Holley 4779 750 cfm Double Pumper

-

Plymouth 65

I'll be in the shop
Joined
Jun 27, 2016
Messages
2,607
Reaction score
2,896
Location
Mich
My Holley 4779 DP carborater 50cc accelerator pump was leaking so I replaced the pump diaphragm. That did stop the leak until recently, started leaking again. Should the cover also be replaced to address the leak? It appears to be coming from between the carborater body and pump cover. Thx, 65'
 
My Holley 4779 DP carborater 50cc accelerator pump was leaking so I replaced the pump diaphragm. That did stop the leak until recently, started leaking again. Should the cover also be replaced to address the leak? It appears to be coming from between the carborater body and pump cover. Thx, 65'
No, usually it is just the pump diaphragm, or the cover is cracked or warped.....
 
Yep, try lapping the pump diaphragm cover on a flat surface with some light sandpaper. Do the same on the bowl side. Unless something's cracked, that should get it to seal. Those pieces are notorious for warping and leaking.
 
Every one that I’ve had leak was at the rivet-spring cup. I’d replace it again.
 
The 50cc accelerator pump arm may be adjusted too tight and it may be bottoming out the diaphragm on the spring causing it to coil bind. This can create a pressure peak from the fuel pressure pushing against the diaphragm when the throttle is slammed open rather than being absorbed by the spring because it is in bind. Because the fuel doesn't compress, it creates these peaks and will rupture the diaphragm. You need to make sure the accelerator pump linkage moves the arm to the diaphragm as soon as the throttle is moved, but that it is just at the point where it just takes up any clearance and nothing else. Loosen the spring-loaded bolt and nut until you have visible clearance and then adjust it till the clearance is eliminated. Do not preload the lever or it can cause the above problem.
 
Mopowers covered my answer.
But what I do when I redo a holley: I have a pane of glass (flat!) that has a half sheet of 80 grit, and half sheet of 320 racer taped to it.
I use it to surface anything that needs to be flat, including accelerator pump housings, and the mating surface on the bowls.
 
The Holleys make better boat anchors because they are made of heavy junk metal, Zinc....same reason they strip threads...
 
The Mopar guys love the Thermoquads, GM guys love their Rochesters, and everyone hates the Autolite carbs. I love all cars, I have GM cars, Ford cars, and Mopars. I like Mopars more than the others, but they were all great. I am the same way with carbs, no brand alliance. The Thermoquad is a mess to deal with to set it up correctly, everything is by bending and then measuring stuff, bend this lever to get so much clearance, bend that lever to get more pump action, bend 4 different levers at the same time to get your choke working, I mean what a mess especially when they have been through previous owners that have messed everything up. The Rochester is a total nightmare on a modified car, you get your big block Olds, Buick or Pontiac, and put a big cam in it, raise the compression, add some headers, an intake and your car will not idle, and to fix it is "simple", just get a bunch of drill bits and start drilling into a ton of passages, there are no real set drill sizes you need to go to (there are some guidelines written by "experts") but they all vary quite a bit in what they recommend, so good luck. A Holley has always worked for me, I drop it on an engine adjust the idle mixture screws and the idle and the car runs. It may need some tunning of the accelerator pump circuit to make it right, but that is really simple. You can get jets, gaskets, needle and seats at your local Auto Zone. Holleys always seem to run rich in the midrange and that is difficult to tune out, but they always idle and work at WOT. Them running rich is what kind of saves the day. The other factory carbs, on modified engines tend to run lean, and that is way more difficult to deal with, especially when you cannot get the engine to idle because it is so damn lean, and finding jets and metering rods is not easy.
 
HSDemon.
News flash for you. Every carb, Holleys, QJs, Carters, Edel etc all have plenty of fuel from the idle cct for a rich idle.
The idle cct jetting is not sized for idle, it is sized for transition; transition to the main cct.
Transition requires MORE fuel because more air is entering the engine. The idle & transition cct are all part of the idle cct & connected by internal passages. The problem then becomes how to reduce the fuel to the idle port because it needs less fuel. That is what the idle mixture screws do, regulate the fuel+air for engine idle. There is always enough idle fuel provided the throttle blades are correctly positioned in the bore, ie about 0.020-0.040" of T slot showing below the blades at idle.
 
HSDemon.
News flash for you. Every carb, Holleys, QJs, Carters, Edel etc all have plenty of fuel from the idle cct for a rich idle.
The idle cct jetting is not sized for idle, it is sized for transition; transition to the main cct.
Transition requires MORE fuel because more air is entering the engine. The idle & transition cct are all part of the idle cct & connected by internal passages. The problem then becomes how to reduce the fuel to the idle port because it needs less fuel. That is what the idle mixture screws do, regulate the fuel+air for engine idle. There is always enough idle fuel provided the throttle blades are correctly positioned in the bore, ie about 0.020-0.040" of T slot showing below the blades at idle.
I have no idea why people that have no clue what they are saying state such easily debunked dribble. Here is a photo from the book written by Cliff Ruggles on how to modify Rochester Carburetors for high performance use. Book is called

How to Rebuild & Modify Rochester Quadrajet Carburetors (S-a Design)​

Cliff literally wrote the book, and he is considered one of the top guys that has ever worked with a Quadrajet. He, not me out of my personal experience, clearly states that the Rochester Carbs have limited idle fuel capabilities and to correct them you need to enlarge certain passages in the in the carburetor (ie drill them out).

Here is a link from his website on dealing with lean conditions of Rochester Qjets, and if you look you will find a ton of threads of people having the same issue:

Lean Idle solutions?

Rochester idle.jpg
 
I installed the new 50cc accelerator pump cover, diaphragm and spring today. Adjusted the arm, fired it up and it's not leaking. I'm definitely more familiar with Carter/Edelbrock carbs but this engine came with the Holley. Car runs great, and I'm learning something new. 65'
 
Demon,
I have the same book. You missed the word 'system'. Idle system. Because the idle jet is part of a system that controls idle AND off idle.
Some emission carbs had limiter caps on the idle mixture screws to stop tampering & rtetain the lean idle for emissions. Do you think the caps were added so that folks could lean the carbs even further? Of course not. Limiter caps were used to stop folks richening the idle because there is enough idle fuel at idle to richen the idle.
If you read Ruggles book, his 'recipes' of engines with different cams etc, at the same time that he talks about enlarging the idle jet, he also enlarges the Idle Down Channel. The IDC [ T-slot restriction on Holleys, Economiser on Carter/Edel ] supplies the transition slot for off idle fuel. It is for the IDC that the idle jet is enlarged, NOT for more idle fuel.
 
This link is from the late John Passini. He was English, hence the use of the word 'weakness' [ he means lean . He has forgotten more about carburetion than Ruggles [ & many other so-called experts ] ever did, or ever will.

img406.jpg
 
-
Back
Top