Confused by carbs... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

dpetteng · 10-08-2015, 08:13 AM

So, I am a new owner of a 1972 C10. I love the truck.

The last few days she has been running on when the ignition is turned off and I have noticed she has a tendancy to get stuck idling faster than she should even when hot.

I took a look at the carb last night and first noticed that it has what I think is an electric choke but I noticed the wire going to it (arrowed) is not positive at any time.

I also noticed that when cold the four butterflies are always closed. I pushed on the throttle cable and the engine reved but the butterflies didn't open. I also noticed that the back butterflies that seem to be connected to the choke do not move.

I am lost, any advice? Should the choke wire be a switched positive?

dpetteng · 10-08-2015, 08:14 AM

Also, on the right hand side of the carb there are two cables, one for the throttle and another one below that seems to disappear down towards the transmission. What is this one for?

**MikeB** · 10-08-2015, 08:47 AM

I don't think 72 trucks used electric choke Q-jets (I know a 69 didn't). If that's true, some previous owner probably installed a later model carb and wired the choke to a 12 volt ignition "on" source. Maybe the wire came loose or a fuse is blown. BTW, I assume you know that 12 volts causes the choke to slowly OPEN, as the current heats up a bi-metal spring.

The rear butterflies on the top won't open until air "demand" by the engine is high enough, like when you are accelerating with your foot to the floor. But just revving the engine in Park or Neutral probably won't open them. If they are connected to the choke linkage, that's probably to make sure they don't open when the choke is on.

**MikeB** · 10-08-2015, 09:08 AM

On another note, if I'm wrong, and it turns out the truck was originally equipped with an electric choke, it could be that an oil pressure switch has to be closed (engine running, good oil pressure) to send 12 volts to the choke. I know that's how the chokes were wired in the late 70s and early 80s, but again, I don't know about 72.

So, a bad oil pressure switch could keep 12 volts from getting to the choke. In fact, I just saw that exact same problem on a 77 El Camino last week.

dpetteng · 10-08-2015, 10:11 AM

Thanks, makes sense.

The bit I am confused about is that the front butterfly is closed when cold and open when hot. How does this happen if the choke has no power?

SierraMtns · 10-08-2015, 12:09 PM

Quote:

Originally Posted by dpetteng

Thanks, makes sense.

The bit I am confused about is that the front butterfly is closed when cold and open when hot. How does this happen if the choke has no power?

Check out this book. Great info. It helped me a lot when I rebuilt my q-jet. Also Cliff is a great guy to talk to on the phone.

http://www.cliffshighperformance.com/buy_book_2.html

Have you looked up the numbers on the carb? Back driver side. These numbers will tell you what year the carb is. Get that info and let us know what it is. There might be something else going on.

http://www.cliffshighperformance.com...carb_ID_2.html

michael bustamante · 10-08-2015, 12:21 PM

the second cable is you shift cable to the transmission. to down shift. you most likely have a turbo 350 tranny

SierraMtns · 10-08-2015, 12:22 PM

Quote:

Originally Posted by dpetteng

Thanks, makes sense.

The bit I am confused about is that the front butterfly is closed when cold and open when hot. How does this happen if the choke has no power?

Quote:

Originally Posted by dpetteng

Also, on the right hand side of the carb there are two cables, one for the throttle and another one below that seems to disappear down towards the transmission. What is this one for?

The other cable goes down to the trans to change shift points.

SierraMtns · 10-08-2015, 12:23 PM

More info.

http://www.cliffshighperformance.com...lectric_1.html

mike16 · 10-08-2015, 06:04 PM

if you like the carb, then jst eplace the electric choke guts with a manual choke set up.

i use a generic manual choke installation kit . they are available through auto parts stores HELP brand of parts.

I dont think they came that year with electric chokes, Understand how it works befor you condemn it or replace it. but if its giving you problems or you want control of the warm up procedure, a manual choke is the way to go. kits are cheap too at about 20 bucks.

Redline novdog · 10-08-2015, 07:43 PM

I have read the post and the symptoms suggest a high carbon combustion chamber/high idle. On the dieseling.I could not see the throttle return spring Needs to be a double spring.on that setup.I have had one the springs break crappy throttle return.When the engine/carb is cold the choke is closed and the rear 2 will only open on demand/vacuum

dpetteng · 10-09-2015, 08:30 AM

Ah makes sense. It definitely has only one return spring. where should the other one go? Does anyone have a pic?

