Hey guys,

Okay, I'm building(having boyfriend built, actually) a big block for my 72 chevelle. One of his friends gives me a 850 Holley double pumper(I think it's a 4781 or something thereabouts), 4 corner idle, mechanical choke, dual float bowl feeds, etc. However, it's been sitting awhile and the front pumper is totally rotted, but I picked up a bigger pump/shot that is the same size as the rear pump. I think it's a 30cc pump, does that sound right? I hear about people putting 50cc pumps on their Holleys, I don't think I need that much, but I'm trying to use the parts that I have in front of me. The questions: can I use this accelerator pump and if I do, don't I have to put in a different cam for it and is this a hard operation? Or is it 10x easier to just purchase the regular pump, spring, gasket for the smaller front pump?

Thanx,

Lindsey

Lindsey,
If the carb has been sitting for a while, you should buy a rebuild kit for it anyway and take it apart, clean everything, and reassemble it with new gaskets etc. All the parts, including the pump diaphram will be in the kit.

Thanx Bill........believe it or not, most of it was cleaned out already and re-assembled with the exception of the accelerator pumps. The carb is ready to go, except for the pumps. It's one of those things where they guy was going to use it, then forgot about the last few things which then rotted in the heat of his garage(or something). The carb doesn't look like it's been in spiderwebs in the corner of a basement. I know Holley sells the complete kit for $75 or so. The thing I was curious about, mainly, was if that rear pump can be used in the front as well, and if so does it need a different cam.

Thank you again,

Lindsey

You can use the 50CC pumps with smaller pump cams and tune it down with them to have a smaller shot. The cams have a pretty big affect on the pumpshot amount and duration.

Lindsey, you want to to to this lisitng,
http://www.holley.com/HiOctn/ProdLine/CarbList.pdf

whoops, my bad, they don;t show pump cam colors there. If you don;t have a pump cam, find a white one, they;re pretty generic, or go to a speed shop and getan assortment of them. The assortment pack may come in handly later

find the carb, and then see which color pump cam cam on both side. Also, I do HIGHLY recommend you take the carb apart, only if as much as the float bowls, blow the holes out ( use a small can of air), check jet sizes, AND power pistons. It should have 80 jets in front, 80 or 78 in back.

Have your boyfriend give the power pistons the old "suck" test to see if they hold vacuum! :D

At least blow through all the holes on the carb. Use an air can chaser after a carb cleaner shot.

Seriously, you want to know all the jet sizes and make sure all the various holes and bleeds are clean as can be before you bolt this thing on.

850 is pretty big. It'll suck on gas. Sounds like a job for a big ol' 850 cfm Qjet.

Holley lists the volume per 10 strokes of a 5cc pump, .015" clearance at WOT:
Position 1:
White - 17cc
Blue - 18cc
Red - 18.5cc
Orange - 19cc
Black - 19cc
Green - 24cc
Pink - 30cc
Brown - 36cc

Position 2

Black - 18cc
White - 19.5cc
Red - 20cc
Blue - 20cc
Orange - 24.5cc
Green - 30cc
Pink - 37.5cc

Holley rates its accelerator pumps by volume per 10 strokes, so a '30cc' pump only pumps 3cc's per stroke, and a '50cc' pump delivers up to 5cc.

Originally posted by 427L88:

Have your boyfriend give the power pistons the old "suck" test to see if they hold vacuum! :D

Lol, he'll like that. I know that you wanted to say something else. :D :D :D :D

I can take a joke!

Thanks for all the help, I'll take it apart and BLOW everything out. The thing actually looks like it's never been used, with the exception of the pumps and some gumming up around the air bleeds.

L

Look, it's wierd. The day my friend and I bonzied the engine swap from a 396 to a 427 ( four hours), I literally sat the 750 carb up on the porch rail. It wasn't windy, but we were moving so fast, who knew. Anyway, we get it all connected up, give it gas and , perfeoct fires right up, idles fine, ready to drive 1.5 hours home. Well, as soon as I got into the throttle a bit it would just lay down. Checked for arcing wires, pulled all the plugs, checked float level.

Long story short, I never saw ANYTHING in that carb, but it performed flawlessly after I removed it, and cleaned everything up. Just a spec of something somewhere you can't see, is all it takes to screw up the metering. Plus, you might find some weird jetting going on in there. Better to find out now while its easy to change.

( BTW, it will usually be a bit better running carb if you drop front jet size a bit, and put a higher rated power valve in. It should make it feel a bit crisper.

And its true, you can test a power valve by sucking it shut.

Keep my e-mail in mind if you want to dump that 4781 Holley - it's a good "old style" double pumper - mine is ready for the parts bin.

Q-jet for driving and reasonble economy.

IMO

MAT