i have a manuel transmission when i begin to move by letting the clutch out the car coughs and sometimes back pops threw the carb. i have just replaced the cam with a comp. 256 which the tech had said would be a good up grade replacement. would the stock rockchester need to be tweeked? any advice would be helpful.

Whats your timing set at? are you using the vacuum advance?

i have a manuel transmission when i begin to move by letting the clutch out the car coughs and sometimes back pops threw the carb. i have just replaced the cam with a comp. 256 which the tech had said would be a good up grade replacement. would the stock rockchester need to be tweeked? any advice would be helpful.


It sounds like you have too much initial advance.

I would double check the spark plug wire firing order and double check the valve lash along with the intial, vacuum and mechanical spark advance timing as Fish and toobroke mentioned. It's obvious that ignition spark is occurring when the intake valve is still open. It's possible that the distributor might be a tooth off when it was reinstalled. Just some things to consider and make sure are where they need to be.

How does the engine idle in neutral? and how does it rev in neutral? I'm just thinking if putting it under load is a key factor or not.

[QUOTE=PalmBeachVette76;506830

It's obvious that ignition spark is occurring when the intake valve is still open.

[/QUOTE]


Exactly. The only way it can backfire through the carburetor is when a spark occurs when an intake valve is still off it's seat. That then ignites the air/fuel mixture in the manifold and then it blows up through the carburetor.

:iagree: with Toobroke, Sounds like the timing is advanced too much. Check the timing and vacuum source for vacuum advance. You might have it hooked to manifold vacuum instead of ported.

Sometimes the balancer (where the timing mark is located) slips on the hub and makes it difficult to get an accurate reading on the timing. If that's the case, you could just retard the timing two degrees and try again.

Another possibility is the transition ports or slots on the carb are not properly lined up. You may have had to increase the idle speed to get the engine to idle properly, thus throwing off the alignment of the transition slots. The primary throttle blades are supposed to line up with these slots. Many mechanics/tuners will drill holes in the throttle blades to compensate for the mis-alignment of the slots, so that they can get the slots to line-up again. These slots provide for a smooth transition from idle circuit to main metering circuit. This is a common problem when putting a larger cam in.

i have a manuel transmission when i begin to move by letting the clutch out the car coughs and sometimes back pops threw the carb. i have just replaced the cam with a comp. 256 which the tech had said would be a good up grade replacement. would the stock rockchester need to be tweeked? any advice would be helpful.


ANY time you get a backfire through the carburetor your ignition timing is too far advanced. What is your timing set to?