383 roadrunner stalls when flooring it

[supernova1969a]

this 71 roadrunner, i have done compression tests 110psi, timing, new coil msd, head job, new orange box, rotor, cap, wires , plugs. runs great on idle circuit, when you get on it , it stalls, and thow fire out exaust. any ideas would be appreciated...thanks

 

[dodge41969]

What carb do you have, Holley,Carter?

 

[supernova1969a]

holley 750 vacuum secondaries.

 

[74sundance]

Hesitation under heavy throttle:
Deadspot & stumble. May backfire or spitback.

1)Defective accelerator pump.

1) Look for dirt in pump nozzles,
swollen cup from bad gas, or check ball missing or stuck.

2) Metering rods or power
valve sticking or binding.

2) Repair as necessary.

3) Vacuum leak.

3) Locate leak and repair.

4) Float level setting very
low.

4) Reset to factory specs.

5) Plugged up fuel filter,
defective fuel pump, or
swollen or kinked lines.

5) Inspect and repair as necessary.

6) Secondary air valve set
wrong.

6) Check & adjust the secondary air valve
spring.

7) Ignition timing retarded.

7) Set to factory specs. Make sure
advancing correctly.

 

[supernova1969a]

Any way I can check fuel volume without a gauge, timing is set to the mark. right now I have vacuum advance hose off distributor, friend from mopar tech said it will work better. ..?.? Any truth to that. Thanks for info will check carb stuff and vacuum readings. Any possibility my valves aren't closing fully? Thanks I also need headers, mine have pin hole leaks.

 

[74sundance]

Any holes or exhaust leaks can cause a backfire due to the oxygen entering the exhaust system....

If your car is backfiring it is most likely due to problems with the fuel-to-oxygen ratio in your vehicle. If there is too much oxygen and too little fuel, or too little oxygen and too much fuel backfire may occur. Generally, too much fuel will result in engine backfires whereas too much oxygen, and thus too little fuel, will result in exhaust backfires...Patch up the holes you have and then go from there if you still have problems.
If your valves wern't closing you would have much lower readings on you comp test at least on one cylinder.

 

[71383bee]

A couple of things I noticed...

110 psi cranking compression is actually very low. I am guessing you have a very low CR.

The orange boxes are known to be prone with lots of problems so having a new one is not always a better thing.

I always thought that backfiring through the exhaust was an overly rich condition while thought the carb was a lean.

If you mean that your timing is set to 0 at idle then you are LONG way off from a good tune to get that thing inline. Mopars LOVE more advance in there timing. Try and advance it to 10 BTDC and see how that works. It could want some more initial in there just to try and burn up the mixture a bit sooner. For an unknown engine I usually set total at 34 and see where initial ends up at idle. USually there is too much advance or it comes in too late so the dizzy will need to be curved.

If this is a mild build the advance should be connected to the ported side of the carb. While doing timing it should not n=be connected and the port should be capped.

It is VERY important to get your timing set dead on before diving into the carb. A lot of perceived carb issues turn out to be timing related.

We need way more info on the build of the motor...are they stock pistons, what size cam, intake single or dual plane, what size and type of carb do you have, where is the timing set for total and initial advance and where does it come all in.

 

[74sundance]

Back-fire
From Wikipedia:

http://en.wikipedia.org/wiki/Back-fire

The service manual calls for 110-150 comp...A few questions:
Was the motor warmed up when you did the test,did you have the plugs all out?
Did you run the test again this time adding some oil to the cylinders?
Were the readings 110 across all cylinders?