The Old Q-Jet Off Idle Stumble Revisited
02-20-2009, 03:58 PM
#1
Team Owner
Thread Starter
The Old Q-Jet Off Idle Stumble Revisited
I have posted before about a minor off-idle problem I have with my 1974. I have ordered and received a Viton accelerator pump just in case modern fuels chewed up the black rubber old and that is an issue. I will poke about in April when I see the car again.
Meanwhile I have had a lot of time to look on-line and try to understand the various issues with the Q-Jet carbs.
In looking at the instructions for the now-defunct Edelbrock Q-Jet I came across this:
Q-JET CARBS WITH EGR VACUUM PORTS
The original carb on some Chevy/GMC trucks and Suburbans has the EGR vacuum port machined higher in the carb venturi than the Edelbrock QJet. This may have an effect on off-idle performance resulting in a flat spot. If you get a flat spot off-idle after installing the Edelbrock Q-Jet carb, you may need to install a vacuum delay valve. A simple test will let you know if the delay valve is needed: a) Temporarily remove and plug the vacuum line to the EGR valve. If the flat spot disappears, you will need to purchase a vacuum delay valve (GM #14020691) and install it in the vacuum line between the carb and the EGR valve. This valve will delay the opening of the EGR valve long enough for the carburetor to transition through the off-idle phase without a stumble, then the EGR circuit will operate normally.
I checked the GM manual/AIM and some archived pics of my setup. I have ports both up on the carb body and on the base plate (see pic). According to the manuals and Edelbrock the upper port is "accessory" and the lower for "EGR." So I have the hose from the thermal vacuum switch in the wrong place! I can't recall why I did things that way. So I guess the question is: Will the current placement of the hose high on the carb body rather than the baseplate cause the off-idle stumble? I have ordered the vacuum delay switch ($12.00) just as a precaution.
Meanwhile I have had a lot of time to look on-line and try to understand the various issues with the Q-Jet carbs.
In looking at the instructions for the now-defunct Edelbrock Q-Jet I came across this:
Q-JET CARBS WITH EGR VACUUM PORTS
The original carb on some Chevy/GMC trucks and Suburbans has the EGR vacuum port machined higher in the carb venturi than the Edelbrock QJet. This may have an effect on off-idle performance resulting in a flat spot. If you get a flat spot off-idle after installing the Edelbrock Q-Jet carb, you may need to install a vacuum delay valve. A simple test will let you know if the delay valve is needed: a) Temporarily remove and plug the vacuum line to the EGR valve. If the flat spot disappears, you will need to purchase a vacuum delay valve (GM #14020691) and install it in the vacuum line between the carb and the EGR valve. This valve will delay the opening of the EGR valve long enough for the carburetor to transition through the off-idle phase without a stumble, then the EGR circuit will operate normally.
I checked the GM manual/AIM and some archived pics of my setup. I have ports both up on the carb body and on the base plate (see pic). According to the manuals and Edelbrock the upper port is "accessory" and the lower for "EGR." So I have the hose from the thermal vacuum switch in the wrong place! I can't recall why I did things that way. So I guess the question is: Will the current placement of the hose high on the carb body rather than the baseplate cause the off-idle stumble? I have ordered the vacuum delay switch ($12.00) just as a precaution.
Last edited by Paul L; 02-20-2009 at 04:04 PM.
02-22-2009, 05:35 PM
#5
Team Owner
Thread Starter
Here is a 1974 intake that I restored a couple of months ago. Came from Florida. What a chore and mess to get the carbon out. That stuff is as hard as a rock.
But it has been completely re-done with all new parts/SS fasteners and ready for installation in spring. This one has passges clean as a whistle now. The same with the distributor. I'll track this problem down.
Last edited by Paul L; 02-22-2009 at 07:23 PM.
