Model A is "bucking." Why dat?

[Tom Wesenberg]

Originally Posted by pat in Santa Cruz

I had a similar problem in the woody. After changing the entire engine, carb, dist, manifold with a set up that was previously proven to run well in my sedan, the problem persisted. The station wagon had a BW overdrive, which I suspected was contributing to the problem. Later on, during a ground up restoration, I found the splines in the drive shaft collar to the BW were worn. I theorized that with the excessive wear, even the slightest hickup in the engine from either a too lean cough when cold or a too rich loping effect from loading up on a long down grade was ricocheting back and forth through the excess drive shaft slop. Having the splines and collars rebuilt solved most of the bucking. There is still some occasional light bucking on long down hill grades. I suspect there is wear inside the BW as a result of the bucking from the worn splines. It could be your U joint and the drive shaft splines are worn too much.

I too have been trying to think of a mechanical reason for the bucking. I think you may have hit on something worth looking at. Also check the rear axles for tight nuts and keyways. I'd put the car in high gear and push it by hand back and forth to check for slop in any of the driveline. Also check that the front and rear axles and wishbones are not loose.

[700rpm]

Originally Posted by Tom Wesenberg

I too have been trying to think of a mechanical reason for the bucking. I think you may have hit on something worth looking at. Also check the rear axles for tight nuts and keyways. I'd put the car in high gear and push it by hand back and forth to check for slop in any of the driveline. Also check that the front and rear axles and wishbones are not loose.

I will check out those things, if for no other reason than it's a good idea; but this problem did not exist before changing out the engine. Nothing in the driveline or axles was altered from what it was with the older engine.

[pat in Santa Cruz]

Originally Posted by 700rpm

I will check out those things, if for no other reason than it's a good idea; but this problem did not exist before changing out the engine. Nothing in the driveline or axles was altered from what it was with the older engine.

are you using the same carburetor, manifold and distributor? Are the clutch disk and pressure plate new?

[RichPA]

Sounds to me the problem is related to idle mixture and throttle plate settings. On decel, the car is slowed by the throttle plate being closed.The fuel being fed is only from the idle circuit, so if the idle mixture is not right and the throttle plate is set too far open you might be drawing fuel from the main jet as well.This may cause a "rich' missfire and therefore a bucking sensation.Maybe the settings are off to compensate for a vacuum leak or timing issue.

[Gary WA]

Another thought the comp jet and cap jet look the same but are different could have put the wrong one in.

[Patrick L.]

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OK, here is my story and I'm sticking to it.. Engine running fine so I have to fix it.. I decided to replace the float[Tillotson] because its been soldered a couple times over the years and I guess I didn't have anything else to do that day.. While replacing the float and valve[I like Grose valves] I give it a quick cleaning and blow dry.. What I initially called the comp jet is apparently the main jet [A9539 or 14050].. It would only spin out about 3 or 4 turns and hang up.. I sprayed it and blew it out and tightened it back up.. Of course I should have left well enough alone cuz it starting to 'buck' upon decel.. I removed the carburetor, drilled out the jet, cleaned the well and installed another clean used one that was lying around..That was the only change, runs fine again..

[700rpm]

UPDATE: I quit driving the car for the winter back in October, so I didn't have any more experience with the bucking. By this spring my new engine was ready and I just got that installed, and the bucking is gone. But re-reading all the responses, I think the problem was in the clutch. All other systems were switched out directly from one engine to the other. That is, distributor, carb, water pump, windshield wiper motor (which was my first guess). The new engine runs smooth and beeyootiful. I am ready for the summer tours!

Hope to see some of you at the NW Regional in Portland in August. I'll be easy to spot: I'm the guy with the '29 Tudor and a big smile on his face.

[roccaas]

Wasn't the purpose of an "over-run" clutch? One that would disengage the engine from the drive train when the vehicle's speed exceeded the speed "set" by the driver's right foot??

[marc hildebrant]

Is the Over Run clutch the same thing as the old "free wheeling" ? My 1933 chev had a free wheeling option that used a clutch to release the transmission to driveshaft connection if the driveshaft wanted to go faster than the transmission output....i.e. to allow coasting .

This option also allowed you to not use the clutch for shifting once you were moving. You let your foot off the gas, and then could shift.

Marc

[Maurer1929]

I have had a similar problem when coasting down a hill or decelerating. What is a Model A (no mods) supposed to sound like when slowing down say from 45 mph to 30 mph when coming to a stop light? Is it supposed to be smooth or choppy? and if it is supposed to be smooth, what makes it choppy. Mine is an old engine.

[Purdy Swoft]

If it is running rich because the gav is opened too far, it will have a lopey idle, that could be called choppy. I run my model A zenith one quarter turn open. If the model B zenith is used ,I run with the gav closed after warm up.

[steve s]

Originally Posted by Maurer1929

I have had a similar problem when coasting down a hill or decelerating. What is a Model A (no mods) supposed to sound like when slowing down say from 45 mph to 30 mph when coming to a stop light? Is it supposed to be smooth or choppy? and if it is supposed to be smooth, what makes it choppy. Mine is an old engine.

Here's the deal.

Two things happen when coasting down a hill or decelerating: (1) The throttle is closed, so the engine is dependent on what little gasoline it can suck through the idle jet system. (2) The engine is being forced to turn over at quite high rpm's relative to the amount of gasoline available to keep all cylinders firing. So, the cylinders are being starved for fuel while being forced to fire rapidly. The result is too-lean fuel/air mixture to support combustion, so the cylinders either misfire or don't fire at all--causing rough running--as lots of unburned fuel is swept out into the exhaust system--where it accumulates until conditions are right for it to be touched off as a backfire.

The problem is strictly fuel/air related, not electrical; electrical problems will lead to similar rough running and backfiring BUT UNDER LOAD.

Possible fixes:

Maybe just opening the GAV will help. I had to open more than one full turn going down the hills in WV.

Maybe your carb float level is set too lean.

Don't rev too high when shifting gears.

Don't downshift or use unnecessarily low gears when backing down from speed or going down hills.

Check diligently for vacuum leaks--too much air being sucked in works the same as too little gasoline being provided.

Hope this helps.

Steve