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MG MGB Technical - Sudden HIF carb failure

Well, I think it's the carb. Drove home from work, no trouble. Wound it up to 75 on the highway, drove home, parked in the garage. Went out the next morning, crank, crank, crank, nothing. Figured ignition and drove the VW to work.

Got a chance to fiddle this weekend, checked/cleaned some connections, nothing. Shot a little ether in the carb, lit right off, then died. did this a couple times, then managed to get around to work the 'manual' choke - a couple fingers across each intake. revved and kept going for some time. even idled on it's own eventually. Moved it out to the barn for an oil change, and same symptoms.

The only thing that seems different than the weeks prior when it was happy is that it's now 80 degrees plus. But that might be coincidental. Any ideas/pointers?

Thanks
Steve
Steve Aichele

Check to see if the dashpot diaphragm is burst.
Iain MacKintosh

I'm not sure what you mean by diaphram - the dashpots have oil, and the piston inside that holds the needle looked sound. Is there some other part to be looking at? Maybe the seal around the edge of the piston?

Steve
Steve Aichele

Pull the fuel line and verify the fuel pump output. You clearly have a fuel supply problem, not necesarily the carbs!
Jeff Schlemmer

SUs have aluminium pistons; Zeniths have rubber diaphragms. Perhaps that's what Ian's thinking about. I'd probably start by checking the fuel delivery from the pump. Take the line off from where it goes into the front(? -- going form memory here) carb and direct it into a bottle. When you turn the key, you should see fuel delivery at a fairly high rate (something like a pint or more in 30 second is what I seem to recall the book stating). Whatever the problem, I think it is unlikely that both carbs developed the same problem at the same time, so it sounds more like fuel delivery to me..... Let us know what you find!
Rob Edwards

Sorry folks, senior moment, of course it's HIFs we're talking about here.
Iain MacKintosh

I immediately thought of a bum pump. Flow was about 10 oz in 15 seconds, so about 2.5 pints +/- per minute. Filter looks clean.

I only checked the front carb - are HIFs plumbed in such away that the shutoff on the front would impede gas supply to the rear (maybe causing a simultaneous failure), or is it more of a pass thru?

Steve
Steve Aichele

It's a pass through.
Rob Edwards

What are the plugs like after cranking for a while with no start? They should smell strongly of fuel but not be wet. If they are wet it is flooded, and you can usually smell this from the cabin anyway, particularly in a garage. Clear this by cranking with zero choke and full throttle ... but fether the throttle the instant it fires! In either case lack of fuel isn't the problem so check the ignition. It's possible the greater volatility ether is allowing a weak spark to ignite the either but not the fuel, but once that ignites the fuel should be able to take over. But an MGB should never need ether to get it started. If there is no fuel smell fuel is not getting through for whatever reason, or there is a gross vacuum leak like a Welch plug fallen out of the ends of the inlet manifold. If one carb is faulty I expect the other to be enough to get it going on 2 1/2 to 3 cylinders, unless (forgive me) the general state of tune is poor.

But you describe your manual choke as being two fingers across the intake. HIFs should have a manual choke control on the dash opening an enrichment jet inside each carb, and in my experience they always need some choke when started from cold even in warm weather. No choke and non-starting is 'normal', to me.
Paul Hunt

Paul-

The 'manual' choke was a small pun on my part, actually using a hand to alter the mixture (maybe it should be digital choke, since they were just fingers).

No offense taken at the poor tune comment - it has certainly been the case often enough. But in this particualr instance everything was (or seemed) pretty solid - timing good, mixture maybe slightly lean (it would need some choke (from the handle on the dash) for several minutes after starting).

I have to admit I didn't check the welch plug, didn't even know those were removable on the manifold, but there was a fair amount of suction through the intakes (as I had my fingers across them...

Maybe it's back to ignition.

Thanks
Steve
Steve Aichele

This thread was discussed on 06/06/2005

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