is it typical for the idle to increase once the car gets hot ? when i start the car idle is around 700 rpm in park and after about an hour of driving the cars idle is at 1100 in park . any suggestion ?

 

Sounds normal, except the hour part that is. The engine is inefficient when cold, thats why a properly adjust carb/choke will enrichen the fuel mixture and have a higher idle at first start up. Or you can just inject it and be done with it! Don't know if this will help or not. Scott

 

is this why i have to warm the car up before i drive it ? because if i just hop in it and go ( when engine`s cold ) the car will stall out any chance it can.

 

:yesnod:

Also the engine will experience less wear when it is at normal operating temp. Sounds like you have adjusted the carb to start the car cold and have it not stall out. Guessing either you have no choke or your choke isn't working/not adjusted properly.

 

Sounds like your choke. I have had a choke problem on my carb for a while. The choke is fine until you drive for a while. I think it sticks open or something. The engine ends up idling at 1500 or so. It make that horrible noise when turning off. I am going to bring it to a pro to get it rebuilt/fixed. i hate carbs. Get a FI unit, you wouldn't have to worry about it again!

 

I had the same problem with my 454. Warm idle was set at 750. If I took it out on the freeway for an hour it would idle at 1000. I had a manual choke so the choke wasn't on in eithe case.

My problem was solved with EFI. :)

 

i was under the impression that that horrible noise was caused by the heat of the engine (only happens when the car temp is over 220) igniting the gas in the carb. on more than one occasion i have seen a cloud of white smoke come from the air filter (acompanied by that nasty sound) but it doesnt happen all the time. can i change the choke on the carb ? the carb is a custom holley 750 . would i be better off replacing the choke myself or taking it back to the builders?

 

High Idle Speed

It's common for idle speed to go up some after driving it for awhile. When the car is cold, the carburetor throttle linkage/plates are "tight". When the carburetor gets hot, the metal expands and there is a slight gap in the throttle/linkage plates. This slight gap allows more air to get to the engine creating a higher idle speed.

With my brand new carburetor, I have about a 150-200 RPM change from cold to hot. I wouldn't be suprised if an older carburetor with worn linkages would have an even larger change.

Engine Run-on

As far as that "bad noise" you are refering to, it's typically called run-on or dieseling. If your idle speed is high enough, your engine is hot, and you have mid-high compression this is something that can happen. When the engine is still in motion when you kill the ignition, it can still create enough pressure for combustion to occur. The bad thing is, this combustion is completely uncontrolled which means you engine can actually turn backwards or even fight back and forth against itself. This is why you will sometimes see smoke come out of your carburetor - the engine was turning backwards.

To fix this you can try several things:

The easiest is to turn of the engine in gear. This is easier to do if you have an automatic but you can do it with a manual too. This way your idle speed will be lower (so it'll be less likely to run-on) and the transmission will help slow the motor down quicker.

The next would be to lower your idle speed to the lowest point where it will still run right in a cold condition. This way your idle speed at hot will be lower and you'll be less likely to run-on.

Next you can install what's called an idle-stop solenoid. They can also be called "A/C idle compensators", "A/C kickers" or something similar. You usually use them to increase the idle speed when you turn on your A/C, so it doesn't lug down the engine. This only works if you don't already use one for your A/C. What you do is set the idle speed so that the throttle plates are completely closed. When you turn your car on, this solenoid engages and opens up the throttle plates to your desired idle speed. The solenoid will have an adjustment to control how far it opens your throttle plates. So then when you turn the car off, the solenoid will close and your throttle plates will snap shut cutting off all the air from the engine. No air = no combustion. This is what I had to do to fix the problem on my car.

If none of this works, then your carb is probably just worn out and needs to be rebuilt/replaced.

Hope this helps.


[Modified by Langadorf, 7:36 PM 9/15/2002]

 

It coud be a combination of a build-up of crud on your carb's throttle shaft and the effect of heat on the throttle return spring. As the spring heats up, it's tension decreases and may not be strong enough to fully close the throttle.