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MG MGB Technical - HIF4 rebuild questions

Well, as posted last week I've begun looking into my HIF4 carb rebuild. I've taken apart one of the 2 Ebay carbs, cleaned it and put it back together. I'm just talking cleaning it here, not a rebuild. When both spare carbs are cleaned I'll see if I can get them installed in my 73B and then rebuild the 2 originals.

Two quick questions from what I've seen so far.

1. The ring gasket on the float bowl cover (Moss 373-635) does not complete the loop by about 1/8". It would seem that the float bowl would leak because of this. Should I put a little sealant in that gap? Remember, this is for temporary use while I rebuild the originals.

2. Is there a way to tell if the floats are cracked or have any holes (submersion test etc.)? This entire operation is taking place because the rear carb has intermittant fuel blow by when the car isn't running (ignition on) and the front carb has intermittant blow by when running.

Thanks,
BH Davis
BH Davis

Unfortunately overflow issues on HIF carbs are much harder to deal with than HSs. Not only do you have to remove the carbs from the engine, and replace the O-ring, but with HSs you can immediately see if there is any dirt in the bottom of the float chamber, and if the levels are different which indicates a float/valve problem.

The float chamber O-ring on an HIF *must* be flexible and in good condition or it will leak. Replace it.

Fuel overflow which happens when the ignition is on but the engine is stopped is due to the float valve not closing, which can be due to dirty fuel, a bad valve or a bad float.

Leaks in plastic floats are usually visible (hold it up against a light) and audible in metal (hold it by your ear) as fuel sloshing around inside when you shake it, or if completely full it won't float! I've had a partially filled float that I could not get any air (under water) or petrol out by either squeezing or heating (I was trying to find the leak in an attempt to seal it), so I'd say there isn't a guaranteed test for a faulty float other than seeing if it does contain fuel.

In some case a momentary particle of dirt in the fuel sticking in a valve can be cleared by disconnecting the fuel pump and running the engine until the float chambers empty, then reconnecting the fuel pump. The resultant rush of fuel through the now wide-open valve can clear it. If the fuel is clean and the float good and the overflow happens again, check the float is hinging freely i.e. no bent hinge pin and replace the valve.

Dirty fuel must be treated at source, either by fitting a fuel filter if only slightly dirty, or cleaning/replacing the tank and flushing the lines if very dirty.

If a carb doesn't overflow with the ignition on and engine stopped, but does when the engine is running, then it could be vibration shaking the valve off its seat. I'd change the valve first, but it could be a sticking float, or possibly some other engine problem causing a harshness which could be affecting even some otherwise good valves.
Paul Hunt 2

This thread was discussed on 25/06/2007

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