Q-Jet help
Thread Starter
Burning Brakes
Joined: Aug 2001
Posts: 1,002
Likes: 0
From: Bay City MI.
Q-Jet help
Having problems with my secondaries opening on the Q-jet, have attempted to adjust the secondary doors by the screw adjuster(looser or tighter) and still not working right. When you hit the gas the seconday opens in first then it won't in second or third, but will open in fourth. My motor has low vacuum ,about 11 inches of vacuum and was told that is my problem. He told me that the carb has something called a dash pot, that controls the secondary opening. Any ideas on this and if this is right how do you adjust it? Thanks!
Le Mans Master
Joined: Mar 2002
Posts: 7,456
Likes: 2
From: Coming home from Luckenbach Texas
One more thing: Do you have a copy of Rochester Carburetors by Doug Roe? If not, get it! 
Thanks to Lars and this book, I'm actually sorta kinda starting to understand QJets!
Thanks to Lars and this book, I'm actually sorta kinda starting to understand QJets!
Advanced
Joined: Aug 2002
Posts: 84
Likes: 0
From: Riockwall Texas
All Q-Jets are mechanical secondaries. The secondary butterfly linkage is directly connected to the promary butterflys. By the placement of the link, the secondary starts later and opens faster than the primary.
Fuel is metered to the secondary by the volume of air flow opening the secondary air dam. Low air flow, minimal fuel to secondaries, more air flow, more fuel. With a correctly set pre load spring, the fuel is metered in line with the engine demand.
If you do not have secondarys in second/third, given that the spring adjustment is correct, there must be a mechanical interference preventing them from opening.
One possible (and probable) cause is the choke lock out of the secondary air dam when the choke is on. Look on the passenger side of the carb and you will see an arm with a notch on the top which moves forward when the choke is on and prevents rotation of the sec air dam until the choke is off. The choke adjustment could be incorrect and preventing sec air dam operation.
Good Luck on your troubleshooting. It is surely a mechanical problem and close attention to detail will trace it down.
Fuel is metered to the secondary by the volume of air flow opening the secondary air dam. Low air flow, minimal fuel to secondaries, more air flow, more fuel. With a correctly set pre load spring, the fuel is metered in line with the engine demand.
If you do not have secondarys in second/third, given that the spring adjustment is correct, there must be a mechanical interference preventing them from opening.
One possible (and probable) cause is the choke lock out of the secondary air dam when the choke is on. Look on the passenger side of the carb and you will see an arm with a notch on the top which moves forward when the choke is on and prevents rotation of the sec air dam until the choke is off. The choke adjustment could be incorrect and preventing sec air dam operation.
Good Luck on your troubleshooting. It is surely a mechanical problem and close attention to detail will trace it down.
Thread Starter
Burning Brakes
Joined: Aug 2001
Posts: 1,002
Likes: 0
From: Bay City MI.
Thanks for the info about the choke, I will look at it this weekend. Seems to make sense about the vacuum, because once you hit it the motor goes to 0 vacuum. In fourth on the highway the secondaries open at will with no hesitation.
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