Hot Rod Forum banner

1 - 9 of 9 Posts

·
Registered
Joined
·
565 Posts
Discussion Starter #1
My old pontiac has an 83 sbc 350 in it. Stock intake and an Edelbrock (weber) carb. It is basic. NO choke (butterfly wired open) no electronic anything, just throttle cable and step down cable. It starts and runs great, with only one problem. When it is hot, the idle speed increases, significantly.
No tach, but I'm guessing 700 rpm goes up to 1000 plus. Acts like sticky linkage, but why only when hot? I'ved fiddled with it several times, but nothing appears to be wrong.
Can something other than linkage cause this?
Thanks guys.
 

·
Registered
Joined
·
565 Posts
Discussion Starter #2
dumb carb/idle question

Duh.. what's a thermal vacuum switch?
the vacuum advance runs off a carb fitting that i assume is plain manifold vacuum. When I say hot, it never gets over 185, the thermostatd setting, but the problem comes after a half hour or more of driving.
 

·
Hates: Liver. Loves: Diesel
Joined
·
5,659 Posts
Sounds to me like the carb is tuned to run well at cold, with the choke wired open, then the throttles are way too far open after its hot. Fast idle can also mean a vacuum leak. First places to check are between the carb and intake and the center ports where the intake meets the heads.

Try tuning it when its hot. Get your initial timing set right then get those throttles closed down to about 500 rpm. Then you'll have a little trouble gettng it started cold, but a few extra pumps from your foot does the trick.

You basically have completely re-worked the carb to work well when cold without a choke. Its not fair to ask it to work right when you've crippled it so much.
 

·
Registered
Joined
·
565 Posts
Discussion Starter #5
dumb carb/idle question

Haven't had it long, and it came this way. Always starts and runs good, so this hasn't been a priority, just a nuisance at stop lights.
Not much of a carb guy, so let me explain... I pump it once or twice before I hit the key. Usually fires right up. I do have to blip the throttle for a bit till it warms up. So I'm not sure I would say its tuned to run cold. To me this is normal for no choke. (I'm inTexas, so cold is a relative term - 60 degrees is cold here) So, I don't understand why having the choke wired open is bad for it when it is warmed up, it just causes the above when it's cold. I can let it sit and idle till it reachs full operating temp (185) and it doesn't act up. Just after 20 minutes or so. The vacuum leak seems plausible, causing it to change after heat has been on it for some time. If I run a vacuum test at cold, just warm, and hot (Hooking it up to the carb vac fitting) should I be getting the same vac at idle at all temps? Seem to remember in the old days we would spray something around the base of the carb when it was running(don't remeber what we used) and a vac leak would be very obvious.
Thanks for the input guys, gives me some things to look at. If my thinking is fuzzy here, or based on 30 year old myths, feel free to straighten me out.
 

·
Registered
Joined
·
60 Posts
This could be a shot in the dark, but i had something similar to happen.
and lived with it for a while... until
While doing a tune up later on, i found a weak/broken spring on my distributor advance weights. It sounds crazy but we guessed that after a little heat soak, temps would eventually get the springs on the distributor. Since springs lose their strength as they warm, it would slip out a little. (we guessed). Don't know if that helped, but if you are still guessing... it is another slim possibility.
 

·
Hates: Liver. Loves: Diesel
Joined
·
5,659 Posts
Test for a vacuum leak using carb cleaner. Spray around suspected areas, including on vacuum hoses and fittings. If you notice a change in idle a second after you spray then you've found it. Using a vacuum gauge would work on a normal car, but since it has such different idles hot and cold, you would be comparing apples and oranges. I also think that with your idle being so high you won't have an accurate reading anyway since the throttles are too far open.

When you describe your starting procedure of pump once or twice and then it starts is very typical of a car WITH a choke. Normally here in SoCal on a car without a choke its a bit more of a chore to get it going that just a couple pumps.

I really think the previous owner got frustrated, wired the choke open, then richened up the idle and opened those throttle blades to compensate. He said, "there, it runs." and called it a day.
 

·
Registered
Joined
·
565 Posts
Discussion Starter #8
Curtis, there you go. It's how he did everything else.
I'm going to chase all these tips this weekend. thanks guys.
 

·
I put up the tools against$300
Joined
·
689 Posts
can you manually adjust the butterflys with your hand once it has warmed up? Just thought it might help with isolating the prob....
 
1 - 9 of 9 Posts
Top