing, I came to a strange conclusion: I don't think is misfires. I think it just fires twice as slow as it should do. it's like I'm having a 2 stroke cylinder and a for stroke one in the same bike :shit:. Some explosions are more powerful than others because sometimes it gets flooded (every other time it just pumps fuel/air mix down the exhaust without exploding it).

If I twist the throttle the revs increase on both cylinders but the ration of 2:1 remains constant. The only place I don't feel the vibrations and specific noise caused by this when riding is close to full throttle. I'm assuming it's not because the explosions even out but probably it just seems that way.

Has anybody heard of anything like this before? If you have any ideas, please don't hold back. I have no clue what might cause this. I am totally stuck. :shutup:

[Ixion]

Yes. It's called four stroking. And it's not uncommon on two smokers. Because there is no exhaust stroke on a two stroke, if the mixture does not fire on a cycle, a lot of it will remain and mix with the mixture on the next cycle.

If the cylinder is running lean, there may be too much air for the amount of fuel on the first cycle. So the cylinder does not fire. Then on the next cycle, some more fuel is drawn in and now there is enough

(Perversely, the same thing can happen with a rich mixture, just reverse fuel and air in the above).

You need to look at your induction system. Either you have an air leak (carbs, joints, seals), or a blockage. The former is more likely . It is unlikely to be over richness because you say that it races at idle, which is diagnostic of a lean mixture.

(Four strokes *can* do the same thing , called eight-stroking, but it is very very rare. Four stroking on a two smoker is common, it's part of the two stroke experience. Thaz why we luvs them :love:)

[xwhatsit]

Yes. It's called four stroking. And it's not uncommon on two smokers. Because there is no exhaust stroke on a two stroke, if the mixture does not fire on a cycle, a lot of it will remain and mix with the mixture on the next cycle.

If the cylinder is running lean, there may be too much air for the amount of fuel on the first cycle. So the cylinder does not fire. Then on the next cycle, some more fuel is drawn in and now there is enough

(Perversely, the same thing can happen with a rich mixture, just reverse fuel and air in the above).

You need to look at your induction system. Either you have an air leak (carbs, joints, seals), or a blockage. The former is more likely . It is unlikely to be over richness because you say that it races at idle, which is diagnostic of a lean mixture.

(Four strokes *can* do the same thing , called eight-stroking, but it is very very rare. Four stroking on a two smoker is common, it's part of the two stroke experience. Thaz why we luvs them :love:)

(O/T I know, but that's what I must be hearing in the occasional decrepit Vespa at uni. Once they get up in RPM it all seems to smooth out, but at certain RPMs and throttle positions they sound like an ancient Honda Dream or Benly.)

As you were.

[centaurus]

I opened the carbs again, I rechecked the slow jet, I flushed the slow jet and it's chamber with compressed air, I flushed all the hoses coming/going from the "faulty" caurburettor with compressed air, I switched the floats between carburettors - still nothing.

What surprises me is that I doesn't seem to run better whether I make the mixture richer or leaner. If I pull the choke (checked that it works) it seems to explode harder and the exhaust gets wet, but still it misses every second beat. if I try to open the air screw too much, it stops firing completely.

I even tried playing with the screw that connect the throttle sliders between the two carburettors. If I'm not mistaking that's the synchronizing control? Still no luck

One thing I noticed today while testing is that the carbs spit back pulverised mixture. the "faulty" carb seems to be spitting more than the other one (this is just an "eye observation" is not something measured in any way). Does this mean I have faulty reed valves or it's something normal? Could this be the source of my problems?

[xwhatsit]

I opened the carbs again, I rechecked the slow jet, I flushed the slow jet and it's chamber with compressed air, I flushed all the hoses coming/going from the "faulty" caurburettor with compressed air, I switched the floats between carburettors - still nothing.

What surprises me is that I doesn't seem to run better whether I make the mixture richer or leaner. If I pull the choke (checked that it works) it seems to explode harder and the exhaust gets wet, but still it misses every second beat. if I try to open the air screw too much, it stops firing completely.

I even tried playing with the screw that connect the throttle sliders between the two carburettors. If I'm not mistaking that's the synchronizing control? Still no luck

One thing I noticed today while testing is that the carbs spit back pulverised mixture. the "faulty" carb seems to be spitting more than the other one (this is just an "eye observation" is not something measured in any way). Does this mean I have faulty reed valves or it's something normal? Could this be the source of my problems?

