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MG MGB Technical - Rough Idle

I have a 1969B, Twin Su carbs, stock ignition. The problem I am having is that when the engine gets hot, the idle starts too get low and eventually the car stalls out,i.e when stopped at a light. It will restart right away. It runs fine when the outside temps are cool. The things I have replaced are the spark plugs, wires, cap,rotor,lead wire in distrubutor, air filters and I have ordered a new ignition coil, as mine seems too get very hot. The heat shields are in place and the floats were replaced by the PO. The fuel pump is new and the fuel filter has been replaced. I have had the ignition timing checked and it seems fine. I think the problem lies in the carbs somewhere but I have no experience with adjusting Su's. Can anyone shine some light on this problem?
Thanks to all,
Frank
Frank Rizzuti

Just throwing some ideas out as I dont have a lot of experience with SU's either. Check the oil in the dash pots to insure they are topped up (use 20w50). You may have to get someone to check to insure they are synchronized and that the jets and what not are set right
william fox

Not uncommon. Fuel pools in the carb throats and gradually richens the mixture. Which is why tuning instructions say to rev the engine for a couple of seconds every 30 secs or so. You might just have to set the idle speed at a compromise, I have to.
Paul Hunt

Another possibility is the needles/jets are starting to wear a bit, adding to the rich condition Paul Hunt speaks of. When I have to sit at a long light, I rev it up to 2500 for a few seconds to "clear their throats" so to speak.
Paul K

may be worn needle&seat or air leak of sorts. lots of places that can happen. do revs fluctuate before staling or does it just die?
Chris Stansfield

I have checked the dashpot oil and its fine, I have topped it up, I also adjusted the idle up a tad, running at about 1050 rpm. I cleaned and lubricated the throttle linkages as well. I tried the carb cleaner trick to find any air leaks, none were found. The revs will slowly go down and at about 500-600rpm the car dies.
I ran the car this evening, and it is very hot here tonight 32c, it didn't stall once. Seems to be getting better after I cleaned and lubricated the throttle linkages. Will have to keep troubleshooting.
Thanks to all who replied.
Frank.
Frank Rizzuti

Just bought a 76 midget. Can anyone tell me what the purpose of the oil resevoir on the top of the carb is for and how it works? I'm an MG newbee. :-)
Glen Reynaud

the fact that the idle gradually drops suggests a gradual buildup of overrich condition rather than a constant one

a common cause of this is leaking needle valves inthe float chamber

in normal running you do not notice this because the usage exceeds the leakage into the float chamber, but at idle it shows up

if you have hs type su's
try letting it happen and then immediately taking off each float chamber lid to see if the level is high

if you have hif su's this cannot be done, and the only thing u can do is to dismantle and examine the needles under magnification - any ridge means wear - you can also do this with hs carbs
chris

glen: The oil at top of the carbs is for the dashpots. It lubricates the piston inside.
william fox

This thread was discussed between 31/07/2002 and 02/08/2002

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