1972 C10 Idle speed - The 1947 - Present Chevrolet & GMC Truck Message Board Network

brown7373 · 08-01-2019, 04:42 PM

I continue to find it impossible to set my idle speed. This is a 4 month old crate 350/290 engine. I an running a non HEI distributor with Pertronix 1 in it, with timing set to 12 degree BTDC and HEAVY advance springs installed. I am using the original Q-jet that is rebuilt and vacuum advance to the distributor is disconnected and plugged. When I set the idle to 650 in gear, all seems fine. When I drive it, or take it out of gear, the engine speed goes over 2000. Timing advances 13 degrees (on my dial back timing light ). The carb linkage still rests on the idle stop, so it isn't a case of the carb opening. I used the same distributor on the old engine and it functioned fine.

If I reduce the idle from where it is racing, it will continue to drop and eventually stall. I can find no vacuum leaks and my booster holds vacuum. I suspect the distributor advances and that is causing the engine speed to increase. If that is what is happening, why would it do that with vacuum advance disconnected and very heavy springs installed on the weights?

**Steeveedee** · 08-01-2019, 05:10 PM

Can we see some pics of the setup? Maybe someone can spot something.

**WorkinLonghorn** · 08-01-2019, 05:35 PM

Vacuum leak? Fast Idle cam in play?

brown7373 · 08-02-2019, 09:58 AM

Fast idle not in play. When the idle increases it is warmed up and off the fast idle cam and the linkage is still on the regular idle stop. Wouldn't a vacuum leak DECREASE the idle? Vacuum advance plugged, vacuum to charcoal canister plugged. PCV is new with new hose and grommet. The more I think about it the more I am leaning toward the distributor. I have heavy springs, but maybe the weights are non stock. I will check today. I also will plug the hose that goes to the left in the picture from the PCV elbow to the charcoal canister. The picture of the silver valve on the left is dummied up and not connected to anything. The pictures show a cold engine that has had the throttle pumped to set the choke. The choke completely closes when warmed up. The throttle linkage stays on the idle stop even when the idle increases. The fast idle cam is NOT in play.

**Steeveedee** · 08-02-2019, 12:40 PM

The vacuum line to the vapor canister is currently plugged? The one that tees in with the PCV? Engine vacuum will pull fuel vapors from the canister as designed. If it's a lot of vapor, and you have reasonably fresh air coming through the PCV valve, it could mix with the fuel vapors. Maybe that's enough to get the idle up?

brown7373 · 08-02-2019, 02:59 PM

I fixed it 90%. Main issue was in the distributor. I changed out the weights and springs with some I knew were factory stock. There were heavy weights that I think was part of a kit to bring advance in quicker. I'm not opposed to that, but when you take it out of gear the rpoms naturally increase, or when rpms increase by normal driving, it shouldn't advance the mechanical and then the rpms keep it advanced.

08-01-2019, 04:42 PM	#1
brown7373 Registered User Join Date: May 2011 Location: Fort Pierce, FL Posts: 430	1972 C10 Idle speed I continue to find it impossible to set my idle speed. This is a 4 month old crate 350/290 engine. I an running a non HEI distributor with Pertronix 1 in it, with timing set to 12 degree BTDC and HEAVY advance springs installed. I am using the original Q-jet that is rebuilt and vacuum advance to the distributor is disconnected and plugged. When I set the idle to 650 in gear, all seems fine. When I drive it, or take it out of gear, the engine speed goes over 2000. Timing advances 13 degrees (on my dial back timing light ). The carb linkage still rests on the idle stop, so it isn't a case of the carb opening. I used the same distributor on the old engine and it functioned fine. If I reduce the idle from where it is racing, it will continue to drop and eventually stall. I can find no vacuum leaks and my booster holds vacuum. I suspect the distributor advances and that is causing the engine speed to increase. If that is what is happening, why would it do that with vacuum advance disconnected and very heavy springs installed on the weights?

08-01-2019, 05:10 PM	#2
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,598	Re: 1972 C10 Idle speed Can we see some pics of the setup? Maybe someone can spot something. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

08-01-2019, 05:35 PM	#3
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,883	Re: 1972 C10 Idle speed Vacuum leak? Fast Idle cam in play? __________________ '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.

08-02-2019, 09:58 AM	#4
brown7373 Registered User Join Date: May 2011 Location: Fort Pierce, FL Posts: 430	Re: 1972 C10 Idle speed Fast idle not in play. When the idle increases it is warmed up and off the fast idle cam and the linkage is still on the regular idle stop. Wouldn't a vacuum leak DECREASE the idle? Vacuum advance plugged, vacuum to charcoal canister plugged. PCV is new with new hose and grommet. The more I think about it the more I am leaning toward the distributor. I have heavy springs, but maybe the weights are non stock. I will check today. I also will plug the hose that goes to the left in the picture from the PCV elbow to the charcoal canister. The picture of the silver valve on the left is dummied up and not connected to anything. The pictures show a cold engine that has had the throttle pumped to set the choke. The choke completely closes when warmed up. The throttle linkage stays on the idle stop even when the idle increases. The fast idle cam is NOT in play. Attached Images

08-02-2019, 12:40 PM	#5
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,598	Re: 1972 C10 Idle speed The vacuum line to the vapor canister is currently plugged? The one that tees in with the PCV? Engine vacuum will pull fuel vapors from the canister as designed. If it's a lot of vapor, and you have reasonably fresh air coming through the PCV valve, it could mix with the fuel vapors. Maybe that's enough to get the idle up? __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

08-02-2019, 02:59 PM	#6
brown7373 Registered User Join Date: May 2011 Location: Fort Pierce, FL Posts: 430	Re: 1972 C10 Idle speed I fixed it 90%. Main issue was in the distributor. I changed out the weights and springs with some I knew were factory stock. There were heavy weights that I think was part of a kit to bring advance in quicker. I'm not opposed to that, but when you take it out of gear the rpoms naturally increase, or when rpms increase by normal driving, it shouldn't advance the mechanical and then the rpms keep it advanced.



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