Q-jet: Idle jumps up post seconday openings
06-22-2013, 02:11 PM
#1
Q-jet: Idle jumps up post seconday openings
This is a puzzler:
After moderate acceleration or WOT, the idle jumps to over 1500 and stays there -- until the engine is shut off and restarted.
It appears the secondary throttle valves stick open just a little. If I grab the shaft and wiggle it, the idle will drop down. Same if I turn the engine off and restart it (therefore I assume the vacuum pressure is a factor).
A friend said a Q-Jet’s lower body warps easily and may cause the shaft to bind but the shaft has PLENTY of play/clearance when the engine is off.
Lube and carb cleaner has not helped.
Anyone dealt with this and prevailed?
After moderate acceleration or WOT, the idle jumps to over 1500 and stays there -- until the engine is shut off and restarted.
It appears the secondary throttle valves stick open just a little. If I grab the shaft and wiggle it, the idle will drop down. Same if I turn the engine off and restart it (therefore I assume the vacuum pressure is a factor).
A friend said a Q-Jet’s lower body warps easily and may cause the shaft to bind but the shaft has PLENTY of play/clearance when the engine is off.
Lube and carb cleaner has not helped.
Anyone dealt with this and prevailed?
06-23-2013, 01:23 AM
#2
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Sounds like the body is warped slightly.
If the secondaries are binding in any way something is outta rack.
You might start by loosening the four corner carb mounting bolts and see if it frees up and then tighten evenly. They don't need to be gorilla gripped onto the intake.
If not I'd remove and inspect from the underside.
You certainly could hit up Lars on this forum.
Marshal
If the secondaries are binding in any way something is outta rack.
You might start by loosening the four corner carb mounting bolts and see if it frees up and then tighten evenly. They don't need to be gorilla gripped onto the intake.
If not I'd remove and inspect from the underside.
You certainly could hit up Lars on this forum.
Marshal
06-23-2013, 12:59 PM
#4
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High idle after secondary opening is a very common problem. It's due to the secondary throttle blades not fully closing and seating in the bores after being opened. It's not from a warped carb - the throttle body does not warp in the secondary throttle blade area.
To fix, first thoroughly clean the edges of the throttle blades (easiest to pull the carb off to do this) and the throttle bores where the blades seal. It's common to get some carbon buildup in this area, and this will prevent the blades from fully closing. Once cleaned well (acetone works well), with the throttle closed, open the secondary airvalve and stick a screwdriver down from the top and crack the secondary throttle blade screws loose 1/4 turn. Just do one blade at a time, and make sure the throttles stay fully closed and seated when doing this. Once you have the 2 screws on one of the blades loosened 1/4 turn, take a screwdriver and simply tap all around the blade, holding the throttle firmly closed while doing this. This will "rattle" the blade into better alignment in the throttle bore to assure that it's seating in the opening. Still holding the throttle firmly closed, snug the 2 screws back up. Repeat for the other throttle blade. I've never seen a carb that was not cured of the high idle issue with this fix. Good luck!
Lars
To fix, first thoroughly clean the edges of the throttle blades (easiest to pull the carb off to do this) and the throttle bores where the blades seal. It's common to get some carbon buildup in this area, and this will prevent the blades from fully closing. Once cleaned well (acetone works well), with the throttle closed, open the secondary airvalve and stick a screwdriver down from the top and crack the secondary throttle blade screws loose 1/4 turn. Just do one blade at a time, and make sure the throttles stay fully closed and seated when doing this. Once you have the 2 screws on one of the blades loosened 1/4 turn, take a screwdriver and simply tap all around the blade, holding the throttle firmly closed while doing this. This will "rattle" the blade into better alignment in the throttle bore to assure that it's seating in the opening. Still holding the throttle firmly closed, snug the 2 screws back up. Repeat for the other throttle blade. I've never seen a carb that was not cured of the high idle issue with this fix. Good luck!
Lars
06-24-2013, 11:47 PM
#5
Lars,
Your advice and experience is greatly appreciated. I was not considering either thing you advised because the throttle was not sticky at all when the motor was off. It only stuck under vacuum and released when the motor was turned off.
I'll do both this weekend.
Thank you,
John
Your advice and experience is greatly appreciated. I was not considering either thing you advised because the throttle was not sticky at all when the motor was off. It only stuck under vacuum and released when the motor was turned off.
I'll do both this weekend.
Thank you,
John
