Cylinder misfire troubleshooting!

**centaurus** · 27th May 2007, 20:28

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** · 30th May 2007, 17:56

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** · 31st May 2007, 12:00

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.

**Racey Rider** · 4th June 2007, 17:02

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** · 4th June 2007, 17:32

Originally Posted by 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?

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** · 4th June 2007, 17:42

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** · 4th June 2007, 17:53

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

**centaurus** · 4th June 2007, 19:03

Originally Posted by Racey Rider

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** · 4th June 2007, 19:03

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** · 4th June 2007, 20:00

Originally Posted by centaurus

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** · 4th June 2007, 20:29

Originally Posted by Racey 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** · 4th June 2007, 20:44

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** · 12th June 2007, 18:09

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** · 12th June 2007, 19:07

Well done mate.

Ya learned heaps, and stuck at it.

**RobH** · 13th June 2007, 10:25

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.