Cub Cadet 125 running hot - back firing

After the fire a couple weeks ago I have replaced a large section of the wiring harness with 12 gauge wire and everything seems to be working correctly - wiring wise.

This cub has had issues for quite a while. It would take forever to warm up and run with any kind of power, but the longer you ran it the better it would run. When warmed up it was EXTREMELY hot
and no amount of idling after use would prevent it from backfiring (very very loud backfire). This engine was rebuilt years ago and after the rebuild I had what I considered a very knowledgable
person (was the small engine man at an IH dealership for years and continues to do small engine repair from his shop) check it out due to the back firing issue. HE couldn't find an issue even after
pulling the head to make sure it didn't have an excess carbon issue. The last couple of years I used the 125 less and less.

Currently it starts extremely easy without using the choke. During the first 10 minutes of use when you set the throttle wide open it does not build up much for RPMs and is extremely under powered.
Once it has been running a while it builds some power up (a lot less than it had) but even running the deck makes the engine labor. With 10 minutes of use if you shut the deck off and let it sit in
place with the throttle wide open it finally runs at full RPMs - but has a noticeable extra backfire like noise going on and the exhaust (factory exhaust for the 125) has a consistent 2 flame.
When you start the deck the RPMs drop by 1/3 and the backfire noise goes away along with the flame. With power so low mowing is almost impossible. The backfire o shut down is not happening now.

I am assuming the exhaust valve has probably been damaged do the backfiring issue and only seals up when it gets hot. I assume the valve issue has gotten worse with time and use and it nolonger seals
up when hot. If I take it apart and check the valves anyone have an idea as to what the excessive heat issue is? I would think it has the timing to far advance but how do you fix that?

Basically I am trying to get this thing to run good enough to sell.

I'm thinking your muffler is clogged. More common in 2 cycle, but I have seen it in 4 cycle engines also. I never found a way to clean them. Take muffler off, see if engine runs like it should if you can take the noise.
50 years working with small engines.
Brad

Let us know what you find out.

I will try this tonight

I am thinking you need to get your engine guy to check the valves and seats. He should have at least gapped them last time.

Another thought is if you are not having to choke it, perhaps the fuel mix is too rich.

Another thought: if your crankcase breather connects to the air cleaner with a rubber tube, try temporarily running with it disconnected and see if performance improves.

***hold everything!: You said this is a 125. So it is a hydro.*** When was the last time the hydro was serviced?****

I just recalled working on a (well used) 108 I have and the engine really struggled just to run the hydro transmission and move itself. Remember your 125 only has 12 hp. available to move itself and run the deck.

dhermesc:

Running the Carb "too lean" will cause overheating also.

Yes but with it that lean I would think I would need the choke to start. I know over advanced timing will make it run hot also.

Not an issue with the hydro - just running the deck stationary makes the engine labor.

Was not the muffler - had a guy come by tonight to look at while I was pulling the muffler. Started it and it ran exactly the same -only louder. Told him the issues and he still paid $250 for it. Its gone.

But even 'stationary' the engine is still having to turn the hydro pump.

I agree with it running lean, that will cause both problems. I think they start on the low speed jet, so that might be set fine, but the high speed need to be opened up. Our Farmall C starts best without any choke, bot then if you don't give it half choke it will die. Once it runs a minute it's fine.