need help with popping thru carb

[dandsgcuda]

I need help from some of the enginge gurus.I am having problems with my engine popping thru the carb at about 4000rpms consistintly. I changed the the heads this winter because my old ones had some valve spring issues.I have checked timing its at 34 degrees all in by 3000, I have torn the carb apart flushed it and rebuilt it,I have tried a different pertronix unit,checked plug wires for resistance,pulled the intake to check lifters and cam for problems and I still can't find the problem.Its slow to rev,has lack of acceration  and when it gets to about 4000rpms it starts to sputter and pop thru the carb.Going to do a compression test tonight.Any suggestions would be much appreciated.Its a 72 340, comp cam deh275,j heads with 2.02 valves,pre-magnum exhaust manifolds,pertronix ignition with a new pertronix coil(ballast removed) holley 750 vacuum carb

[djwhog]

Could be an intake valve not closing, a very weak spring or could still be a cam going flat will do that too.

Also what about cam timing? Has it every run OK? Maybe the cam timing is way off buddy?


Do  a compression test 1st to see and make sure the engine is OK, if it is  then most likely it is electrical. But lets look at the basics 1st.

Lastly you dont have a plugged exhaust system do you?

Dumb ??, are you setting the base timing with vac line disc from dist? 

let us know sorry!

[MEK-Dangerfield]

Maybe it's running lean.   

[dandsgcuda]

the timing is set with the vacuum advance disconnected.I have 12 volts to the pertronix .It ran fine last year. timing chain is set to stock specifications(dot to dot)

[MEK-Dangerfield]

How do your plugs look? I'm thinking a lean condition. What carb do you have?

[Moparal]

A bad ballast resistor can cause that problem to, if it is going bad or the wrong ohm

[dandsgcuda]

compression test shows 175-190lbs in all cylinders.The ballast resistor was removed when I installed the pertronix.I used one spark plug as a standard to test the spark.3 and 7 were not quite as bright as the rest.Also when you let off the key it would get britter.all the plugs look a medium tan. ohms test showed that they had about the same resistance as ones approximently the same length.Tommorow I will run a lead straight from the battery to the positive post of the coil.

[Chryco Psycho]

going out on a limb but virtually everyone connects the red wire from the pertronics to the coil terminal , this is wrong , it has to go to a 12 source such as the blue wire at the ballast , the power at the coil is 6-8 v typically & will not power the pertronics properly

[hooD]

is the fuel filter clean....and how sure are you about the fuel pump being 100%?

[06Daytona]

I had popping from my carb this morning. It turned out to be a few vacuum leaks at the gaskets. It idles fine, but when I put some gas to it it started popping like mini backfires.

[dandsgcuda]

update I finely was able to get back to it.I changed plugs and it does'nt seem to pop thru the carb any more.its still slow to rev and is still hitting a wall at 4000.the pertronix is attached to the coil along with the original blue wire.Measursd voltage at coli is 11.75.I have a new set of plug wires that I am going to put on tonight.

[dandsgcuda]

Well put the plug wires on and it rev to 4500 real slow and popped agian.Iam at a loss.I am going to pull the valve covers off tommrow and check the rocker movement to verify that I don't have a flat cam.

[UKcuda]

I know you said the dots are lined up, but it does sound like it could be bad cam timing as djwhog speculated (above).  Could it have skipped a tooth ?

I had symptoms exactly like you describe on my '71 340 and it turned out to be the cam was a tooth off.

The cam had been installed by the previous owner - a USAF engineer !!?

I remember on mine I could make it pop back through the carb. even when I tried to rev. it in neutral.

[72cudamaan]

  sure sounds like cam timing

[dandsgcuda]

pulled the valve covers off and watched the motor spin over.Everything is moving like it should so the only thing left to do is pull the front of the motor apart and double check the valve timing at the chain.This sucks summer finally gets here in Oregon and I am tearing the motor apart agian.Oh well you have to love'em hate'm and some point.