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MG MGB Technical - Float chambers sloppy

My 71 roadster recently when on a run occasionally started to run only on two cylinders, both from the same carb.
On inspection it appeared to be a faulty or partly blocked grosse type needle valve.
Sorted it and decided to fit some new rubber fuel hose, I decide to flush through before connecting up. To my supprise hardly any fuel was coming through. On removing the in line plastic fuel filter that I changed about two weeks ago, fuel came through normally.
I have been using one of these for the 10 years in the B with no problem.

On inspection I noticed just how sloppy the float chambers are on the carburettors, is this normal,it was only once the fuel pipes were disconnected that it was so noticeable.
Would this leak fuel.
Trevor Harvey

Trevor - "To my supprise hardly any fuel was coming through. On removing the in line plastic fuel filter that I changed about two weeks ago, fuel came through normally."
That clogged coil is telling you that you need to inspect the fuel tank for rust. Cheers - Dave
D.W. DuBois

I dont think theres a problem with the tank, it is not that old, also the filter is see through and still looked clean. I think either the filter was faulty or maybe the pump is getting weak and has not the power to push the fuel through the filter as I believe they only operate at 2lb.pressure.
Trevor Harvey

The pump must be very weak if it failed to push through the filter, in which case I'd expect fuel starvation now. Maybe it was something in the inlet or outlet so not visible in the body of the filter.

Really you need to do a delivery check - filter fitted or not as you wish. With the delivery pipe removed from a carb and directed into a container (note that it may spurt if the ignition has been on recently), turn on the ignition, and it should deliver a minimum of half an Imperial pint in 30 secs - and in practice at least double that - in a continuous series of pulses with minimal bubbling.
paulh4

Trevor,

The clear plastic fuel filter I nstalled here is directional. If yours is the same you may have it wrong way around?

Regards
Roger
R Taylor

Direction indicated, but isn't that more to do with catching any debris in the outside container where it is visible, rather then inside the filter medium where it wouldn't be? Surely it doesn't contain a one-way valve?
paulh4

The filter was fitted in the direction the arrow showedso I am sure it was the correct way round.
With the filter off, the pump was delivering approximately what you suggest Paul.
I had one of these filters on previously and though I never had a reason to check if it restricted the fuel delivery, it caused no problem.
Trevor Harvey

Is there any apparent restriction if you blow gently through the filter?
paulh4

Thinking about the way it ran in the past with the previous filter fitted. On the odd occasion when I might have driven the motor abit faster than normal, maybe past 3000rpm, I would get a bit of missfiring, did not bother about it as I thought it could be down to anything, and I rarely drove it that hard.
Now with no fuel filter fitted it revs past this with no missfire at all. Would be interesting to know if others have had problems with these small inline plastic filters.
With the electric mg fuel pumps only operating at 2lb pressure, any restriction could cause a problem. Trev
Trevor Harvey

The V8 (with HIF) has a plastic filter and that certainly doesn't restrict. Maybe yours is faulty.

It's not so much the pressure that is important with the SU (although too much can overwhelm the float valves and flood the carbs) as volume, hence the minimum half pint in 30 secs - with a filter in circuit. If yours is about that, then I can see that you could get starvation at wider throttle openings. But that is the minimum, if it's only just above that, i.e. significantly less than a quarter of a pint in 30 secs, then you do have a problem with either pump, lines or filter.

If you have no missfire now, with no filter, then you need to do the delivery check both with and without filter, and if those are significantly different then do it again with a new filter - if you want to keep one.
paulh4

Sloppy float chambers? What do that mean?
J MacDonald

J mcdonald. With the fuel lines and overflow pipes removed from the float chamber of the carbs . The float chambers are very loose and are not held in an upright position. Its only once the pipes are attached that the chambers are held.
Trevor Harvey

There should always be some movement of the float chambers relative to the main carb body on HS carbs, including with all the plumbing attached. But as Chris has said elsewhere it should not be sloppy as in rattling around. If it does that with the pipes removed, then the rubber grommet AUD 2677 has failed.
paulh4

AUD 2677 front carb, AUD 2676 rear carb.
Chris at Octarine Services

Thanks Paul / Chris that is what I wanted to know.
Trevor Harvey

This thread was discussed between 24/04/2017 and 30/04/2017

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