ok, I'm getting fed up with this rebuilt carb already........

on my '65 L76 I have the correct Holley 2818 carb. Most may remember I sent it out for a complete rebuild earlier in the winter and it came back with a non-working choke which I had to repair myself and leaking (which it never did before the rebuild) which again I had to fix myself.

I've been fighting another constant problem in addition since it's come back and this one has me baffled.
I'll set the idle to the perfect idle speed for my motor - approx 850rpm. it will run and idle great for that day. The next time I take the car out and the car warms up the idle is up to around 1100-1200rpm. I'll reset it back to 850-900rpm. The next time the car is driven the idle is back to 1100-1200rpm again.
I've reset the d*mn thing 7-8 times already and only driven the car less than a dozen times this season! Pretty much every time I go to drive it I need to reset it.
Last time I drove the car it was idling high again so when I got home I put the idle back to 850-900rpm.
I just went out to the garage tonight to check everything on the car over for the cruise I plan on taking in it tomorrow and when i started it and after the choke came off the idle was down to about 550-600rpm which is way too low for an L76 to idle. I figure it's low because I had to reset it down after it was too high the last time I drove it.

With the spring on the idle screw there is plenty of tension so I don't think the idle screw itself is moving so why is this doing this and constantly changing the idle speed on me each time I drive the car.
This is getting to be very annoying even if it does only take a minute to readjust it. I shouldn't have to do this everytime I want to drive the car!

 

you mention the spring tnesion - is it s new spring? if not, consider replacing it

 

Are you setting the fast idle scrw or the mechanical stop idle screw? Is the fast idle cam free to move? Is the choke pull off dropping out correctly? Do you have an intermittent vacuum leak allowing extra air in speeding up the idle?

 

Rick, I have no idea.
It didn't do this anywhere nearly as bad before it was rebuilt - I only had to reset the idle about twice a season before, not everytime I drive it like I do now.
I haven't a clue if the spring was replaced or not when it was rebuilt

 

rgs
i'm adjusting the mechanical stop idle screw on the drivers side of the carb.

yes, the choke is coming off completely - don't even get me started on what I had to do to get the choke working after the carb came back from rebuild but the choke now operates perfectly and it's not hanging up on the choke fast idle cam.

hmmmm, I didn't think about an intermittent vacuum leak. Not even sure how I would try to track that down. It doesn't SEEM to be acting like that would be the problem though as when I reset the idle it does fine for a while but after driving a while it just starts to creep up by itself until it's back up to about 1100-1200 and than it just won't drop back down unless I readjust the idle screw again. If it was an intermittent vacuum leak it would come and go wouldn't it? this comes on after driving a bit and stays high until I adjust it again.

 

The RPM range your mentioning seem to be in "steps", I would check the fast idle cam and make sure it isnt sticking. One step on the cam may be around the 300 rpm jump your experiencing.

My .02¢

Chuck

 

Chuck
Ok, that's two votes for the fast idle cam so I'll check it again, but I'm 99.5% sure the entire choke system is now working perfectly and I don't think that's it but I'll check it again anyway.

A main reason I don't think it's the fast idle cam though is that once I readjust the idle to proper level it will run correct and idle correctly for a while - maybe 25-50 miles?? than I'll see it start to climb back up again. If the choke is alreadycompletely off and she is warmed up and idling correctly than than it starts to bump the idle up by itself it wouyld have already down completely off the fast idle can already anyway so how would it get hung back up on there by itself if the car hadn't been shut down and the choke back on because the motor was cold?

 

I would take the carb off and put it on a flat surface (a mirror or glass) and see if the base is flat. Check the butterflys, see if they hang up when you pull in and out on the shaft. Another thought, could the secondarys be opening up a little sticking. Move them by hand and see if you can get it change idle and not drop back. Just things that I have run into from time to time.
Bob

 

Bob

thanks for the suggestions. I do know for a fact since I had to go over and redo a lot of the stuff myself after it came back from the rebuild that all the surfaces are perfectly flat including the base. I checked all the surfaces with a straight edge. the Butterflys are not sticking, but I will check the secondaries - or at least i'll try to if I can figure out what to look for.
having to deal with certain areas of the carb already myself to fix things I'm getting famaialr with the areas and parts I had to work on but other areas of the card I'm still unfamilar with and the proper workings and operation of them.

I guess if nothing else, as long as things on this carb keep screwing up i'll keep learning more about working on it...........

 

You had mentioned in your initial post that the idle had gone both up and down. If it only creeps up, then something is continually opening up, Either a worsening vacuum leak or the idle screw turning in. The spring tension would cause a loose idle screw to back out, dropping the idle, although vibration can cause some strange things. A continually worsening vacuum leak would eventually lean out the mixture and you would eventually close the throttle plate completely. Are you adjusting the idle mixture screws also to compensate for a lean mixture?

The only vacuum used on these cars is vacuum advance and optionally power assist brakes. To check for a vacuum leak in the advance system, disconnect the advance tube at the manifold. Using a known good vacuum pump, pump the advance line to the distributor down. Watch the vacuum gauge. It should hold vacuum.

If you suspect the power brake booster, disconnect it and plug it. Drive it with manual brakes and see if you have an issue. Be careful of the extra effort needed to stop!

 

A couple more things to check on;
Take the distributor cap and rotor off and check for free movement of centrifugal advance weights and springs and lube lightly (!!!!), if they feel sticky.
As mentioned above check secondary throttle plates and spring including connection to diaphragm to make sure it is returning to closed position each time the diaphragm is released. The throttle stop screw on the Sec. side is critical on mine and the ST manual adjustment Spec. won't work on it either.
Check the idle bleed holes primary and secondary - clean with spray carb cleaner; sometimes it helps to do it with the engine running.
Hope some of this helps - Good Luck.

