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MG MGB Technical - Hesitation/stall at WOT

First things first.
76B with a 25d, Marina manifold and an HIF6 carb, stock head and bore, totally de-smogged.

Car used to run very well, then the float failed. Since replacing the float I have been running at the maximum rich setting. When running at WOT it stops all combustion (so it seems) but the tach continues to correctly register the speed at which the engine is rotating. If I set the choke to maximum it runs fine, other than the high idle.

I initially set the float at 1mm under the straight edge. I went in and checked it again this spring (I replaced the float late fall, just before salt season) and it was indeed 1 mm below the straight edge.

I did some general maintenance, just to be sure - valves set with Micrometer to 13mm cold, ran a can of seafoam through the intake manifold and then changed the oil, replaced all 3 (carb to carb heater, carb heater to throttle return spring mounting plate, mounting plate to intake)gaskets and checked for any vacuum leaks - manifold to head, throttle bushings. I even plugged the carb manifold (yes, I know, but that's another story) advance port and any other orifice between the air filter and the head, all tight - no leaks.

Tonight I lowered the float another mm and remounted it with another new gasket and changed the fuel filter, but no change in performance. Idle is fine, but the idle stop is just barely making contact with the body of the carb.

Is the float setting different for the Marina manifold?

What am I missing here?
C Holm

Changing the manifold and particularly the carb will definitely change the breathing and hence engine characteristics.

I don't think float height has that much effect. When I had a float leak and was turning the fuel pump on and off with the overdrive switch, letting the float chambers empty and start to misfire before turning the pump back on again for a couple of seconds, it ran fine until they *did* empty. OTOH if this only started when you changed the float then either it is an incorrect or faulty float, or you disturbed something else when changing it, like dislodged the jet.

The fact that you have it set to maximum rich (screw all the way in?) indicates insufficient fuel supply for the air flow, and if it has to be set that far to one end then who knows how the mixture will vary as the air flow increases. It can also indicate a vacuum leak of course, try propane or carb cleaner sparayed around the manifold to head and carb to manifold flanges.

Also check fuel delivery, although if pulling the choke clears the problem then it is a carb mixture problem and not fuel starvation.
Paul Hunt

I'm pretty sure I got the right float. I ordered it from Joe Curto and he verified it as correct based on the carb and number on the failed float.

I even set the fuel pressure regulator up to 4 lbs without any changes in lack of performance.

Could the failed float have caused something to be moved or jammed by excess fuel being forced into the float chamber? This stumbling showed up at the same time as the fuel smell/failed float.
C Holm

It sure comes off to me as an intake system air leak. Follow Paul's advice and check all around the carb base as well as the intake manifold with propane. RAY
RAY

I've already done that.
Carl H

4psi for an SU sounds too high, all the figures I've seen previously quote 1 to 2psi. But it depends on what pump you have, if it is an SU then it will be delivering the correct pressure, below your regulator setting regardless.

I'm not familiar with a single HIF6 on an MGB, but I know they exist i.e. it should work. If you are certain there are no vacuum leaks at the flanges, or anywhere else on the manifold (the Welch plug on the ends has been known to cause problems), then given that pulling the choke seems to clear it it is definitely looking like fuel starvation inside the carb and not fuel starvation from the pump (but I'd still check delivery anyway). In which case it can only be incorrect or mis-placed parts. With only one to look at, instead of two to compare, spotting the problem could be difficult.

Fuel delivery should be at least one Imperial pint per minute from the delivery pipe taken off the carb, and in practice closer to two. From an SU pump it should be in a steady stream of pulses with minimal bubbles.
Paul Hunt

I generally ran 1.5 psi in the past, but cranked it up to 4 (released it) to see if more pressure would have any positive or negative effect, which it did not.

I see from the float chamber cover that the 90degree elbow may have a range of rotation, where as mine does not rotate at all. I hesitate to force it as there is no replacement handy. Should this move?
Carl H

anybody?
.

Solution to my issue:

The float chamber seal was leaking and the rear filter I had added had loosened and was varnished internally.

Still doesn't explain why it ran better under full choke, but it works well now.
.

I can't help but think that a bit of dirt has gotten in there (maybe
when you fiddled with the float) and has traveled up and is
fouling the mixture needle (starving, too lean, etc.).

Maybe a partial tear-down and blow-out with compressed air
is in order.
Daniel Wong

It is fixed now - nothing more to do.
.

This thread was discussed between 13/05/2009 and 10/06/2009

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