Q-Jet Carb Base Gasket Tech Info: The "Hot Slot" Manifold Problem
12-14-2016, 10:51 AM
#21
Team Owner
Where does this exhaust gas come from? assuming that the intake only has a connection to the intake side of the heads, I don't understand where the exhaust comes from.
I have a 1969 350 that came without an original intake. I purchased an intake (69 350) that has the hot slot. and purchased a 69 QuadraJet. Did I receive a wrong part? Can it still be used with the above mentioned method for sealing off the port (either with the proper gaskets or with plugs)?
I have a 1969 350 that came without an original intake. I purchased an intake (69 350) that has the hot slot. and purchased a 69 QuadraJet. Did I receive a wrong part? Can it still be used with the above mentioned method for sealing off the port (either with the proper gaskets or with plugs)?
The first is a pic of a bone stock '61 283 with holes marked; the second is the intake off an original 63 327....you can see where the heat passed darkened the paint at the runners.
Last edited by Frankie the Fink; 12-14-2016 at 10:53 AM.
12-14-2016, 10:56 AM
#22
also a stock GM holley intake manifold with a hot slot will not seal with a non OEM holley carb because the base plate will not cover the hot slot.. you need to plug the holes in the hot slot to prevent a exhaust leak there.
12-14-2016, 11:03 AM
#23
on the 3X2 BBC carb set up you need to make sure the gasket is position correctly on the center carb gasket or you will have a vacuum leak.
Last edited by PAmotorman; 12-14-2016 at 11:17 AM.
12-14-2016, 12:05 PM
#24
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Thank you all for clearing that up!
so the "crossover" is the source.
I found that there was quite a bit of Carbon (chunks like coal) in the drip plate on the underside of the intake. I was able to get it cleaned out but wonder if this is a common occurrence? or Do I have a crack that is venting crossover gasses into the drip plate... since the drip plate is riveted in place, I was unable to get a good look between it and the bottom of the intake...
so the "crossover" is the source.
I found that there was quite a bit of Carbon (chunks like coal) in the drip plate on the underside of the intake. I was able to get it cleaned out but wonder if this is a common occurrence? or Do I have a crack that is venting crossover gasses into the drip plate... since the drip plate is riveted in place, I was unable to get a good look between it and the bottom of the intake...
12-14-2016, 01:36 PM
#25
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Thank you all for clearing that up!
so the "crossover" is the source.
I found that there was quite a bit of Carbon (chunks like coal) in the drip plate on the underside of the intake. I was able to get it cleaned out but wonder if this is a common occurrence? or Do I have a crack that is venting crossover gasses into the drip plate... since the drip plate is riveted in place, I was unable to get a good look between it and the bottom of the intake...
so the "crossover" is the source.
I found that there was quite a bit of Carbon (chunks like coal) in the drip plate on the underside of the intake. I was able to get it cleaned out but wonder if this is a common occurrence? or Do I have a crack that is venting crossover gasses into the drip plate... since the drip plate is riveted in place, I was unable to get a good look between it and the bottom of the intake...
https://www.corvetteforum.com/forums...-manifold.html
12-15-2016, 07:46 AM
#27
Team Owner
Plugging the hot slot does not prevent the intake runners from passing the exhaust heat below the carb in the intake. It can help a bit (and prevent some carb base corrosion is incorrect gaskets are used) but blocking off the runner is the best approach if you're fighting fuel perc and the like... BTW - you need to wire open or eliminate the passenger side exhaust heat riser valve if you do this..
12-15-2016, 05:00 PM
#28
Not fighting fuel perc here. I did use the intake gasket runner block off inserts, but I don't expect them to last. I'd like to eliminate the heat riser valve completely. Is there a spacer available to take it's place?
12-15-2016, 05:02 PM
#29
Team Owner
Yes - all the vendors sell them...its just a triangular-ish hollow spacer.... Prob got 'em at Jegs and Summit too...
12-15-2016, 05:11 PM
#30
if you use stainless steel for the block offs they will out last the car.
12-15-2016, 05:37 PM
#31
They came in the Fel Pro $350.00 premium gasket kit. I assembled the engine, but I don't remember what the material was. Shiny metal, so I guess maybe stainless. That was the last thing on my mind at the time.
