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Discussion Starter · #1 ·
I have a souped up 302,when its running and rev it up it will backfire out the carb.Now when u advance the dizzy and rev it up it will stop backfiring.I can't leave it advanced for the engine will turn over slow and hard and will not start.So when u turn the dizzy back it will start and idle nice till u rev it up?
 

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Discussion Starter · #2 ·
I have roller rockers could I have one on the intake valve to tight leaving the valve open?I just replaced a bent intake valve in my left head.How can I tell what clylinder is backfiring too?
 

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IMO, the tight rocker shouldn't be the problem, or it would do it all the time. My guess would be something with your distributor, maybe the vacuum or mechanical advance inside. Those are intended to adjust with the RPM/load put on the engine. If they are not working correctly, it can cause problems like you described. Has this problem always been there or did it pop up after a change you made?
 

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Start with the basics: Do a compression test. Hook up a vacuum gauge and look for a steady reading; if needle wavers, it's a bad sign. Spark plugs not fouled. Good plug wires. No cracks in distributor cap. Good coil, putting out a strong spark. Check and verify all carb settings: accelerator pump linkage, float level, and jet sizes. Popping or spitting back is usually a sign of a lean mixture or weak spark. Make sure the accelerator pump nozzles are giving a good squirt as the throttle is opened. Too much timing advance can mask this problem. Also, make sure the mechanical advance in the distributor is working.
 

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Discussion Starter · #6 ·
Ok I just did a compression test and the left side all run around 150(thats the side that I just replaced the valve in).Now the right side were all around 120 on the gauge.Should all the compression #'s be around the same on both sides?All the plugs were sooty black Like its been running real rich.The carb is a 670 truck avenger,is this carb to big?I still have to check my duraspark dizzy for advance.
 

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That really doesn't sound right in regards to the compression test. Do you have access to a leakdown tester?

What distributor are you running in it? If it is a factory distributor it could be putting a huge amount of advance in w/ the mechanical and vacuum.

Carb is not too large.
 

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Discussion Starter · #8 ·
It is the factory distributor.I don't have a leakdown tester yet x-mas is coming.but why would all the cylinders read 120 on my tester on that side(5,6,7and
.The other side had 150 in all the cylinders(1,2,3and4).Could the head gasket be leaking between all the cylinders?This is very puzzling.
 

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Although the head gasket could be suspect because it covers all cylinders, I don't think I've ever had one drop cylinder pressure across a whole bank. Especially with equal results.

I wonder if your compression gauge might have an issue? Did you prop the carb throttle wide open when doing the compression test? Normally some variance is seen between individual cylinders. I think I would try it all again.
 

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sounds like to me after doing the left bank the battery was run down a bit after cranking on the first 4 holes !
 

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It is possible one head been shaved more than the other sometime in its life
 

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Discussion Starter · #12 ·
Mustang42782,Idid the rigth side first! 1bad6t,I sent my stock heads to powerheads and they port & polished them and put bigger 194&160 valves in them and they did not shave them.When i just had my valve replaced thats all they did,So now I don't know what to do.Could one of my head gaskets be thicker then the other because I only had to change one of them when I repalced my valve?
 
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