Perplexing Weber issue

[tommott]

Got an odd Weber issue with one of my newly rebuilt Weber 40 IDA 3C carbs on my 914/6 swap that I was wondering if some of you can help me out with?

Starting the car is a bit tricky but nothing too unususal for a carbed car. Once started the car runs reasonably well; it idles nicely and takes throttle decently well for a car with un-synced carbs at this point. I've taken it for two test drives thus far and both times after about a mile or so the car starts starving for fuel. After the second time it happened I was able to ascertain that passenger carb was not shooting fuel out of the accel jets. After popping off the top of the carb my suspicions were cemented as both bowls were completely dry.

Now to the crux of the matter......why is this happening? The bowls are filling initially on start up, but then just drain out and don't fill back up under load while driving. I checked to make sure that the carb was getting some sort of fuel delivery with the top of the carb off,...it was. If the car sits for a couple minutes it seems to then repeat the cycle.

My first thought would be the needle valves, but they are both new valves out of the rebuild kit and it seems unlikely that they both would be acting up as BOTH bowls are emptying and not filling back up.

Second thought was float levels, but this seems somewhat unlikely as well as it would not explain why the floats fill up initially but not thereafter. Seems like it should be consistently inconsistent if the float levels are off if you catch my drift.

The most plausible explanation I can come up with at this point is weak fuel pressure. The fuel pump may be strong enough to get the carb filled on start up, but once it gets hot and is working under load it pressure capacity may be weakening. Also the car sat for a good bit while in the possession of a buddy of mine and when I checked the fuel filter in front of the pump yesterday it was FULL of crap and crud. Some of this crap and crud definitely got past the filter as the carbs had a lot of junk in them too....hence the rebuild.

I'm going to start off tomorrow by testing, and perhaps swapping, fuel pumps. Was wondering if any of you Pelicanites might have some additional ideas of what else I should check while I'm literally elbow deep in Porsche tomorrow?

As always all input will be greatly appreciated

[911_Dude]

I would check fuel pressure and float bowl level. Both need to be right.

[1976 TN Porsche]

I agree. It sounds like a fuel delivery problem. The fuel pressure needs to be 3.0/3.5 Lbs. with a good regulator. You should check the float level with a flout gauge. There is a good float gauge for sale here on Pelican. Also you can contact Paul Abbout here on pelican ( 1Quicks ) or at www.perfomanceoriented.com 530-899-8371 He can help you. He is Good Man. Craig.

[1QuickS]

Crud in the fuel filter means crud in the tank or deteriorating fuel lines. If car sat and crud passed thru filter then the first carb would get most of line crud ahead of second carb...maybe. Even with a t-fitting the carb with the straightest flow path will get the early shot of crud and the second carb would be a bit less likely to clog the fuel strainers in the banjo fittings. I'd pull the banjos fittings and check the fuel screens there for crud. You will want to pull the fuel screen in the fuel tank to check that it is clean as well.

Fuel delivery under light load shouldn't be an issue for any fuel delivery system offered through Pelican. If you have a big motor with big power potential then you may want to review your fuel delivery system for adequacy.

[Daves911L]

I'd be suspicious of the screen inside the fuel tank. There is a very fine mesh brass screen "sock" over the fuel line fitting inside the tank. Over the years, it will get a coating of crud/varnish/slime (call it whatver you want). This will impede the flow rate of fuel from the tank to the pump. I'm theorizing it leaks enough fuel through the crud to charge up the fuel line and allow you to fire up and run OK for a bit. But gradually the supply runs farther and farther behind demand. You shut off and it builds up OK again. To clean this "sock" you have to unscrew the fuel line fitting from the bottom rear of the tank (great fun, especially with a full tank!). It will look like about half an old cigar when you pull it out. Clean, scrub and soak overnight in vinegar and it will be a shiny mesh filter once again.

[Daves911L]

Oh, sorry, I just noticed you are talking about a 914. I don't know if they have the same screen in the tank as a 911. If not, please disregard.

[eastbay]

Maybe neither here nor there, but had the same problem on a non P-car.
Turned out it was a new air cleaner blocking the carb vent enough to slow the fuel bowl fill rate to a crawl.


I just angle cut the vent and was good to go.

[tommott]

Thanks all for the responses. I've narrowed the issue down to either a fuel pump that goes out after warming up or a clog in the fuel tank.

I put a fuel pressure gauge in front of the offending carb. Upon start up the fuel pressure was reading around 4.5 lbs. Drove the car for a good 10 minutes, the farthest its made up until this point though I was much less aggressive on the throttle, and the car ran reasonably well. Pulled into the garage and the fuel pressure was reading just about 0. The gauge never read that low, even when not installed. Shortly thereafter the car puttered out and died, not sure if it was good timing on my part, or if the car sitting stagnant running had something to do with the loss of pressure. Within a minute of the car puttering out and dying on me I restarted it and it was instantaneously back up around 4.5 lbs of fuel pressure.

I have taken the fuel pump off and will be inspecting/testing it out later today. My hunch now is leaning more towards a clogged pickup in the fuel tank. If the fuel pump was losing power as it got hot it seems somewhat implausible that it would right itself after cooling down for only a matter of seconds. The pump in question is one of the red Holley deals.....I'm half tempted to throw on one of those cheaper Mr. Gasket pumps and seeing of that makes a difference as well.

I'm now thinking that it is more likely that a bunch of crap is in the gas tank floating around and as pressure builds in the tank the fuel pump slowly pulls the crap down to the tank's pickup. As soon as pressure is relived, ie the car being shut off, the crap then floats back off the pickup line.

Now this is all conjecture at this point as I am not familiar with the innards of a 914 fuel tank. Not sure if there is a sock-like filter, like what is found on a 911 tank, that could capture this type of crud. I'll guess I'll have to do some research over on the 914 forum or bounce over to 914 world, and culminate the tests on the fuel pump, prior to popping the tank out.

[eastbay]

Try running it without the gas cap in place, you could be pulling a vacuum on the tank if it is not vented.