My car has an original distributor and carb on it. I have the idle set to a smooth and steady 650rpm. Sometimes after driving and coming to a stop, the idle will be raised to around 750rpm and stay there, again smooth and steady. No big deal, but I would like it to idle consistently where I've set it. I have to back off the idle stop screw to again set it at 650rpm. The distributor is not moving, the accelerator lever is against the idle stop, there is no bindage in the throttle linkage and the fast idle cam is not engaging causing the increase in rpm when this happens. I removed the distributor weights and lubricated them, which didn't help. Assuming this isn't being caused by the carb, could this be caused by something sticking in the distributor, perhaps the plate that the points attach to not returning to its' original position for some reason?
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Re: '66 350 HP IDLE SPEED
Rob, I have the same problem on my 62 except the stuck speed is a little higher than yours. I found that the bellcrank on the carb. or what ever it is called that the return spring connects to is sticking open a little when hot. I can get out of my car and have it idling at a higher than normal RPM and pull the lever back and the engine idles at a normal idle speed. I moved my return spring to a different position to give it more return force. Kicking the acc. pedal also gets it to go back to normal idle.
do not know what is wrong. could be my return spring does not have enough force (new Paragon spring) or something in the shaft of the carb. Anybody have any ideas?
Thanks, Chris -
Re: '66 350 HP IDLE SPEED
Chris,
Sounds like you can see what causes your idle rpm increase. There doesn't seem to be any visible reason for my idle increase.Comment
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Re: '66 350 HP IDLE SPEED
I just updated my 66 327/350 distributor with a VC-1765 vac can as per Duke's recommendations. Installed new points, condenser, etc. and timed it as it should be. Runs so good it scares me. But, after I run it hard and it is hot, it will idle about 100 to 150 rpms high. It will settle down after about 30 seconds. My next project is to rebuild the carb as it smells rich all of the time. I hope this takes care of the fluctuating idle.Comment
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Re: '66 350 HP IDLE SPEED
From my AFB experience, throttle shaft bore wear can cause varying idle speed every time you return to idle - not sure if this is an issue with Holleys, but I expect it is.
AFB's also have a "hot idle compensator" - a bimetallic spring with a small orifice/valve located between the secondary throttle bodies that opens when it gets hot to admit more air and offset an overly rich mixture due to fuel percolation. Of course, this will also increase idle speed. I don't know if Holleys have and equivalent device.
IMO 650 is WAAAAAY too low an idle speed for L-79. I recommend 750-800.
I recommend the VC-1765 as "optional" for L-79, particularly if there is a part throttle or transient detonation problem.
The OE "236" vacuum can provides max. vacuum advance at 8" as does the VC1810, which is equivalent to OE. The VC-1765 doesn't provide max vacuum advance until 12", but since the L-79 pulls about 14-15" at (my) recommended 750-800 idle, it meets the "2" less than idle vacuum" criterion to keep the can pulled to the limit at my recommended idle speed.
The Chevrolet recommended idle speeds for SHP engines are unrealistically low IMO, and since idle speed is a subjective call, my criterion is "the lowest speed commensurate with acceptable idle quality" and "idle quality" itself is subjective, so different strokes for different folks, but you DEFINITELY want to make sure the idle speed vacuum keeps the vacuum can pulled to the limit with 2" margin.
DukeComment
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Re: '66 350 HP IDLE SPEED
Is your distributor an original ..1153, and what manufacturer makes the VC-1765 vacuum advance? Your fluctuating idle sounds very much like mine. I rebuilt my carb to original specs(jet size, etc.) Maybe I'm trying to correct something that is normal. Thanks...Comment
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Re: '66 350 HP IDLE SPEED
Duke,
I had a '66 mustang with CA emissions, with an original autolite 4V carb which had a "hot idle compensator". I don't see one on my holley 3367.Comment
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Re: '66 350 HP IDLE SPEED
Rob,
Same thing happens on my 67 with the newly rebuilt 3810. I found that the fast idle cam was sticking a little and wouldn't go down all the way. A shot of Tri-flo on the cam shaft solved that. But I find that after a run when she's hot the idle is still a bit elevated. Could it be that the alternator is no longer charging, and there's less resistance to turning?
Jerry Fuccillo
#42179Jerry Fuccillo
1967 327/300 Convertible since 1968Comment
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Re: '66 350 HP IDLE SPEED
Jerry,
There's no mechanical interference of any kind with the carb. I wasn't aware that there was less resistance to the alternator turning whether it was charging or not.Comment
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Re: '66 350 HP IDLE SPEED
My distributor is an 1150 but correctly dated. Kind of upset me, but oh well. That's the 327/300 distributor. Not only did I want to put the correct can in, I figured since it's been there for almost 40 years, it couldn't hurt to replace it with a new one. You can buy the vac can at your local NAPA dealer.Comment
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Re: '66 350 HP IDLE SPEED
Cannot really see it. It does return to the isdle stop screw but enough slop somewhere so that I can pull it back enough to lower the idle. HMM, there is a engineering word for that slop I cannot come up with!!! Oh yea Hysterisis (SP)
Per a later post is sounds like I may have some wear/binding in the shaft. I think I will call one of the rebuilders.
ChrisComment
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Re: '66 350 HP IDLE SPEED
Duke, is this shaft bore wear repairable? What does it take?
Thanks, ChrisComment
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My experience
A 3367 Holley has no hot idle compensation system.
A Holley primary butterfly shaft penetrates the baseplate without bushings. Over time these shaft holes become egg-shaped. Such wear allows the butterfly plates to wander a bit in the venturi bores. Sometimes they close slightly off center and don't seat well. If a stronger return spring gives you a consistent idle speed, then this is your problem.
Most old Corvette distributors have aftermarket Zoom mechanical advance springs installed. These springs are a weak suck compared to the originals. Weak springs don't always pull the advance weights back fully on return to idle. Use a timing light to see if your springs are so weak that they sometimes start the mechanical advance at or below idle speed.Comment
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Re: '66 350 HP IDLE SPEED
The repair is to remove the butterflies and throttle shaft, bore them out for a suitable size repair bushing, and reassemble.
I believe some of the carburetor restoration specialists can do this.
DukeComment
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