tri carb vapor lock

[Galen]

I have a re-built engine and re- built tri-carb with 500 miles. I am experiencing vapor lock at the carbs preventing the flow of gas to the carbs. I have been through the entire fuel system and upon pulling the fuel lines I get a back pressure. The result is a cutting out at 3000 rpms. Any ideas or suggestions on how to fix this?

Thanks Bruce Galen

 

[GregW]

Do you have the heat shields installed under the carbs? I here more tales about vapor lock happening where the fuel pump sits over the exhaust pipe.

 

[Galen]

I replaced the asbestos panels with hardiplank. I suspect that heat is the issue here and possibly the fuel lines are at issue. I have just standard rubber fuel hose. You know the british did not spend extra money on manufacturing these cars but they did go to the extra expense of braided fuel lines for some reason. The fuel line is also at the highest point of the fuel system and air in the lines is the issue.

 

[AH100M]

Is the vent on your gas cap clear? Just a thought.

 

[HEALEYJAG]

Do not use gas with ANY alcohol additives or what used to be gasahol. I prefer high octane Sunoco only. Had big vapor lock with my V-12 e-type, only to figure out it happened when I used the wrong gas.

Pete

 

[Galen]

The fuel pump is on the driver's side opposite the muffler.

I use the highest gas octane which all of the types in Texas have ethonal.

I have completely isolated the fuel tank, changed fuel pumps, etc. Checked static flow every where and no flow issues except under load.

I have run down the road watching the clear in-line gas filter as I go thru the gears pumping from a 1 gallon gas jug. (This takes 2 people and a wide unoccupied street)

There exists a back pressure from the carbs because I can see bubbles blowing back into the filter.

The filter is not filling with gas because of the back pressure. This eliminates all issues past the fuel pump.

There is vapor lock because when I pull the fuel hose from the back carb (tri carb) there is back pressure. (Whoosh- air)

I am going to change the fuel lines at the carbs to braided line and if that doesn't work Im going to try a 7 psi fuel pump (regulated to 5 psi).

 

[GregW]

The filter is not filling with gas because of the back pressure.

I don't think I've ever heard (I know I've never seen) those filters ever filling up. They shouldn't be made see-thru. :eeek:

 

[Patrick67BJ8]

The fuel pump is on the driver's side opposite the muffler.

I use the highest gas octane which all of the types in Texas have ethonal.

I have completely isolated the fuel tank, changed fuel pumps, etc. Checked static flow every where and no flow issues except under load.

I have run down the road watching the clear in-line gas filter as I go thru the gears pumping from a 1 gallon gas jug. (This takes 2 people and a wide unoccupied street)

There exists a back pressure from the carbs because I can see bubbles blowing back into the filter.

The filter is not filling with gas because of the back pressure. This eliminates all issues past the fuel pump.

There is vapor lock because when I pull the fuel hose from the back carb (tri carb) there is back pressure. (Whoosh- air)

I am going to change the fuel lines at the carbs to braided line and if that doesn't work Im going to try a 7 psi fuel pump (regulated to 5 psi).

A higher pressure pump regulated to an even higher 5.0psi, than 3.5psi max for SU's might not be a good thing. When whatever is broke, fixes itself, you'll have other problems. Just my 1.5psi thought.
Patrick

 

[GregW]

There is vapor lock because when I pull the fuel hose from the back carb (tri carb) there is back pressure. (Whoosh- air)

I don't know much about SUs, but isn't there a vent in the float bowl that should keep that from happening?

 

[SideShifter Tri-Carb]

There is vapor lock because when I pull the fuel hose from the back carb (tri carb) there is back pressure. (Whoosh- air)

I don't know much about SUs, but isn't there a vent in the float bowl that should keep that from happening?

No, it has the check valve connected to the float. Gas would also come out if it did. The only way the check valve opens is if the gas level is too high or it's stuck

 

[tahoe healey]

Actually, there is a washer that has a cloverleaf internal pattern under the center bolt. I'm guessing it is a vent of sorts.
Are you using a vented gas cap?

 

[Galen]

Yes I am using a vented gas cap. But I have eliminated all potential issues from the carbs back to the fuel tank. the back pressure is at the carbs.

 

[AH100M]

Wow. Curious one.

A woosh back through the fuel line means that the air that's in the carb bowls has no place to go as it's being displaced by fuel being pumped in. You need a vent there someplace, like tahoe healey said above.

 

[richch]

We experienced a similar situation when getting a fully restored tricarb ready for the road a few years ago. Turned out to be a really bizzare situation.... Even though the carbs had been fully rebuilt by a professional, we finally found an old blockage in the vent tube on one of the float lids. It had been previously inhabited by some sort of insect which had virtually crawled in and mummified there, blocking the air passage. It was about the size of a large grain of rice but had to be dug out with a dental pick. Problem was solved.

 

[Patrick67BJ8]

We experienced a similar situation when getting a fully restored tricarb ready for the road a few years ago. Turned out to be a really bizzare situation.... Even though the carbs had been fully rebuilt by a professional, we finally found an old blockage in the vent tube on one of the float lids. It had been previously inhabited by some sort of insect which had virtually crawled in and mummified there, blocking the air passage. It was about the size of a large grain of rice but had to be dug out with a dental pick. Problem was solved.

I'm betting my money on this solution since I've heard of this happening once before.
Patrick

 

[Ed_K]

The top cover of the fuel bowl has to have a vent in it or;
1.. The fuel level would never be able to rise high enough to float the float up to close the little valve to stop the gas flow into a full fuel bowl.
2.. The build up of pressure in the top of the fuel bowl would force fuel out into the carb when it wasn't needed making for a very rich mixture entering the engine when at partial throttle.
3.. The build up of pressure in the top of the fuel bowl would slow down or stop the fuel in the fuel line from entering the carb until the pressure was released. This is the only way a rush of air would come out of a disconnected fuel line as described.
.....
PS ... I have one of those glass inline filters and it is always 1/2 filled with air at the top of the glass when the engine is running.

Ed

 

[tahoe healey]

Don't forget that 85% of carb problems are ignition.

 

[AH100M]

Carburetor is a French word meaning "Don't touch it".

Usually true.

 

[Ed_K]

I forgot to mention that unless you take the cover off the fuel bowl to the rear carb, you don't really know for certain that any fuel is entering the rear carb. Maybe the screen is blocked with trash or perhaps the float valve body is blocked with trash.
Ed

 

[Galen]

Thanks for all of the ideas. I will report back tomorrow on my success.