Weber carb filling cylinder with fuel when off

[supersignman]

I have a 1969 911T with Weber carbs. I recently noticed a fuel leak around the accelerator pump housings and figured I'd better to replace the accl pump gaskets before I burned it down. The engine ran fine so I decided to see if this would fix the problem without a full rebuild of the carbs. After replacing the gaskets I tried to crank the car and noticed when looking down into the carbs that the back two accelerator pump nozzles were dripping a steady stream of fuel when the ignition was off.

At this point I also notice the accelerator pump housings were still leaking and actually appeared warped. I decided to a full rebuild on the carbs and I also purchased some new pump housings from our host. The pump housings no longer leak but I still have one of the accelerator pump nozzles dripping fuel when the ignition is off. It appears that it drips until the residual pressure from the fuel pump is bled off but it is filling up that cylinder with a substantial amount of fuel.

The kit I used came with new needle valves so they have been replaced. I'm assuming something isn't sealing properly but I don't know where to look.

[Joe Bob]

Float level checked?

[supersignman]

Joe Bob, I have not checked the float level. Thinking about it, I'm kid of embarrassed because that sounds like a pretty obvious thing to do. Since it was running so well before and the floats looked good I assumed they were set correctly. I'll check that next.

[john walker's workshop]

Clean the needle valve too. New ones have issues. Maybe put used ones back in.

[gwmac]

PMOs will do that if the air cleaner base plate is installed backwards covering a small hole.

[supersignman]

I just took apart the carb and realized that one of my accelerator pump nozzles (different than the one leaking fuel) is cracked at the top and it's squirting fuel out the back side. Our host doesn't list those nozzles. Does anyone know where I can order one? Also, our host says the float level adjustment gauge takes 2 - 8 days for shipping plus transit time. Can I get one anywhere else quicker?

Also, thanks GWmac. I have an aftermarket air cleaner base so I will check that too.

[john walker's workshop]

Parts k l a s s i k

[Locker537]

Call Larry at Weber Carb Parts, PMO Carburetors and Parts, Porsche Posters, engine oil testing

[75 911s]

I just had this exact same issue. Float chamber (for 2 and 3 cyl) on the drivers side was evacuating after sitting and overflowing into cyl 2 and 3 causing massive backfires upon start and also hot start issues.

I ordered the needles from the eBay italy guy and I think they are junk. The reason I think this is because I was having one that was sticking, and now this one is overflowing so I'm thinking it's sticking too. Some of his stuff is fine, the gaskets and needles are trash imho.

I replaced with two new needles from PMO (via larry youroil.net) Haven't had a chance to verify the problem is solved yet, but just telling you my process.

So check the needle, then check the float level after you put it back together, I would get it closer to the bottom line rather than in the middle to start off with.

I'm also using phenolic spacers and two gaskets on the manifold, and my other chambers are not boiling over, so I think it's a flow problem with fuel.

[pmax]

After fixing this, don’t forget to dump the oil.

[TimT]

Does anyone else remember "grose jets" float valve replacements? they were the shizzle in the '80s... I wonder if they are still available? i just did a cursory search..

[lateapex911]

Originally Posted by TimT

Does anyone else remember "grose jets" float valve replacements? they were the shizzle in the '80s... I wonder if they are still available? i just did a cursory search..

I used those on my race car about 15 years ago at the advice of a carb guru. Some swear by them. They were less than impressive in my use. Cost me a race at the ARRCs (American Road Race of Champions...SCCA thing).

needle valves are funny, I have kinda just decided that I run what works, new? old? meh, as long as it holds pressure with the pump on and the car not running it's all good LOL.

[1QuickS]

Fuel needle valves were failing at the rate of 8% for the 200 I pressure checked for PMO. I now buy Viton tipped ones from Alfa1750 and I just had one fail during test running my latest Weber project.

The accelerator pump top cover is routinely warped but I have re-surfaced nearly 1000 of them and they seal just fine without resorting to replacement. Just press the pin out & sand the sealing surface flat.

The main body of the accelerator pump has three "nipples" on the inner chamber that are normally closed during steady state operation. If the throttle is depressed rapidly from a closed or nearly closed position then these nipples are exposed by fuel passing from the outer chamber into the inner chamber. An aluminum disc is pressed against a diaphragm gasket that keeps these nipples sealed against fuel flow until acceleration is demanded.

If the back surface of the accelerator pump body is not flat and if the aluminum disc is not flat then there is the possibility of a leak past a nipple which could allow fuel to pass from the outer chamber up the fuel gallery feeding the squirter nozzle. It is easy to surface the back of the pump body but you must first remove the three brass tubes. This is easily accomplished by insertion of a metal rod into the brass tube to support it while grasping it with side-cutting wire cutters. Then, lever the tube out. Sand the back flat and be sure there are no imperfections in the nipples and that the two little cast-in-place fingers are also flat.

Once all sealing surfaces are flat then the only fuel that can pass through the squirter nozzle is the column of fuel in the gallery from the inner chamber of the accelerator pump to the squirter nozzle. Fuel in the main (outer chamber) will return to the fuel level in the float bowl, the check valve in the bottom of the float bowl that feeds the accelerator circuit is "Open" to back-flow when there is not acceleration demand.

The last statement is meant to be clearly understood: Heat rise in the carbs will not cause an undue amount of fuel to pass through the squirter nozzles if the diaphragm gasket for the inner chamber of the accelerator pump body and its associated sealing surfaces are in good order as discussed above.

Also, float level does not affect this, if fuel level is too high then it will be draining out the auxiliary venturi long before it comes out the squirter nozzle.

[supersignman]

Thanks for all the input everyone! Parts k l a s s i k that John recommended had everything I needed. My background is from the stock car racing world with Holley carbs so I'm still learning about the good Porsche suppliers and they look like they've got some hard-to-find parts. Thanks Paul for the tip above. It sounds like you've worked on these carbs a time or two! I'll definitely take a close look at the pump body. I'm guessing it's important that aluminum disc is perfectly flat too.

[911 SLANT]

When I know my car is going to be parked for the day or couple. I unplug fuel relay and let my PMOs run out of fuel from bowls. Plus makes a great theft deterant.

[supersignman]

Mystery solved! One of the new needle valves was bad! I figured out which one was feeding that accelerator pump and replaced it with one of the old ones and it's working perfectly now. Paul commented above that he found an 8% failure rate on aftermarket needle valves. That's approx 1 in 12. If you need four valves to rebuild your carbs then on average 1 out of 3 rebuilds is going to get a bad valve! That's not good!

Thanks to Pmax also for reminding me to change the oil. Extremely important!

Many thanks once again to all you Pelicanites for the great tips!!!!

[GG Allin]

Originally Posted by gwmac

PMOs will do that if the air cleaner base plate is installed backwards covering a small hole.

Glad the OP solved his problem, that way I'm not hijacking here. But the above quoted post got me thinking. I got my car running briefly back in August. My driver side carb was doing the same thing, dumping fuel when that car was shut off. So I went out and checked the air cleaner base plate. I believe it was on wrong so I swapped it around. I won't know until Spring if this solved my problem.

Before:



After: