I recently traded a friend, my not so loved Edelbrock vacuum secondary, for his Holley 650 double pumper. Boy was that a mistake! I've even taken the carb to a very well respected old timer who builds race engines and knows Holleys inside and out. He played with it, but it still has a HORRIBLE stumble just off idle.

Here is the setup:
ZZ3 crate engine based off casting numbers.
700R4
3.73s

Idles great, RUNS great, but can't get the in between right. They ship new with 28's. I jumped to 35's and the car gets so bad it just stalls or completely dies. Currently it's 25 primary 28 secondary.

We have gone up and down several squirter sizes with no resolution (got worse with larger sizes). HEI base timing had no effect including spring weights, so it isn't ignition related. Which I already knew, as the problem follows the carb and goes away with a vacuum secondary carb (borrowed).

The more I research, the more I read people warning against double pumpers with auto trans.

Any suggestions BESIDES changing the carb? This carb is brand new and I've sunk money and too much time in it to give up. Im waaaaaayyyy too stubborn for that.

Thanks

I went the dbl pumper route many years ago with a 4 speed and hated it. Went with a Holley Vacuum secondary and life has been good ever since...

A Holley mechanical secondary carb seldom works on an automatic transmission. Get a Holley vacuum secondary carb.

It stumbles even in park/neutral though, so how would it behave any different on a manual trans car?

Well, I hate to step in when a race tuner cant get it right, but I'll try.

First, ensure you can just see the transfer slots peeking out from the secondary butterflys. The transfer slots being almost exposed helps quicken the fuel flow and aids in the transition from primary to secondary.
Second: set the fuel level properly with the carb HOT. Hopefully you have one or both bowls too high.

Third . If it still stumbles, put a 095 or 105 power valve in the primary metering block. I assume the ZZ has plenty of idle vacuum?

Fourth, you may need to play with pump cams to control the timing of the acccelerator pump shot.



EDIT: BUT FIRST.... assume your "old timer did this" but I had a Holley "crap out" by sitting on a fence rail while we swapped engines. Apparently, some airborne debris had found its way into one of the air bleeds, so when you were at part throttle, it ran like crap. Carb cleaner down those air bleeds fixed it. ( after I chased my tail in circles for a bit trying to figure it out. ) One reason I "saran wrap" any carb that will be sitting for a while anymore.

Wow...all these great running auto cars with dp’s must all be flukes

Gimme me a break

I will second exactly what 427L88 stated.
Power valves and air bleeds.

New carb means nothing.
So much stuff out there today that missed the quality control step ..you could have shavings or millings in the passages.

It is NOT a double pumper thing.
I have a double pumper on my 350 and it has been on 4 other vehicles and has trumped all carbs it was tested against and all of them had automatics.

Mine has an 095 power valve and it is a 750 DP worked perfectly on a car with 2.41 gears and automatic believe it or not.

Your carb needs taken all apart if the carb cleaner down the bleeds and power valve does not fix it.

Question this of idle stumble is it when you are just driving around normal or when you drive around normal or just when you really step on has. What stall speed converter (rpms wise) do you have in car? Oh I see it stall even in neutral. Check the power valve do you have a vacuum gauge. I think stock the carbs come with a 6.5 power valve. If this is a stock crate motor you might need to put a higher power valve in like say an 8.5 or so as your vacuum is too strong and not allow it to open quick enough. If you have a vaccum gauge that could help. I think the general rule of thumb is whatever you do at idle (vacuum) the power should be 2 or 3 points less than that. So say you idle at 11 pounds of vacuum then an 8.5 power valve is in order. The power valve adds about 8-10 jet sizes when open so its like another double pumper shot in a way. So once you go below the 8.5 psi vacuum in this example more gas comes in the from the power valve. The power valve is typically only in the front part of the carb.

