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 )
-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.

[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... 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... 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

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.