Cylinder misfire troubleshooting!

[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 . 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.

[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 )

[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 )

(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

[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

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.

[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.

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?

[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?

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?

[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?

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.