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Moss Supercharger and Fuel Overflow

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1970RedMGB Avatar
1970RedMGB Silver Member Thomas P
Lombard, IL, USA   USA
1969 MG MGB GT
1970 MG MGB
I have been having an issue getting my supercharged engine dialed in and today I was able to identify the problem. This is the newest version of Moss's supercharger with the HS6 carb.

The symptom was during cruise, the car would begin to run rough and then die. I could press in the clutch, let the engine die, then release the clutch to use the momentum of the car to re-start the engine. The engine ALWAYS restarted without issue. I suspected fuel but started with the ignition system to make sure everything was up to snuff - and it was. So I replaced the generic fuel pump with an electronic SU pump - and the problem persisted. I put on a good fuel pressure regulator - and the problem persisted. I bought an expensive fuel pressure gauge from McMaster-Carr to check the fuel pressure - dead on at 3.25lbs. Yet the problem persisted. I checked the float - it was in good shape and is the black newer style. Float level was right about 1/8".

I was able to recreate the issue this morning in my driveway while I was looking for oil leaks. I put the car on ramps, put UV dye in the engine, and started it up. After it warmed up some I ran the engine at about 3000 rpm to let the leaks commence. The engine started to run rough and then fuel started coming out of the overflow pipe (I have mine plumbed to exit about where the end of the transmission is). While this was happening I checked the fuel pressure gauge and it was still at 3.25 lbs.

It seems like this is NOT an issue of too much fuel pressure, but of the float vibrating/resonating and causing it to allow too much fuel in, which then floods the carb (causing the engine to run rough) and overflowing the vent.

Any suggestions on how to fix this? Fall is definitely in the air here in Chicago and I can feel my driving season ticking away...

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LaVerne Avatar
LaVerne LaVerne Downey
Fruita, Colorado, USA   USA
1954 MG TF "Green Hornet"
1969 MG MGB
I had an issue with fuel overflow on hard right hand cornering. A better orientation of the float bowl would have had it in a front to back hinge. That said my issue was later determined to be a faulty float as the metal broke in two at the 90 degree bend. In your case I would suggest removing the float valve and check it for debri .

Ottodesmo Otto Sleeking
Amsterdam, NH, Netherlands   NLD
If the pressure stays the same it must be the needle in the bowl - if that gets stuck this is what you get in terms of symptoms. Had it once with an old motorbike.

Otto

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1970RedMGB Avatar
1970RedMGB Silver Member Thomas P
Lombard, IL, USA   USA
1969 MG MGB GT
1970 MG MGB
The needle looks fine. I'll change it anyway but if the problem persists I am going to try adding a small amount of weight to the float to change it's resonant frequency.

1970RedMGB Avatar
1970RedMGB Silver Member Thomas P
Lombard, IL, USA   USA
1969 MG MGB GT
1970 MG MGB
FIXED!

I decided I was not going to replace the needle and seat. Instead, I wedged a 1/4" nut between the float and float arm. The small amount of extra weight was not enough to sink the float but I think it added enough mass that it prevented the float from doing whatever it was doing.

It sounds dumb but it works. If that means it's still dumb I'm ok with that as long as I can drive the car. smiling smiley

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