Skip to main content

My car has a stumble when accelerating off of idle.  It is most pronounced when the engine is still warming up and mostly goes away when the engine is hot.   It has a Holley 750 vacuum secondary carb.  The 351C engine has an Edelbrock Torker intake manifold, a hydraulic roller cam, 1970 closed chamber heads, and dished pistons.  I read an article about this at Holley's website and they sort of suggest a larger nozzle for the accelerator pump.  Your suggestions to remedy this issue are appreciated.


Images (1)
  • mceclip0
Last edited by stevebuchanan
Original Post

Replies sorted oldest to newest

If you are getting an occasional backfire, it is probably a lean condition.

It’s pretty easy to change the squirter nozzle (or remove it, and carefully drill the orifices slightly larger with a hand (pin) drill).

Before you do that, you want to ensure that there is no slop / delay in your accelerator pump linkage.  As soon as you give it the gas, you want that pump shot to begin.

There are also more “aggressive” accelerator pump cams (and positioning on the linkage) you can play with.

If it didn’t stumble before, and recently started, I would bet on your accelerator pump adjustment.

Easiest to evaluate/check that is with the carb off the manifold.

Good Luck


Last edited by rocky

Steve - when I installed my QuickFuel 600 last year, out of the box it had a stumble off idle.  I followed all the QF (owned by Holley) videos and everything I could read on tuning it.   Two things solve my issue:  1 - adjust the accelerator pump linkage to zero or a couple thousands of interference.  Do NOT preload the pump by adjusting the linkage too far.  2 - buy a set of new pump cams (get a set of Holley cams that come in all the colors) and look and the graphs online of how each cam changes the shot and duration.  A guy on the GT40s forum gave me this advice and I only had to step up one cam each on my primary and secondary (I have a double pumper car).   Once I did that and dialed in the idle screw settings to maximize the amount of vacuum I was pulling, the car is like butter.   Starts right up and at cold, has complete throttle response.   For street driving, you should not have to drill anything out.   I started going down the path of getting bigger nozzles and was warned to stay away from that.   Hope this helps.

Hey Steve,  where are you located?  Do you live in California?  Our crappy gas has so much alcohol and other corrosive agents in it that it will dissolve the accelerator pump's rubber diaphragm in short order.  I've had to replace mine twice within about 6-7 years.  If you replace your accelerator pump, don't buy the standard black rubber pump that all the auto parts stores stock.  Get one that's suitable for the gas - I think the green or blue versions are more resistant to our crappy gasohol.

Fair disclosure, I'm far from a carb expert.  But I spent hours tuning the formerly Holley became Summit carb (750 cfm, I had the vacuum secondary version) for my 80 F-250 Windsor swap with some cam.  I had the same issue.  I did also upsize jets, mostly secondaries, which in my case also seemed to help some though logic eventually kicked in and I knew I was already plenty big there :^).   But the most dramatic improvement in my case came from lightening the accelerator pump spring (I think I ended up going with the lightest from the kit of those I had bought, it was a skinny white one as I recall).  Still wasn't perfect, but MUCH improved and "quickly."  Obviously your mileage may vary, as they say.

I had a stumble when I had a Edelbrock 625 cfm with vacuum secondaries. I installed a Holly demon 625 cfm that has an adjustment to keep the vacuum secondaries from stumbling when accelerating after coasting and it ram great out of the box. I only had to adjust the air mixture screws some. I think the Edelbrock AVS does the same and is easier the access the air mixture screws as they are angled up more than the Holly demon ones.

Here is what Holley says:  "Adjust the spring-loaded bolt and nut until the clearance is eliminated. This should require only a slight adjustment to minimize the clearance. Do not preload the lever. Excess clearance at idle is a very common occurrence that takes a minute to repair and when adjusted drastically improves drivability."

I would close the gap so that when the throttle is just opened, the pump arm begins to move but not preload.  You are very close.

Once checked, I would put a vacuum gauge on the car, warm it up, and reset the idle screws to maximize the vacuum reading.  On mine, I got 17 inches of vacuum.  

What color cam is in the carb?  I would get a set of pump cams and if you still have a stumble, just slip in the next larger one and see how that changes it.   They are a 2 second change.

I took out a small amount of slack in the accelerator pump linkage and the problem immediately went away.  I have the accelerator pump cam assortment and might try a little more fine tuning with those.

Thank you all for your help.

That's great news Steve!  I recall when I adjusted mine and cured the same issue, I was amazed at what a difference a very tiny adjustment made to the car's overall drivability.  Congrats!

Add Reply

Link copied to your clipboard.