Weber Type 31 or 151 on a Sprint?

[Cadence]

Thank you Richard. The needle jet swop was a fail. The car ran well and I stopped a few times on a 20 minute run to check for fuel leaks - all looked good until I got home.

For circa 30 seconds there was no leak. Then just as I was feeling optimistic, the first drip appeared and it’s on both chokes of the forward most carb. The smell was pretty strong so I left the car in the drive overnight to clear the fumes.

However………,further research on this forum threw up raft of folk experiencing a similar issue. This is a reply by fatboyoz in 2014

I had a similar problem that eventually started leaving a puddle of fuel under the engine. It used to manifest itself a few minutes after shutting down the engine after having driven the car.
The following is what was suggested to me. I followed this procedure which cured the leak.
Regards,
Colin.


Throttle lever housing cover plate dribbles:

? The throttle lever housing cover plate is between the two barrels and is fixed with two countersunk screws.
a) Remove the screws and plate. A full sealing gasket should be in place. Remove it and check if the chamber is wet with fuel or shows signs of this.
b) Fuel can enter the chamber via the pump rod hole or the return spring hole. Yes I realise this should not be the case BUT do you have ANY clearance in the butterfly shaft bearing on either side of the chamber. Of course you do even if it?s only small.
c) When you are pulling 5000 to 6000 + rpm and lift off the throttle you can appreciate many inches of vacuum generated must suck from anywhere it can. Herein lies our problem. Fuel is sloshing around in our float bowl through corners and clearances around pump rod and throttle return spring hole are considerable. The vacuum pulls fuel into the chamber but there it just builds up to gradually dribble over our distributor or hot exhaust as the case may be.

Solution to throttle lever housing cover plate dribbles -

Drill a 1.5 mm ?breather hole? in the cover plate and match drill the gasket. The hole should be level with the top screw and half way to the edge to miss the housing cast edges.
The small hole stops the effect of the vacuum but also keeps the dirt out of the throttle lever chamber. Screw the plate back on for dry, dribble free motoring.

This really is an incredibly simple fix for a rather obscure problem. It took a lot of reasoning and close inspection to analyse what was actually happening but investigation is part of the problem solving. It was only after I fixed the problem that a carburetor repair person suggested that this was probably why Weber modified some plates in the 80?s with a 8 ? 10 mm hole in the middle. Unfortunately Messers Weber don?t tell everyone why they did this modification.

NOTE: Obviously all of these fuel checks are better if carried out shortly after a run so wet fuel can be seen when the carburetors are removed.

Elanner, who had the problem later answered

To close out my leak problem: Colin’s advice was the clue to everything and put me on the right track.

After a 20 mile run I removed the carbs and the accelerator pump lever covers. There was a teaspoon or so of petrol sloshing around behind the cover of one carb and a few drops in the other. I gave them a good cleaning and drilled a 3/16? hole in each cover.

Bingo, three weeks and several good drives later, no leaks from the pump lever covers and dry under the flexible mounts. And now it?s also dry around the fuel pump and the coil/distributor wiring. Whoo hoo!

I’m mostly a bit disgusted with myself for not fixing this ages ago. Really, I’m happy to futz with minor stuff and ignore leaking carbs?

This seems to be a worthwhile 10 minute enhancement whenever the Webers are off.

Thanks again guys for your help with this.

Attached is photo upload by elanner showing the holes in the plates.

Going to give that a try tomorrow - hopefully a quick win!