Cub 125 - issues low power - Next step

dhermesc · Oct 3, 2022

I installed a new carb on this 125 - I believe the old one had clogged fuel jets. The new one fired right up and has good power immediately - but the backfiring issue is still there. I was messing with it in the dark and noticed after only a minute of running it is blowing a flame out the exhaust. After a few more minutes the base of the muffler started to glow a nice shade of red. At shut off it backfired through the carb.

This is after pulling the head and cleaning a massive amount carbon. The head and piston top were cleaned and the head machined flat. The original headgasket does not appear to have been blown.

old · Oct 3, 2022

Vale stems maybe cover with carbon also and if so they can tend to stick open A BIT. or maybe the flywheel key is partly sheered so the timing is a bit off or it is running lean

dhermesc · Oct 4, 2022

The exhaust valve was also replaced.

old · Oct 4, 2022

Was the guide area cleaned up?

DoubleO7 · Oct 4, 2022

(quoted from post at 10:14:10 10/04/22) The exhaust valve was also replaced.

How about the valve seat also?

R Bedell · Oct 5, 2022

dhermesc"

After a few more minutes the base of the muffler started to glow a nice shade of red.

Try turning the Main Carb Jet to a richer position. Sounds like you are running too lean.

T.R.K. · Oct 6, 2022

If the muffler is getting red hot your carburetor is set too lean. Open the main mix screw. If it is an original style carburetor, the screw should be the bigger one on top. (not the one at an angle.) Try backing it off 1/4 turn. If no change, back it off in 1/4 or 1/8 turn increments. The engine will let you know when its getting too much fuel. You may also have to open the idle mix screw some too.

You did adjust the valves when you put the new ones in, didn't you?

dhermesc · Oct 11, 2022

I am assuming the massive carbon build up we cleaned was from running to too rich - right? I haven't had time to mess with this thing since I last posted. Now that I think about it the engine may have also been over revving as I do know the throttle cable needs to be reset. The intake valve (and stem) was cleaned and the exhaust valve was replaced. The inspection plate area was also cleaned - huge amounts of carbon build up everywhere on this little engine.

I will try adjusting the engine to run slower then adjust the carb to possibly run richer. It also needs a fuel filter installed as the gas from the fuel tank is rust colored (no actual rust seen).

R Bedell · Oct 12, 2022

dhermesc:

Now that I think about it the engine may have also been over revving as I do know the throttle cable needs to be reset.

The Governor, if properly adjusted, should NOT allow that to happen, regardless of throttle cable position,

Cub 125 - issues low power - Next step

dhermesc

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old

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dhermesc

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old

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DoubleO7

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R Bedell

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T.R.K.

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dhermesc

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R Bedell

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