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MG MGB Technical - mystery gas leak

The adventure continues..........1973 B, HIF carbs. Had a great run yesterday with the temp gage saying 1/2 way between N & H as expected. After stopping for 15 minuets, turned on the key and heard the fuel pump "ticking" away. It did not stop, or slow down as usual and I spilled gas from (?) on the ground. After further cooling, the car started and ran fine! Where did the gas come from as I see no overflow pipes. Thanks, Tom
Tom in HOT upstate NY

Tom check all the fuel lines. I had the same issue and dicovered all the rubber lines were brittle and cracked.

cheers

gary

79 mgb
gnhansen

Tom: one or both of the carbs is overflowing gas into the charcoal cannister and the gas is emptying out the bottom of the cannister. The bad news is, your car may start and run fine a few times, but the overflow condition will probably return. You'll have to remove the carbs and investigate.
willieL

Remove the hoses from the vent/overflow ports on the carbs, and switch on the ignition with a *cold* engine! Then you will see which carb needs attention.

On HIFs the main petrol inlet is on the front carb, then passes stright through to come out of that carb inline with the inlet, and go into the rear carb still inline. Those ports are closer to the wing. The overflow ports are the other two - facing forwards on the front carb and rearwards on the rear, closer to the engine than the fuel ports.
Paul Hunt

Thanks for the answers/opinions on the mystery gas leak. I think "willieL" is on the right track as everything in or on the car is new or renewed during the 3 year restoration.

As the problem no longer exists, I am thinking that the awful heat that day may have been a factor in the event. Any thoughts or cures? WIll a few ounces of "Marvel Mystery Oil" keep the carbs "lubricated"? ( Thats an old MGT* trick that solved a stuck valve on the TC).Thanks, Tom
Tom

The worst problems are intermittent problems and HIF carb flooding is up there on the list. My original set of HIFs flooded on every 4th of 5th drive. Got tired of that (suspected a bad needle valve) and replaced them with a set of new Burlen HIFs. As my bad luck would have it, one of the new ones started flooding because of a leak in the float. Fixed that by putting in the float from the old HIF!
willieL

How about Gross Valves? Tom
Tom in beautiful upstate NY

Recent reports on Grose valves haven't been that good.

I'd suggest get some new Genuine SU (i.e. Burlen) needle valves and put in. One caveat on those though -- I've seen several recently, both on my own car and on others, wherein the plastic needle body had a little bit of casting flash on the "fins" that center it in jet. The flash causes it to stick (either open or closed -- this could even be the problem with your current ones if they're new within the last year or two). A few strokes on a piece of fine sandpaper or with the edge of shape knife is enough to remove it. The goal is that the needle slides easily up & down in the jet tube without much side-to-side movement.

HTH!
Rob Edwards

Any oil in an SU carb would be in the piston damper, which is totally isolated from the float valves, so nothing to do with overflowing. If heat is an issue with the float valves then they are marginal and should be replaced.
Paul Hunt

Did you replace the fuel lines? Gas tank?
James

OK folks, it happened again tonight. I came home on the hook. It appears that the rear carb float valve does not shut off the fuel when it is "hot". Any one else been here done that? Are the Moss valves any good? Thanks, Tom
Tom

Check the float very carefully for fuel. I had intermittent flooding on one of my V8 HIFs, despite changing the float valves twice (a real pain on HIFs compared to HSs). It was only when it happened again and I took them off for a third time that I noticed fuel in the float. If it done it every time I would have checked the float more carefully in the beginning, but it wasn't so I didn't. Squeezing and heating didn't get any fuel out, so I presume there must have been a hair-line crack somewhere. Since then it has been fine.

Grose 'jets' are not recommended any more as the materials and/or quality seem to have deteriorated. Conversely viton-tipped conventional valves have become much better.
Paul Hunt

There have been loads of posts on this problem with the HIF, but the HSU seems not to suffer so much. I tried the grosse jets, waste of money, had new floats and needles fitted by a MG specialist, waste of money. Finally I bought a pair of second hand HSUs off a scrap GT, and have had no problems since. even if you do, sorting the float chamber takes a few minutes on a HSU not the hour+ for HIFs.
c cummins

This thread was discussed between 27/04/2009 and 07/05/2009

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