Holley Double Pumper question
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
Holley Double Pumper question
I'm having a little bit of a problem setting the idle on this puppy. Its sitting atop a edelbrock EPS performer and the car is side-piped. Its a 76 L-48. When we fire it up, the idle does not rest at a constant RPM, instead it revs in a cycle. We've of course messed with the idle screws and the float, and have even played with the timing some.
I know this is a popular carb to use so I would expect many people to have it. Would there be anything else that would settle the idle that we just aren't thinking of? 650 cubes.
I know this is a popular carb to use so I would expect many people to have it. Would there be anything else that would settle the idle that we just aren't thinking of? 650 cubes.
Thread Starter
Melting Slicks
Joined: May 2006
Posts: 3,438
Likes: 41
From: Raleigh NC
St. Jude Donor '13-'14
Ok, what is the vacuum tube coming from the master cylinder? The tubing on there now is a cheap piece of tubing that actually gets sucked flat when the motor is running.
Burning Brakes
Joined: Nov 2000
Posts: 948
Likes: 0
From: Johnston Iowa
Is this a stock motor? Do you know how much vacuum the motor pulls at idle? This should not be an issue but I would check the vacuum and make for sure its not pulling open you power valve. What do your plugs look like? I would still suspect a vacuum leak most likely.
Le Mans Master
Joined: Oct 2000
Posts: 8,897
Likes: 1,353
From: Santa Ana CA
I suspect that the secondaries are not seating correctly when the throttle is released. (This actually used to happen to me with my dp.) Check the setting on the secondary side for proper slot exposure and be sure that there is no binding of the linkage.
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Team Owner
Joined: Jun 2004
Posts: 20,216
Likes: 4
From: Smile, it's just the internet.
Sounds like a lean condition to me.
Adjust primary throttle idle stop so that it allows about .020" of the transfer slot to shot (see this from the under side). Close secondary slots. Put carb back on car.
Set your timing to factory specs for the engine. This affects vacuum, which affects signal to the carb, which affects A/F ratio.
Something in the range of 30-36 degree BTDC after all the mechanical advance is in (depends on dist. when this happens. Just run the engine up and watch the timing mark. When it stops advancing, you're at max mech. advance. Do this with the vacuum advance disconnected and plugged.)
Once the timing is set, put a vacuum gauge on the manifold and adjust each of the idle mixture screws for highest, steadiest vacuum.
Adjust idle speed at SECONDARY set screw, so as not to disrupt the relationship to the transfer slot at the primary bores.
Recheck all idle mixture screws using vacuum gauge.
Readjust idle speed, if necessary.
Adjust primary throttle idle stop so that it allows about .020" of the transfer slot to shot (see this from the under side). Close secondary slots. Put carb back on car.
Set your timing to factory specs for the engine. This affects vacuum, which affects signal to the carb, which affects A/F ratio.
Something in the range of 30-36 degree BTDC after all the mechanical advance is in (depends on dist. when this happens. Just run the engine up and watch the timing mark. When it stops advancing, you're at max mech. advance. Do this with the vacuum advance disconnected and plugged.)
Once the timing is set, put a vacuum gauge on the manifold and adjust each of the idle mixture screws for highest, steadiest vacuum.
Adjust idle speed at SECONDARY set screw, so as not to disrupt the relationship to the transfer slot at the primary bores.
Recheck all idle mixture screws using vacuum gauge.
Readjust idle speed, if necessary.
Last edited by CGGorman; May 24, 2006 at 09:47 PM.
Le Mans Master
Joined: Oct 2000
Posts: 8,897
Likes: 1,353
From: Santa Ana CA
Originally Posted by CGGorman
Sounds like a lean condition to me.
Once the timing is set, put a vacuum gauge on the manifold and adjust each of the four idle mixture screws for highest, steadiest vacuum.
Adjust idle speed at SECONDARY set screw.
Recheck all four idle screw using vacuum gauge.
Readjust idle if necessary.
Once the timing is set, put a vacuum gauge on the manifold and adjust each of the four idle mixture screws for highest, steadiest vacuum.
Adjust idle speed at SECONDARY set screw.
Recheck all four idle screw using vacuum gauge.
Readjust idle if necessary.
Team Owner
Joined: Jun 2004
Posts: 20,216
Likes: 4
From: Smile, it's just the internet.
He said it's a Holley Double Pumper. To the best of my knowledge, all 4150,4150HP,4160,and 4500 series Holleys have secondary idle speed screws. All but the 4160 have four corner idle mixture screws, AFAIK.
I believe that covers all the common models someone might call a double pumper
If not, I apologize. I went back and clarified my instructions a bit.
I believe that covers all the common models someone might call a double pumper
If not, I apologize. I went back and clarified my instructions a bit.
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by My White 79
Best way to find vacuum leaks is with a can of carb cleaner. Spray anything coming off the manifold one at a time, if your idle picks up and smooths out for a second or two, you've found a leak.
I would not recommend carb cleaner.
You can use a propane torch.... DO NOT LIGHT IT, simply open the valve then pass it around any area that may cause a vacuum leak....If the RPM increases you have found a vacuum leak
Intermediate
Joined: Jun 2004
Posts: 34
Likes: 0
From: Raleigh NC
The carb is a holley 4150. The problem now is that we have found the vacuum leak but cant get the idle down. The idle screws are in all the way and it is still idling too high. Is the carb just getting too much gas?
Safety Car
Joined: May 2005
Posts: 4,176
Likes: 7
From: Jennings LA
:
also check the primary side the throttle plates have to be balanced. a good rule is the slot exposed by the throttle plates should be square. there is a stop screw to adjust this. Lars has some good info on this 
Originally Posted by toddalin
I suspect that the secondaries are not seating correctly when the throttle is released. (This actually used to happen to me with my dp.) Check the setting on the secondary side for proper slot exposure and be sure that there is no binding of the linkage.
Team Owner
Joined: Jun 2004
Posts: 20,216
Likes: 4
From: Smile, it's just the internet.
Originally Posted by ajrothm
If you have all 4 idle air bleeds screwed all the way in and its still running, then you still have a vacuum leak.
and/or you may have one or the other throttle plates too far open, because it has to be pulling the fuel from the transfer slot if the mixture screws are closed.