 

rgs
it's mostly just eventually creeping back up. when it was down tonight i think that was because I had to turn it down from being too high last time I drove it but why it was WAY down I don't understand as when I reset it last time it was reset to the proper idle speed than turned off and not started again until tonight.

no, I'm not having to touch the mixture screws at all, just the idle.

I don't have power brakes so I know the only vacuum is for the can on the distributor for the advance. When it was suggested to check for vacuum leaks I assumed it meant for leaks in places like the base plate of the carb but I do know that's flat and true. I know the vacuum can is good - the distributor was also recently rebuilt and the guy who rebuilt it for me knows what he's doing and actually decided to reuse my original can rather than the Echlin new 1810 can I had on there as he said it was more accurate for my application and he checked it out and said it was still good and holding vacuum as per his testing on it. I suppose it's worth another check though - you never know.

continuing on the idea of a vacuum leak possibility though, it doesn't make sense to me based on how it's acting unless i'm missing something here:
i adjust it and it's fine, as I drive the car it eventually creeps up to a higher idle speed. since it takes a while for that to happen, say 25-50 miles before it starts to creep up again perhaps it heat causing some metal surface to warp or expand causing a slight vacuum leak or air to get into the carb causing the idle to increase? but than if thats the case the idle should go back to "normal" once the motor cools down until it heats back up again shouldn't it? it doesn't do that - it stays high until I readjust it down again by turning the idle screw than after another 25-50 miles it creeps up again by itself.

I agree with you on the idle screw spring tension - a loose spring would cause it to back down lowering the idle, i can't see it increasing the idle so i THINK I can rule that one out.
That only leaves the one vacuum line to worry about from the carb to the vacuum can and the can checked out fine and holding vacuum at the distributor rebuilder.

man, maybe it's just getting late and I'm getting tired but nw i'm even more confused about how this is happening than i was before because I think I can rule out certain areas or causes but it's still doing it.......

why can't I ever run into SIMPLE problems ??

 

67
the distributor is recently rebuild completely by someone who really knows Corvette distributors.
you and W1ctc suggested checking the secondaries so I will check those out.
thanks

 

Barry,

Welcome to the Wonderful World of Holley Carbs. I have had the same problem on every carb that I have had on my 66. When I orginally rebuilt the engine I purchased a new direct replacement carb for the 390 hp 427 from Holley. It ran like s--t. Worked on it for a number of years. Changed bowls, etc. Never got it right. I bought a new 7667 double pumper for mine and put in on the engine. It does the same thing. Not sure why. I did originally have a heat shield on the manifold and don't remember the idle fluctuating with that on. I had to take it off though because my Weind manifold sits to high and with the spacer and extra gasket, it caused the hood to bulge. Now I get the fluctuation in the idle again. Mine is not as drastic as yours but I have to believe over time it will get there. Let me know if you ever solve the puzzle. Wish I could help but I haven't a clue as to what the problem might be and I sure as heck don't want to send you down the wrong path by telling you some of the things I did and show that I know nothing at all about these units.

Steve

 

If it's an intermittent problem it may be as simple as the ground braid connection to the accelerator rod not allowing the rod to return all the way.

 

I have had the same problem, or very similar problem, and what I found after going through all sorts of hoops like you are doing, was that the throttle plates (butterflys) were hitting the carb gasket VERY slightly causing them to hang up and do pretty much the way you describe. I removed the carb and pushed the gasket slightly toward the front of the engine, tight against the studs, and my problem was solved !
It took lots of trial and error before I found my problem. Good luck with yours. I'm sure you'll find the problem before long. There are some great folks on this forum and a lot of knowledge. Somehow, it'll get solved !!!
Tom

 

Steve & Tom, well it's good to know at least that I'm not the only one to ever exoerience this odd problem.

Tom, I'll check the butterflys as this was also suggested a few times by other posters.

Garth, I actually thought of that the very first thing but the rod is moveing fine and the ground connection isn't hitting or hanging up anywhere.

Since I readjusted the idle last night I went on a cruise up to a show today and round trip was approx 120-140 miles and at least for today the problem has not re-occured. I made it there and back with the idle staying where it should be. God, I hate intermittent problems!

 

Barry, check the torque on your carb and your intake manifold. I suffered a similar bout of creeping idle last year. I know you've had your intake off recently, have you re-checked the torque?

P.S. Congratulations on your showing today.

 

If it's any consolation, I've got the same problem on my Holley 650 double pumper, and just started happening this week. I had the air cleaner off to clean the engine - I can't imagine that I accidentally did something to cause this, but now it's idling between 1000 and 1100

I had to tighten the air cleaner down quite a bit to the point where I couldn't move the air cleaner assembly with my hand - not sure whether this could have anything to do with it.

What do I need to adjust - the curb idle speed screw? Holley has a diagram on their instructions but I've never done this before.

Barry - keep us updated. Hope your idle holds.

 

Larry
opps....... no I haven't checked either the intake or the carb bolts since I put everything back together again back in Jan and Feb.
Guess that'sd stupid of me.
At least that's something easy to check and I can do that in the morning.
may not get a chance to check the other areas suggested for a few more days.

Thanks - did you take a 1st place today? you car was looking as great as always. That motor is HOT looking. :beer:

SMR, yes, if you need to adjust your idle, there will be a idle set screw on the side of the carb, drivers side, by the linkage. just give it small maybe 1/8 turns or so until you get your idle back where yo need it. on mine it doesn't take much turning of the screw at all to get it back where it needs to be