Spitting back could be because of the four-stroking Ixion mentioned, due to unburnt mixture still being in the cylinder (if I understand the cycle correctly).

Listen to Ixion -- he's a smart fellow, my engine actually goes because of his smarts. Check for air leaks, don't worry about your carb jets for now.

Don't know how helpful this will be, but talcum powder or similar can help with air leaks on exposed surfaces. Tracked down an issue in the lawnmower with this :yes:

[centaurus]

Spitting back could be because of the four-stroking Ixion mentioned, due to unburnt mixture still being in the cylinder (if I understand the cycle correctly).

Don't know how helpful this will be, but talcum powder or similar can help with air leaks on exposed surfaces. Tracked down an issue in the lawnmower with this :yes:

The high idling problem was only temporary and was solved the next time I opened the carbs. Probably I didn't assemble or install them carefully. So now it comes down' to the normal idle revs after releasing the throttle. So I'm trying to figure out if the four stroking is because the mixture is too rich or too lean.

- checked the carbs: fuel level ok (even swapped the floats between carbs)
- checked the slow jet (cleaned with compressed air and switched the jets between carbs)
- checked that the hoses connected to the carb were not clogged
- checked the seal between the carbs and the crankcase - it looks in very good condition (no cracs, no scratches, on the rubber boot, good sealing towards the crankcase

At this point I have no reason to assume a problem with the fuel delivery (too much or too little). The only thing could be some sort of air leak (in or out). But from where? Is there anything that I left unchecked? I'm at the end of my wits. Anybody has any ideeas about other tests that I can do or ways of testing for air leaks?

[tri boy]

Hi centaurus,
I'm not familiar with your specific induction system, but you could try the following to diagnose a seal air leak in your cases:
Drain, and save your crankcase oil.
Fill with an oil like Castor oil. (to promote a noticeable exhaust aroma).
If your notice a change in the exhaust smell, you my have a crank/bearing seal leak. Good luck.:yes:

[centaurus]

Hi centaurus,
I'm not familiar with your specific induction system, but you could try the following to diagnose a seal air leak in your cases:
Drain, and save your crankcase oil.
Fill with an oil like Castor oil. (to promote a noticeable exhaust aroma).
If your notice a change in the exhaust smell, you my have a crank/bearing seal leak. Good luck.:yes:

Hi tri boy,

Thanks for the advice. However, my bike is a 2 stoke - so it doesn't have any oil in the crank case. It only has oil in the gear box. I've already tested that I have no leak from the gearbox oil into the crankcase (where the fuel mix resides before entering the cylinder).

I am suspecting an air leak (inwards or outwards) and trying to find whether I have an air leak and if so, where.

[tri boy]

Hi tri boy,

Thanks for the advice. However, my bike is a 2 stoke - so it doesn't have any oil in the crank case. It only has oil in the gear box. I've already tested that I have no leak from the gearbox oil into the crankcase (where the fuel mix resides before entering the cylinder).

I am suspecting an air leak (inwards or outwards) and trying to find whether I have an air leak and if so, where.

I did realize its a stroker. Lol.
I'm interested to know how you checked the crankshaft seal condition, as these can be tricky to diagnose. Also, isn't there a "dry" seal on a rotor or electrical side of the crank?:mellow:

[centaurus]

I did realize its a stroker. Lol.
I'm interested to know how you checked the crankshaft seal condition, as these can be tricky to diagnose. Also, isn't there a "dry" seal on a rotor or electrical side of the crank?:mellow:

That seal has actually been broken. The bike kept dying after 1/2h or riding and kept fouling the bottom cylinder spark plug. It took me quite some time to diagnoze it correctly and a few months to find a suitable replacement oil seal (honda does not produce it anymore :( ).

I replaced the oil seal almost two months ago. Since then I kept track of the oil level in the gearbox - it has been constant. Plus, there is no oil fouling on the plug. It is wet but with petrol. That and the fact that the exhaust pipe is wet (I think it's petrol also, it doesn't look like oil), makes me lean towards the "too rich mix" possibility rather than "too lean mix".

