1969 mustang 302 clevor breaking up at high rpm

Hi All,
I recently got my 69 mustang with a 302 clevor (302 block with 351 clevaland 2v heads) up and running pretty well. Having one really strange issue though and cant figure it out. If i get to about 4,000 rpm it starts to really break up and fall apart sounds like really bad misfiring and when i let off gas i get some popping out the exhaust, BUT if i really give it gas and don't coast at high rpm it will rev all the way up no problem and no breaking up until i let of the gas and attempt to cruise at high rpm above 4000. I've tried playing with the timing tried retarding it which seemed to make it better (before it was breaking up around 2800 rpm and took off distributor vacuum advance).
Carb is a 570 holley street avenger double pumper
Pertronix flame thrower coil and distributor
Fuel pump is mechanical holley pump
Everything runs great unless i slowly accelerate into the 4000 range.

Thanks!

Mitch

So I tried to retard it more but then if I rev it quickly, it backfires out the carb.

I checked the power valve based on the Holley procedure and it seems to be working as it should.


Thanks,

Mitch

rbohm said:

alright you might still have too much timing in the motor. where did you set it initially? i like 10-14 degrees for most of my engines. another isue to check into is the ignition box and the ignition coil. either one can be breaking up under load. same with the distributor cap, look for carbon tracking and cracking. also check the rotor to make sure the voltage isnt leaking out to the distributor shaft, and yes i have seen that happen.

it also sounds like you have a lean fuel mixture as well. might be the float level is too low, jetting not big enough, power valve not operating properly, etc.

and then there is the possibility that the start solenoid is screwed up and shorting out from time to time. remember that ALL the electricity for the car goes through that item.

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2Blue2 said:

Check the engine block and battery grounds too.




+1 on the coil going bad.

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Yup you guys were right! Coil was still using the resistor wire and not getting a full 12v which the pertronix unit requires. Now revs great all day long after switching to a full switched 12v source.

Thanks again!