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Author Topic: Q-Jet swap problem  (Read 6487 times)
snowballfisher
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« on: January 07, 2006, 09:52:11 AM »

I am trying to do away with a Holley Q-jet replacement and go back with a Q-jet.  I found one finally but installed it and I have a huge vacuum leak at the front base of the carb to intake.  The intake has a long skinny port going from the left to right and the carb doesn't seem to match it.  The carb hangs over enough in front of the intake that it exposes some sort of port on the bottom of the carb.  Will all Q-jet's not interchange?  I had always heard that they would.  I also could only find a thin carb base gasket to fit both intake and carb.  When I got the car it had a thin and a thick one mounted on it.  I couldn't find a thick mounting gasket that would cover all of my ports in the intake and carb.  Should I give up on this carb or keep at it?  I believe that the Q-jet I have is post 1975.  I was told by a Holley specialist to throw my Holley away because it was such a poor design and parts are pretty expensive for it.  We agreed that I should try to find a Q-jet for originality, tuneabitlity and hood clearance.  I just need to know if I should be able to find gaskets to make this project work, or try to find an older Q-jet?
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JohnZ
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« Reply #1 on: January 07, 2006, 12:52:55 PM »

Can't help you with the mismatch issue, but if your intake has the "hot-slot" like the one in the photo below (that's an intake for a Holley, but the same "hot-slot" design was used on many cast-iron Q-Jet intakes), I recommend plugging the holes at each end of the slot with core plugs as shown in the photo. That slot is connected to the exhaust crossover passage, and allows hot exhaust gases to heat the carb base at cold start for improved atomization of the air-fuel mixture, with a stainless steel heat baffle to protect the carb base from direct impingement of the hot gases. Unfortunately, that heat also fries/distorts the carb base and can cause the bottom well plugs to fall out of the Q-Jet float bowl, resulting in engine fires. GM recalled millions of Q-Jet-equipped cars with this "hot-slot" design in 1969 due to the fire problem, and abandoned the "hot-slot" design in 1970.

What's the casting number on your intake?
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'69 Z/28
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snowballfisher
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« Reply #2 on: January 07, 2006, 06:20:37 PM »

My casting # is 3931087.  I broke down the engine casting # and I believe it was a 1968 396 out of a p/u.  I just can't believe that this newer style Q-Jet won't fit it right.  It was enough of a leak that it was leaking fuel out of the front of carb where it mates to the intake.  I guess I could take some pics if it would maybe help with getting some answers.  I do appreciate the info on the hotslot.  I kind of figured it was connected to the crossover port because mine is almost carboned shut.  Can you tell me if a normal setup with a Holley or Q-Jet on an intake like the one in your pic uses a thick spacer gasket or did they just used the thin heat-shield gasket?  Also do you know what size brass core plugs you used on the holes?
----I've attached a file with a pic showing the problem with my installation!
« Last Edit: January 07, 2006, 07:14:51 PM by snowballfisher » Logged
sam
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« Reply #3 on: January 07, 2006, 11:49:13 PM »

I would say you have to find the right gasket. They should all fit. Especially those years.
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CNorton
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« Reply #4 on: January 08, 2006, 09:16:08 AM »

I agree with Sam.  The early carburetors should fit the early manifolds with little regard for the original application, big or small-block.  If raw gasoline is leaking onto the manifold I'd look for another problem (such as loose well plugs on the bottom of the carburetor).  Quadrajets have a well-known tendency to leak raw fuel from those plugs.  If it is merely a vacuum leak you may need to look for one of the Felpro gaskets that are at least .200"-.250" thick and incorporate a plastic bushing around each of the bolt holes.  I've also seen some of the early combinations that included a thin stainless steel shim gasket between the manifold and the thick composition gasket.  The steel shim kept the hot gasses from the cross-over from eating up the composition gasket.

