Backfire Issue
 

Ok I have a 350 motor all forged internals, corvette 74-75 cc heads, airgap manifold, and msd ignition. my question is this, while in gear driving down the road under a heavy load the car tends to stutter and backfire through the carb, it feels almost like driving at top rpm's and not shifting.

I think I may have an intake valve stuck open on number 8cyclinder, not sure besides timing and valves what other issues may cause this problem?

Boy,that could be a lot of things,first thing I'd do is pull off the valve covers and check all the valve springs and rockers.
You may need to start and idle the engine and look at each valve for proper action,could be a weak or sticking lifter too.
Don't drive it anymore till you make sure there is no damage there,it would be a real bummer to suck a valve and destroy the whole engine.
Then if you didn't find any issue pull all the spark plugs and do a compression test.
Still no issue turn the crank back and forth with a socket and watch the dizzy rotor (cap off of course) to see if it is moving with the crank and cam,too much slack may mean a bad timing chain.
Of course before doing all that double check your ignition for burnt plug wires and what not,maybe even a bad spark plug could cause that issue.
Is there any mechanical noise associated with it cutting out?
Any lifter tic at idle?
Car run good at lower rpms?
Stick or auto? solid lifters or hydraulic? missed a shift recently and winged up the rpms till valve float? that could bend a pushrod.

How did you come to that conclusion?
Those are good suggestions by Y2K, however I would start with an easier and less intrusive thing to check. First, I have a couple questions...
1) What carb do you have?
2) Is the choke working correctly and opening all the way when hot? If the choke is not opening up, the engine will run rich when it doesn't want to.
3) Is your carb's float at the right level? If it's set too low, you will be starving the engine at higher rpm's. And if too high, it will flood the engine.
Either #2 or 3 could make the engine kick, buck, or stutter if they are out of adjustment.

Yeah for sure,seen a lot of messed up Holleys from people that shouldn't be allowed to touch a screwdriver.:D

Well today I ran a compression test, and obtained a reading of 150 for cyclinders 1-7 and a reading of 90 for cyclinder number 8, another thing that makes me think 8 has a stuck valve is I hear a slight tapping noise from under that valve cover. to answer your questions though, I have a recently rebuilt edelbrock 1406, 600cfm electric choke carb, i also have a holley high flow 110gph fuel pump and a moroso fuel pressure regulator set at the number 4 setting with a msd street fire dist, and msd 8.5mm wires

You might get lucky once you get the valve cover off. It could be a loose adjustment on the rocker arm. I have even seen a push rod launch itself through the end of a worn stock steel rocker arm. Anything is possible.

hahah, that would have been something to see! I think my rockers are good, I have a hydralic roller cam with roller lifters and 1.5 roller rockers, runs smooth as can be except for the stutter its having, I think your right I think its either a under tightened or over tightened intake valve....

Just an update for everyone, esp. Camaro 69, I took off my pass. side valve cover loosened the intake stud for number 8 cyclinder fired up the car and then took another look found out number 4 intake not even moving so i tightened that down, and wouldnt ya know it it stopped backfiring through the carb.

Now I have another issue i was missing an exhaust bolt on my reducer fixed that but i now have a backfire issue through the exhaust on the driver side, but only when driving, while sitting and reving now problems only when in gear driving. Any suggestions, maybe a too tight valve?

first off whats your experience with motors? sounds like all the rockers need to be readjusted. some hp hydraulic lifters recommend a setting that differs from the stock way of doing it. if u know the brand and type of lifter used u should be able to get the info from the business website.

wow that's a lot of work.. good luck mate..