Fuel leaking into carb

[Shaun]

Hi All,
I'm having some problems with my 1963 Mk II 3.8 with running rich and the idle speed too fast. Tried adjusting everything and have gone back to the book setting for mixture etc but still it runs too fast and cant be slowed down and seems top be running rich particularly on the rear carb.
During fettling I had the air intake removed from the carbs and noticed that when the engine is not running and has been stopped for a short time, petrol rises and starts slowly dripping from the intake side of the rear carb. I removed the top of the float chamber and checked the float and float valve and all appears to be working ok but when I reassembled and then switched the ignition on (without starting the engine) petrol again started leaking from the carb inlet. This sounds like it is the cause of the richness and possibly the fast idle but how do I stop it? There does not appear to be any fuel dripping from the overflow type tube the is attached to the top of the float chamber which is where I would have thought excess fuel would go if the needle valve was leaking?
Any advice greatly appreciated.

[Tintin]

Persistent fast idle sounds like an air intake leak, or the throttle not returning to fully close the butterflies. I would check the inlet manifold etc for air leaks and the throttle return setting. As to fuel, have you checked the mixture/jet height and the float level height? Burlen Fuel Systems have a very good guide on tuning/setting SU's on their web site - definitely worth a look.

Robbie

[Shaun]

Thanks for the response Robbie,
I had checked and adjusted the mixture/jet height and float level when I first investigated but the fuel still continued to rise. Last night I again dismantled the float chamber for another closer look and discovered that the brass seating of the needle valve was not fully tight. This was allowing fuel to leak past the threads even though the needle valve was sealed. This effectively allowed the fuel to bypass the needle valve. I used a thin walled socket to fully tighten the brass seat fitting and re-assembled the float chamber and the problem seems to be solved, the fuel is no longer rising in the carb with the ignition on and when the fuel line pressure is up the pump stops altogether and does not tick every few seconds as it was doing before. The idle speed is ok also although by the time I got to this stage it was late at night so I have not run the engine up to normal operating temperature yet. Fingers crossed!

[piman]

Hello Shaun,

that sounds as if you have found the problem as fuel should not be visible in the carburettor throat.
The over flow pipe may be blocked but I also seem to remember there is a special washer (scalloped cut outs on the inside diameter) used on the overflow banjo fitting on the lid and a plain washer, these need to be assembled correctly for the overflow to work? You need to refer to a manual as I have forgotten which order they are assembled.

Alec

[PPV]

I found that my overflow pipe was blocked with dirt from never being cleaned since installation I suppose. I rodded it with a stiff wire after straightening it gently. It certainly helped sorted a running problem I had with my car at the time.
It does not takemuch to stop the valve from seating either, a little bit of wear or a bit of dirt stuck there. Just have to be careful with all the soft fittings.