The other seal (on the left side) is dry and if I would've had a problem there, I would have seen the fuel mix dripping out, plus the problem would have been on the top cylinder (which is on the left)

The problem is that I've testes pretty much everything, and the problem keeps staying with one cylinder. The other keeps running smoothly doesn't matter what I do. The only thing that I haven't done (and I hope I won't have to) is actually switch the whole carburettors among them.


*********************************************
Just to make sure everybody is clear about the situation:
The high idling was a temporary problem and it disappeared the first time I took off the carburettors again. Probably was an installation error from my part with led to some air leak. The initial issue which still stands is the fact that the bottom cylinder fires only every second time.
*********************************************

[tri boy]

Sounds like you have done just about everything.
You said you swapped coils/leads/caps etc. I'm just wondering if the primary wires in the harness that connect to your lower cyl coil have a partial break/contact fault. This may explain the wet plug and erratic firing.
Also, when the fault developed had you done anything different with the bike previously that could contribute to the fault, ie fairing removal, washed it?:mellow:

[centaurus]

[Edit] Sorry, double post.

[centaurus]

Sounds like you have done just about everything.
You said you swapped coils/leads/caps etc. I'm just wondering if the primary wires in the harness that connect to your lower cyl coil have a partial break/contact fault. This may explain the wet plug and erratic firing.
Also, when the fault developed had you done anything different with the bike previously that could contribute to the fault, ie fairing removal, washed it?:mellow:

I measured the resistance of the primary wires from the ECU to the coils and they are in one piece. Also, with the engine running, I tried taking off the plug cap and I saw blue sparks (quite powerful) and they seem to be at the correct frequency (however, this is an "eye measurement", so I can't vouch for it).

Also, the misfire is every second beat, it's not erratic. It's just like I have two different engines, each running at a different speed.

After fixing the seal, I noticed the bottom cylinder exhaust was smoking, but the bike was running fine. After a while, it kept deteriorating slowly (more and more smoke), until I started feeling running rough when I was off the throttle. That was the point when I started investigating seriously and I noticed there was a misfire, and the smoke was probably partially burned fuel mix.

During various tests, the smoke has come and gone (I'm assuming that the mix got richer and leaner), but the misfiring remained constant: every two fires from the top cylinder, one beat from the bottom cylinder.

I took it to a shop which proved to be more useless than me. They actually made it worse, so my that option is off the list also.

[tri boy]

Well, all I can is that you have given it a good check over, and with persistence,
you will find the fault.(then wonder how you didn't see it earlier)
Keep going.you'll find it.:yes:

[centaurus]

Thanks for the confidence tri boy. I really hope you're right.

Going back to my previous question. Does anybody know if fuel mist coming back from the carbs when opening the throttle is normal? If it isn't (then I assume bad reed valves), could this be the cause of my issue?

[centaurus]

I've been playing with different setups on my bike in the last days. I even wrapped the vacuum hoses leaving from the bad cylinder with electrical duct tape to make sure there are no undetected cracks.

After quite a lot of testing, I found that with the air screw completely closedon the carburettor corresponding to the bad cylinder and the exhaust valve on the same cylinder set to a much more closed position than normal, the bike runs tons better. It's still not running like it should, but if fires almost every time at idle.

The only problem is that the exhaust of the bad cylinder is wet with petrol. This creates a little bit of contradiction:
- If I open the air screw (to make the idle mixture more lean) the cylinder goes back to misfiring, and if I open it two turns (close to the factory setting), the engine stops firing completely
- on the other hand the exhaust is wet with unburned petrol (it's definitely not oil and I checked that the coolant level doesn't drop)

Also, closing the exhaust valve made a lot of difference, so it's a mixture problem or an exhaust valve problem? And if it's a mixture problem, it's too lean or too rich?

I haven't played with exhaust valves before so I don't know how to check it they are ok or not, but when I cleaned the exhaust port of the cylinder, both valves were moving properly, didn't look melted/distorted/broken and were quite clean. They looked to be in order.

I have to mention that the misfiring appears in the first 1/4 of the throttle. This means that if the cause is the fuel mix, I can excludes from the list of suspects all the jets except the slow jet and pretty much can exclude the float/fuel lever too.

Anybody has any ideeas?

