Q-jet problem?
Thread Starter
Drifting
Joined: Mar 2002
Posts: 1,810
Likes: 0
From: Indian River MI
Q-jet problem?
72 350 Q-jet carb. Runs great until flooring it. At 3300 to 3500 rpm's it stumbles and finally will backfire through the carb. Once pass this, it will run to red line but with lack of power.
Fresh tune up. Rebuilt carb.
What would you do next?
Fresh tune up. Rebuilt carb.
What would you do next?
Drifting
Joined: Nov 2001
Posts: 1,469
Likes: 6
From: 68 427 4.11s Roadster
Not sure of the backfire but would try tightening the airvalve more.
Secondaries opening too early result in a bog and then the engine uses the increased air flow. Yours sound way too loose.
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Tiny screw on the shaft is spring tensioner.
Tiny allen lock screw under the shaft. Very tiny.
Hold screw driver in place and loosen lock screw.
Tighten (or loosen) screw driver not more than 1/8 of a turn- hold and lock set screw.
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Try tightening the screw 1/8 turn per test. If it goes away then your just about where you want to be. Loosen 1/16 of turn which is nearly impossible to see but you get the idea. If this does it you will adjust right around this spot till your happy. Or just leave it where you don't have any bog.
This test requires WOT blasts. You only need to test this past your bog point, it may change.
Secondaries opening too early result in a bog and then the engine uses the increased air flow. Yours sound way too loose.
-------------
Tiny screw on the shaft is spring tensioner.
Tiny allen lock screw under the shaft. Very tiny.
Hold screw driver in place and loosen lock screw.
Tighten (or loosen) screw driver not more than 1/8 of a turn- hold and lock set screw.
-------------
Try tightening the screw 1/8 turn per test. If it goes away then your just about where you want to be. Loosen 1/16 of turn which is nearly impossible to see but you get the idea. If this does it you will adjust right around this spot till your happy. Or just leave it where you don't have any bog.
This test requires WOT blasts. You only need to test this past your bog point, it may change.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Backfire through the carb is a lean condition. That rpm range is where the air valve begins to open. You may want to go a step or two richer on the secondary metering rods. Also make sure the small holes at the leading edge of the air valve door are functioning. These are a tip in fuel enrichment feature of most Q-jets. If your carb has these but they don't work it will cause the problem you describe.
BigBlockk
Later.....
BigBlockk
Later.....
