High Idle - The 1947 - Present Chevrolet & GMC Truck Message Board Network

TexanC3 · 10-27-2007, 08:44 AM

I am having issues with my 71 with a 350 and q-jet. When I frist start it revs to 2700rpm and stays there for atleast 5-10 min. If I drive it for a while it will finally idle normally. My temperature gauge does not currently work, so really don't know when it is at operating temp. Is there a way to bypass the "choke", and would I want to do this?

droppedgmc · 10-27-2007, 08:52 AM

is it an electric choke? disconnect the hot wire and remove the choke plate

**1970cstblazer** · 10-27-2007, 08:59 AM

The problem you have is very simple to correct. On the passengers side of the carb on the bottom, there is a fast idle screw. It is attached to the throttle shaft. When the engine is warmed up, the fast idle engine speed should be set for 1800 rpm in park. It sounds like the choke is operating normally, just that someone overdid the fast idle speed.

**WorkinLonghorn** · 10-27-2007, 09:14 AM

Yes , and after a few minutes stab the peddle once and it will come back down.
BTW,the factory service manual for 1969 shows the "fast idle" for a 350 with Q-jet to be 2400 RPM.Maybe it's lower for 1971 as mentioned above.

TexanC3 · 10-27-2007, 09:15 AM

Quote:

Originally Posted by 1970cstblazer

the fast idle engine speed should be set for 1800 rpm in park.

Sorry for my ignorance, but how does the carb know when to go to fast idle?

TexanC3 · 10-27-2007, 09:37 AM

I found this website that has info..http://www.jetchip.com/carbtechinfo.asp

"Fast idle screw adjustment:

The fast idle screw adjusts the idle speed when the vehicle is cold started and the choke is on. Use a flat head screwdriver to adjust the fast idle speed screw shown in photo. Turn the screw clockwise for a higher idle speed, counter clockwise for a lower idle speed."

**WorkinLonghorn** · 10-27-2007, 09:42 AM

Quote:

Sorry for my ignorance, but how does the carb know when to go to fast idle?

There's a little "cam" that rigs the idle to be higher until the choke pull-off mechanism acts on it as the engine warms up. On my '69 it is an element that expands from the heat,on yours I guess it is electronically activated.

northerngmc · 10-27-2007, 12:33 PM

I adjusted mine just last week and it sure does run better when cold.

I also found that my choke linkage had 15 years of dirt and gunk on it and was sticking. So I sprayed down the choke linkage with penetrating oil to clean it up. That helped as well.

So without a tach in the truck how can I tell the rpm of the engine?

68gmsee · 10-27-2007, 01:13 PM

So without a tach in the truck how can I tell the rpm of the engine?

This is similar to what I have to check mine...

http://www.amazon.com/Actron-CP7605-...3331942&sr=8-2

northerngmc · 10-27-2007, 02:40 PM

Thanks.

10-27-2007, 08:44 AM	#1
TexanC3 Registered User Join Date: Apr 2006 Location: Katy, Texas 77450 Posts: 31	High Idle I am having issues with my 71 with a 350 and q-jet. When I frist start it revs to 2700rpm and stays there for atleast 5-10 min. If I drive it for a while it will finally idle normally. My temperature gauge does not currently work, so really don't know when it is at operating temp. Is there a way to bypass the "choke", and would I want to do this? Last edited by TexanC3; 10-27-2007 at 08:45 AM.

10-27-2007, 08:52 AM	#2
droppedgmc Truck Enthusiast Join Date: Dec 2006 Location: Arlington, WA Posts: 1,346	Re: High Idle is it an electric choke? disconnect the hot wire and remove the choke plate __________________ '85 swb

10-27-2007, 08:59 AM	#3
1970cstblazer All stock and staying that way Join Date: Jul 2003 Location: Elkland, PA Posts: 1,827	Re: High Idle The problem you have is very simple to correct. On the passengers side of the carb on the bottom, there is a fast idle screw. It is attached to the throttle shaft. When the engine is warmed up, the fast idle engine speed should be set for 1800 rpm in park. It sounds like the choke is operating normally, just that someone overdid the fast idle speed. __________________ 1970 K/5 Blazer CST 4WD, Medium Bronze, 93k ACT. miles, 350, 4 speed, rear positraction, 16.5" x 8.25" HD wheel option, tilt, tach, vacuum, AM/FM, manual throttle...Dad ordered and purchased new 4/70. Currently frame off restoration finally getting close to completion.. 1972 Cheyenne Super 20 2WD, DK Blue/White, 90K ACT. miles, 402, TH400, 4.10 open, tilt, tach, vacuum, A/C, AM/FM, manual throttle.. A mostly original paint never rusted Texas survivor... 2017 Sierra 1500 SLT 4WD, Black, 45k miles, 5.3, 6L80E, 3.42 LS, 20" polished wheels, everything but moonroof and 6.2... 2019 Canyon SLT 4WD, White, 62k miles, 3.6, 8L45, 3.23 LS, 18" wheels

10-27-2007, 09:14 AM	#4
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: High Idle Yes , and after a few minutes stab the peddle once and it will come back down. BTW,the factory service manual for 1969 shows the "fast idle" for a 350 with Q-jet to be 2400 RPM.Maybe it's lower for 1971 as mentioned above. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, stereo speakers, 2nd owner, Work-Truck supreme. Last edited by WorkinLonghorn; 10-27-2007 at 09:19 AM.

10-27-2007, 09:37 AM	#6
TexanC3 Registered User Join Date: Apr 2006 Location: Katy, Texas 77450 Posts: 31	Re: High Idle I found this website that has info..http://www.jetchip.com/carbtechinfo.asp "Fast idle screw adjustment: The fast idle screw adjusts the idle speed when the vehicle is cold started and the choke is on. Use a flat head screwdriver to adjust the fast idle speed screw shown in photo. Turn the screw clockwise for a higher idle speed, counter clockwise for a lower idle speed."

10-27-2007, 12:33 PM	#8
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Re: High Idle I adjusted mine just last week and it sure does run better when cold. I also found that my choke linkage had 15 years of dirt and gunk on it and was sticking. So I sprayed down the choke linkage with penetrating oil to clean it up. That helped as well. So without a tach in the truck how can I tell the rpm of the engine? __________________ Blaine 59 GMC 9314 a.k.a. Whoopee 68 C20 50th Anniversary a.k.a. The Rat http://www.flickr.com/photos/northerngmc/ http://www.members.tripod.com/P-wee/index_032.htm 1.61803 is one H of a lot better than 3.14159, no really it is!

10-27-2007, 01:13 PM	#9
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: High Idle So without a tach in the truck how can I tell the rpm of the engine? This is similar to what I have to check mine... http://www.amazon.com/Actron-CP7605-...3331942&sr=8-2 Last edited by 68gmsee; 10-27-2007 at 01:15 PM.

10-27-2007, 02:40 PM	#10
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Re: High Idle Thanks. __________________ Blaine 59 GMC 9314 a.k.a. Whoopee 68 C20 50th Anniversary a.k.a. The Rat http://www.flickr.com/photos/northerngmc/ http://www.members.tripod.com/P-wee/index_032.htm 1.61803 is one H of a lot better than 3.14159, no really it is!

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