CH 25s engine misfires under load

[hakelly]

This is on a 2002 Toro Dingo 425 with CH25s Kohler, ex-rental, rode hard and put away wet.
It starts, idles and runs fine @ no load ( maybe the occasional miss at full throttle). But when I put it under load it bucks and bawls with heavy misses. To me it sounds like true backfiring (through carb) as opposed to afterfire through exhaust. I can operate smoothly enough at 1/4 - 1/3 throttle but this is not good for a hydraulic drive.
The condition seems to improve slightly with warm-up, esp. in winter, but it's getting worse overall. I did all the fuel things: new fuel pump, new filter, cleaned carb. I didn't think it was fuel anyway because the condition happens immediately when opening the throttle, not like draining the bowl of fuel. Compression is 185, exactly the same both sides.
Can't be spark breakdown due to heat because it happens cold or hot. Choking the carb just makes things worse. The plugs look normal. No smoking or excessive soot in exhaust pipe. Carb governor linkage operates freely. I'm not sure which particular ign. system this has (CDI of some sort?) but the flywheel key is in place and nothing is wobbly there.
At this point I am thinking valve issues. Not burnt but maybe loose seat or bad lifters. Any thoughts from you guys before I take the plunge? Any other checks I can do?
I love that Dingo and really need to bring her back to life.

 

[logan01]

I went through a similar ordeal with my old rough cut mower. Turned out to be the right side plug wire had cracked and was arcing against some metal as the mower bounced around. Tape fixed it for a while. I then retaped, cut and split a piece of fuel line and put over it, taped and zip tied that. Works fine. Might want to check for something shorting.

 

[hakelly]

Thanks, I'll take a look at those wires, maybe tonight after sundown.

 

[shiftsuper175607]

Thanks, I'll take a look at those wires, maybe tonight after sundown.

Cold or hot can be relative. The engine can get hot in the winter too.

You said the problem is not heat related.

 

[hakelly]

I was saying this happens at startup before the engine gets hot, as well as at operating temp. In other words, the condition does not change appreciably, in fact if you look at my post I said there seems to be some slight improvement on warm up, just as you would generally expect in a normal engine.

 

[ILENGINE]

Lets start by getting the specification number off of the engine model tag, so we can figure out what ignition system you have. Could be a ignition module that is cutting out under load. Faulty lifter would rattle in most cases because they are hydraulic, unless the oil pressure is over pumping them causing it to lift the valves off the seats, Most of that would be related to weight of oil in engine.

 

[hakelly]

I expected two ID stickers on the flywheel housing but there's only one and nothing anywhere else that I can find:

Family: 1 KHXS 7252GC
725 cc
plus an emission compliance #

I don't know how I figured it was a CH25s engine, maybe from Toro. (Dingo yr. 2002, mod #22307, ser #220000165 if that's any help)

The crank end has a threaded hole for a bolt (and washer?) but the flywheel is attached with what looks like one of those Browning 3-bolt tapered hubs.

 

[ILENGINE]

Your Kohler spec number is 68688 and should have a spark advance module. To determine which ignition system you have you will need to remove the blower housing and see how many wires connect to the ignition modules. You should have the two wire ignition modules. If you have the one wire modules somebody may have screwed up and try to convert an analog system to the new DSAI system which will cause performance problems. which could be a possibility.

 

[hakelly]

Thanks for that spec #.
It's two wires, brown and yellow. Kohler diagram says yellow means analog SAM, pink means digital.
Two lower shroud screws are nigh inaccessible. Getting the housing all the way off and messing with anything behind it would pretty much mean pulling the engine.
Also, forget what I said about Browning pulley in previous post. Sometimes I'm stupid.

 

[hakelly]

If this is not a valve/cylinder issue, what it really sounds and feels like to me is bad timing. However it is related to load and I don't know how this CDI system operates.
The other thing could be bad plug wires but I don't see anything obvious, and they seem to be integral with the CDI unit anyway so not directly replaceable.