Mild vacuum leak, replace HS4 carbs?

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Dave Linkson
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Mild vacuum leak, replace HS4 carbs?

Post by Dave Linkson »

Hi
Have been informed a number of times in the past by expert classic MG mechs that carb spindles need replacing, the odd flat spot has felt barely perceptible due to a vacuum leak, although idle does vary a little, (about 850 to 1000 RPM with cooling fan cutting in and out) and in the past haven't found them easy to tune with a Gunsons, and have just gone on as is, but now feel I'm doing the engine an injustice which was rebuilt 5k miles ago, and would the running really benefit more if I did have the carbs repaired/replaced? is it worth doing now I can afford to throw some money at this or just wasting my money? Has anyone here experienced this situation? Cheers..
Ian Fozzard
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Re: Mild vacuum leak, replace HS4 carbs?

Post by Ian Fozzard »

I'm a little unsure of the question you are asking, but my experience suggests that a fully functioning set of carbs is well worth the investment.
I have always got away with fitting new spindles and jets into the original bodies. The spindles may still be a little loose if the aluminium bodies are worn, but you will have to judge that acceptability on your carbs. With biased needles, the jets will be worn oval, and will benefit from replacement.
A rebuild kit from Burlem fuel services should have most of what you need.
My other advice would be to get your distributor rebuilt and calibrated by the Distributor Doctor. I made the mistake many years ago, believing that if I set the base timing all would be well, and indeed everything worked. But the advance curve was far from optimum.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
Dave Linkson
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Re: Mild vacuum leak, replace HS4 carbs?

Post by Dave Linkson »

Thanks for the reply..
Everything else on the GT is fine timing, tappets and the dissy replaced in recent years with a CSI electronic effort..
Ian Fozzard
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Re: Mild vacuum leak, replace HS4 carbs?

Post by Ian Fozzard »

I have a Gunson mixture tuning set, but they are of limited use I think. They can set the mixture at idle, but that is assuming the needles are correct for the full range of throttle settings.
I have found that getting the carbs balanced, and then checking the mixture settings by performance checking (stopwatch!) and checking plugs after a full throttle run was best. Switch off after high speed/load and coast to a stop. Remove a plug and see how it looks. Avoid a weak mixture at full throttle!
If you have the funds, a rolling road check does all this for you........

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear
Vic Butler
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Re: Mild vacuum leak, replace HS4 carbs?

Post by Vic Butler »

Dave, what year is your MG?
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Dave Linkson
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Re: Mild vacuum leak, replace HS4 carbs?

Post by Dave Linkson »

Vic
1972 blaze
Dave Linkson
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Re: Mild vacuum leak, replace HS4 carbs?

Post by Dave Linkson »

Ian
I have done the plug check in the past, remember the ridge of the threads being a tad sooty and centre electrode a bit grey, so maybe slightly weak? Maybe due to k&n filters & (as fitted by po years ago along with sports exhaust) and not helped by vaccum leak at carb spindles?

Have read that fitting richer AAA needles with just the K&Ns isn't justifiable, and I'd be concerned it would then be too rich..

I had a rolling road session done on it about 6 years ago after having new engine fitted, didn't seem to make much difference if any..
Randy Forbes
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Re: Mild vacuum leak, replace HS4 carbs?

Post by Randy Forbes »

I must agree with Ian, in that the brass spindles carry most of the wear, and replacing them will show considerable improvement.

If that doesn't stop the air getting past the spindles, my next choice would be to fit the .005" oversize throttle shafts (using the appropriate, piloted reamer on your existing bushes fitted to the body).

I've also rebuilt SUs by replacing the original bushes with new ones, reamed to size & alignment, but it IS NOT a fun, easy or quick job__by far, I'd rather go with either of the above jobs before resorting to this one.

While brand new SUs can be bought, they're neither inexpensive, nor do they look exactly like the originals.

I too never had satisfactory results using Gunson's see-through sparkplug; you're only checking the mixture at idle, so taking a lot on faith. I'll bet mine hasn't been screwed into a cylinder head in >35 years. Given the technology available today, talking wide-band O2 sensors & meters, a few friends could pool their monies and make up a test-fixture to clamp to the tailpipe and give the car a few runs up the motorway, making some stops as needed to fine-tune the settings. Get a bunch of you together and run tests on several cars during the same outing (ideally ONE__or more__of you would already be a First Class Tuning Wizard ;) ).

Good luck, have fun, and Safety Fast!
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