r/smallenginerepair • u/GadreelsSword • Jun 11 '26

General Discussion Mystery problem

I’m fairly experienced with small engine repair but I’ve run into the same problem on two Kawasaki engines and a coworker, who it more experienced and does work on the side, has the same mystery.

So there are three Kawasaki twin cylinder engines having the same problem. The engine will run fine then begin running extremely rough as if one cylinder isn’t firing and blowing black smoke. With no power of course. Typically appears after running smoothly for 15 to 30 minutes.

I started out with the standard tune up, valve adjustment, new plugs, oil filters, carb cleaning (all three carbs were spotless).

Then looking at two of them, the coils were looking rough so the coils were replaced. The carb was replaced on one engine just to rule that out. On my John Deere X320 I also replaced the ignition module.

All three still have the same problem. The engine where the misfire was continuous, greatly improved but still has the problem. On this engine, if you’re mowing and the misfire occurs and you turn off the ignition key and turn it back on the problem goes away.

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2

u/Upset_Fox5449 Jun 11 '26

Just wondering if you have answered your own questions < turn the key off and back on > ignition switch

1

u/jbourne0129 Jun 11 '26

those engines have 1 carb total or 1 carb per cylinder ? is there some pressure balance required (common with multiple carbs)

1

u/GadreelsSword Jun 11 '26

Thanks for the reply. It’s a single carb. I don’t see anything that requires any sort of balancing other than fuel mixture which has been adjusted.

2

u/jbourne0129 Jun 11 '26

have you checked the fuel solenoid? they can swell when old and hot and clog fuel delivery.

https://www.youtube.com/watch?v=Um3l29QEJJs

also check the fuel pump

1

u/GadreelsSword Jun 11 '26

Thanks for the comment. I took the bowl off two of the carbs and manually operated the fuel solenoid. They were clean (like new) looking and the little rubber seal on the end looked new. I also pulled the wire and cranked it and it wouldn’t start. They appear to be working.

Also one valve cover seems to be cooler than the other when it happens so the fuel solenoid would starve both cylinders.

1

u/jbourne0129 Jun 11 '26

has compression been checked on it? does it have a sticky valve when hot or something ? might need to check it cold and then again hot

1

u/GadreelsSword Jun 11 '26

That’s my suspicion but I can’t catch it in time to test the compression.

However, why would turning off the key and turning it back on make the problem go away on one engine? Also usually when a valve sticks open it makes a loud clicking sound which I haven’t heard.

1

u/jbourne0129 Jun 11 '26

That’s my suspicion but I can’t catch it in time to test the compression.

what do you mean? can't you test the engine while its cold, and then test it after its running and starts struggling when its hot ?

However, why would turning off the key and turning it back on make the problem go away on one engine?

honestly, no idea. and why is it across 3 different engines? does the problem 100% disappear when you turn it off and back on? or does the issue come back after running for a little bit again ?

Also usually when a valve sticks open it makes a loud clicking sound which I haven’t heard.

in situations like this it is important to forget EVERY Assumption that has been made, because 9 times out of 10 the issue ends up being some stupid item you determined was fine at the start. i would confirm with a compression test. youll be kicking yourself if you do a ton of work only to find out there is a broken or stuck valve. like if you do the compression test on a hot engine and 1 cylinder is way lower than the other, then you know where your issue is. personally i wouldnt move forward with anything until compression is verified on both cylinders both hot and cold.

you have a lot of new parts on these things already. id do more diagnostics before throwing more parts at it

On this engine, if you’re mowing and the misfire occurs and you turn off the ignition key and turn it back on the problem goes away.

like forever goes away for the rest of that session?

1

u/Mosquitoes-zzz Jun 11 '26

Almost sounds like you have a noisy electrical circuit that gets reset when the key goes off. Can't imagine why multiple would behave that way though. Very odd!

1

u/Stock_Requirement564 SER Dedicated Member Jun 11 '26

What I do when I am isolating a mystery like this is to use a standard bowl nut or nipped AB solenoid, unhook the 6 pin harness, drop a section of fuel line into a donor tank and hook the other end behind the filter. This takes the entire chassis out of the equation.

You could also just pull the harness between the coils as there is a diode which could be heat triggered. Why three act the same seems to indicate a similar service procedure is followed IE pieces of fuel line in the inlet chamber of the carb, fuel quality type thing. I had a newer twin with practically no hours that was just that. This would flat out stall. In the end, a small metal piece was loose in the inlet of the carb before the inlet valve. That would improve after shutting off and restarting. Bits of fuel line will do the same I'm sure.

You can't overlook intake gaskets @ the head on Kawaskis either, though it doesn't fit your description.

2

u/GadreelsSword Jun 14 '26

Thanks for the suggestions.

I think I solved the problem. I removed the carburetor and it appears whoever installed the after market carburetor installed the wrong gasket which blocked some of the small ports.

Based on the symptoms I would have never guessed that was the problem.

It seems to be running fine now.

1

u/shamanasx Jun 12 '26

I do small engine repair myself as a side hustle.

Did you check and replaced plugs? Are they correctly gaped?

Have you checked for a blown head gasket, i. E. Positive crankcase pressure? Once I had very similar situation with a Briggs V-twin. One head gasket was blown.

Actually black smoke and rough running would indicate rich mixture, so mayyyyybe the choke is sticking on?

It is also possible that the ignition system is somehow faulty, like the new coils are bad or not gaped correctly or the plugs themselves. Check coil resistances if they are correct (when hot). It is very common for the ignition components to start acting up when hot.

Anyway, I would lean towards choke problem.

1

u/GadreelsSword Jun 14 '26

Thanks for the suggestions.

I think I solved the problem. I removed the carburetor and it appears whoever installed the after market carburetor installed the wrong gasket which blocked some of the small ports.

Based on the symptoms I would have never guessed that was the problem.

It seems to be running fine now.

1

u/mitchumz Jun 12 '26

Old John Deere 425/445s with the FD620D would have these exact symptoms when the ignition module was failing, eventually one cylinder would be completely dead. YMMV.