quadrajet idle mixture screws all the way in - The 1947 - Present Chevrolet & GMC Truck Message Board Network

chris71 · 05-10-2010, 08:16 AM

hey guys,
I have a bone stock 71 qaudrajet on a bone stock 40,000 original mileage 350. had the carb rebuilt about a year ago and I just getting the engine back together because I had the cylinder heads redone (hardened seats, one peice valves). still has the the original points style dist, the PO did the new cap, points, condenser routine. well i got her together and she is idling ok, a little rough but not bad (seems like she has a cam when she doesnt) but the main problem is a hesitation/sputter under part throttle, WOT she seems good, at highway cruise she seems fine, so i went to adjust the idle mixture screws, and did the turn in 1/2 turn until highest vacuum, but i was able to turn the screws all the way in with no change in vacuum and she will still idle okay. the timing is at 8* and the vacuum is 15, i havent got a chance to check the dwell yet, because i cant find my meter. I know a new brass float was installed, along with the rebuild. does anyone know what may be going on here?

P.S. please no qaudrajet bashing or go buy a holley/edlebrock talk, i want to keep this engine/carb/dist. setup all original and numbers matching like it is. thanks in advance for your help.

pumpkin · 05-10-2010, 09:57 AM

Souds like maybee the float is set to high.

68gmsee · 05-10-2010, 10:44 AM

Carefully check for vacuum leaks including the pcv valve and the base of the carb.

These trucks have been around for over 40 years and I'm amazed when one of them is all original since there were a lot of us that tinkered with cars and trucks back then.

The quadrajet/rochester is probably one of the most difficult carbs to get just right. It's all I've ever had on my GM cars and trucks and I've rebuilt a few but mostly just cleaning and replacing parts from kits. I also have found out that not many shops know how to rebuild these carbs. I would suggest a google search and even a YouTube search to learn as much about them as you can. Example below.

http://www.carcraft.com/howto/57178/index.html

http://www.gmcws.org/Tech/quadrajet/...iles/frame.htm

slowgo72 · 05-10-2010, 12:51 PM

funny i have the same problem on my 2 barrel rochester and rebuilt it 2 times...
been through all the timing checks nad all ok.. have not had any luck with mine either..

mdwilf · 05-10-2010, 01:45 PM

I've fought some similar problems - here are a few things to check. 1. if you have too much fuel pressure, it will overwhelm the carb & it won't be depending on the low idle "circuit". 2 Look at the idle speed set screw - if it's cranked way down, you might be idling off it instead of the "low idle" & it is possible that the low idle passages (which are very small) are clogged. I had a freshly remaned carb that had set for about a year & I had to use my air gun to blow them out - carb cleaner wouldn't do it.
Do a search for a diagram & you'll see which ones they are.

http://www.highperformancepontiac.co...ing/index.html

//vetteworks.tripod.com/qjet.htm

05-10-2010, 08:16 AM	#1
chris71 Registered User Join Date: Oct 2007 Location: Sykesville MD Posts: 357	quadrajet idle mixture screws all the way in hey guys, I have a bone stock 71 qaudrajet on a bone stock 40,000 original mileage 350. had the carb rebuilt about a year ago and I just getting the engine back together because I had the cylinder heads redone (hardened seats, one peice valves). still has the the original points style dist, the PO did the new cap, points, condenser routine. well i got her together and she is idling ok, a little rough but not bad (seems like she has a cam when she doesnt) but the main problem is a hesitation/sputter under part throttle, WOT she seems good, at highway cruise she seems fine, so i went to adjust the idle mixture screws, and did the turn in 1/2 turn until highest vacuum, but i was able to turn the screws all the way in with no change in vacuum and she will still idle okay. the timing is at 8* and the vacuum is 15, i havent got a chance to check the dwell yet, because i cant find my meter. I know a new brass float was installed, along with the rebuild. does anyone know what may be going on here? P.S. please no qaudrajet bashing or go buy a holley/edlebrock talk, i want to keep this engine/carb/dist. setup all original and numbers matching like it is. thanks in advance for your help. __________________ '55 3100 2nd series short-bed stepside '70 Corvette coupe stingray '71 Chevelle 2-dr coupe SS clone '71 C10 long-bed fleetside '83 long-haired brunette wife

05-10-2010, 09:57 AM	#2
pumpkin Registered User Join Date: Apr 2006 Location: East Sooke B.C. Canada Posts: 543	Re: quadrajet idle mixture screws all the way in Souds like maybee the float is set to high.

05-10-2010, 10:44 AM	#3
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: quadrajet idle mixture screws all the way in Carefully check for vacuum leaks including the pcv valve and the base of the carb. These trucks have been around for over 40 years and I'm amazed when one of them is all original since there were a lot of us that tinkered with cars and trucks back then. The quadrajet/rochester is probably one of the most difficult carbs to get just right. It's all I've ever had on my GM cars and trucks and I've rebuilt a few but mostly just cleaning and replacing parts from kits. I also have found out that not many shops know how to rebuild these carbs. I would suggest a google search and even a YouTube search to learn as much about them as you can. Example below. http://www.carcraft.com/howto/57178/index.html http://www.gmcws.org/Tech/quadrajet/...iles/frame.htm

05-10-2010, 12:51 PM	#4
slowgo72 Registered User Join Date: Mar 2007 Location: kentucky Posts: 14	Re: quadrajet idle mixture screws all the way in funny i have the same problem on my 2 barrel rochester and rebuilt it 2 times... been through all the timing checks nad all ok.. have not had any luck with mine either.. __________________

05-10-2010, 01:45 PM	#5
mdwilf Registered User Join Date: Feb 2009 Location: Jefferson City, MO Posts: 83	Re: quadrajet idle mixture screws all the way in I've fought some similar problems - here are a few things to check. 1. if you have too much fuel pressure, it will overwhelm the carb & it won't be depending on the low idle "circuit". 2 Look at the idle speed set screw - if it's cranked way down, you might be idling off it instead of the "low idle" & it is possible that the low idle passages (which are very small) are clogged. I had a freshly remaned carb that had set for about a year & I had to use my air gun to blow them out - carb cleaner wouldn't do it. Do a search for a diagram & you'll see which ones they are. http://www.highperformancepontiac.co...ing/index.html //vetteworks.tripod.com/qjet.htm __________________ What I do in my spare time: http://rosebudsgarage.shutterfly.com



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