webers perculating

[ein 6er]

just read the post about the weber insulators on the early 911 engines.

i have a 67 2.0 in my 914/6 (w/40mm IDTP13C). i recently cleaned and rebuilt the carbs, (basicly just new gaskets and new needle valves). after going for a long drive i pulled the air filters and notice that fuel was perculating up and out of the emulsion tubes.

as directed in the haynes weber manual i set and adjusted the float level with vernier calapers.

before the cleaning, i drove this car cross country in the summer and never had a problem with vapor lock or hard starting and never heard the carbs perculating.

so ... would the float level be wrong? i've double checked it a couple of times. when i adjusted the level to the manual's specs for triple choke types, i noticed that i had to bend the float tab up quite a bit to get it to the recomended dimension. bending the tab up would make the fuel level lower and less likely to perculate, wouldn't it??

i noticed that on the float the number 25.5 was stamped. is that the needle valve dimension or just the float size?

now ... after this event i checked the float level again and it hasn't happened again. but ... i don't need a fire, so i guess the insurance of the insulaters wouldn't hurt.

help is appreciated!!

doug waters

[Joe911]

This is a common problem with Webers. A good source of information of causes and cures (improvement anyway) of this weakness of the Weber design is described in UP-FIXEN volume 8 pages 107-8.

Here is the related text.

“There are several problems associated with the quality of gasoline available nowadays: clogged injectors due to lack of detergency and presence of gross amounts of trash in the gas, vapor lock and fires caused by increased vapor pressure, gross water which leads to the replacement of complete injection systems. The main cause of carburetor fires is float bowl percolation. In almost every case, someone drives his car for a while, stops and comes back after 15 minutes to an hour or so, then starts up his car and there is either an immediate or delayed fire. Sometimes there is no fire but enough gas has evaporated so that the car is difficult to start. What happens is that there is a heat-soak up the manifolds to the carbs, which boils the extremely thin and volatile fuel …... If you own a carbureted 911 driven on the street, there are a number of measures you can take to cope with this problem. Venting the top covers of the carbs along with insulator plates under the manifolds are the primary solutions. Venting the top cover works by releasing the percolated gas through small breather holes before it can rush out through the secondary venturis and down the carb throats.”

PMO sells kits for performing the venting operation. My Webers are currently soaking in carb cleaner and will get the fix over the next week.

[ein 6er]

thanks for the info joe!!

although i've drained and cleaned the tank and lines, i'm still getting some rust past the filter. i'll be putting an additional filter inline.

thanks again!!

doug

[Tyson Schmidt]

You really need to set your float level with the proper float level gauge. The method you used will get you in the ballpark.

The gauge screws into the side of the float bowl, and allows you to see the actual fuel level through a glass vial. To adjust the float level, you add or remove shims from under the needle/seat screw.

[T Bird]

I agree with Tyson. The gauge (P 226a) is sold by PMO and is recommended by Porsche in the Workshop Manual for setting float levels. It screws into the four float bowl plugs and has lines scribed into the vial portion. The fuel level needs to be set between the lines. I have tried to use the gauge with the engine running, but the vibration of the engine makes it difficult to get an accurate read on the level, as the gas tends to slosh around in the vial.

All you need to do after you screw in the gauge is turn the ignition key until you energize the fuel pump. The vial will fill up and show you exactly where the float level is. Then add/subtract shims as Tyson notes. If you order the gauge from PMO, make sure to order at least 4 new brass crush washers to fit under the float bowl plugs. If you try to reuse the old ones, you will likely experience leaks.

Properly adjusting the float levels is critical, not only to prevent percolation, but because the float levels determine when the main circuits kick in. You want the floats all set correctly and evenly to ensure that fuel is introduced through the main circuits at exactly the same time in each barrel.
Good luck.

[ein 6er]

hey, thanks for the info guys!!

if you are familiar with the haynes manual, basiclly it tells you to set and chock the float to a specific level, measured with venier calipers, then bend the float's tab to match the needle valve's spec. when doing this i had to bend the tab up quite a bit. i think the manual says to set the tab at 18mm below the bowl's rim, the old measurement was about 25mm. unless i'm wrong (>very< possible) that would mean the fuel level was higher before. and remember, they ran fine before i rebuilt them, that is until they clogged with rust.

i recently bought this car and i have little history on the engine. the carbs are from the 914/6 engine, (i think). i found that the idle jets are smaller than stock, .45 compared to .55 (i think, i don't have the manual here) for the '67 2.0 that's in it now. i don't know what any of the POs did to the carbs. that is why i was wondering about the #s on the float, (25.5), that is about what the float tab was set to before hand (just guessing here). is there such a thing as a stock float? would you change floats like you would change jets? i didn't have to add or subtract any shims to get the needle valves into spec.

anyway, i drove the car a lot today with no perculating problems. the carbs are running pretty good except for erratic idleing. i checked for manifold leaks and i know i still have the rust in the fuel issue to resolve.

i'll be looking for the proper jets, the insulators and the gauge.

thanks again for the info, my knowledge of the webers is limited to the manual and it doesn't seem to tell the whole story, so any additional info helps.

doug