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MG MGB Technical - 75 b won't start when warmed up

I have a 75 B with a twin su's It starts great when cold but is running rich Once warmed up it runs rougher and eventually dies and is very hard to restart, and runs even richer if I get it started [lot's of black smoke]

I switched back to the twin carbs because I could not get my single Weber from running rich as well.
Could this be timing, timing chain, valves, distributor??

Any ideas would be appreciated

wally armstrong

yes it could be all of them

set up/check/adjust in this order - tappets, CB points, plugs, timing, mixture

if you adjust any item in that chain then you need to check and adjust if required all the following items in the chain

your Driver's Handbook will tell you how to do this work and give you gaps and settings for a standard car but bear in mind these are a good starting point but your car could have many variations including wear in or worn components, lack of service/lubrication and lack of use that could mean the timing setting isn't exactly as per book

is the car serviced and used frequently, are the air filters clean, CB points gapped and lubed as required, good condition plugs, engine oil and filter clean as these will help with the running
Nigel Atkins

Does your car still have the stock fuel pump or has it been replaced with an aftermarket model? If it has been replaced, it could be the root cause of your problems due to excessive fuel pressure overcoming the floats in your carburetors. RAY
rjm RAY

Wally. I agree with Ray. The first thing to check is the fuel pump pressure. Both the SU and the Weber DGV need about 3.5 psi max pressure. If there is more, they will run rich. You need to hook up a fuel pressure gauge to the inlet line to the carbs and see what you actually have present. Excessive fuel pump pressure is the most probable cause of the problem you describe.

"Could this be timing, timing chain, valves, distributor??"

None of these are commonly associate with the problem of running rich. /

Timing can be over advanced or retarded. When over advanced, the piston is fighting the burning mixture which results in over heating and burned pistons. When retarded, the fuel is still burning when the exhaust stroke happens resulting in over heating and possible burned exhaust valves. In both cases, engine power is significantly reduced.

Timing chain problems result in the intake and exhaust valves operating at an incorrect time resulting in loss of engine power.

Valves?? If the cam is worn, they will not fully open meaning that a less than adequate amount of fuel will enter the cylinder on the intake stroke, or the burned fuel will not be fully exhausted, thus less room for the fresh fuel to be drawn into the cylinder on the intake stroke. Result--loss of engine power.

Distributor. See "Timing" above.

None of these areas are, to my knowledge, capable of causing the carbs to be excessively rich. A non-sparking plug, perhaps as a result of an air leak at the intake manifold, can cause a cylinder to burn the fuel/air mixture inefficiently, but this is not a symptom of the carbs "running rich" which would be indicated by blackened coatings on all of the spark plugs.

As Ray notes, the fuel pump, offering up excessive pressure, is the most common cause of the problem you describe. Especially since it is common to two very different sets of carbs.

Les Bengtson

If it's flooding from too high a fuel pressure, the carbs will overflow with ignition on, pump running but not the engine. Are these SU's HF or HIF?
Allan Reeling

apologies - yeap not a good thing to put 'could be all of them' I really meant it could be all or none of them or combination for the poor starting when warm - but I didn't put that, rushing again

I didn't think about the pump either :)

and I should have thought that if you'd changed to a Webber then you might have made other changes

here's my thinking on checking the set up, I think it's good to start with a solid reliable base, if the tappets, CB points, plugs and timing are already set OK you'll just be confirming it or adjustments where required so that when you get to the fuel and sort that you've already set the rest and don't need to start again and possibly be adjusting the carbs twice
Nigel Atkins

The rich running is obviously the thing to sort out, as that will make it stall as well as difficult to hot start.

If you still have the SU pump then turning on the ignition but *not* starting the engine, the pump shouldn't click more than once every 30 secs once it has done a few clicks to pressurise the system.

If it's clicking more often than that it could be the inlet one-way valve leaking back (which wouldn't cause a rich mixture, more likely fuel starvation) or one or both carb float chambers leaking.

If it still has the charcoal canister then it could be quite a long time before there is any overflowing fuel dripping on the ground, so temporarily remove the overflow hoses from the ports and see if it leaks from the carbs then.

Also be sure you have the fuel and overflow hoses on the correct ports!
Paul Hunt

thank you all for the very good comments

I will get out to the shop today and start with the fuel pressure as the pump does not click more often then it should
If that doesn't work I will take out the pertronic ignition and put some regular points back in
wally armstrong

Interesting dilema! And interesting answers.

If it is the pump that is flowing too much you could resolve it by installing a fuel pressure regulator valve. They cost around 35 GBP and regulate the fuel to flow a steady equal pressure.

As far as I can remember off the top of my head the PSI should be around 3.2/3.6 Although Im sure someone here will know the correct PSI for your carbs.

Good luck in solving your problem :)

JS Ahlund

The problem with installing a fuel pressure regulator is that a lot of the ones on the market today aren't very accurate. You can install one and think that you have the pressure set to the proper limit, only to find out later on that it's still way too high for your carburetors. RAY
rjm RAY

This thread was discussed between 28/08/2013 and 29/08/2013

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