The Stovebolt Site Forums The Shop Area Driveline bucking at slow speed (1949 3100)

I have a question for the knowledgeable people on here. When I slow down the truck, "bucks". Does it worse in 3rd gear, but also in 2nd. Not as noticeable in 1st. What would cause this? Clutch, rear end? My 1938 doesn't do this. It just chugs along. Thanks, Grady.

It is a 216 in it. 3 on the column.

Sounds like you are lugging the engine, or it's misfiring from too much fuel on coast-down.

It seems more like it is the drive line. Like something isn't tight together. It does it on level road, in towns going slow.

Any wear/play/slop in the drive train will show up to a certain degree as you describe. I would check universal joints, splined shafts, ring/pinion backlash, transmission wear.

Have show tomorrow and sat. Guess next week I can check things. How would I go about doing this? Jack up back end so rear wheels are off the ground and turn to see if there is play in the drive line? Thanks.

Jacking the rear axle up to check for backlash is a good start. I would look at the differential and floating axles closer than the transmission for this slop.

When you are driving too slowly in third gear (or even in second), your throttle is almost closed. Vacuum will be high, making your engine strain to get enough fuel...which it will not be able to do...which in turn means your engine can not operate efficiently. And this will cause a bucking/jerking sensation as some cylinders can fire and others are denied fuel and fire poorly if at all. This action will naturally be felt in your drive train. Things in your drive train must be really loose to cause this to happen and if you engine is spinning efficiently it is pretty uncommon for this to happen unless you've broken a ring gear or something. Try shifting back to first and raising your rpm to 1000 or so. I don't believe you'll have that problem then.

I will try that tomorrow going to show and report back.

Just jacked up one side. Wheel will move approximately 2 to 3 inches back and forth. When I jack both off the floor, I can spin the wheels even with it in gear? Wouldn't the engine compression stop the wheels from spinning?

Disregard the last statement as I understand the other wheel goes the other way.

Park the truck on a level surface, block the wheels so the truck can't roll in either direction, put transmission in neutral, crawl under the truck and starting at the rear axle try to turn the driveshaft in both directions. Oh wait, you probably have a torque tube drive, that won't work.

You might consider leaving the truck in gear, preferrably 1st, jack up one side of the rear axle so that just one tire is up off the ground. Try rotating the "free" tire in either direction and observe amount of back lash you obtain. If you observe a lot of backlash you'll at least know there may be an issue in the driveline.

Dusty, I did that this afternoon. Have about 2 to 3 inches of movement. I'd that acceptable?

this

When my truck does the same thing as you describe, it is fixed by either cleaning the points or by replacing the accelerator pump diaphragm which has developed a pin hole in it, leading to an over-rich condition. I always start by cleaning the points. That is easier and free.

Will clean points this morning before leaving and see what happens. I do have another new rebuilt carb so can do that tomorrow if problem persists.

Usually, points problem get worse as the engine turns faster.

The slack you're seeing is probably right on the ragged edge of "too much". I'd suggest removing the rear cover plate of the differential and inspecting the ring and pinion gears, and especially the spider gear cross pin for slack or noticeable wear. If the rear end lube hasn't been changed recently, inspecting the gears is a good excuse to service it. If you don't see any noticeable abnormalities, the next step is to unbolt the torque tube ball pivot at the back of the transmission and inspect the universal joint. If you choose to disconnect the torque tube, be sure to support the truck on jack stands at the frame rails, not the axle tubes. Weight on the rear end housing causes some pretty big downforce on the torque tube, and the tube can slam down as the U joint is unbolted. Safety first!
Jerry

I would do that.

Cleaned the point before trip. Seemed better. Going to change carb, hopefully this weekend. Will also do checks Jerry suggested. That will be next week. Thanks all for info. Will report back when completed.

Finally got time to change carb. It runs alot better. Bucking seems better too. Thanks for the help.
Grady