VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc 1955 engine miss

I've been experiencing a slight miss in the 265 engine of my 55 Bel Air. It's so slight I have to be on a smooth road to make sure that I'm not experiencing minor bumps in the road. It occurs at speeds of 40 mph and above. It's a slight miss or surging sensation. It doesn't miss while accelerating. I recently replaced the gas tank because there was a large amount of rust in it and the rust repeatedly clogged the fuel filter. With the new tank the filter clogging is no longer a problem. Previously, I thought this miss was caused by the rust in a the fuel. Now I'm not sure what's causing the problem. I replaced the ignition wires yesterday and the miss is still there. I know a weak coil can cause a miss, and weak valve springs can as well. And a vacuum leak could be the cause. I guess it could also be caused by a dirty carburetor or trash in a fuel line. How do I proceed from here? What should I check next?
Thanks,
Sid

A lean mixture can cause what they call surging at mid speeds. Does the engine have a 2 barrel or a 4 barrel?
I assume it is a Carter if a 4 Bbl.

Hello Gene, it's a 2 barrel. I failed to mention I also gapped the plugs and points to specs.
Sid

If you are in an area where non-ethanol fuel is not available, it may be the lack of energy in the ethanol fuel.

Often, necessary to open up the main metering jets 0.002 to compensate for the lower energy content of the fuel.

BUT, test before drilling!!!

Reset the carburetor float 1/16 inch too high. This is approximately equivalent to one jet size. See if the hesitation disappears.

If yes, reset the float to spec, and drill the main metering jets 0.002 oversize.

If no, reset the float to spec, and look elsewhere for the problem.

Jon.

Also check the size of the main jets. Should be stamped 51 indicating .051" size of opening.
Make sure heat riser is free and heat openings (exhaust passages) under base of carburetor are free of carbon.

Thanks Gene and Jon, my plan is to convert to a four barrel. I have already located the Carter carburetor, correct 55 four barrel intake and correct air cleaner. But I wanted to make sure this miss problem was resolved before I swapped intakes, because if the problem is valve springs then I have to remove the heads. I noticed in looking through records that came with the car, that the prior owner did not install new valve springs when the heads were redone 20 years ago. Is there a way to rule out a weak valve spring problem before proceeding with carburetor issues?
Thanks,
Sid

Weak valve springs will cause the valves to not close completely at high engine speeds (Valve float)and limit the top speed of the car. If it will get up to 50 MPH in low range with not bucking the springs should be OK.
Actually the shift point at full throttle is 55 MPH if the Power Glide linkage is adjusted correctly the the arburetor is actually opening all the way.
A 1955 shuld reach 60 MPH in low range which is 4800 RPM before the valves float.

Note that ere was two types of Carter 4 barrels in 1955. The early had a bell crank mounted on a stud on the left side of the carb. Late had the pedal linkage rod connected directly to the lever on the side of the carb. Generally the early also had the air cleaner with the round snout and the late the air cleaner had the oval snout.

Thanks Gene, The car has a three on the tree. It will accelerate strongly in all three gears with no evidence of miss. On a recent 120 mile trip I only noticed the miss when in high gear with light pressure on the accelerator. It occurred at 40, 50, and 60 mph. When passing it easily did 70. It may have been missing then, but on 2 lane blacktop I was more concerned with getting around the slower car.
Sid

I would say your valve springs are OK.
For engine speed the standard trans. will be about equal to low gear of a PowerGlide so it should get up to 50 MPH smoothly in 2nd gear. If ti will springs are ok.

iWOULD SUSPECT THE PROBLEM IS EITHER A VACUUM LEAK OR LEAN FUEL MIXTURE BOTH WHICH SHOULD BE CHANGED WITH THE NEW INTAKE AND CARB.

Thanks Gene. I will proceed with the plan.
Sid

Check the timing and the distributor with the vacuum advance, as well.

Charlie

4 barrel is installed now. Miss is gone. Thanks for all your help guys.
Sid