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I am chasing a high-idle issue with a Holley 4801 (1971 LT-1) and it appears that the primary throttle bodies are "sticking" to the bore at idle. This is preventing them from closing properly and resulting in a high-idle. I just installed a new Lokar throttle cable with double-spring return, but it still won't return to full idle. If I blip the throttle, I can get a little better return sometimes, but it is random. If I manually close the throttle, I can get the carb to return to a proper idle, but it feels like the throttle bodies are binding up against the bore a bit.
I searched around and it seems like worn bushings on the throttle shaft COULD be the cause of the problem, but I wanted to throw this out there to see if there were any other ideas before I start tearing things apart.
Are you sure the throttle plates arent hitting a carburetor to intake gasket that is interfering with full closure?
The accelerator pump arm on the float bowl isnt rubbing against anything beneath it?
Are you sure the throttle plates arent hitting a carburetor to intake gasket that is interfering with full closure?
The accelerator pump arm on the float bowl isnt rubbing against anything beneath it?
Good points, and I've thought about the intake gasket, but I'm not seeing any catch-point at least from looking down through the bore. I also don't see any impediment to the accelerator pump arm.
have you taken the carb off and verified the primary throttle blades are accurately centered and properly adjusted? very common problem on those...there's a few threads on here about that, as well as lars holley tuning paper if you email him...believe they are SUPPOSED to stick when throttle is fully closed--but they are not supposed to ever fully close. must be properly adjusted...
have you taken the carb off and verified the primary throttle blades are accurately centered and properly adjusted? very common problem on those...there's a few threads on here about that, as well as lars holley tuning paper if you email him...believe they are SUPPOSED to stick when throttle is fully closed--but they are not supposed to ever fully close. must be properly adjusted...
Thanks -- I have not done this yet. I wasn't aware that they are supposed to "stick". I have read a number of Lars' tuning papers but didn't see anything on this topic. I will have to research on how to center/adjust these as I wasn't aware there was anywhere you could do this.
you center them from the bottom. the secondary's adjustment, if too far off, will force the primary's to try and close too much to compensate. in a nutshell, adjust the rears first, setting their idle stop screw to 1/2 turn from fully closed, then verify the butterfly's are centered and adjust idle from there. if the butterfly's are off you have to pull the carb and base plate to slightly loosen and re-align. too much secondary opening causes idle issues that get exacerbated by trying to close the primary's to compensate.
I plan to remove the carb this weekend and inspect the secondary stop and expect that I will (should) replace the base gasket while I'm at it. I've searched everywhere, but what is the part number for the base gasket to this carb? The holley catalog has good xref info for lots of other parts, but I cannot determine which base gasket to get.
I just had this problem and it ended up being the accelerator cable hanging up. I had the carb off 3 times before I realized that the cable needed to be adjusted so that it had a little slack in it so that it would return back to idle.
To tell you what gasket you need requires to know which intake you have, 4 equal size holes, one large square hole, two oval holes?
I don't know as I haven't pull the carb yet, but it's a stock Holley 4801 for a 71 LT-1. I was searching around some more this morning and found some threads that suggest that a Holley 108-12 is the correct base gasket for this carb. Can anyone confirm that?
Originally Posted by Crimson Thunder
I just had this problem and it ended up being the accelerator cable hanging up. I had the carb off 3 times before I realized that the cable needed to be adjusted so that it had a little slack in it so that it would return back to idle.
I thought the same and just installed a new Lokar since the old cable was a bit long and it needed to be replaced.
you have no idea what manufacturer your intake manifold is? The 108-12 is a square insulated gasket, not what I would use with a 4 hole intake.
This one is more likely https://www.summitracing.com/parts/hly-108-58
you have no idea what manufacturer your intake manifold is? The 108-12 is a square insulated gasket, not what I would use with a 4 hole intake.
This one is more likely https://www.summitracing.com/parts/hly-108-58
Thanks -- I believe it is a Winters aluminum manifold, but all I really know is that it's stock for the 1971 LT-1. This thread has more details on the exact manifold casting: https://www.corvetteforum.com/forums...t1-1971-a.html
These threads led me to think it should be a 108-12:
Thanks for confirming. I don't think the gasket on there right now is the correct one (not home at the moment), but I wanted to be sure I have the right one ready to go when I pull the carb to take a look at the secondary idle stop screw.
I pulled the carb as soon as I got home this evening and look a closer look at how completely the primary/secondary throttles are closing. Here's a pic showing ZERO daylight through the primaries but a fair amount through the secondaries -- I assume this is a bit too much on the secondaries and a probable cause of a high idle?:
You are correct looks like they are open too much.
On the passenger side of the carburetor, in the base plate is the secondary idle stop screw. The slot for the screw is on the bottom of the throttle body. Loosen the screw so the secondary plates completely close. Then turn the screw so it just touches the linkage and turn 1/2 turn.
You are correct looks like they are open too much.
On the passenger side of the carburetor, in the base plate is the secondary idle stop screw. The slot for the screw is on the bottom of the throttle body. Loosen the screw so the secondary plates completely close. Then turn the screw so it just touches the linkage and turn 1/2 turn.
Awesome - thanks for the 2nd set of eyes. I'll back that secondary idle stop screw down as you've stated and see how it goes. I also determined that the current gasket was NOT the correct one, and it's stuck pretty good to the carb base. So need to get that cleaned-up before I install the thicker 108-12.
Holley throttle sticking at idle
