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MG MGB Technical - HS4 float

A pal has been having overflowing from one carb which moves with the float. On removing and weighing both (which are Stayups) one is 6.8 gm and the other is 14.5 gm.

Obviously not correct but I wondered if anyone had a good standard float handy that they could weigh for me.
paulh4

I don’t know what a stayup one is but had to change one of the clear plastic ones last year as it was half full of fuel and causing running problems. Could find nowhere that fuel was getting in to it and six months latter it was still half full of fuel.
Trevor Harvey

Typical StayUp for HS4 attached. They reputedly use a closed-cell foam body with a smooth skin in theory making it 'puncture proof' ... but they have been known to sink! When a purchaser complained to the supplier he was told he must have damaged it when fitting and wanted nothing more to do with it.

I had an HIF float suddenly cause fuel to pour out not just drip, brown plastic so I couldn't see fuel but could hear it sloshing about. Despite heating it and hoping to find the leak that way (vague thoughts about being a cheap-skate and repairing it ...) I never did.


paulh4

I don't really like the supposedly sink proof micrphyl floats, They do give trouble, there only needs to be a scratch or a pinhole in them and you get what you have there. They do give up eventually anyway without being damaged.
If you have one that is over double the weight of the other it needs to go in the bin.
I have a new one in a sealed plastic bag which I don't really want to open but the float and bag combined weighs just on 5.9 including the bag-- so probably 5.5 or less
HIF's as you say are a pain -you can be driving around forever with one half sunk and not realise until it finally sinks a bit further and spills fuel.
Cheers
willy
William Revit

Thanks Willy.
paulh4

Paul,
our kitchen scales only resolve to the nearest gramme but the one I've weighed is 6g! I can do it again tomorrow on my workshop scales that will measure to .001g if required but I would guess that with Willy's number and mine you can deduce the weight to be circa 6g +-0.5g

Your 14.5g would appear to be faulty and the other slightly over weight. If I were you, I'd be checking how well the lighter one floats and probably adjust the level to compensate for the additional weight.

As you know but for those on here that might not ;O), the float sets the fuel level in the jet and if the stay-up is heavier it will need to be displaced by more fuel to lift to the required height. This will cause the fuel level in the bowl to be higher than standard and, if high enough, flood out the overflow.

Best of...
MGmike



M McAndrew

There's a pattern here - 2 new and unused, out of the bag, each weighs 7.00gms on the kitchen scales. I won't be using them, as I changed my mind and stayed with the old plastic floats. I'll send 'em down to you for fishing floats, Willy o' the south!
John.
J P Hall

Sorry Paul - new unused standard plastic float weighs 7.00gms as well. John.

J P Hall

Thanks both. Generally concurs with Joe Curto who measured both types at just over 7gm. Yes a heavier float will sit lower (ship displacement and Archimedes Principle) so raising the level of fuel in the chamber and the jet which is something else that can be measured with HS lids off.

But is that discrepancy enough to cause the carb to overflow? The overflow is higher than the jet/bridge so fuel will come out of the jet first, and in theory it will only come out of the overflow if the float is failing to close the valve and the amount coming through exceeds that being used by the engine and trickling into the inlet manifold. Whilst obviously wrong I'm not convinced yet it is the cause of all his problems until he replaces it with a 7gm one and gets to drive it a bit.

Incidentally a high level in the float chamber could well be the cause of 'vaporisation' and difficult hot starting in summer that people complain about. They blame ethanol fuel, but there must be many hundreds like me who don't get that even in the hottest weather.
paulh4

I'm thinking the 14.5 g one probably isn't rising much at all,at least without enough force to hold the needle shut causing the fuel level to keep increasing.
The gap between the meetering needle and jet is only a thou. or so so not much fuel can escape there so out the overflow she goes-----
William Revit

This thread was discussed between 16/01/2026 and 20/01/2026

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