Hesitation Blues - or - Holley Accelerator Pump Cams... which one?

Not sure about this one gang...

From an idle, when I mash the loud pedal to the carpet, what is an acceptable response?

1) The exact nano-second I stab it the back tires break loose and the car lurches forward.

2) A slight (very slight, mind you) hesitation, then the beast roars and the back tires break loose.

3) The neighbors call the cops from the holeshots I'm doing in front of the house while swapping accelerator pump cams and trying them all out.

So, what is the concept for the profile of the accelerator pump shot here y'all? It's my 462. Should there be a huge instant shot, then it fades? Should it be a gradual shot, working up to a bigger shot at the end? What does the 455 like?

Thanks!

 

You have to remember that a double-pumper is more-or-less made for a high-stalled race engine and not a Riv. That being said..

It sounds to me like you need a longer shot. If it sags after mashing it to the floor either the pump volume is too small for the needed length of pump shot, or *maybe* you can get away with smaller nozzles and that would make the pump's current volume last longer. By the way, I ain't an expert with Holleys, but might be something to try until an expert responds.

Now if I could fix that off-idle stumble on mine.....

 

Thanks Adam.

I have 35 nozzles in it now. Going smaller to 31s makes it a little worse, and going to 25s make it REALLY worse.

That isn't to say that with smaller nozzels and a change in cam it couldn't be corrected... but I'd rather not have to make 50 changes to get it dialed in. I'm hoping someone can give me an idea on what cam and squirters they use.

 

I know what your going through

Hey Jeff, I know what your going through. I havea 37 in the front and a 40 on the rear. I use the pink cam I fouind that it's a little bit long for me. I still get a stumble but at 1200 rpm it's great. Mechnicals are just dumping gas. Not to great for the Rivs. Good Luck Chris

 

Hey Jeff,

Your car's weight and gears (assuming there tall) are going to play into this ALOT. If your have very slight hesition with the setup now, I'd call it good and leave it alone.

It's been my experiance in the past (messing with a few motors (even chevys ou: ) that every car is going to need pretty much a custom setup. There doesn't seem to be one fine tuned setup that works for several motors. Weight, Stall, cam profile, headflow, all change how much or how little squirt is required from the DP.

I'd say of your close with the 35's and tired of messing with it, leave it be for a bit. If you get ansty again, move to the next larger squirter Since you already know smaller is worse).

My personal setup with the SC is a 40 in front and a 35 in the rear.............

Not sure why it liked the larger one in the front, but it does.

 

Thanks guys!

I should have mentioned... it is a single pumper, vacuum secondary 4150 carb off a 1965 427 Corvette. Jet Performance has given it a Stage2 build. So I have jets in the rear, but no pump and no power valve.

I'm pretty happy with how it takes off, but was just wondering what "normal" would be.

Thanks again!

 

Hi Guys,

Yardley....No one has mentioned this but I would bet you probably have a 30cc accelerator pump in the front bowl? If so...going to a 50cc would probably be to-the-good for your application. It certainly worked wonders in the past for my old 850DP in that regard several years ago.

Just a though....

 

Kerry,
What do you recommend to look at for an off-idle stumble/hesitation?

Yardley,
Knowing it's vacuum secondary now:

If its falling on its face AFTER it takes off when you put it to the wood, I'd guess you have too light a spring in the secondary diaphragm. My thinking is that if it can handle a quick stomp the shot is fine, but the secondary opening is what would make it fall over after the initial gust of air. A good test would be to fatten the secondary jets up a few sizes and see if it goes away. I know that it will be too rich overall, but it will give you an idea if your looking in the right place. replace the right jets and concentrate on secondary diaphragm springs.

Also, don't some of the customized Holleys have a screw that adjusts the secondary opening rate by restricting the passage to the secondary diaphragm?

 

Don't know, but they probably do. I know mine doesn't.

Kerry,
What's involved with changing the accelerator pump?

Thanks!

 

Jeff, you do need the 50cc pump. The 30cc is too small and barely adequite for a near stock application. The carb has to come off and you just change out the pump assembly. Also make sure the power valve is the proper size.

 

Adam....truthfully I've found in the past that an off-idle stumble or hesitation can usually be (9 times out of 10 maybe) an ignition problem....especially if your car is still sporting a breaker points distributor! Also make sure you are getting total advance in the distributor. This is more of a problem with an original distributor that can bind-up or seize altogether. Be sure and check the limiter bushing too as those deteriorate over time as they are just nylon that easily gets brittle from heat.

After that look at the above mentioned carb potential problems that can exist such as:

a weak pump shot....

low volume pump shot (again, a 30cc accelerator pump...go to 50cc)...

squirter being to big and "dumping" thus cutting down on "duration" of pump shot that may be needed.....

squirter being to small and limiting volume needed when called upon.....

then there's the pump cam selection...

Make sure all the air bleeds and such are clean and debris free.

Probably nothing here that you weren't aware of already but maybe it can helps other's who are not.o No:

Hope this helps in some way...

 

Holley ignition (better not be weak!), Timing locked at 37*, BG Silver Claw, which I think has the big pumps already and Dominator style bowls.

Guess it's most likely pump cam or squirter time. Geez the ones in it look to be pouring it in pretty good though. o No:

Thanks for the advice Kerry.

 

Thanks guys. The 50cc shot and probably the hollow squirter screw will be making their way onto the Riv soon!

 

For what it's worth, I had severe hesitation just off idle. I changed to 50cc accelerator pumps ( front & back ) and changed the squirters. ( 35 up front and 37 in back, with the hollow screws. ) The package comes with two pump cams. I used the dark brown one. It has a very fast ramp. My off idle acceleration problems are all gone. With a street car weighing in at 4000+ lbs. and a 1.65 sixty foot, it was all worth it and more. Good luck Jeff!! :TU:
John Biggs

 

Thanks John! I should pick up the stuff tomorrow.