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float bowl boiling over

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
250
Location
Michigan
Corvette
1977 light blue
I've been having an issue with E10 boiling out of my float bowl after the engine is warm It's a basic l48, with an aftermarket cam. My 77 has basically the same engine, with an Edlebrock intake. I do not have this with the 77, but I do with the 79. Ive heard the coolant passage that goes under the carb can possibly cause this, and blocking this passage will keep things cooler. I've been running straight RV gas, and it doesn't boil. The E10 has a lower boiling point. Straight RV gas is over a dollar more, and not universally available. Any input as to how I can solve this would be appreciated.
 

Tom Bryant

Well-known member
Administrator
Joined
Nov 9, 2000
Messages
7,279
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
The passage under the carb is not a coolant passage. It's an exhaust passage that heats the carb during warmup when the heat riser valve is closed to get the carb and choke warmed up quicker. Many aftermarket intake gasket sets will have a steel block-off plate that you can position over that opening on both heads to stop the heat from reaching the bottom of the carb. This will slow down choke warmup and you might have to set your choke accordingly so it will open all the way. Another thing to consider is an insulating carb spacer either made of wood fiber or a Phenolic material. Summit, Jegs and others have a multitude of choices but keep in mind that you have limited hood clearance. You might need to stick with something in the 1/4 inch range. Wad up a ball of aluminum foil and lay it on the tallest part of the air cleaner and close the hood. The amount this crushes down to will tell you how much room you have to add a spacer.

Another thing to check is that the heat riser valve is opening when warm, assuming that you still have the factory manifolds and not headers. These are known for sticking. If yours has the vacuum diaphragm actuated one and not the thermal spring type check that the diaphragm is working. If it is stuck closed almost all of the exhaust from that side will be passing under the carb to exit out the other side making it very hot.

Tom
 

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
250
Location
Michigan
Corvette
1977 light blue
The passage under the carb is not a coolant passage. It's an exhaust passage that heats the carb during warmup when the heat riser valve is closed to get the carb and choke warmed up quicker. Many aftermarket intake gasket sets will have a steel block-off plate that you can position over that opening on both heads to stop the heat from reaching the bottom of the carb. This will slow down choke warmup and you might have to set your choke accordingly so it will open all the way. Another thing to consider is an insulating carb spacer either made of wood fiber or a Phenolic material. Summit, Jegs and others have a multitude of choices but keep in mind that you have limited hood clearance. You might need to stick with something in the 1/4 inch range. Wad up a ball of aluminum foil and lay it on the tallest part of the air cleaner and close the hood. The amount this crushes down to will tell you how much room you have to add a spacer.

Another thing to check is that the heat riser valve is opening when warm, assuming that you still have the factory manifolds and not headers. These are known for sticking. If yours has the vacuum diaphragm actuated one and not the thermal spring type check that the diaphragm is working. If it is stuck closed almost all of the exhaust from that side will be passing under the carb to exit out the other side making it very hot.

Tom
That's very interesting. Now I remember what I heard. It's exhaust, not coolant. I'll check the heat riser issue also. I was thinking of changing the gaskets to keep the float bowl cooler.
Thank you Tom
 

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
250
Location
Michigan
Corvette
1977 light blue
That's very interesting. Now I remember what I heard. It's exhaust, not coolant. I'll check the heat riser issue also. I was thinking of changing the gaskets to keep the float bowl cooler.
Thank you Tom
Tom, I have a heat riser, but there is no linkage attached to it. I can rotate it clockwise, counterclockwise. It was rotated to the left (counterclockwise). Can you tell me which way opens and closes the heat riser? I have a feeling that is my problem. If I can keep it open, then my issue maybe solved. The thing is, it wasn't doing this last spring, but it started last summer. I just thought it was summer heat.
 

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
250
Location
Michigan
Corvette
1977 light blue
Tom, I have a heat riser, but there is no linkage attached to it. I can rotate it clockwise, counterclockwise. It was rotated to the left (counterclockwise). Can you tell me which way opens and closes the heat riser? I have a feeling that is my problem. If I can keep it open, then my issue maybe solved. The thing is, it wasn't doing this last spring, but it started last summer. I just thought it was summer heat.
Alright, I found my heat riser from my 77, and it was in the open position. The heat riser was not stuck closed. I was hoping this was the issue, but It's not.
 

stevolwevol

Well-known member
Joined
May 28, 2015
Messages
250
Location
Michigan
Corvette
1977 light blue
So I ruled the hear riser out. I'm thinking about blocking the passage that warms the carb with a new intake gasket and blocking the ports. If this cools the float bowl, then maybe this will be something I should do. I did not have this with my 77 even with the stock intake, and they are essentially the same engine. If I do nothing, I can resume running just on RV straight gas. geesh!
 

Tom Bryant

Well-known member
Administrator
Joined
Nov 9, 2000
Messages
7,279
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
Sorry I've been away for a few days. It's hard to figure why one car will boil the fuel in the carb and another with the same setup won't. Since you know which way is closed have you observed if the valve is being forced closed when the engine revs? If the actuator and rod are not connected or missing I'd attach a spring or wire it so it can't close. With this possibility eliminated I'd add an insulator spacer under the carb first because it's easiest, and then put the block off plates in the intake gaskets if that doesn't cure it.
 

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