[tri boy]

Hi centaurus,
Bloody thing still giving you the run around? How were the reed valves? no cracked/bent/poorly seated ones?
You may have to return things to standard settings to avoid a "cross over of symtoms".
Sounds like the wet cylinder is the red flag you need to concentrate on.
Electrics/fuelling, and compression can all cause this.
Sorry I can't be much more help. Without being there/hearing/experiencing the fault, theres not much else I should say.
Maybe try asking a few more bike mechanics in your area. Most good mechanics hate it if they can't sort out a problem.(pride, detective leanings etc), so I'm sure someone will take up the challenge without charging like a wounded Bull.
Good luck. Hang in there. You'll be wiser from the experience.:yes:

[Racey Rider]

How long have you had this bike?
Was it running good, then this problem developed?
Or have you just brought it, and are trying to get it running right?

[centaurus]

How long have you had this bike?
Was it running good, then this problem developed?
Or have you just brought it, and are trying to get it running right?

When I bought it, it was running fine. After a few weeks the oil seal between the crank case and the gearbox cracked and it started fouling the plug and killing the engine.

After I replaced the oil seal, I noticed that the bike wasn't smoking equally, but it was running fine. About three weeks later (and a few hunderd kms on the bike), the cylinder started misfiring gradually until I couldn't ride it properly anymore.

So, to summarize, when I bought it it was running fine, but I haven't had it for a long time.

[centaurus]

I took off the reed valves today. They were in good nick. All the flaps were in one piece, no cracks or other marks of usage. There was some gunk, so disassembled and cleaned them properly and put them back.

After putting everything back and getting everything back to default (air screw, power valves, etc...), there was an improvement. At higher rpm's (let's say 3000 and above) it seems to fire correctly. At the idle it still misfires, less than before, but doesn't run smooth as it should be. If I close the air screw, I can make it run better at idle (1200-1300 rpm) but when riding it, it misfires at higher rpms and the exhaust gets wet with unburned fuel.

Conclusion: as far as I can tell, at this moment, the problem appears only at idle. As soon as the rpm's rise, the cylinder doesn't misfire anymore, and I'm out of ideas as what to check next.

[Racey Rider]

What Fuel Octane are you running? Should be 91 yes?

[centaurus]

What Fuel Octane are you running? Should be 91 yes?

Usually 95. Rarely, when I have the chance, I use 98. I've been told that being a 2 stroke, I should use the highest octane possible.

[Conquiztador]

I hate these problems. U check, double check and triple check things and still same problem.

From my experience with 2 stroke bikes the "4 stroking" is almost always caused by too rich fuel mixture. Not enuf fuel they just dont want to run.

So here a list of things (that U probably have already checked, but...) that I have not seen mentioned here:

- Aircleaner. If it is blocked then U will get too much fuel.
- Needle in float. If it leaks U will get to high level in the carbi and too rich mixture. (Can u swap the 2 carbies??)
- Carbi position. Is it straight or leaning towards the cylinder? Could cause fuel to flow in to cylinder (U mentioned that the problem was in the lower cylinder)

Thats all I can think of right now.

But post your problem here: http://the-stable.lancs.ac.uk/~esarie/honda/techbrd.htm

No need to be a member, just post your question and you have some replies quite soon.

I have gotten some very good tips from the guys on there. And U might find that they know where U can get a manual too.

Good luck

[Racey Rider]

Usually 95. Rarely, when I have the chance, I use 98. I've been told that being a 2 stroke, I should use the highest octane possible.

Often people grab hold of Half a truth, and make it law for all situations.

My understanding is, all normal Jap road bikes of the last few decades are designed to run on 91 Oct. (Yes, there will be exception. But the NSR250 isn't one of them as far as I know) But I don't know much.

A change from 91 to something else will change the burn rate of the fuel. I have proved before on my own 250cc 2 stroke, this can make quite a difference to combustion. Unless someone has done performance mods to the motor, there should be no need to run higher Octane than 91. This simple change back to 91 could be all that is needed. Probably not, but it's an easy thing to try before looking at other possible issues.

But see what Ixion thinks.

At least it would give you more of an original starting point for problem diagnostics.
Just a thought.

Racey

[The Lone Rider]

Often people grab hold of Half a truth, and make it law for all situations.

My understanding is, all normal Jap road bikes of the last few decades are designed to run on 91 Oct. (Yes, there will be exception. But the NSR250 isn't one of them as far as I know) But I don't know much.

