JD 140- stumbles at full rpms

Thought I'd try here, as a few other guys have been working on their small engines lately it seems. I picked up an early ('68) JD 140 garden tractor this spring. I've worked a few bugs out, and it runs ok, but at high idle it stumbles until I bring the throttle back down to about 1/2. When I got it, it didn't seem to run right at all. I rebuilt the carb, and installed new points and a condenser. I also put new rubber fuel line and a new fuel filter on it. I didn't think it ran fast enough, so after getting a photo tach to check it with, it was only running around 2400 RPM instead of the 3600 or so that the book calls for.

It's a Kohler 301 12 HP engine (serial is 3395)

I got it to get at least close to 3600 now after putting a heli-coil in the hole that holds the throttle cable clamp and putting the throttle cable in what I assume is the proper hole on the linkage, given the wear on that hole and not the one it was in.


The main problem I'm having now, is that it stumbles like it just isn't getting enough gas at full throttle, but using the choke makes it worse. I can get it there if I slowly increase speed, but as soon as I try to move it or engage the deck it starts to stumble again. Seems to get a little better as it runs and gets warmer.

Where should I go next? Sick of cutting with a Crapsman... and I miss having the luxury of my parents 425 w/ 60" deck.

Donovan from Wisconsin

When you replaced the breaker points did you follow the procedure in the service manual and set the point gap using a timing light? (On those engines point gap determines ignition timing.)

BTW, you said you replaced the "fuel filter", which leaves me a bit confused because that GT had NO fuel filter when it left the Horicon Works.

Below is a link to the official Kohler service manual for that engine, you can find all sorts of info and troubleshooting info there.
K301 service manual

Previous owner replaced the sediment bowl with a flow through filter. I have the sediment bowl in a bag of parts I got with it, but the gasket is rotted and I haven't gotten a new one yet to bother putting it back in.

I also got a JD service manual with the tractor. I don't have a timing light, but I replaced them and set them with the static procedure in the book. I'll check out the link, and see about procuring a timing light in the mean time.

It's got potential, once I get all the little bugs worked out of it.

Donovan from Wisconsin

I've had some small engines that won't work
with paper in line filters. Try using no
filter or get an in line screen filter from
mower shop.

The fuel tank is not very high above the carb, if the "filter" is anything but a screen like the in-line ones Briggs used, it MAY not be flowing enough gasoline, in that gravity system, and there's NO fuel pump.

As to the sediment bowl, the later 140's just had a shutoff valve with a screen sticking up into the tank and that's what the sediment bowl subs to, if you check with the dealer's parts department and try to get a replacment.

If the filter isn't made for gravity flow it likely won't work for you.Some small engines won't run for 5 minutes at idle with the wrong filter installed.

Use an o-ring to seal the sediment bowl.

How are the throttle shaft and bushings? Is there wear causing a vacuum leak?

Might revisit the carb, check the mid range circuit. Inside the bore of the carb body, where the throttle plate closes (almost) against the bore, there are 2 small holes. The one closest to the mount flange is the idle circuit. The next one slightly inside the throttle plate is the mid range circuit. Make sure that passage is clear. It feeds through the main jet circuit. Also make sure the throttle plate is flat, not bent or installed crooked on the shaft.

The carb certainly isn't new, but it was a replacement the previous owner installed because the original one was in really bad shape. The throttle shaft had lots of wobble, and it wouldn't start when he bought it so he found a used one.

I had spray carb cleaner coming out all out the internal passages I could find, and I also soaked it in the chem dip carb cleaner for around an hour. Didn't want to over do it as this stuff will eat aluminum if left too long. Carb came out really nice and shiny, as a bonus.

This winter I'm tempted to pull the carb apart again and see about getting the throttle shaft out to measure the bore it goes through. The screws are peened on the back side, and both the original carb and the replacement are like this so I'm guessing it's supposed to be like that. I had enough issues with the choke shaft, as those little brass screws twisted right off, and I had to drill them out and chase the holes with a small tap. Didn't feel like fighting the throttle shaft at that time so I left it alone.

I did spray some ether on/around the carb when it was running rough, but it didn't seem to change anything.

Today's project was getting a door back on the mud room. Was going to replace it maybe 2 months ago, but then other stuff took priority. Hopefully I can get back at it this week some time.

Donovan from Wisconsin