02-26-2017, 05:15 PM
#32
Hey Lars, I have an old holley 650 double pumper on my 327/350 vette. Its not been running too well lately and I was thinking of having my original Q jet rebuilt and install that. Are you still rebuilding Q jets? Do you think buying a new holly double pumper is a better choice then a referbed Q jet? I drive around town mostly but I like the Hollys performance when i want to get on it.
02-26-2017, 07:40 PM
#33
when I was rebuilding carbs and someone had a holley spread bore to rebuild I would ask if they had the original Q jet. if they did I told them I would rebuild the Q jet and if you did not like it better than the holley I would rebuild the holley free. never had to do that. 
02-26-2017, 08:53 PM
#34
Melting Slicks
[QUOTE=greggome;1594180302]...I have an old holley 650 double pumper on my 327/350 vette. Its not been running too well lately and I was thinking of having my original Q jet rebuilt and install that....
Unless your 327/350 Corvette is a C3 (1968 in particular), it was not born with a Quadrajet.
Unless your 327/350 Corvette is a C3 (1968 in particular), it was not born with a Quadrajet.
02-26-2017, 10:42 PM
#35
Its a 68 and yes I have the original Q jet which was on the car when my brother purchased it new.
[QUOTE=SI67;1594181847]
[QUOTE=SI67;1594181847]
01-24-2018, 09:33 PM
#36
Instructor
Wow, what an incredibly intriguing and helpful thread. Thanks to Lars and everyone else for increasing my understanding. I have the "smiley face" intake manifold and someone in the past has installed the 3-piece gasket set (stainless in the middle), but both the bottom AND top gasket are smiley. Any problem with the top also having the hot slot? FYI, someone in that past has indeed plugged the exhaust gas holes in the aluminum manifold. I'm thinking I'm OK since the holes are plugged but of am open to comments. Thanks!
01-25-2018, 11:06 AM
#37
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The "factory" setup only used 2 of the 3 gaskets: The bottom hi-temp gasket with the smiley-groove, and the top stainless shield. The carb was mounted directly to the stainless shield so it would heat the bottom of the carb up for better cold-weather driveability. Since a hot carb doesn't do much for performance, the top gasket is added to insulate the carb base a little from the hot stainless shield, so it doesn't matter much if it has a slot in it as long as it prevents the carb from touching the hot shield. In your case, with the plugs in the slot, it becomes irrelevant, as long as you have a complete seal at the carb base. 
Lars
Lars
01-25-2018, 12:10 PM
#38
Instructor
The "factory" setup only used 2 of the 3 gaskets: The bottom hi-temp gasket with the smiley-groove, and the top stainless shield. The carb was mounted directly to the stainless shield so it would heat the bottom of the carb up for better cold-weather driveability. Since a hot carb doesn't do much for performance, the top gasket is added to insulate the carb base a little from the hot stainless shield, so it doesn't matter much if it has a slot in it as long as it prevents the carb from touching the hot shield. In your case, with the plugs in the slot, it becomes irrelevant, as long as you have a complete seal at the carb base.
Lars
Lars
01-26-2018, 07:35 PM
#39
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I installed the same sandwich Lars pictured on my '67 GTO back in 2010, after dealing with percolation issues for years. I also re-installed the disconnected fuel return system on the car. No more percolation, even on 100 degree Fresno summer days. Many people don't even know that the steel plate must be used, and simply install a single gasket. Not a great plan. Great article, as usual.....thank you, Lars.
07-28-2018, 12:59 PM
#40
Posting my misfortune for others to learn from. The story. I have a 1968 327 in an 59 chevy pickup or did have. I opted to use the freeze plug method to block them off. I used the 7/16 size recommended. I prepped the holes by wire brushing to bare metal, the holes where not pitted or damaged. I tapped them home with a dab of high heat sealer. They were very snug or so I thought. I let it sit for a couple days. Put the carb back on and started it up. Engine ran for about 4-5 minutes then pop pop and then a series of loud clunks, I quickly shut it off. the silence was filled with the sound of coolant gurgling and cascading down through the engine. This just happened so I haven't assessed the damage but I'm sure it isn't good.