Can't figure out missfire

[tkos115]

So... finally put a new head on this engine. Made almost no difference in how it runs. I literally can't think of anything else. Compression is good, no leaking valves, perfect spark, swapped carbs, coils and plugs, all new gaskets. The cylinder bore looks good and has no scoring or abnormal wear marks.

Anyone else have an idea? I'm about to give up on it.

 

[sgkent]

take a breath. Are you able to determine which side is now misfiring, or are both?

 

[tkos115]

It seems like the LH head (of referenced from sitting on the seat) is the issue. But if I run it and clamp of the fuel to the RH carbutor it will stop popping and back firing once the fuel is burned out of it. It will run still but definitely a difference in power and sound. If I keep it running and remove the clamp it will go right back to popping and slight misfiring again through the exhaust, sometimes worse than it was. It does the exact same thing if I do it to the other carb but it won't drop off as much in speed or sound. If I pull the plug per cylinder it's pretty much the same result. LH head seems to make less of a difference when the plug wire or fuel is off. It seems like it misfires as soon as both cylinders are active, but pulling a plug wire or turning the fuel off to one of them will make it stop popping and missing.

 

[tkos115]

It seems to go away when using the mower deck but the idle will start to surge once it gets hot. It seems like it's getting too much fuel maybe. Using the choke never seems to make it any better. It will start to run rougher when too much choke is on. I've cleaned and tried new carbs too with no change.

Also wondering.. can a bad muffler cause any of this? I only ask because I can hear a baffle or something rattling around when it's running and the exhaust seems to be louder than it should be.

 

[slomo]

But if I run it and clamp of the fuel to the RH carbutor it will stop popping and back firing once the fuel is burned out of it.

Sounds like a leaking carb needle/seat. I would pressure test both carbs.

I would load test the ignition coils also. Confirm you have proper spark.

 

[tkos115]

That was my thought too. Did that and it held pressure fine, as well as vacuum. No drop offs or leaks, I also put new carbs on with no change either.
I used a spark tester similar to the one in the video. It has an adjustable gap and it will jump the gap at almost the furthest setting. Spark color is whitish blue on both coils.

 

[slomo]

That was my thought too. Did that and it held pressure fine, as well as vacuum. No drop offs or leaks, I also put new carbs on with no change either.
I used a spark tester similar to the one in the video. It has an adjustable gap and it will jump the gap at almost the furthest setting. Spark color is whitish blue on both coils.

How long did you test the carbs with press/vac?

So it sounds like you've done it all. New carbs, tested old ones. Is the fuel tank spotless in the bottom? Does this engine have a fuel pump? Should have good fuel flow AT the carbs. Carb linkages out of whack? Meaning one cyl set to 3k and the other just off idle?

You've missed something. All these new parts you've thrown at it. You will need to test all these new parts. New doesn't mean new anymore.

 

[slomo]

When it's running it seems to run a bit rich and it pops and backfires a bit, mostly at idle speeds.

Lack of fuel, carb needle sticking open or vacuum leaks.

If I take off the plug wire on the left head it almost makes no difference in the way it runs. But it does stop poping.

You just found a suspect cylinder.

Compression when cranking is around 70psi. The right side is actually less at about 55psi.

This is a little low, still should run. Valves not set proper? Blown head gaskets? This you said you put a new head on.

Flywheel key just a touch off and about to shear? Flywheel torqued to proper spec?

Do what I do when I get a new to me used mower. Look everything over.

Are the block cooling fins clean and clear? As in looking like a new engine? Some engines have metal pans that run around the bottom of the block to funnel cooling air. Look under these pans for anything that shouldn't be there.

 

[tkos115]

So I ended up replacing both cylinder head gaskets anyway as I had the other side from a kit. I lapped all the valves and set them to spec. I have noticed now, that even when its cold at idle and just above it will hunt/surge and pop still and it didn't used to do that. I can't find any vacuum leaks and replaced all intake seals. I will check the flywheel key again but when I looked before it seemed fine. All the cooling fins are clean and I had all the shrouds off and cleaned them as they were a little dirty. I tested both carbs for leaks and to make sure they are holding and seem good.

Will let you know when I check the flywheel again.

Thanks for your time.

 

[slomo]

that even when its cold at idle and just above it will hunt/surge and pop still

Too much or not enough fuel. Possible vacuum leak/s. You ruled out weak spark right?

Long shot is flywheel key off just a touch creating improper ignition timing.