Weber Carbs and Fuel Back Pressure

[73911guy]

Hey everyone,

Well, as some of you know, I inherited a '73 911T from my Dad. The car hasn't been driven or started for some time. In July, I tried to crank it over with no success. So, Slammed1000 and I pulled and replaced the plugs to find that the cylinders had filled with fuel. When we went to start it, the same thing happened. We pulled the Webers (40's) and they will be sent out to be rebuilt.

Tonight, there was a lingering scent of gas, and I went outside to find that the regulator was dripping fuel - probably around one BIG drip per second. I relieved the pressure by pulling the fuel cap. Here's the question: Why is there pressure building in the fuel tank and down the fuel lines? The fuel pump has been disconnected for some time. Do you guys have any suggestions?

This dripping of fuel might be the cause of the carbs dumping fuel into the cylinders upon startup.

On to the Webers...

The gaskets were replaced about seven years ago. I would like to have them rebuilt. Supertec and Bieker seem to be on the top of the list. Do you guys have any others?

Also, I want to put a set of PMO Fuel Logs on there. Do you guys have experience with these? ---If you have a set for sale (AN or barbs doesn't matter), send me a PM---

Thanks for your help in advance!

Chris

[Eagledriver]

The PMO fuel rails are very nice. You could try to clean and reseal the carbs yourself. The float valves are probably sticking open so the fuel runs through and floods the engine. There are kits with all the parts you need to rebuild the carbs. The main thing is to get all the passages and jets cleaned out. There is a lot of varnish in there after sitting for several years. Your fuel tank vent system sounds like it might be clogged. The tank heats up and the fuel tank gets pressurized. It must be able to vent or it will put pressure on the system and drip out like you are seeing. You also probably need to replace all the rubber lines after all these years.

-Andy

[barney911rs]

Make sure you don't have the fuel pressure set too high. 3.5 psi is plenty. I don't have any for sale, but I do have a set of the PMO rails and I love them. On my race car I have all my oil and fuel lines are AN so that is what I got, I think the barbed ends for a street car would be easiest to work with.

Not sure why you would of had pressure in the lines. I've never seen that.

[JohnJL]

You can do it yourself with the appropriate kits. Pelican kits are good though mine didnt have the top paper gasket included that another source included in their kit..

Lots of books and help here.

Off the top of my head, I would think a slow leak like you have would still alow fuel past the needle valves even if they were new. Is the fuel siphoning?

Yes, PMO logs are nice.

[911pcars]

Well, as some of you know, I inherited a '73 911T from my Dad. The car hasn't been driven or started for some time. In July, I tried to crank it over with no success. So, Slammed1000 and I pulled and replaced the plugs to find that the cylinders had filled with fuel. When we went to start it, the same thing happened. We pulled the Webers (40's) and they will be sent out to be rebuilt.

Tonight, there was a lingering scent of gas, and I went outside to find that the regulator was dripping fuel - probably around one BIG drip per second. I relieved the pressure by pulling the fuel cap. Here's the question: Why is there pressure building in the fuel tank and down the fuel lines? The fuel pump has been disconnected for some time. Do you guys have any suggestions?

This dripping of fuel might be the cause of the carbs dumping fuel into the cylinders upon startup.

Webers already removed, correct? High ambient air temperature can increase the air pressure/fuel volume inside the tank and force fuel to escape. That's my guess for pressure not created by man. Unpressurized fuel normally can't enter the engine unless the float bowl fuel level is set too high or the float needle valves aren't closing.

On to the Webers...
The gaskets were replaced about seven years ago. I would like to have them rebuilt. Supertec and Bieker seem to be on the top of the list. Do you guys have any others?

Many repair shops can rebuild Webers; it's also possible via DIY. It's not difficult if you're thorough and understand the necessary steps, the parts and the circuits. There are plenty of books on the subject. BTW, ancient fuel lingering in small passages isn't good. However, if the throttle shafts are worn, Eurometrix, Bieker and, I assume Supertec, can rebush the throttle shaft bores and also rebuild to spec.

Also, I want to put a set of PMO Fuel Logs on there. Do you guys have experience with these? ---If you have a set for sale (AN or barbs doesn't matter), send me a PM---

I'd save the $100+ cost for the PMO fuel logs until later unless you locate a deal. Apply it toward the carb rebuild invoice. New fuel filters (one per carb), intact hose and working hose clamps should be sufficient until you get the engine to run normally. It's good to look down the road, but realize the engine may have some issues that will require inspection, correction and/or repair before you get to this point.

Since fuel has washed the lube from the cylinder walls and piston rings, squirt some ATF or engine oil into each spark plug hole to prevent the parts from corroding and sharing molecules with each other. Does the engine rotate manually (wrench on crank pulley)? If it rotates, have you cranked it with the starter? If so, have you tested the engine for compression?

Let us know your progress.

Sherwood

[73911guy]

Thanks for the help, everyone.

Yes, the carbs are already off the engine.

The engine turned over freely when the plugs where out. The starter was replaced with a new Tilton last summer and has less than 50 miles on it.

Just to clarify the information, the car was running at the end of '06. My Dad owned the car since '88 and the engine was rebuilt by Bob Farmer around '95. After my Dad passed this year, I inherited the car.

The carbs are just a personal thing. I'm not mechanically inclined and my time is limited - college.

How should the gas be venting? If the tank is venting properly, why would pressure build up?

We replaced the fuel pump with a Holley a few years back. Wouldn't the regulator keep the fuel pressure at the desired rate even if the pump was oversized?

[911pcars]

"How should the gas be venting? If the tank is venting properly, why would pressure build up?"

I think the tank vents to atmosphere in '73. Not sure. You should confirm. Perhaps a later, emissions-spec sealed gas cap was substituted.

Could be what Andy suggests. Perhaps the return fuel line to the tank is blocked. However, if the fuel pump is removed, there shouldn't be any restriction or pressure on the fuel unless it's created on the return line. Maybe a combination of factors? Check fuel line pressure. Should be 3-3.5 psi. Regulators aren't perfect.

Sorry I can't assist more.

Sherwood

[73911guy]

Thanks for your help, Sherwood. I really appreciate it.