351 4V Cleveland (351C) in a 1965 Mustang (Yes, that is right) with a lumpy roller cam and single plane Edelbrock intake with a Holley 4777-5 double pumper 650 carburetor. C-4 automatic transmission with stock lock up converter (I know, it will be going away soon!). The huge 4V Cleveland heads with a single plane intake and mild street cam (224 intake and 232 exhaust at 0.050) makes for under performing driving conditions in the lower rpms.
If you attempt to accelerate too hard off idle, to the point where the secondaries kick in, the car bogs and stumbles to the point where it dies unless you lift your foot off the pedal. Quarter or part throttle acceleration is fine and acceleration once the engine is up to 3000 rpm’s and over is fine; no stumbles. Backfiring would occur occasionally when the engine was cold.
The Thought Process:
From what I read online and heard from my friends at Hi-Tech Motorsport, who specializes in racing engines in Elk River MN, about the problem, most likely the engine was not getting enough fuel when you stomp on the throttle from a low rpm. After two weeks of reading online here and there and some bedtime reading in a Holley carburetor book I purchased, I removed the carb and cleaned it all up in my garage. I verified that the 30cc power valve was not damaged and the accelerator pump arms where properly adjusted with the spring. While I had the carburetor off I checked the accelerator pump squirters and noticed the front was a number 31 and the back was a number 35. According to sources I found online, the 4777-5 Holley double pumper originally came with number 28’s in both the front and rear, so somebody must have changed these already (I’m pretty certain I acquired the carb second hand). Because the squirters were already larger than normal, I was hesitant that another increase in squirter size was the solution.
|It was obvious to me the problem lied in the secondaries. I purchased a larger 40cc accelerator pump squirter and the screw with hole in the middle as holley recommended for any squirter above 39cc.|
While I was surprised my little 358ci engine required a 40cc accelerator pump squirter, it certainly did the trick. It definitely makes a difference if the engine is well warmed up after 15 minutes of driving. I may swap in a 42cc squirter in the coming weeks to see if there is any additional improvement when the engine isn’t completely warmed up.