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MG MGB Technical - Fuel blowing out of carb?

I suffered a broken carb body on my '67 BGT, so I replaced both with a spare set I have on hand. While tuning with the air cleaners off, I saw something I never noticed before. When I give it some throttle, a bit of fuel spray comes out of the carb toward the air cleaner. Is this normal?
Steve Simmons

Steve - Have you checked your manifold vacuum? Your symptom sounds the same as what I had many years ago when an inlet valve started leaking badly on a PB we had. there should be no pressure in the carb throat to blow fuel out toward the air filter and the only thing that I can think of that might do that is an inlet valve that isn't sealing properly. The other thing that might do this is a long overlap of the inlet and exhaust valves - do you have an exotic cam in the engine? Cheers - Dave
David DuBois

Yes, it can happen, usually the spray is sucked back into the carbs and burned . i have been a passenger in aweber equipped mini cooper S and seen this.
Stan Best

To dive into more detail...

The car was running perfectly until the carb broke. I got it home by keeping the revs up at traffic lights (major intake leak!) and all was fine.

I swapped out the carbs and now the car is very low on power, as if the throttle only opens half way. It runs fine otherwise.

The vacuum gauge reads normal while driving, and reads from the intake manifold.

Nothing else on the car has changed.

Perhaps it's normal but I've never noticed it before on any of the MGs. It's a small mist of spray that gets the intake of the carb wet and immediately dries.
Steve Simmons

Oh, forgot to answer... the car has a Crane cam with a nearly stock grind. Nothing aggressive.
Steve Simmons

Steve,

I've not seen a properly set up car with SUs spray fuel. I would do a compression check. Keeping the revs up with a major intake leak may have resulted in a very lean condition, and burned an exhaust valve or seat, making the engine a three cylinder. It will idle fine, but be low on power.

Aside from that, Skip Burns discusses in some detail some of the pitfalls in setting SU carbs. One of the things he talks about is a non-synchronized throttles. Check out this link and go straight to the bottom and start with 'Abnormalities' http://www.chicagolandmgclub.com/techtips/mgt/tune_up2.html

I hope this helps,
dave
Dave Braun

Thanks Dave, I hadn't considered that possibility. When I was driving home that night, I had no idea that the carb was cracked. It just felt like an ignition problem. I hope I didn't damage a valve since I just had a head job done last year!
Steve Simmons

Followup:

In an incredible stroke of coincidence, my timing advanced itself somehow and was off the scale. Rebalanced the carbs and set the timing, and all seems well.

Thanks for the input!
Steve Simmons

This thread was discussed between 07/02/2008 and 12/02/2008

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