VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc 1950-216.5 stumble

I have a 50 Chevy Styleline Special with the 216.5. The car has been in my family since new. I have taken care of it for about 50 years, but have an issue I have not been able to resolve. Would like words of wisdom. The problem is that the car runs rough just above idle and stumbles randomly during steady state operation in any gear. Just for the record: the engine was professionally rebuilt about 20 years ago and has run great since, until this issue. The valve clearances are set, carb was professional put to specs recently, distributor was bench tested and put to specs recently, points and condenser are recent, dwell is on the money, plugs correct, new distributor cap and rotor, fuel is good and clear as evident by the two glass viewing bowls, fuel pump seems to supply proper fuel, timing is correct, mechanical advance operates by turning the distributor, vacuum advance turns distributor and has no leaks, starts and idles perfectly.

As I advance RPM off of idle, it runs a little rough. In driving, this rough running manifests as a random stumble. If I accelerate hard, it does not stumble.

I know this car very well, as I have taken care of it for 50 years, but this one has me stumped. Any suggestions would be greatly appreciated. If anyone who really knows these engines would prefer to talk, I would be happy to do that and answer any questions. Thanks much. Bill

I ASSUME it has the original low cover Rochester carburetor.???

The symptoms suggest the power valve assembly is not operating properly. It is regulated by engine vacuum so hard acceleration dumps a lot of raw gas into the intake overcoming the stumble.

The carb is the original. I had the stumble prior to having the carb professionally rebuilt to spec. I just wanted to make sure it was all to spec however it did not change the issue I am having with stumble. Thanks for the questions.

The Rochester type B is infamous for driveability issues. This is just one of many.

An interesting fact is that when the originals with the original steel fuel valve got quite a few miles, the fuel valve would have a groove, allowing poor seating and richer running; thereby masking the original problem. Rebuilding with the better neopreme tipped fuel valve returned the carb to its original issue.

Many 216/235 owners replaced the troublesome Rochester type B carbs with the far superior Carter type YF carbs in the 1950's.

Low energy deathanol has not helped, as the issue with the B was a lean condition. The energy content of the bio-fuel is lower, thus adding to the issue.

You could try the following test:

Readjust the float such that the fuel level in the bowl is 1/16 inch higher that normal. This is approximately the same as using a main metering jet one size (0.002 inch). If this helps the problems, then you could either acquire an oversize fuel jet, or bore yours slightly oversize. Running the fuel level too high is only for testing purposes, and gives an idea of the result of a modification without actually making the modification. The fuel level should NOT be left too high.

Personally, I would follow the lead of many Chevrolet 6 cylinder owners, and replace the B with a Carter YF. Unless you vehicle is a numbers-matching show-car. The YF, although far superior to the B, is incorrect, and would lose points in a judged car show. Carter sold more than a million of the Chevrolet-specific Rochester B replacements.

Jon.