Good luck.
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JohnZ
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« Reply #5 on: January 08, 2006, 03:28:22 PM »

The original configuration on intakes with the "hot-slot" had the gasket on the manifold, then the stainless baffle, then the carb; was used on many '66-'69 iron-intake combinations.
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'69 Z/28
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snowballfisher
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« Reply #6 on: January 08, 2006, 10:20:24 PM »

Okay guys, I'm confident that this thing is almost solved.  It seems like a gasket or two too many, but I think we're going to have to use the following... a thin gasket that matches the intake, then a steel shim gasket that blocks the hot slot, then a thick composition gasket to get the carb to seal to.  These last two entries really helped a lot.  I think this combination will work well.  All I have installed right now is the thin gasket, then the comp. gasket.  But I could see how you need something in between to seal off all of that heat.  I also still have a tiny gap at the frt. drivers side where the hot slot was longer than the gasket.  If I get that shim in there, it has the correct shape to cover what remains open.  Now if I can just get this stupid Q-jet to work.  I fought it for a while today and finally pulled it back off.  The secondaries couldn't open because someone had pinched the baseplate gasket too tightly when they rebuilt it last and the secondary throttle plates were sticking on the cardboard like material whenever they tried to open.  It's just one headache after another, but I'm getting there with everyone's help from the forum.  I really appreciate it guys!
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67rs350ss
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« Reply #7 on: February 22, 2006, 08:20:46 PM »

From my own experience the thin metal plate has to go on the manifold first then the gasket then the carb. If you put a gasket over those heat passages it will get cooked.
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cib12
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« Reply #8 on: January 15, 2007, 07:36:44 PM »

67rs350ss is correct . if everthing is in place correctly everthing will work fine( i have worked on ............. in 35 years( millions do dollars of engineering should not go to waste- and always match up ss plate and gasket to the base of carb and also the intake surface. another clue is to always use the studs and not use bolts to fasten the carb(over the long term it does make a differance)
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JohnZ
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« Reply #9 on: January 16, 2007, 11:53:10 AM »

67rs350ss is correct .

No, he's not. The stainless baffle goes between the carb and the gasket, as shown in the Assembly Manual; the correct gasket has a slot in it that matches the "hot-slot" in the intake manifold.
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'69 Z/28
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camaronut
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« Reply #10 on: January 17, 2007, 10:33:36 PM »

Johnz hit it on the head....I followed the assembly manual, but........

I used another slotted gasket between the carb and metal plate for additional sealing.   

Just to make sure...
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Wallace
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« Reply #11 on: March 15, 2007, 10:14:46 AM »

I had the same problem with my 396.  I finally removed the intake manifold and plugged the exhaust crossover ports between the heads and manifold with metal plates which were included in the gasket kit.  I then went back to one thick gasket under the carb.  I live in the  Arizona desert, so the more heat I keep from my intake manifold and carb the better.

Wallace
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click
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« Reply #12 on: March 17, 2007, 01:35:33 PM »

Plugging those holes in the intake was the first thing my mechanic did when I went back from Edelbrock and Holley to cast iron and Qjet. Runs just fine. Smiley

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Click is Jim , central Minn.  Moderator at Team Camaro www.camaros.net
lcmc
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« Reply #13 on: March 23, 2007, 04:21:49 PM »

I just replaced the carb on my car with a correct dated carb. When I took the old carb off it just had the thin metal plate between intake and carb. When I put replacement carb on I put the metal plate on first then a gasket. I had a terrible vacuum leak so I removed tha gasket and just left the metal plate in. It seems just fine now. Is this right?
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Danny
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1970 Nova L78 9300 original miles
1968RSZ28
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« Reply #14 on: March 23, 2007, 04:42:47 PM »

No, this is the correct way:

The stainless baffle goes between the carb and the gasket, as shown in the Assembly Manual; the correct gasket has a slot in it that matches the "hot-slot" in the intake manifold.

Paul
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