Acceleration Smooth but Deceleration Rough

M

Maurer1929

Member
When my 1929 Model A is under load such as going up a hill or slight incline, the acceleration is very smooth. I typically drive on the open road at 45-50 mph and my engine is stock. However, when I am going down a hill and coasting in 3rd gear, the engine seems choppy, not smooth as with a modern car. Is this normal for when a Model A decelerates? or is this a sympton of something that I can fix?
 
In compairing it to a modern car assuming it has an automatic trans. the engine is freewheeling on an incline thus almost in an idle. The A engine with its trans is directly connected to the wheels and acting as a break to the engine. My opinion. Sorry if I didn't explain it quite right.
Pete
 
My 31 [stock] is also rough when going down hill, I thinks it's pretty much normal with an "A"
 
Here's a theory that's been rattling around in my head. Engine gurus can confirm or deny...

When decelerating down a hill, your big heavy flywheel (which is essentially connected to the rear wheels) becomes a bully that forces the pistons at its pace. That is different than accelerating, where the pistons drive the flywheel. Different mechanical forces are at play.

When the flywheel is "driving" the engine, it pulls pistons down and then pushes them back up.

In one direction, all the bearing surfaces are pulled apart by .002 because of the suction on the piston in the head. In the other direction, the bearings are all pushed together because of the compression on the piston. Multiple bearing tolerances add up.

It is the repeated "pulling apart" and "pushing together" of multiple bearing surfaces that is the vibration we feel. You do not feel this during acceleration because the bearing surfaces are always seated in one position. They do not come apart.

Is this anywhere near close?
 
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Newshirt, The bearing clearance thing is also there when pulling, at the top of the exhaust stroke and bottom of the intake stroke. .002" clearance is not enough to cause vibration, anyway. The vibration could be worn bearings or ring/pinion in the differential (decelleration puts a different load on the gears) or u-joint. Also try retarding the spark when you slow down, and play with the GAV. Is your idle speed where it should be, or is it set too high? Try shutting off the ignition when slowing down to see if the vibration goes away. WARNING! This can fill the muffler with unburned gas vapor and if you turn the ignition back while still coasting, it may go boom! and split the muffler open (Can you believe teenagers used to do this on purpose?). Come to a stop and let it air out a few minutes, or turn off the gas shutoff valve and let it run out to slow down and check the vibration.
 
Vibration, if any, should be the same or close whether accelerating or decelerating. I would suspect a bad universal joint if the engine is in good shape. As others have noted, it could be the mains or rods but, I don't think so. Still, it is hard to diagnose online.
 
Ray, I like to think that you are very close. When accellerating the force is mainly downward. Intake is the only time the crank is not totally bottomed. Compression, power and exhast all have a down force. During decelleration the flywheel becomes the driving mass instead of the driven mass. Intake is restricted and doesn't allow a fuel charge sufficient enough to allow the power stroke required to maintain speed and to keep the crank cradled in the bearings. If there is any imbalance it will manifest itself more when the bottoming force is removed. It is like the rod knock in an old engine, accelleration keeps the rod quite, decelleration does the same, but keeping the engine speed close to the driving speed and suddenly you are blessed with an eight piece castanet ensemble.
 
Simple test

There is a simple test you can perform to determine if the vibration is due to engine or driveline problems. Start up the vehicle and while parked in neutral rev up the engine to "highway" speeds. Then abruptly take your foot off of the accelerator. If there is a vibration similar to the one you experience when decelerating down a hill then you have a problem with the engine. If the vibration is not as pronounced then you should look for driveline issues. All Model As will have some vibration when decelerating.


Bob
 
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