[Tucker in KFalls]

I'm hoping to ask the experts, I'm a carburetor novice but trying to learn. I have a 1964 GTO with factory tri-power. I have a small amount of fuel weeping out of the center carb from the throttle body by the throttle shaft on the driver side AFTER the engine is switched off for 5 to 10 minutes. The front and rear carbs do not leak. The center carb will leave a few drips on my intake manifold. It also weeps just in front of the throttle shaft and collects by the intake stud and nut. Maybe from the upper gasket?

When the engine is shut off after 5 minutes or so vapor will begin to appear in the center carb and then I am able to see a several slow drips that collect onto the throttle blades. Mainly on the driver side throttle blades.

I've read that this happens as the fuel heats and expands creating pressure in the line from the pump raising the float level above the main discharge nozzle dripping onto the plates which are closed and then exists out beside the throttle shaft.

The car has always been hard to start when it's warm after sitting but otherwise runs great and starts well when cold.

The question is how to fix it? I've installed phelonic spacers but that didn't help. I've seen in other posts possibly a return vapor fuel line or a possibly a float adjustment. Thanks for your feedback!

[JSchmitz]

I would test the needle and seat in the carb. Use a hand vacuum pump with the carb upside down. If it doesn't hold vacuum, replace and retest. the end carbs are seeing the same pressure as the center carb. If they don't drip, the center shouldn't either. Also make sure the center carb float isn't bad. Verify float adjustment also.

[Kenth]

First check for original style and size of inlet valve (.101" viton tip).
Check float settings (23/32" and 1 3/4" drop).
Check float bowl vent.
Check fuel pressure a while after shutoff, (have the meter connected while driving until the engine reaches normal temp.)
A remedy could be a fuel filter or a fuel pump with vapor return.
But, then you would have to add a return fuel line to the tank and a pickup in tank with a fitting for this line.

My solution expering the same issue 30 years ago was mount an electric fuel pump with no check valve (Carter P4070 5psi) that would make fuel expand in the fuel line, from the hot engine, between the mech pump and inlet valves in the carbs.

HTH

[pfilean]

Regardless of what you decide about the needle/seat and float level adjustments also check the wear on the throttle shaft. The drivers side where you see the leaking gets more load as it is the side with the linkage and return springs. That side will wear more and many need a new shaft and/or bushing. Get with Dick Boneske or pontiactripower.

You could also consider heat isolation phenolic spacers under the carbs to reduce heat expansion of the fuel.

[Tucker in KFalls]

Thanks everybody, I appreciate your input! I did install the phenolic spacers but it didn't seem to help with the flooding while the engine is shut off.

Wouldn't a needle/seat or float issue give me issues while the car is running also?

Thanks again for all your help, this gives me something to go on.

[Kenth]

The fuel does not expand in the line during travel as speed wind and fan wind cool down the temperature in the engine compartment.
Heat isolating spacers under carbs will not help the heated fuel lines during shut-off.
And, vapor lock can not occur in the carburetos since they are ventilated to atmosphere at all times.
In any or all cases it does not hurt to use the correct inlet valves and floats that are correctly adjusted.

[JSchmitz]

Originally Posted by Tucker in KFalls

Thanks everybody, I appreciate your input! I did install the phenolic spacers but it didn't seem to help with the flooding while the engine is shut off.

Wouldn't a needle/seat or float issue give me issues while the car is running also?

Thanks again for all your help, this gives me something to go on.

The float, or needle/seat, problems would potentially make it run rich. But you might not see a problem. Look down the center carb with it idling. See if it's dripping fuel from the venturis.

[carbking]

Kenth has nailed the problem.

Check out this link to the "troubleshooting" section of my website:

https://www.thecarburetorshop.com/Tr...g.htm#Fuelleak

Jon

[mike w]

This is usually related to fuel pressure. Fuel will not flow backwards through your pump. Under hood heat will expand fuel in line and increase pressure.

Try putting a gauge on the line and watch pressure raise after engine is shut off ; it will.

These carbs can only handle 4-5 pounds of fuel pressure. Most new mechanical pumps push 6-8 #.

A good fuel regulator should solve this issue. Holley makes a good 1-4# regulator for less than $50.

Your throttleshaft may be worn, but replacing it will not solve your problem. Fuel should not be dripping onto the throttle blades after shut off.

[Dick Boneske]

Mike is 100% correct. The fuel pump has a one way check valve. After the engine is shut off, the trapped fuel in the lines between the pump and carbs expands from the heat under the hood. If the pressure exceeds 6 psi, the needle/seat will not stay closed. I have seen this occur on several Tri-Powers.

