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MG MGB Technical - Carbs piston stuck down

Maybe some of you ran into this before.
A couple weeks ago I drove the '69 B into town to meet the rest of the club and the car ran fine. When I next fired it up to drive 1/2 block to pick up a stragler it was running on two cylinders. So we left the car there. On returning at the end of the day I drove it home and it still ran like crap.
Yesterday after cleaning and gapping the plugs I took off the air cleaners and found the rear carb piston was stuck in down position. I don't how it became stuck but I figure it not raising, cylinders 3 & 4 got flooded with fuel and the front carb was feeding just cyls. 1&2. Cleaned the piston & dashpot, added a bit of oil to damper and it started and ran fine. Took it on the road and cleaned out the cylinders.
Still don't know why it stuck all of a sudden.
Sound like a plausable reason for my misfire?
Geoff Kimler

I think you need to re-centre the jet.
Art Pearse

With my carbs you can loosen the 3 dashpot screws and it gives you the wriggle room to centre the needle in the jet. The lifting pin lets you check when you have it right
Stan Best

I have the floating needle type. The jet still looks centered. Will check out some more though. Maybe the damper plunger jammed.
Thanks.
Geoff Kimler

Certainly the only thing that could hold the dashpot down would be the needle, unlest the inside of the dahspot itself was ver dirty. if the needle is running central, could it have been a massive bit of dirt? Mike
J.M. Doust

Geof-
Check to see if the damper rod is slightly bent.
Stephen Strange

After 40 some years, a build up of residue of fuel and damper oil is bound to form inside the suction chamber. If the car is running fine since you cleaned the suction chamber and piston, I think that you have found your problem. RAY
rjm RAY

Dirt or a burr can stop the dash pot sitting square on the carb body. When I re-build S U's I always put some fine wet and dry on a sheet of glass and give the dash pot a light emerying and then clean the inside, the carb body and the piston with petrol or carb cleaner and a cloth. When assembled (without the damper)lift the pistons with your fingers and let them drop. They should both hit the bridge at the same time with a "clunk".
Allan Reeling

This thread was discussed between 11/04/2011 and 15/04/2011

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