Holley carburetor bog and hestitation fixed 3


The Application:
351c
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.

The Problem:

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.

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. squirter and screw squirter location
I replaced the orange pump cam in the front with the brown cam that allows a longer duration and higher lift, which allows more fuel when the throttle is pushed further. This should aid the secondaries and help with a little overlap of fuel when I mash the throttle. Also, I put the blue cam in the rear for the secondaries because it has a more aggressive ramp and will shoot more fuel in quicker than other pump cams when the secondaries are activated. medium-pump-cam-intro pump cams

 

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.


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3 thoughts on “Holley carburetor bog and hestitation fixed

  • Kevin O'Neill

    Great post. I have a 355 Chevy in a 86 Monte Carlo.
    Bought a new 4150-4779 750cfm Holley. Stuck it on my Airgap intake and its bogging right off the line. I have a 100 gph Holley fuel pump. 041 64cc 202/160 heads. .480 liftcam at 280 at 50. I was thinking of putting in the 50cc diaphram and see if that would solve the problem. Like you, I’m surprised a mechanical 750cfm would need more fuel on a relatively small motor. I’m now thinking after reading your post that just replacing the daphram ain’t going to fix it. I’m thinking the 50cc pump kit, Holley # 510-20-11 is what I’m looking for. Any thoughts or suggestions would be greatly appreciated.
    Thanx.
    Kevin

  • Dan

    I have gone through three 600 Holley carbs I bought from Summit Racing on my 16 year old daughter’s stock 302 1970 Mustang Convertible.

    The car bogs right off of idle driving it normally. All three carbs did the same thing. I’ve tried everything to make this carb work, including Holley Tech Support, changing cam position, diaphragm, Squirters, jets, etc. Nothing has helped. Everyone I speak with has the same problems with Holley carbs.

    Why are folks having problems with Holleys? I used to buy 3310 Holleys for just about every street car I had, and they worked fine right out of the box!!

    I have a car now that my 16 year old can’t drive to school!

    • Matty P.

      Dan,
      I suffered with the same issue on 2 seperate cars. The first one had a flat cam lobe. The seconomy had two problems causing a pop through the car, going from idle to WOT. I didn’t have enough initial timing as well as my accelerator pump linkage was out of adjustment. I know your question is older, and I wasn’t sure if you were still having your issue.