Q-Jet Idle
Thread Starter
Drifting
Joined: Mar 2006
Posts: 1,526
Likes: 2
From: Oak Ridge NC
Q-Jet Idle
I was doing some tuning now that I have an open exhaust system.
I was looking in the carb while it was idling and I noticed fuel dripping from one of the veturies. I don't recall seeing this before. I also noticed that my airhorn gasket is a little wet on that side.
I am getting a little black smoke when I flick the throttle.
Is my float set too high?
I was looking in the carb while it was idling and I noticed fuel dripping from one of the veturies. I don't recall seeing this before. I also noticed that my airhorn gasket is a little wet on that side.
I am getting a little black smoke when I flick the throttle.
Is my float set too high?
Pro
Joined: Nov 2004
Posts: 607
Likes: 0
From: North Alabama
I was doing some tuning now that I have an open exhaust system.
I was looking in the carb while it was idling and I noticed fuel dripping from one of the veturies. I don't recall seeing this before. I also noticed that my airhorn gasket is a little wet on that side.
I am getting a little black smoke when I flick the throttle.
Is my float set too high?
I was looking in the carb while it was idling and I noticed fuel dripping from one of the veturies. I don't recall seeing this before. I also noticed that my airhorn gasket is a little wet on that side.
I am getting a little black smoke when I flick the throttle.
Is my float set too high?
Instructor
Joined: Jun 2004
Posts: 220
Likes: 0
Had the same issue with mine. I down loaded all of Lars' papers on Q-jets and checked all the settings. In my case the power valve height was not properly set, propably because I was messing with the adjuster when I didnt know what it was for! Lars set me straight.
Tech Contributor
Joined: Aug 1999
Posts: 14,375
Likes: 6,375
From: At my Bar drinking and wrenching in Lafayette Colorado
Fuel dripping out the main discharge nozzle at idle is not right and can be indicative of several issues:
Most common is high float level or a sunk float. Replace your float with a new NitroFill float from NAPA (part number 2-440 for 69-74 and 2-442 for 1975+). Set float level to .375 for the pre-75 carbs and set to .420" for 75+.
If it still drips, it's usually because the throttle is open too far. Be sure your timing is set up right (36 total) and run your vacuum advance off direct manifold vacuum instead of ported. This will allow you to close down the idle speed screw.
If the carb is running lean (as it may be with the open exhaust), this will cause you to have to crank the idle speed screw in a little, and this will cause the drip. Check the power piston seated height and make sure it's not too low (lean). The inner brass sleeve on the power piston should be about .020" - .030" above the top lip of the plastic retaining collar when fully seated. Also, make sure the power piston is staying seated at idle (stick a probe down the vent tube with the engine running to verify the piston is seated).
If you post your carb number and jet/rod combo, I can take a look at it and give you an opinion on your setup.
Most common is high float level or a sunk float. Replace your float with a new NitroFill float from NAPA (part number 2-440 for 69-74 and 2-442 for 1975+). Set float level to .375 for the pre-75 carbs and set to .420" for 75+.
If it still drips, it's usually because the throttle is open too far. Be sure your timing is set up right (36 total) and run your vacuum advance off direct manifold vacuum instead of ported. This will allow you to close down the idle speed screw.
If the carb is running lean (as it may be with the open exhaust), this will cause you to have to crank the idle speed screw in a little, and this will cause the drip. Check the power piston seated height and make sure it's not too low (lean). The inner brass sleeve on the power piston should be about .020" - .030" above the top lip of the plastic retaining collar when fully seated. Also, make sure the power piston is staying seated at idle (stick a probe down the vent tube with the engine running to verify the piston is seated).
If you post your carb number and jet/rod combo, I can take a look at it and give you an opinion on your setup.
Thread Starter
Drifting
Joined: Mar 2006
Posts: 1,526
Likes: 2
From: Oak Ridge NC
Lars,
7040205. Before I adjusted it 78/49 AX. I changed it to 78/48 CC on your advice after I put the flowthrough exhaust on--stock manifolds with dynomax.