Not sure what to tell you on the carb but I do want to mention that you need to make absolutely sure you have the TV cable working right to the trans. Most Holleys need a special linkage adapter for those transmissions. If it is not moving the cable correctly you can easily burn the trans up

Many on here have tried the Holley Street Demon and love it.

It's basically an improved quadrajet design. The 750 is perfect for a street big block that doesn't have a big cam.

https://www.summitracing.com/parts/dem-1904

BTW, Swisher is much more experienced than I at this, so truth right there. ALSO.... reading forums hardly qualifies as research, too many know nothings that think they do, so DIS-infomation, like you cant use a DP with an auto trans, is much too prevalent. FakeBook is the worst at this, but we do live in the DIS-Information Age , so alter "research" appropriately. Maybe its my age, but the level of bullchips is deafening these days, in all realms. ( I fear we are digressing into a nation of liars, cheaters and thieves)

Buy some Castle "Dragonfire" ( the only combustible carb spray left AFAIK) , and blast those air bleeds FIRST. Then buy a "105" PV and throw it in the primary side. Holley using 065 stock is a "dumb it down" move, as most systems respond well to a bump in power valve opening on the primary side. ALSO, you can switch the font pump cam to the "winter" position to give it more shot earlier. Intuitively, I think the issue in in the cams. If I recall , blue and yellow cams are pretty early shots. I have a spec sheet at home I can post if you''d like, direct from Holley.

The Pump Cams "time" the shot, while the squirters time "duration" of the shot. What is weird is I use 25's on my tripower, as it has 3 squirters, but man , they be small. I suspect, your pump cam is off, meaning the timing of the shot is early or late. Remember the squirters only give it time delay, while the cams "time it".

PS, a vacuum gauge really helps dial in a Holley, esp with setting the power valve.

So this was a known good running Holley 650, or one in need of a rebuild?????

I don't know how much time, money, and patience the OP has....but while the DP can eventually work out OK, the fact the dropping a borrowed vacuum secondary carb on the car and it performs well should speak volumes, IMHO.

Personally, I'd sell the DP and buy a Holley 3310 or a Street Avenger 670.

If the "old guy" who is regarded as good with Holleys can't get it to run, how is the OP going to do so?

How much time will be spent in the process, most of which will likely be futile?

This could be a great learning experience....but it will take lots of time and probably be very frustrating.

Contrast that to buying a new carb.....and selling the old one to help offset the cost.

Geez, as I read the facts, he claims he doesnt want to buy a new carb. He may, in fact, get to know as much about this crude metering mechanism as the old timer does, if he chooses to do it the "hard way".

Doesnt anyone like to tinker anymore?> Its how we used to learn! I say the OP gets all INTIMATE with this casting and makes the SOB work! Then he can go back to the old timer and say, " Hey man, I got it working, with a 095 PV and blue pump cam on the primary side".

Yet, I did just buy a new .223 resizing die when I got a case inextricably stuck in my old one. ... so even I "punt" once in a while.....

My two cents are that he's already tinkered with the DP...and failed.

My additional two cents are that someone who knows what he is doing can't get the DP to work.

The OP has already spent considerable time and effort trying to get the DP to work....and has not been successful. (Pound head on wall here!!!!)

Back in 1990, I spent all summer chasing down what turned out to be a turd Holley carb. Many, many hours and time wasted....when I had neither. I would have been SO much better off buying a new carb.

Like the OP, I had a friend with a working, vacuum secondary carb....and problem solved.

My point is that there simply comes a point when you are time and money ahead going a different route, much as your pride may not agree with it.

Here is holley's answer. QUESTION How do I know if a vacuum or mechanical carburetor is best for me?

ANSWER For street cars the vacuum secondary carburetor works best on midweight or heavyweight cars with an automatic transmission. They are more forgiving than a Double Pumper is because they work by sensing engine load. The mechanical secondary carburetor is best on a lighter car with radical camshaft and a lower gear and manual transmission or on a car that is going to be used for racing purposes.