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Cub 125 - issues low power - back firing

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dhermesc

Well-known Member
We have had this cub for about 10 years. It has always had an issue with back firing on shut down and low power at start up. We had a guy that (former small engine repair man for IH dealership) that tried to adjust the timing but it made no difference. The engine starts fairly easy but has extremely low power until it has run about 5 minutes - when good and hot it runs good and has the expected amount of power. At start up up it can barely pull the tractor and rider around and has virtually no power to run the deck - no misses or cut outs, just extremely low power. On shut down no matter how long you let it idle to cool off it sounds like a shotgun going off.

My son pulled the head - he thought the head gasket might be leaking but nothing was obvious. He cleaned the carbon up with a brass wire brush (there was a lot) replaced the exhaust valve and reassembled with a new muffler (old muffler was horribly carboned up). Started it - and it runs exactly as it did before he touched it - no power when cold, good power when hot, huge backfire on shut down.

What are we missing?
 
Check the advance mechanism in distributor. It may be stuck or worn.
With all the carbon, sounds like it is running very rich.
 
Low power at startup makes me think that the generating system is stealing power until the battery is fully charged. The backfiring sounds like spark plug is of too high heat range.
 
Does choke help with power ?? If so, clean the carb. If not the fuel pump may be weak and slow to pull fuel from the tank.
 
Backfire: When turning it off, do not pull the throttle down. Cut it off from near wide open. See if it can blow that last bit of gas on through the muffler before it can explode.

Check/adjust valves. I am guessing your son adjusted them when he had the head off. It wouldn't hurt to adjust them again. I am guessing he lapped the valves while he was in there.

Richen the fuel mixture. Since that engine should have an adjustable carburetor, with throttle open, open the main mixture as much as you can before it begins to 'load up' from too much fuel, then turn it back down just a tiny bit. Do the same to the idle circuit. A richer fuel mixture will make the engine run a tiny bit cooler.

Low power: Most everything was mentioned except if it hasn't had a set of rings recently, it may be due for a set.

You are running Hytrans (Or whatever it was that IH recommended) in the hydro, aren't you? What you may be experiencing is the hydro taking all the power the engine can give until it's oil gets warm and flows easier. I have a 109 that really struggles just to move itself.
 
Once the engine is hot the low power issue goes away. It actually has pretty good power when engine is hot. Does not appear to burn excessive amounts of oil so I don't think the rings are in too bad of shape.
 
I was thinking that might be the case. Carb is extremely old and has been neglected at times in the past.
 
I have a K301. Check the obvious. Check the points and the gap. Check that the spark is strong and not weak. Check the timing at the little viewport. Check that the gas flow to the fuel pump is good. Then check the flow from the pump to the carb is a strong pumping action. Check that the governor is free and adjusted correctly. The old carbs have both an idle and high speed adjustment. Adjust both to the specs. A free manual is available for those engines at the Kohler site. Lastly, the carb may have to be removed and cleaned. Hopefully, this will resolve the problem or highlight it.
 
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