Q-Jet problems - Team Camaro Tech
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post #1 of 12 (permalink) Old Apr 21st, 01, 11:40 AM Thread Starter
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My 396 is giving me fits trying to get it to idle correctly. Here's what i have, 396 with mild RV grind cam, Melling MTC-6 I think, stock intake with the crossover for the choke, turbo 350 trans,with stock converter, and the stock Q-Jet, It will not idle below 1200 rpm for nothing! If I put my hand over the choke horn the rpm's go up and it idles fine. I have checked for vaccum leaks and cannot find any, I even changed the baseplate on the carb, thinking it may have a crack somewhere,I am thinking it may be the baseplate because of the stainless steel shim type gasket that installs for the heat crossover, which goes on first if I remember right? anybody have any suggestions as to what my next move would be? and no I don't want to put a Holley on it

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69 rallye green X77 Z/28,1967 SS 396 Conv. 1974 c-10 454 swb
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post #2 of 12 (permalink) Old Apr 21st, 01, 03:05 PM
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You have a giant vacuum leak. Did you change anything before the problem started? Or did it just start all of a sudden.

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post #3 of 12 (permalink) Old Apr 23rd, 01, 02:39 AM Thread Starter
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Car never has idled properly since the rebuild, Have been running it just to move in and out of shop, sometimes it will idle around 1000 rpm and then it will just fall off and not idle at all. I haven't done any modifications to the carb or engine, but the idle has always been a problem. Could the heat crossover be causing the trouble?
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post #4 of 12 (permalink) Old Apr 23rd, 01, 09:06 PM
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Pull the carb off and look at the gasket around the cross over.If it is leaking,you will be able to see the exhaust residue on the gasket.Check the manifold for warping,especially near the crossover.I have a customer with a 69 truck.We couldnt keep a base plate gasket in it,it would burn up.The manifold was warped.It had the same symptoms that you are describing.

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68 z28 ,68 rs 327 ,73 454 vette, 2 goofy kids
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post #5 of 12 (permalink) Old Apr 23rd, 01, 10:36 PM
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Does the car have power brakes? Try plugging all of the vaccuum ports on the carb (a brass screw in plug works good for the power brake port...if it has one) and see if there is any difference. How about a T fitting on one of the manifold runners? You definately have a vac leak somewhere. If nothing else, try a different q-jet and see if it makes any difference.

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375hp 78 Chevy truck
77 Chevy Nova
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post #6 of 12 (permalink) Old Apr 24th, 01, 02:43 AM Thread Starter
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So Dale, does the stainless steel gasket? shim? the thin piece go on the intake first? I have installed it both ways to try and eliminate the leak. What did you do for the guy with the 69 truck, which btw is what my 396 came out of, a 69 3/4 ton . I am going to get a known good Q-jet and install tonite and see what happens. Can I block off the exaust crossover to try and eliminate a potential leak or can the crossover cause a vaccum leak? btw, the engine seems to idle better when its is cold, then gets worse from there, finally when at full temp, it will not idle below 1000 rpm and its a little rough at that rpm. thanks for the imput, ain't going to let this get this best of me yet

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69 rallye green X77 Z/28,1967 SS 396 Conv. 1974 c-10 454 swb
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post #7 of 12 (permalink) Old Apr 24th, 01, 06:30 PM
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Sorry I bailed on that one.After 4 or 5 burnt up/sucked in gaskets,we installed a performer intake.Our problem was the gasket would get burned up by the exhaust.As far as I see, there has to be a gasket between the intake andstainless gasket.I tried filling the passageways with red silocone,lasted longer,but still burnt through.I also tried red silocone and then the stainless gasket,then a regular gasket.The problem was a warped intake right in the middle of the primary blades,kinda like the intake surface was eroded. Check to see if the gasket is not sealing,it's usually pretty obvious.

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68 z28 ,68 rs 327 ,73 454 vette, 2 goofy kids
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post #8 of 12 (permalink) Old Apr 25th, 01, 02:50 AM Thread Starter
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What about inserting a thin stainless shim between the intake and head on both sides of the intake where the heat crossover is located, seems I remember reading about this in one of the mags years ago. Thinking if I eliminate the sourse of the exaust heat I may be able to settle the idle down somewhat. Would that affect the function of the choke during mild weather starting 60-90 degrees, Boy, sure gets cold here in the winter thanks for the replies

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69 rallye green X77 Z/28,1967 SS 396 Conv. 1974 c-10 454 swb
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post #9 of 12 (permalink) Old Apr 25th, 01, 06:13 AM
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Make sure your heat riser is not stuck closed. That can cause your burning up of the gasket problem.

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post #10 of 12 (permalink) Old Apr 25th, 01, 07:22 AM Thread Starter
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no heat riser, I gutted it and use it for a spacer, I was thinking of getting one of the carb heatshields and trying it, what do you think?

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69 rallye green X77 Z/28,1967 SS 396 Conv. 1974 c-10 454 swb
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post #11 of 12 (permalink) Old Apr 25th, 01, 09:29 AM
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dale68z has the right idea somewhere in the manifolds life it may have had a bad heat riser and it got fried. Clean the manifold and put a straight edge across the carb gasket surfaces do it in every direction this will tell you how much warpage there is. It is pretty hard to warp that big a piece of iron but exhaust heat can do it if it's concentrated for a long enough period.

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post #12 of 12 (permalink) Old Apr 27th, 01, 03:10 PM
 
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Something to try involves some testing. You might try seeing which cyls are not firing. This carb and manifold is a dual plane system. The driver's side port will feed the cyls 1,4,6,7. The pass side will feed cyls2,3,5,8. A simple blocked port in the carb could cause this situation. Please do not be blind to the obvious. A vacuum leak could cause this, but so could carb problems. A piece of charcoal could hav eentered the carb through the bowl vent from the cannister. Good luck. Ray
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