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Replacing Qjet with ??

1K views 8 replies 8 participants last post by  BlackStrat67 
#1 ·
383 sbc 10.5:1, comp xe274 234/242@.050, dart iron eagle platinum 200cc heads, edelbrock performer rpm intake, th350 w/2400 stall, 3.31 posi in a 1967 chevelle. Street use, but i like to beat the crap out of it.

Right now i'm running one of the 800cfm qjet w/ 74 primary jets, 44L primary rods, DA secondary rods, .070 air bleeds, .090 idle adjustment, .036 idle tubes. It's crazy rich, like fouling plugs and dropping cylinders at idle. Fuel pressure is at 5psi and i cant see it leaking anywhere. I'm guessing i made some air bleed bigger than it was supposed to be. All in all i'm tired of messing with it and want to replace it w/ a holley or something else like a quick fuel technology carb. I called the quick fuel tech line and they recommended a 680 vacuum secondary unit. That sounded small to me but i wanted to know what some of you guys thought.
 
#3 ·
Hey - You didn't mention if you put a smaller/lighter spring under the Q-jet's Power Piston. If your engine has any lope at all, the Power Piston is likely staying in the "up" position (full rich) at idle causing that eye-watering, over-rich condition.

Edelbrock sells an assortment pack of Power Piston springs. (Edelbrock #350-1994) Many speedshops carry them or you can order from Jeg's for $6.99
http://www.jegs.com/i/Edelbrock/350/1994/10002/-1?parentProductId=753848

If in doubt, you can remove the PP spring altogether and reinstall the PP without it. Restart your engine and see if it still exhibits the over rich idle condition. If the mixture is now normal, it's indeed too stiff of a spring and the lower idle vacuum isn't strong enough to keep the PP pulled down at idle.

If you're using the stock spring, it's probably opening the PP at anything below 10"-12" of engine vacuum. The Springs in the Edelbrock kit lower that to 4, 5, 6 or 8" of vacuum. Hook up a vacuum gauge at idle and see what reading you've got. Start with a spring rated 4-5 inches less than your idle vacuum. That way the PP will stay closed (down) at idle, but will open (move up) when you hit the gas, giving you the enrichment you need for acceleration but giving you the leaner setting for idle. Hope this helps!
Kevin
 
#6 ·
Suggest you buy one of the books out there on tuning Qjets. Best street carb you can have if you have it tuned right. Your engine is very seldom going to need all 800 cfm cruising on the street but it's there if you need it.
Cranking up the initial is a good idea. Do that and tune your carb and I think you'll be very happy.
 
#9 ·
Thanks for all the info. I did get carried away w/ the drill bits. It worked really well for a week or there abouts, always a touch rich, but it gradually got more and more rich as time went on. The performance was awesome, but it is where it is now. I thought the same thing about the power piston coming up early b/c of lack of vacuum and stuck a small screw driver down the vent tube to see if i could push it down at all but it was bottomed out. I'd like to make it work to save the money. I'm running 16 degrees initial timing and made it so im at 36 total mechanical advance. How would i go about resizing the idle circuit orifices? I thought that was the problem just didn't know how to make the holes smaller.

Also i just broke the nose off the starter this morning and broke 3 teeth off the flexplate. I didn't have the brace on it, didn't have any shims in it b/c everything lined up well. it was a brand new NAPA starter w/ a new flexplate, on the car for a year or so. So while replacing the flexplate i was thinking about replacing the converter as well.
 
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