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MG MGB Technical - HIF4 leaks...

I have just got my rebuilt engine / OD tranny and HIF4 carbs going (all rebuilt), and I am having a leak issue with the carbs. The rear carb in particular will be fine with the engine off and fuel pump on (fills up OK, no leaks). When I start it, at low RPM's (under 1500), the fuel flow rate of the carb doesn't appear to be high enough to stop it from leaking out of the vent tube (with the vent plumbing disconnected for troubleshooting). At speeds higher than that it won't leak at all.

Now, I have dissasembled it twice, checked the float height, and confirmed the needle is sealing all the way. If I invert it, I can't blow into the fuel inlet, confirming that the needle is correctly seated. Any ideas? I can't run it for long enough to try to adjust the mixture for fear of it leaking gas on the exhaust!
Dave Whitehead

Have you checked to see if the float has fuel in it? Remove the float and shake it to check for fuel. The float chamber should stop filling when the float level is reached regardless of engine speed. Since the fuel flow isn't being stopped you have a defective float, dirt in the needle valve, binding float or a misadjusted float. Good luck, Clifton
Clifton Gordon

Just been there last month with newly rebuilt HIF-4's leaking through the bowl vent. Would only leak after turning over the engine. Finally cured the problem by replacing the SU needle valve with a Grouse Jet w/one washer under the valve. ($8 Moss) A few washers come with the valves. Have not had any problems since.
GM Chohaney

"If I invert it, I can't blow into the fuel inlet, confirming that the needle is correctly seated."

But how much pressure will your lungs provide? As much as your fuel pump? I can't recall, and don't keep a pressure gauge at my desk. But I think you can only generate a couple of psi, where your fuel pump may be pushing at 4 psi or even more.

As you go through troubleshooting, check your fuel pump pressure. If it's more than 1.5 - 2 lb. then that's more than the SU is built to overcome. You may need a fuel pressure regulator. Don't bother ordering one by mail. You can get them at any parts store, and they're generic.

As to why it only leaks when the motor is on? I'm no electrical whiz. But I'll bet that a pump running on battery-only gets less voltage than one running on alternator+battery. Maybe this difference is just enough to push your pump's output over what the float and needle can handle.

Best of luck.
Matt Kulka

If you have an SU pump fuel only flows under return spring pressure so voltage is imaterial. The fact that at higher speeds you get no overflow indicates seepage rather than significant flow through the valve. If the overflow only occurs when the engine is running and not when stopped when the ignition is on then I would suspect vibration is the problem and a marginal float or valve. Replacing the valve, possibly the float as well, should cure it. If not an SU then excessive pressure may well be the cause.

"Grouse Jet" - I like it, sounds like a good reason for not fitting them, as many have testified in the past :o)
Paul Hunt

Hi Paul,

humour ???

"Grouse Jet" - I like it, sounds like a good reason for not fitting them, as many have testified in the past :o)

Regards .
Renou

"Grouse - to complain or grumble". As in a sign commonly seen on a low beam in an English pub - "Duck or Grouse". The humour in *that* being two double-meanings: a duck and a grouse are both birds and unless you 'duck' or lower you head as you pass under the beam you are likely to bump it which will cause you to complain or grumble ...
Paul Hunt

Thanks all. I'll update once I get it going. Unfortunately, I can't look at it tonight, but I will get another chance this weekend. Once I get the carbs sorted, it's back to the body work! The good news is that the clutch, which was stuck, has freed itself, so I think I'm good to go (once I have a floor again!).

Dave
Dave Whitehead

Update:
I have measured the fuel pressure now, and found it to be 5.5 psi, both engine running, and ignition only on. Obviously this is too high, and is causing failure of needle seal. I haven't pulled the pump yet, but it does appear to be the original SU. It is just about the only part of the car I haven't had apart, so this was to be expected!

Is the pump pressure adjustable, or does this imply that the return spring has been replaced with one of higher strength?

Dave
Dave Whitehead

This thread was discussed between 12/05/2004 and 15/05/2004

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