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MG MGB Technical - Drawing Air?

I have a new built engine with new HS4 carbs and Facet pump and fuel reg. fitted.

The problem is I have a tick over of 780 and sits at this few a couple of mins and then tick over rises to 1240 for about 30 seconds then drops back to 780 for a while then starts to rise again, Ive disconnected the fuel reg and put the pipe directly on the carbs thinking it was the fuel reg playing up, but the same happens again.

What would cause this, my thoughts is it must be drawing air from somewhere but I cant think where (cracked inlet manifold maybe)

Any thoughts welcome!
K Harris

Take an unlit propane torch and use it to check the intake gasket. If there is a leak the engine will speed up when it gets a whif of the propane.
John H

Do these carbs have the poppet valves in the butterfly plates? They are reknowned for causing problems, I've soldered mine shut.

A rich or weak mixture on correctly adjusted carbs would normally cause the idle speed to reduce, so it is unlikely to be too much (the usual problem with aftermarket pumps hence the pressure reducing valve) or too little fuel. A vacuum leak is the most likely cause, but coming and going regularly would be unusual. If you don't have propane carb cleaner has a similar effect, albeit (paradoxically) a bit messier.
Paul Hunt 2

Today I run the car using gravity feed from a bottle held aboth the car and the engine ran smoothley. I have a Filter King Reg. fitted as well which I set at 1.5 psi and watching the pressure gauge drop to almost nothing then rise back up to pressure at 1.5. The revs went up with the rise in pressure.

I think the problem lies with the fuel Reg not keeping a constant pressure (sets a max. but not a minimum) going to try another one, and see if that has any effect.
K Harris


A pressure regulator will only control the maximum pressure. The fuel pump is what controls the minimum pressure.


Larry Hallanger

Thank you for your help, it turns out its a lazy fuel pump, with no pressure it has full fuel flow and runs freely out of the pipe, As soon I stuck a gauge on just the fuel pipe it only managed 1.5 psi instead of 5+ I would expect from a Facet Red Top.

Changing the pump hopefully Ive cracked the problem (so many new parts are faulty) I will let you know how I get!!
K Harris

This thread was discussed between 23/10/2006 and 25/10/2006

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