Holley Support is no Help!

[AM.MSCL]

I have a Holley 750 Double Pumper with Vac 2ndaries on my 400 Pontiac in my 69 Firebird. I have a 4 speed manual trans.
The car starts and idles at 900 rpms just fine when cold.
I get on the highway and drive a few miles at 70 mph which is 3200 rpms for the engine; now the car idles at 1200 rpms. I can park the car with it running and push the throttle lever in the direction that will reduce the rpms back to 900. Prior to pushing the throttle lever I make sure that there is no throttle cable hanging to cause the issue. Goose the throttle and it goes right back to 1200 rpms.
Next part of this craziness if I do not get on the high and just drive around the local streets and never get above 50 mph which is about 2500 rpms the idle will go back to 900.
Holley keeps telling me I have a vacuum leak which I have verified that I do not. I have taken Berryman's Chemtool and sprayed lightly around the intake and carb and no rpm change what so ever.
I honestly think something in the Carb is sticking after it has been around 3000 rpms for a while but can not get any help from Holley to check what.
Does anyone have any clues what is going on?
thanks
Ray

[exwestracer]

I think the key is heat. You said it acts fine when cold. right? Also, if you putter around and don't hammer it it's ok?

I gotta think that as the aluminum base plate warms up and expands you are getting an interference issue with the throttle blades. You may not feel it, but it doesn't take much to raise the rpm like that. If you can get it back to 900, it's not a vacuum leak, UNLESS you are way off on the tune and have the idle set screw cranked in far enough to uncover the port vacuum slits in the throats.

Try running a thick gasket, or better yet, and plastic spacer under the carb if you have hood clearance. If you want to check the carb, pull it off and look very carefully at the edges of the bores where the throttle shafts go through. If you see any evidence of the blades rubbing the bores, it's getting stuck.

Hope this helps.

[cp87]

what if you were to run an extremely heavy throttle springs? just curious if the problem is still there with a heavy return spring. What kind of choke set up are you running?

[AM.MSCL]

I have a dual spring return throttle spring setup. I have an electric choke.

[Vegas69]

Getting a repeatable idle with a holley is tough. Mine will range from 1000-1200 hot. You need to make sure your rear throttle plates are contributing to your idle speed. You can start by taking both front and rear plates to square one and then turning the adjustment screws equally until you get to your target idle. The problem with only adjusting the fronts is you can easily get into the transition slots and it pulls fuel through them. When this happens, the idle doesn't want to go back down to the idle circuits only. Initial timing will also help your engine idle more consistently. Additional initial will pull more vaccum. That in turn means less throttle blad angle. What distibutor and initial timing are you running? I run 26 in mine with a total of 35. Custom bushings are available.

[Northeast Rod Run]

There is always the possibility of an internal vacuum leak too.

[ProTouring442]

Originally Posted by AM.MSCL

I have a Holley 750 Double Pumper with Vac 2ndaries on my 400 Pontiac in my 69 Firebird. I have a 4 speed manual trans.
The car starts and idles at 900 rpms just fine when cold.
I get on the highway and drive a few miles at 70 mph which is 3200 rpms for the engine; now the car idles at 1200 rpms. I can park the car with it running and push the throttle lever in the direction that will reduce the rpms back to 900. Prior to pushing the throttle lever I make sure that there is no throttle cable hanging to cause the issue. Goose the throttle and it goes right back to 1200 rpms.
Next part of this craziness if I do not get on the high and just drive around the local streets and never get above 50 mph which is about 2500 rpms the idle will go back to 900.
Holley keeps telling me I have a vacuum leak which I have verified that I do not. I have taken Berryman's Chemtool and sprayed lightly around the intake and carb and no rpm change what so ever.
I honestly think something in the Carb is sticking after it has been around 3000 rpms for a while but can not get any help from Holley to check what.
Does anyone have any clues what is going on?
thanks
Ray

Obviously I am not there, but my bet is that it's running lean when hot. Now the $64,000 question is, why? How far are the primaries open to set idle? If it's got a pretty good cam in it, then, as has already been suggested, you are probably into the transition function. If I recall correctly, the solution for this is to drill a small hole in (I think?) each primary throttle blade. This allows air to enter the engine without affecting the transition circuits.

Shiny Side Up!
Bill

[GregWeld]

Not sure why a "vacuum leak" would be intermittent.... regardless of how you ran the engine -- seems to me you'd either have a leak or you wouldn't.

If you can pull the throttle blades back to get to your target idle.... regardless of how you drove it... then to me it's saying you're hanging up on the throttle blades AND OR -- have you checked your float settings etc? Could you be a little on the high side and once you have some heat in the engine/carb you're pushing fuel in. BUT if you can, as stated above, always return to the idle you want by moving ("pushing on") your throttle linkage then it's a linkage/throttle blade issue not a fuel or vacuum issue. Since you have a dual spring return it should certainly be closing the linkage. So take a look at the STOP when this happens before you touch anything and see if there's a gap between the stop and your linkage. If there's no gap -- then you have a fueling issue that you'll need to solve. A combination of relatively high idle speed (900) where you may just be on the cusp of uncovering the transition slots and just a barely sticking issue could be you're problem. But I think you would feel a sticking blade, especially if you can reset with your hand.

I would NOT recommend holes in the primaries. That is an issue/fix for wild ass cams and high idle speeds which is a cover up solution when you really MIGHT (since we don't know what carb you have) need a different carb with changeable air bleeds etc. Not being rude to the poster that mentioned this - but holes in the primaries is an old skool backyard remedy. Yeah it can work - but you're killing the carb for any other use on a different combo it. Yeah it can be repaired - but it is still just a cover for a proper fix IMHO.

Your float check should be done when the engine is HOT.... because todays fuels expand more than old stuff did.

Also -- Holley had a serious problem with some bad metal for awhile. The new gas formulas where eating the metering blocks etc. so have you looked at that? Have you had the bowls off the carb for inspection?

[AM.MSCL]

On suggestions of a few different people on different boards I did some debugging last night.
I reset the Electric choke back to the manufacture default settings
Got the car out and reproduced the problem
With the car running:
I disconnected the throttle cable = no change
manually ran the rpms up to 3000 by hand with the throttle lever and let it re-establish idle = no change
Backed the Fast Idle screw until it no longer was touching to be causing it to hold the idle up = no change
manually ran the rpms up to 3000 by hand with the throttle lever and let it re-establish idle = no change
Took a long screw driver and tried to close the secondary butter-files; they would not close any more = no change
Added a 3rd return spring to the throttle lever but it is connected to another spot from the original 2 springs to make sure there was no interference with them = no change
Again took some B-12 Chemtool and spot shot different areas all around the carb base and intake = no increase or decrease surge
one spot shot into the primary of the carb and a did get a rpm increase for a split second
Looked around the choke area of the carb to try and determine if something was hanging or sticking but did not see anything obvious.
That was all the time I had last night; hopefully this weekend I will get to check the choke area more.