Exhaust manifolds glowing red. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

desert-rat · 08-27-2023, 11:05 PM

First start up after rebuild.
2,000 RPM break-in and noticed they were glowing. So I shut it off. I think the carburetor needs to be tuned because I can't get the RPM past 2,000 when WOT.
Just glad I was doing this in the evening outside my garage. I was able to notice the glow.

**Steeveedee** · 08-27-2023, 11:47 PM

Lean mixture. In general, high idle speeds will give a lean mixture unless the choke is closed slightly during break-in. Closing the choke a little will cause the speed to increase. You just need to use something to hold it slightly closed at an optimum idle speed, since the automatic choke will be hot.

RustyPile · 08-28-2023, 12:22 AM

Timing too retarded will cause the same thing.. Spark fires late and the burn process hasn't completed when the exhaust valves open.

1967GMCC20 · 08-28-2023, 09:45 AM

Vacuum leak too. Inspect above notes, and correct.

desert-rat · 08-30-2023, 03:37 AM

So I finished the cam break-in. All went well. The timing was off so now that's been adjusted. Except the rear main seal is dripping oil like crazy. So I drove the truck to the shop that rebuilt the motor. The shop is gonna keep the truck over the weekend to repair the oil leak. Once that's done, gonna flush the water and refill with coolant, slap the new cowl hood on, and enjoy.

**53burb** · 08-30-2023, 11:35 PM

so it was just the timing being off? Was it a significant amount?

**Rust_never_sleeps** · 08-31-2023, 02:47 AM

Quote:

Originally Posted by 1967GMCC20

Vacuum leak too. Inspect above notes, and correct.

Is that just because it leans out the mix, or some other factor?

desert-rat · 08-31-2023, 06:39 AM

Quote:

Originally Posted by 53burb

so it was just the timing being off? Was it a significant amount?

Not sure if bad timing had anything to do with the glowing manifolds. Not sure what's a significant amount for the timing, but it was hard to start, idle was rough, the idle was almost like a drag car, very little power. Adjusted the distributor and the carburetor multiple times. Checked timing with a strobe. Got it to idle around 700. Now she purrs like a cat.

**53burb** · 08-31-2023, 09:31 PM

cool...thx for the update

**MikeB** · 09-01-2023, 11:20 AM

Was just one part of one manifold glowing red? That could mean a problem only with that cylinder. Have to wonder what the spark plug, valve, and top of piston look like.

Let us know when you find the culprit.

**maverickmk** · 09-06-2023, 08:25 PM

Quote:

Originally Posted by Rust_never_sleeps

Is that just because it leans out the mix, or some other factor?

A carb meters the fuel by "sensing/reading" the incoming flow of air. If there is air be introduced into the intake after the carb it will cause a lean condition because it is getting more air than the carb is calibrating for.

**Rust_never_sleeps** · 09-06-2023, 09:47 PM

Quote:

Originally Posted by maverickmk

A carb meters the fuel by "sensing/reading" the incoming flow of air. If there is air be introduced into the intake after the carb it will cause a lean condition because it is getting more air than the carb is calibrating for.

yep, just wasn't sure if there was some other effect

**maverickmk** · 09-06-2023, 11:49 PM

Quote:

Originally Posted by Rust_never_sleeps

yep, just wasn't sure if there was some other effect

Gotcha' - Sorry, I didn't mean to be condescending

**Rust_never_sleeps** · 09-07-2023, 12:40 AM

Quote:

Originally Posted by maverickmk

Gotcha' - Sorry, I didn't mean to be condescending

no sweat

**MikeB** · 09-07-2023, 09:57 AM

Quote:

Originally Posted by maverickmk

Gotcha' - Sorry, I didn't mean to be condescending

Not condescending at all. But a good explanation of a lean condition caused by a vacuum leak.

08-27-2023, 11:05 PM	#1
desert-rat Registered User Join Date: Jan 2013 Location: 3,018 feet above the SoCal/Mojave Desert region. Posts: 371	Exhaust manifolds glowing red. First start up after rebuild. 2,000 RPM break-in and noticed they were glowing. So I shut it off. I think the carburetor needs to be tuned because I can't get the RPM past 2,000 when WOT. Just glad I was doing this in the evening outside my garage. I was able to notice the glow. Attached Images

08-27-2023, 11:47 PM	#2
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,596	Re: Exhaust manifolds glowing red. Lean mixture. In general, high idle speeds will give a lean mixture unless the choke is closed slightly during break-in. Closing the choke a little will cause the speed to increase. You just need to use something to hold it slightly closed at an optimum idle speed, since the automatic choke will be hot. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

08-28-2023, 12:22 AM	#3
RustyPile Registered User Join Date: Dec 2017 Location: Elkhart, Texas Posts: 1,809	Re: Exhaust manifolds glowing red. Timing too retarded will cause the same thing.. Spark fires late and the burn process hasn't completed when the exhaust valves open.

08-28-2023, 09:45 AM	#4
1967GMCC20 Registered User Join Date: Apr 2022 Location: Medina, Ohio Posts: 224	Re: Exhaust manifolds glowing red. Vacuum leak too. Inspect above notes, and correct.

08-30-2023, 03:37 AM	#5
desert-rat Registered User Join Date: Jan 2013 Location: 3,018 feet above the SoCal/Mojave Desert region. Posts: 371	Re: Exhaust manifolds glowing red. So I finished the cam break-in. All went well. The timing was off so now that's been adjusted. Except the rear main seal is dripping oil like crazy. So I drove the truck to the shop that rebuilt the motor. The shop is gonna keep the truck over the weekend to repair the oil leak. Once that's done, gonna flush the water and refill with coolant, slap the new cowl hood on, and enjoy.

08-30-2023, 11:35 PM	#6
53burb Senior Member Join Date: May 2004 Location: Queen Creek, AZ Posts: 1,007	Re: Exhaust manifolds glowing red. so it was just the timing being off? Was it a significant amount?

08-31-2023, 09:31 PM	#9
53burb Senior Member Join Date: May 2004 Location: Queen Creek, AZ Posts: 1,007	Re: Exhaust manifolds glowing red. cool...thx for the update

09-01-2023, 11:20 AM	#10
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,568	Re: Exhaust manifolds glowing red. Was just one part of one manifold glowing red? That could mean a problem only with that cylinder. Have to wonder what the spark plug, valve, and top of piston look like. Let us know when you find the culprit. __________________ Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!



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