Fast idle increase slowly
Thread Starter
Instructor
Joined: Feb 2022
Posts: 232
Likes: 98
From: Switzerland
Fast idle increase slowly
Hi together,
I have a bit trouble with my fast idle speed. I have a factory discovered choke on the q-jet and the idle stop solenoid for a 72.
I changed the carb and the gasket and set the carb to follow specs:
Idle screw on carb is set to minimum before stalling (550-600rpm). Idle stop solenoid is set to 800rpm. Fast idle is set 1350rpm.
Timing is also checked to GM 350 spec for a 72 vette.
After a while I retorque the carb screws, while I had some leaks around the carb (new gasket compression).
After a nother while, when I start the car correctly; one pump on the pedal, the choke set and close correctly, start the car with only a bit cranking, pull off valve on the carb works, is my fast idle a bit low, maybe 900rpm, but increase after a while to 1100rpm and in the best case to 1300rpm.
I know I have some small vaccum leaks on the headlight and my vaccum canister is disconected. Also I know my valve stem seals are bad.
Could this have an impact?
I have a bit trouble with my fast idle speed. I have a factory discovered choke on the q-jet and the idle stop solenoid for a 72.
I changed the carb and the gasket and set the carb to follow specs:
Idle screw on carb is set to minimum before stalling (550-600rpm). Idle stop solenoid is set to 800rpm. Fast idle is set 1350rpm.
Timing is also checked to GM 350 spec for a 72 vette.
After a while I retorque the carb screws, while I had some leaks around the carb (new gasket compression).
After a nother while, when I start the car correctly; one pump on the pedal, the choke set and close correctly, start the car with only a bit cranking, pull off valve on the carb works, is my fast idle a bit low, maybe 900rpm, but increase after a while to 1100rpm and in the best case to 1300rpm.
I know I have some small vaccum leaks on the headlight and my vaccum canister is disconected. Also I know my valve stem seals are bad.
Could this have an impact?
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
First thing to do is to disregard the factory GM timing spec. You want 36 degrees total at or around 3000 rpm.......
Second thing to do is to disconnect the entire vacuum system at the carb, cap it, and see if anything changes......uae an unlit propane torch to "sniff" out any vacuum leaks in the intake area......if there is a leak, the idle will change as it sucks the propane in....
Jebby
Second thing to do is to disconnect the entire vacuum system at the carb, cap it, and see if anything changes......uae an unlit propane torch to "sniff" out any vacuum leaks in the intake area......if there is a leak, the idle will change as it sucks the propane in....
Jebby
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
It is perfectly normal for the fast idle rpm to increase a few hundred rpm from the initial cold-start. On the carbs I set up, I will typically set initial fast idle rpm to 1270. After about 1 minute, the rpm will increase to around 1350 - 1400 before the choke fast idle cam drops down one notch to then lower the rpm a little. Your situation sounds normal.
Lars
Lars
Thread Starter
Instructor
Joined: Feb 2022
Posts: 232
Likes: 98
From: Switzerland
Thanks, Lars for the info, that I didn't know.
I think I set my fast idle a bit higher like your recommendation to 1270rpm, but for safety, I will test with carb starting spray for some leaks.
I have another situation where I am a little unsure. My idle also increases by 100-200rpm between after the fast idle and when the car has its operating temperature (for example: after a 20-minute ride). That's also normal? Otherwise the idle is always constant. I have had set my idle when the car had the complete operating temperature.
I think I set my fast idle a bit higher like your recommendation to 1270rpm, but for safety, I will test with carb starting spray for some leaks.
I have another situation where I am a little unsure. My idle also increases by 100-200rpm between after the fast idle and when the car has its operating temperature (for example: after a 20-minute ride). That's also normal? Otherwise the idle is always constant. I have had set my idle when the car had the complete operating temperature.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Idle rpm will change slightly any time engine temperature is changing. Idle rpm will not be stabilized until the engine has reached its stable operating temperature, which usually takes about 15 minutes. This ain't no computer-controlled idle speed system....
When I test carbs on my test engine here in the shop, I have to run the engine at least 20 minutes to assure that engine temp, with associated idle rpm, has stabilized to allow final setting of hot idle rpm.
Lars
When I test carbs on my test engine here in the shop, I have to run the engine at least 20 minutes to assure that engine temp, with associated idle rpm, has stabilized to allow final setting of hot idle rpm.
Lars