A change from 91 to something else will change the burn rate of the fuel. I have proved before on my own 250cc 2 stroke, this can make quite a difference to combustion. Unless someone has done performance mods to the motor, there should be no need to run higher Octane than 91. This simple change back to 91 could be all that is needed. Probably not, but it's an easy thing to try before looking at other possible issues.

But see what Ixion thinks.

At least it would give you more of an original starting point for problem diagnostics.
Just a thought.

Racey

Does this apply to 4 strokes as well? I'm running 95 and it coughed/small backfired a bit even before I put on the new pipes.

[Racey Rider]

The same principle of the slower burn rate of the higher octane fuels having a small affect on combustion timing could show that symptom.

But probably something else.....

[centaurus]

Last week, I got crossed with the bike so bad for making me so much trouble that I decided to check EVERYTHING again. When I checked the oil level the gearbox, I noticed there was almost no oil at all.

Turns out there was a mixed problem: Initially there was no oil leak, but the carb needed cleaning. While I was struggling with the carb, a foreign metal object in the gearbox got stuck between the sprocket and the oil seal and chewed it's way through the oil seal. When I finished with the carbs, the oil seal wasn't airtight anymore, so the problem reappeared, only with a different cause.

luckily for me the "foreign metal object" didn't touch anything else in the gearbox, so no need to replace other parts. Turns out it was a washer that got lost inside (I really can't imagine how it got there). I am sure it's nothing that was once part of the gearbox. It's clearly an external object. On the other hand, my clutch cover was dismounted for a few months while I was searching for a compatible oil seal, so who knows...)

I replaced the oil seal and the bike runs much better now. Unfortunately, it's an "inside" oil seal, so me forcing it from the outside didn't do too much good - it still leaks a little bit making the bike misfire from time to time when the engine is on load.

I will have to take it to a shop to split the crank case and replace the seals. Unfortunately I don't neither the tools nor the space or the time for taking the engine out of the bike.

Even if technically I didn't fix it, I've learned a lot about my bike in the last month or so. Thanks a lot guys for all your valuable input.

[tri boy]

Well done mate.:yes:
Ya learned heaps, and stuck at it.:first:

[RobH]

Hi
I thought I'd add my experiences. I've got an MC18/2J with similar problems here in the UK. It was having idling and acceleration problems. Initially I thought it was the top end because the front cylinder was well worn. I did a top end rebuild and found the front cylinder exhaust bridge had a hairline crack. After welding and replating it still doesn't run well. The carbs were clean and part of the rebuild included new plugs coils lead and new front plug cap. Still no joy so I took it to a local 2 stroke specialist. They had it for a week, spent plenty of time cleaning the carbs, check and rolling road testing. They had it running ok for a while, but when I collected it a couple of days later, it was back to normal. The cabs were off again and the jets throughly cleaned again. Ran for a few minutes ok then duff again. Swapping plugs over help for a while. Took it out for a 40 mile run. Still rough at low revs and resistanting reving out. The bike pulled well up hill and it would get over 100, but no powerband rush. Disconnect the rear rear cap and the front cylinder fires ok. The bikes quite happy to tick over on one cylinder for a while. The engine starts to fire better over 3k. I've got a new CDI unit on order from a UK supplier David Silver www.davidsilverpares.co.uk who specialise in New-Old spares for £125. I'll post my findings whn I get the part.

[centaurus]

Hi RobH. Your problem sounds quite interesting. Please keep us informed of your progress.

[delusionz]

i put 95 & 98 in my 2 smoke once and all it did was make it run slower, usa uses 87 octane, 93 is their premium... using an octane level over the manufacturer specification is doing nothing but retarding your ignition timing in effect. your mix probably doesn't spark in time

[FruitLooPs]

Thread resurrection!

I've got this exact same problem on an NSR250 R2J race bike I just got, i'm hoping its not the oil seal. :( Runs on both cylinders, with varying amounts of smoke from none to normal 2-stroke amounts then one will die and come back or vice versa. Leaves dots of oil on the ground thats been suspended in the exhaust!

It's running 25:1 ratio premix which was what it was meant to run apparently. It doesn't idle high or seem lean in the least, but it will bog and stall if i rev it past 9k rpm or so.


Also regarding the above, octanes are different type (RON Vs MON I think) between American and NZ. 87 octane for them is like 91 for us