Thanks for asking. I bolted a 90 GT mustang roller motor into my 66. It has headman shorty headers, RPM Air Gap intake, mallory unilite distributor with old style coil and a holley mechanical secondary carb. I replaced the timing chain with a new one.

The carb was heavily modified. The air horn was machined off and I suspect the jetting has been altered as well. The carb appears to be a 650 or 600.

Timing is set at 34 degrees max with an idle timing of 9 degrees or so.

The problem is an intermittent explosion in the headers at higher rpm under load. I am assuming that there is unburnt gas exiting the cylinder or a valve timing problem. I would suspect a valve timing problem to be consistent.

Today, I plan on replacing the old spark plugs and checking for compression.

I was hoping someone can give me some guideline jetting specs. Maybe from a stock holley carb. Main jet, accelerator pump cam, etc.

Thanks for the help.

 

Might want to post this in it's own thread. It'll get more attention.

 

Pull the plugs and take a look, tell us what you find. Keep track of what plug comes from which hole, and dont drop them.

 

I pulled the plugs. 5 and 6 were black, the rest were the correct brown/tan color. I have 180 compression in 1, 175 in 3 and 170 psi in the other 4 cylinders.

I replaced the plugs and gapped them to .050. I have the same miss. I'll post this to its' own thread.