10-08-2015, 08:13 AM	#1
dpetteng Registered User Join Date: Aug 2015 Location: Philly, PA Posts: 17	Confused by carbs... So, I am a new owner of a 1972 C10. I love the truck. The last few days she has been running on when the ignition is turned off and I have noticed she has a tendancy to get stuck idling faster than she should even when hot. I took a look at the carb last night and first noticed that it has what I think is an electric choke but I noticed the wire going to it (arrowed) is not positive at any time. I also noticed that when cold the four butterflies are always closed. I pushed on the throttle cable and the engine reved but the butterflies didn't open. I also noticed that the back butterflies that seem to be connected to the choke do not move. I am lost, any advice? Should the choke wire be a switched positive? Attached Images Last edited by dpetteng; 10-08-2015 at 08:37 AM.

10-08-2015, 08:14 AM	#2
dpetteng Registered User Join Date: Aug 2015 Location: Philly, PA Posts: 17	Re: Confused by carbs... Also, on the right hand side of the carb there are two cables, one for the throttle and another one below that seems to disappear down towards the transmission. What is this one for? Last edited by dpetteng; 10-08-2015 at 08:37 AM.

10-08-2015, 08:47 AM	#3
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,458	Re: Confused by carbs... I don't think 72 trucks used electric choke Q-jets (I know a 69 didn't). If that's true, some previous owner probably installed a later model carb and wired the choke to a 12 volt ignition "on" source. Maybe the wire came loose or a fuse is blown. BTW, I assume you know that 12 volts causes the choke to slowly OPEN, as the current heats up a bi-metal spring. The rear butterflies on the top won't open until air "demand" by the engine is high enough, like when you are accelerating with your foot to the floor. But just revving the engine in Park or Neutral probably won't open them. If they are connected to the choke linkage, that's probably to make sure they don't open when the choke is on. __________________ 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! Last edited by MikeB; 10-08-2015 at 08:54 AM.

10-08-2015, 09:08 AM	#4
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,458	Re: Confused by carbs... On another note, if I'm wrong, and it turns out the truck was originally equipped with an electric choke, it could be that an oil pressure switch has to be closed (engine running, good oil pressure) to send 12 volts to the choke. I know that's how the chokes were wired in the late 70s and early 80s, but again, I don't know about 72. So, a bad oil pressure switch could keep 12 volts from getting to the choke. In fact, I just saw that exact same problem on a 77 El Camino last week. __________________ 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!

10-08-2015, 10:11 AM	#5
dpetteng Registered User Join Date: Aug 2015 Location: Philly, PA Posts: 17	Re: Confused by carbs... Thanks, makes sense. The bit I am confused about is that the front butterfly is closed when cold and open when hot. How does this happen if the choke has no power?

10-08-2015, 12:21 PM	#7
michael bustamante Registered User Join Date: Sep 2007 Location: Albuquerque NM Posts: 7,841	Re: Confused by carbs... the second cable is you shift cable to the transmission. to down shift. you most likely have a turbo 350 tranny __________________ 1967custom

10-08-2015, 12:23 PM	#9
SierraMtns Registered User Join Date: Jun 2014 Location: Shingle Springs, CA Posts: 869	Re: Confused by carbs... More info. http://www.cliffshighperformance.com...lectric_1.html

10-08-2015, 06:04 PM	#10
mike16 Registered User Join Date: Dec 2009 Location: bisbee, arizona Posts: 1,529	Re: Confused by carbs... if you like the carb, then jst eplace the electric choke guts with a manual choke set up. i use a generic manual choke installation kit . they are available through auto parts stores HELP brand of parts. I dont think they came that year with electric chokes, Understand how it works befor you condemn it or replace it. but if its giving you problems or you want control of the warm up procedure, a manual choke is the way to go. kits are cheap too at about 20 bucks.

10-08-2015, 07:43 PM	#11
Redline novdog Registered User Join Date: Jan 2012 Location: Washington,Il Posts: 632	Re: Confused by carbs... I have read the post and the symptoms suggest a high carbon combustion chamber/high idle. On the dieseling.I could not see the throttle return spring Needs to be a double spring.on that setup.I have had one the springs break crappy throttle return.When the engine/carb is cold the choke is closed and the rear 2 will only open on demand/vacuum __________________ It builds on my intelligence when I make a mistake, Boy am I building on it! '67 C-10,'70 Nova SS

10-09-2015, 08:30 AM	#12
dpetteng Registered User Join Date: Aug 2015 Location: Philly, PA Posts: 17	Re: Confused by carbs... Ah makes sense. It definitely has only one return spring. where should the other one go? Does anyone have a pic?



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