Cub 128 surging - stumped

[MattC]

This motor was running fine. I came out to mow and the float had failed. I couldn’t source another walbro float, so i swapped to a spare carter I had laying around.

I replaced the throttle - governor linkage at the same time as the po had used a zip tie. At this point I fired the tractor up and it began surging. I have tried the following:

Rebuilding the carb
Swapping to a known good carb off another machine
Reducing governor spring tension
Tuning the carbs for ages
Regapping the spark plug
Setting the points
Replacing the fuel line
Replaced carb to manifold gasket
Sprayed propane around looking for vacuum leaks

Motor idles great, but surges from just off idle to full throttle. Surges under load too.

What else should I try? Could I have messed something up switching the governor throttle linkage? I’m going to reset the governor shaft next, but it doesn’t make sense that it would be out of adjustment spontaneously.

Any help is greatly appreciated.

[MattC]

I reset the governed shaft, no change. When it bogs it’s rich, sometimes it will blow some black smoke out when bogging. Then surges, stabilized, then bogs again. Incredibly frustrating.

[ol'George]

This motor was running fine. I came out to mow and the float had failed.
And just what failed?
Did it have a hole in it and sunk like the Titanic?
Did it stick in the up/off position and no fuel flowed into the bowl?
Did it not press the needle into the seat for a positive fuel shut off and the carb leaked like a drunk in the restroom?
These things can be fixed.
Surging is caused by:
A lean condition,
A rich condition,
or linkage not adjusted properly.
There is a delicate balance of the throttle spring working against the gov linkage to keep the rpm's at the desired setting.
Are the fuel/air adjustments responding to changes on the "known good carb?
If not,
obviously you have a linkage problem.

You haven't said what you actually did when you "replaced the throttle- gov.linkage?
a new piece? the end? used a coat hanger? found something off whatever that looked close?
These things matter, and without my crystal ball I have no idea how to help you.
I deflect to others--

[MattC]

Hey George,

I replaced the linkage with a good used one from a 124. The float leaked and sunk like the titanic.

The carb is responding to changes in mix. It either lean surges or rich surges.

[cooperino]

Originally Posted by MattC

Hey George,

I replaced the linkage with a good used one from a 124. The float leaked and sunk like the titanic.

The carb is responding to changes in mix. It either lean surges or rich surges.

Matt... Can you snap a couple pics of carb and linkage to post?

[MattC]

So, I decided to clean the points again. That got rid of the surging, but now it is loading up. Running a bit rough. I’m going to swap out the condenser tomorrow, as that’s the last of the ignition that I haven’t changed. I’m wondering if the condenser is bad, and is eating up my points.

Thanks for the help all.

[MattC]

Update for the next guy. It was a bad condenser. Points were eaten up. Replaced the points because they had chipped, installed a new condenser and all is well.