Shrike

Ok, I've been driving the vette this week trying to get it inspected and I am having the same problems as before. It usually (but not always) goes like this: If I set the idle to 8-900 rpm when the engine is warmed up, it stays there until I shut it off and restart (like getting gas, or going into the store). Then it idles about 3-500 rpm lower. If I then set the idle at 8-900 and drive it for a while especially on the highway, the idle will be at 12-1500 rpm.
I initially thought it was something with the carb, but as I think back on it the same thing was happening with the original carb. Could the distributor be messed up and let the timing change slightly? Or something else?
Thanks in advance,
Scott

sb69coupe

Sounds like either a worn throttle shaft or a vacuum leak to me.

Boofers

:iagree: Make sure your primary throttle blades close all the way. Compare how closed the blades are with the throttle return springs on and off. If your shaft (or throttle plate) is worn then the return springs pull the shaft sideways and the plates don't close all the way. Might only happen when things are warm. You might also want to check out Lars papers on QJET tuning. Good Luck!

Shrike

Definitely not a vacuum leak. I have disconnected the entire vacuum system (manual brakes) and sprayed silicon spray all around the carb base while it was running; no change in idle speed. I guess the intake could have a leak...I'll check that tonite. I just bought the carb (remanufactured) from a reputable shop, I hope they didn't sell me one that was no good. How do I check the throttle shaft?

EDIT: just saw that last one. I have checked the throttle shaft before, I remember someone telling me that. It is tight, no slop. Also, its not the original carb, its a holley spreadbore.


[Modified by Shrike, 9:56 AM 4/28/2004]

Shrike

So the consensus is vacuum leak?
Unlikely that the distributor is doing anything?
Is it normal for the intake manifold to develope a leak? Can I spray silicon around the perimeter of the intake while it is running and see if idle changes? Or is there a better way? Stock cast iron manifold btw.
thanks again

Brettmc

I wonder if it could be your timing. Have you checked that? If you weights stick and unstick, you'll get a fluctuating idle.

Brett :thumbs:

Shrike

I have had the whole top of the distributor apart; new cap, coil, rotor, springs, vac advance unit, and module. I cleaned and lubed everything while I was in there, but I never took the distributor out of the engine. I thought that maybe the gear might be worn, or there might be a lot of play and that was affecting the timing.
I don't know.
I'm wondering now if I shouldn't just pull the intake, clean it and put new gaskets on. Hell, I should just replace it while i'm at it. :confused:

mvftw

I have a similar problem. 1 thing I'm going to do is replace the vacuum advance weight bushings, I know there worn (25 yrs old), maybe that's throwing off the timing.


[Modified by mvftw, 3:11 PM 4/28/2004]

Boofers

Maybe you have a vacuum leak in your vacuum lines like for the headlamps, etc. Easy enough to check by disconnecting that stuff. Get a vacuum guage and timing light if you're really hungry.


[Modified by Boofers, 2:18 PM 4/28/2004]

IRON WILL

You are not alone. I have been fighting this for awhile on my car. I bought a new carb because the throttle shaft was worn out on the old one. Also because some of my engine mods required it. I am having alot of trouble getting the idle set. In the driveway I can get it to stay at 6-700 rpms I drive the car a bit and it goes up to 900. It was alot worse than that. Its an auto so I try to keep the idle r's down so it doesn't bang into gear. I have checked linkage and change return springs , carb adjustments, distributor parts, looked for all vacuum leaks. I am at a loss right now. I am too the point where I will never again buy a carburetor. EFI is next, then I can have all sorts of electrical problems, I can't wait.
I am pretty sure the problem lies within my base plate and linkage. If i put slight pressure on the throttle linkage the idle will come down. I think there might be something with the electric choke adjustment that could be causeing the throttle to hang up. I will continue to mess with it till I get it right.

Shrike

mvftw- I'll give that a shot, its pretty much the only thing not replaced in there.
Boofers- already did that. I disconnected the entire vacuum system. The only vacuum is the vac advance and I replaced the hose and the vac advance canister. Everything else is capped off.
IRON WILL- I hear ya, I'm thinkin I'd rather play with TPI! But, that'll have to wait. No choke on mine, so I know thats not the problem. It's the little stuff that drives me nuts :mad

Boofers

Ya I hear ya, but its stuff like this that makes you learn the most! :cheers: