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MG MGB Technical - Valves sticking

More 79 MGB woes.
Well.... after fixing the fuel pump problems, we now have no start in the morning when it's cold. After letting it sit in the hot sun it starts and we have the wonderful clacking sound with some backfiring from the exhaust. We checked the valve clearance and set at .33mm when cold, per the manuals, (we actually set them at .35), and that did not seem to do the trick.
Is it possible the valves are stuck in the closed position and is causing the no start problem?
The car had been sitting but this came after two days of it running while working on the fuel pump problems.


Elizabeth




Elizabeth

I doubt if the valves are stuck closed because you wouldn't have been able to set the valve clearances
in the first place if this was so. You did use the "Rule Of Nine" when you set the valve clearances, yes?
(...set valve #1 when #8 is open; set # 2 when # 7 is open, set #3 when #6 is open, etc.).

Check the spark plugs. Are they black, or sooty, and/or wet? Do they smell "fuely"?

Could be a badly leaking carburettor float needle valve(s) - and the condition was acerbated when the
fuel pump was rejuvenated. Also, maybe some debris in the fuel line had worked loose when the fuel
pump was being worked on and got caught in the float needle valve.

While there - check the pivots on the floats for wear, looseness, or slop. Check the float level setting
when you reassemble everything.

A leaky needle valve, or a bad float, can allow raw fuel to dribble uncontrollably into the combustion
chambers.

Unlike gases - liquids do not compress.

The clattering noise occurs when the pistons cannot compress the excess raw fuel in the combustion
chambers ...bordering on achieving a "hydrostatic lock-up". This is REALLY bad and, if so, you can
actually crack a piston or two, or three, or four ...or goof a rod bearing, or bend a rod or two, or bend
a valve(s) or pushrod(s) etc.. None of this is any good.

The backfiring comes from the excess fuel finding it's way into the exhaust system - and is ignited in
there from a stray ignition flame traveling past an open exhaust valve.

Put it all together...and you've got a clattering, backfiring car.

The car starts better when the ambiant temp is warm because the excess fuel has a better chance to
evaporate.

When cold (mornings), the excess fuel has a lesser chance to become vaporized and the car becomes more difficult to start.

My little 2¢ theory.
Daniel Wong

The valve clearances should be .38mm cold, and it is easier to set them cold than hot. Too small a clearance will cause valve burning. It is highly unlikely that the valves are sticking closed without seriously and irreversibly damaging the valve gear. They could be sticking open but simply allowing the car to warm up in the sun for a while is highly unlikely to affect this, IMHO. When it won't start check you have a spark at each plug when cranking. After cranking for a bit look at each plug - wet means flooded i.e. too much choke or a very rich mixture, and you can usually smell this around the car, especially in a garage. No fuel smell (on the plugs) means no fuel getting through. There should be a strong fuel smell short of being wet. A overly rich mixture is more likely to start when cold and less so when warm, a weak mixture or no choke is more likely not to start when cold but will do so when warmed.
Paul Hunt

Well we rechecked and reset the valve clearances when dead cold and lo and behold the clacking is now gone. They were too loose.
We are still stalling out from gas starvation though after thinking that was solved. We removed the front fuel filter for a test and found that helped a little but it's still stalling out after a mile. It starts up but will not run so towing became the ongoing agenda of the day. We found that the output from the new fuel pump, a low pressure (2.5-4 lbs.) Facet pump purchased from Napa, is also half the required 2 pints per minute. Next step is to check for blockage in the lines and check the needle and float in the carb as you suggested Daniel. Is this pump sufficient? It was previously suggested by another. We installed a new filter in the back also but maybe it's too small.
Any other suggestions is welcomed.


Elizabeth
Elizabeth

Is your fuel tank filler cap vented or not - sounds like a vacuum is building up in the tank as you pump fuel.

When it stalls, try removing the filler cap - a big inrush of air points to the fuel cap!
Chris Betson

We tried this but there is no inrush of air when the cap is removed but will try again.



Elizabeth

Cool mornings = dampness. Warm sun = dry. Check your high tension wires and distributor cap. Might have a second problem there. Old, leaky plug wires, or a bad cap or rotor.
R. L Carleen

Whilst the AUF300 pump is required to deliver 2 pints per minute in a test rig the earlier HP-type pump only delivers half that and is more than adequate for standard twin SUs. Did you check delivery by removing one of the fuel feed hoses from a carb? If so then a pinched pipe is not the problem so if it really is fuel (and not ignition) then the problem must be in the carbs. But that implies both carbs, so sounds more like incorrect assembly than anything else. Do you have the overflow ports open to atmosphere or capped? If capped the float chamber will not fill.
Paul Hunt

This thread was discussed between 30/06/2003 and 03/07/2003

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