Weird Q-Jet problem
Thread Starter
Safety Car
Joined: Apr 2002
Posts: 3,551
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From: No link, No dupe
Weird Q-Jet problem
Over the last two days, I've had a strange problem with the Q-Jet.
In normal driving, it's fine, it idles good, and part-throttle acceleration is good, cold or hot.
But, the problem I am having is when I simulate drag-strip conditions with my G-Tech. I was on some backroads, testing my 0-60mph time. I put it in first, and accelerated strongly to about 6,000rom. But, when I shifted to second, the car takes a nosedive and the engine runs rough. When I lifted off the throttle, the condition went away. Immediately afterwards, while still in second, I could floor it again and it reved fine. :crazy:
This problem also occurs if I am winding second out, and shifting to third under full acceleration - nosedive and rough running engine.
What in the world could be causing this? This seems to be a very strange problem, especially when it was fine a few days ago.
I don't know if this has anything to do with it, but I removed the Q-Jet last weekend to try out a Holley Carburetor. But, when I replaced the Q-Jet, I went out for a few G-Tech runs with no problems. It was maybe 3 days later when this crap started.
Thanks for any comments or suggestions :seeya :chevy
In normal driving, it's fine, it idles good, and part-throttle acceleration is good, cold or hot.
But, the problem I am having is when I simulate drag-strip conditions with my G-Tech. I was on some backroads, testing my 0-60mph time. I put it in first, and accelerated strongly to about 6,000rom. But, when I shifted to second, the car takes a nosedive and the engine runs rough. When I lifted off the throttle, the condition went away. Immediately afterwards, while still in second, I could floor it again and it reved fine. :crazy:
This problem also occurs if I am winding second out, and shifting to third under full acceleration - nosedive and rough running engine.
What in the world could be causing this? This seems to be a very strange problem, especially when it was fine a few days ago.
I don't know if this has anything to do with it, but I removed the Q-Jet last weekend to try out a Holley Carburetor. But, when I replaced the Q-Jet, I went out for a few G-Tech runs with no problems. It was maybe 3 days later when this crap started.
Thanks for any comments or suggestions :seeya :chevy
Melting Slicks
Joined: Feb 2000
Posts: 3,052
Likes: 0
From: Vancouver BC, Canada
Re: Weird Q-Jet problem (shotgun_000)
i sorta had a similar problem last summer. i raised the float slightly and it went away. but i've recently noticed a new high rpm stutter... thinking filter or pickup sock maybe causing a restriction. ???
Drifting
Joined: Aug 2002
Posts: 1,731
Likes: 22
From: Marietta Ga.
Re: Weird Q-Jet problem (shotgun_000)
Thread Starter
Safety Car
Joined: Apr 2002
Posts: 3,551
Likes: 0
From: No link, No dupe
Re: Weird Q-Jet problem (72GACRZR)
Thanks, that's one awesome tip sheet!
From reading, seems like I must have a problem with the secondaries. It still puzzles me as to why it's only during a shift under WOT.
At the worst, I've got a Holley spreadbore that I've been dying to try out. Once I do that, maybe I'll donate the Q-Jet to the forum loaner program :cheers:
From reading, seems like I must have a problem with the secondaries. It still puzzles me as to why it's only during a shift under WOT.
At the worst, I've got a Holley spreadbore that I've been dying to try out. Once I do that, maybe I'll donate the Q-Jet to the forum loaner program :cheers:
Race Director
Joined: Oct 2000
Posts: 15,132
Likes: 79
From: central FL, near the beach.
Re: Weird Q-Jet problem (shotgun_000)
i suspect it is the fuel pump.
easy to find out.
Put a T in the line and tape a gauge to the windshield for a test. watch it when u nail 2nd. report the PSI :cheers:
easy to find out.
Put a T in the line and tape a gauge to the windshield for a test. watch it when u nail 2nd. report the PSI :cheers:
Heel & Toe
Joined: Apr 2003
Posts: 22
Likes: 0
Re: Weird Q-Jet problem (shotgun_000)
The Qjet secondary air valves have a "pump shot" type of feature to provide fuel just as the air valve starts to open (look for a small hole just above the air valve). It is limited in fuel volume and it sounds to me like that system is causing your problem (if you are shifting just as the secondaries are opening). It is common for the supply tubes, which are only press fit into the air horn, to fall out. When you remove the air horn (can do this with the carb still on the car) you should see 4 tubes hanging down from it. If one has fallen out, it will be in a well in the metering body (the one that holds the fuel for the transition).
Another possible cause for this condition is the air valve control spring being a bit too loose, allowing the valve to pop open too fast. I suggest you run to 3000 rpm in second gear and THEN floor it and run to full rpm. You should get a clear bog (tempory) if the secondary air valves are not getting enough fuel as they open. If it pulls hard all the way, try it again but just let off the gas a bit as you hear / feel the secondaries coming on. If it bogs then, tighten the spring 1/8 turn.
Hope this helps.
Rich
Another possible cause for this condition is the air valve control spring being a bit too loose, allowing the valve to pop open too fast. I suggest you run to 3000 rpm in second gear and THEN floor it and run to full rpm. You should get a clear bog (tempory) if the secondary air valves are not getting enough fuel as they open. If it pulls hard all the way, try it again but just let off the gas a bit as you hear / feel the secondaries coming on. If it bogs then, tighten the spring 1/8 turn.
Hope this helps.
Rich
Tech Contributor
Joined: Aug 1999
Posts: 14,384
Likes: 6,409
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Weird Q-Jet problem (shotgun_000)
shotgun -
2 possibilities: Either secondary airvalve spring adjustment or a lean-out cause by inadequate fuel delivery. The latter usually happens at the 2nd gear shift point or part-way into 2nd gear.
2 possibilities: Either secondary airvalve spring adjustment or a lean-out cause by inadequate fuel delivery. The latter usually happens at the 2nd gear shift point or part-way into 2nd gear.
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Thread Starter
Safety Car
Joined: Apr 2002
Posts: 3,551
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From: No link, No dupe
Re: Weird Q-Jet problem (lars)
shotgun -
2 possibilities: Either secondary airvalve spring adjustment or a lean-out cause by inadequate fuel delivery. The latter usually happens at the 2nd gear shift point or part-way into 2nd gear.
2 possibilities: Either secondary airvalve spring adjustment or a lean-out cause by inadequate fuel delivery. The latter usually happens at the 2nd gear shift point or part-way into 2nd gear.
I made some progress. After doing a visual check, I found a kink in one of the metal fuel lines. I think it's been there for awhile, but when I was swapping carbs I may have moved the line just enough to start causing a problem. I replaced that section of fuel line. Afterwards, I went out for another full throttle blast, and the problems seems to be gone :party:
I am keeping in mind that this could be a coincidence, and that the problem might come back.
Thanks Lars, and everyone else for the help :thumbs: :cheers:
Burning Brakes
Joined: Apr 2003
Posts: 844
Likes: 0
From: Central KY
Re: Weird Q-Jet problem (shotgun_000)
Funny story - I had a '69 Nova SS that did this. Turned out that the throttle was touching the distributor coil wire and shorting out the ignition when I floored it! Be sure you don't have a similar issue before tearing into the carb...
Tech Contributor
Joined: Aug 1999
Posts: 14,384
Likes: 6,409
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Weird Q-Jet problem (shotgun_000)
Kinked fuel lines - both in the metal lines and in the rubber hoses - are very common problems on these cars, and this will cause the problem you describe. Good job tracking it down - I'll bet it solves your problem.