I changed and adjusted the the float ( NAPA 2-440 at about .375) this afternoon and reset the idle. I am getting 16" vacuum at 700RPM. Power piston height is about .025. It is fully seated at idle.
Dripping is gone. Old float was too high..about .2.
I have weaned the car off of a race gase/pump gas blend(blended at about 100 octane). The advance is 32 total in at 2500. I get pinging at 34.
Idle and vacuum is pretty stable. But I am still getting the puff of black smoke when flicking the throttle hard a couple of times. Could it be that I should go back to 48 primaries?
I have the TCS installed, so no vacuum advance at idle. The vacuum can is stock 12 degrees and only comes in in 3rd/4th gear. Perhaps I should bypass the TCS, reset the idle and see if the smoke is still there.
What do you think?
Paul
7040205. Before I adjusted it 78/49 AX. I changed it to 78/48 CC on your advice after I put the flowthrough exhaust on--stock manifolds with dynomax.
I changed and adjusted the the float ( NAPA 2-440 at about .375) this afternoon and reset the idle. I am getting 16" vacuum at 700RPM. Power piston height is about .025. It is fully seated at idle.
Dripping is gone. Old float was too high..about .2.
I have weaned the car off of a race gase/pump gas blend(blended at about 100 octane). The advance is 32 total in at 2500. I get pinging at 34.
Idle and vacuum is pretty stable. But I am still getting the puff of black smoke when flicking the throttle hard a couple of times. Could it be that I should go back to 48 primaries?
I have the TCS installed, so no vacuum advance at idle. The vacuum can is stock 12 degrees and only comes in in 3rd/4th gear. Perhaps I should bypass the TCS, reset the idle and see if the smoke is still there.
What do you think?
Paul
Tech Contributor
Joined: Aug 1999
Posts: 14,375
Likes: 6,375
From: At my Bar drinking and wrenching in Lafayette Colorado
Paul -
Put some direct manifold vacuum to your vacuum advance and see how the engine responds. Also, knock the roll pin out of your accel pump lever so that the accel pump is disconnected - then flick the throttle and see if you get a bad bog or stumble. If not, and the black puff goes away, you're running rich. You can either lean it out a little or set up the accel pump on the outer hole in the lever.
Put some direct manifold vacuum to your vacuum advance and see how the engine responds. Also, knock the roll pin out of your accel pump lever so that the accel pump is disconnected - then flick the throttle and see if you get a bad bog or stumble. If not, and the black puff goes away, you're running rich. You can either lean it out a little or set up the accel pump on the outer hole in the lever.
Thread Starter
Drifting
Joined: Mar 2006
Posts: 1,526
Likes: 2
From: Oak Ridge NC
Paul -
Put some direct manifold vacuum to your vacuum advance and see how the engine responds. Also, knock the roll pin out of your accel pump lever so that the accel pump is disconnected - then flick the throttle and see if you get a bad bog or stumble. If not, and the black puff goes away, you're running rich. You can either lean it out a little or set up the accel pump on the outer hole in the lever.
Put some direct manifold vacuum to your vacuum advance and see how the engine responds. Also, knock the roll pin out of your accel pump lever so that the accel pump is disconnected - then flick the throttle and see if you get a bad bog or stumble. If not, and the black puff goes away, you're running rich. You can either lean it out a little or set up the accel pump on the outer hole in the lever.
I disconnected the pump lever. Partial flicking did not produce a bad bog or stumble. Full flicking(to WOT) did produce a temporary bog but it almost immediately picked up. So I put the rod in the outer hole. This eliminated the black smoke. However, I lose power power at WOT at about 4000RPM. I think it might be that the secondaries are not kicking in...or they are kicking in but there is not enough gas.
I re-routed the vacuum hose to intake manifold and plugged the open carb vacuum ports. I have to really put the idle mixture screws in before I can get any response out of the Idle speed screw. Doesn't idle very well...too quiet
(possibly too lean?) I get the noticable rythmic thump when I am running my normal set up with the TCS. There doesn't appear to be any difference in performace when at WOT.I was thinking of going back with the TCS set-up and reset my secondaries and see if that solves my problem.
What do you think?