Adding a return vent like the A/C cars have is not a simple task. Setting the float levels slightly lower than spec sometimes cures this issue, You may also be aware that the center carb (pre '66) has a smaller float than end carbs. The float bowl on the small center carbs is the same size as the end carbs. Try using a larger float in the center carb. This results in added resistance to fuel pressure overcoming the needle/seat. I've done this with success.

Originally Posted by mike w

This is usually related to fuel pressure. Fuel will not flow backwards through your pump. Under hood heat will expand fuel in line and increase pressure.

Try putting a gauge on the line and watch pressure raise after engine is shut off ; it will.

These carbs can only handle 4-5 pounds of fuel pressure. Most new mechanical pumps push 6-8 #.

A good fuel regulator should solve this issue. Holley makes a good 1-4# regulator for less than $50.

Your throttleshaft may be worn, but replacing it will not solve your problem. Fuel should not be dripping onto the throttle blades after shut off.

[Tucker in KFalls]

Probably a silly question but where on the fuel line assembly for a stock 64 set up do I put a fuel pressure gauge &/or install a fuel regulator. Just after the fuel filter before the line going to the fuel block on the front carburetor?

Thanks again everyone for your expertise! I've have a lot of good information to go on.

Originally Posted by mike w

This is usually related to fuel pressure. Fuel will not flow backwards through your pump. Under hood heat will expand fuel in line and increase pressure.

Try putting a gauge on the line and watch pressure raise after engine is shut off ; it will.

These carbs can only handle 4-5 pounds of fuel pressure. Most new mechanical pumps push 6-8 #.

A good fuel regulator should solve this issue. Holley makes a good 1-4# regulator for less than $50.

Your throttleshaft may be worn, but replacing it will not solve your problem. Fuel should not be dripping onto the throttle blades after shut off.

[Dick Boneske]

I suggest temporarily installing a gage in whichever line that is easiest to access. It doesn’t matter in which line the gage is installed, since they’re all connected together. Since the pressure is in the range of 5-7 psi, rubber fuel hose & hose clamps will suffice to connect a gage.

If you have a section of 5/16” flared tubing, cut it to a length that will allow you to connect it to one of the carb fuel inlets with hoses.

I don’t know of an ideal permanent location for a fuel pressure regulator. Unless you add a return line to the tank, I’m not sure a fuel pressure regulator will solve the problem.

[Tucker in KFalls]

Thanks Dick, and thank you for the suggestion about the larger float in the center carb. It is interesting that my front and rear carb doesn't have the same issue (that I've noticed) as my center carb with the smaller float.

[Kenth]

Center carb has a larger .101" inlet seat, smaller float and higher float setting than the end carbs .086" inlet seats, larger floats and lower float settings, thus the center carb is more sensitive for higher fuel pressure.
Center carb is the weak link in the chain so to speak.

[carbking]

Originally Posted by Kenth

Center carb has a larger .101" inlet seat, smaller float and higher float setting than the end carbs .086" inlet seats, larger floats and lower float settings, thus the center carb is more sensitive for higher fuel pressure.
Center carb is the weak link in the chain so to speak.

If the center carb was rebuilt with a FLAPS kit, that orifice could be 0.118.

Jon

[Dick Boneske]

Laws of physics tell us that the larger the orifice in the needle/seat, all other things being equal, the less pressure it takes to lift the needle off the seat.

A Tri-Power setup has three needle/seats controlling fuel into the carbs, not like a Quadrajet that has only one needle/seat. The amount of fuel allowed is three times that of a quadrajet, assuming the orifices are equal. Normally, the orifice in a Quadrajet is typically .135" vs the recommended center carb orifice of .101". But the two end carbs at .086" will allow much more flow than a single .135" orifice.

I use the Tomco needle/seat assemblies that have a flat rubber disc that shuts off fuel when the float exerts pressure on it. I am aware that many rebuilders frown on this style of needle/seat, but I've restored hundreds of Tri-Powers using this style of needle/seat with excellent results. In the past, there were similar flat disc needle/seat assemblies that often leaked and flooded the engine.

Previous to the late '90's, I used conventional needle/seats and had many instances of leakage, requiring disassembly of the carb to clear the problem. The rubber disc style needle/seat is not as prone to debris interfering with a perfect seal. I don't know why, but that is what I've experienced. Previously, with converntional needle/seats, flooding at initial startup was very common, occurring in about one out of three startups.