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MG MGB Technical - Sticking carb piston

Novice needs help. Managed to rebuild brake master cylinder, new ignition parts and 1969B "Old Dan" is on the road with a few problems. This after about six years idle.

Starting is very slow (sometimes 15 tries) and think carbs might be the trouble. Dual SUs and the rear carb drops fine with metallic clink when lifter is raised... also holding lifter stalls engine.

Front carb piston seems to be stuck. Lifter has no effect on running. With damper out, you can see piston lift in rear carb but piston seems to be in uppermost position in front carb and never moves.

Question: Can I remove cover of carb chamber with damper, et. al., to check piston? Obviously only three bolts in chamber top. And once I take off chamber, how might I get the piston unstuck. I've tried tapping it lightly through the damper opening, but with no avail.

And might this be my reason for slow starting? I hate to get back into the ignition system for fear I'll be stuck in the garage with no fire for another six years.

Thanks for any help and advice. I'll worry about the brake light problem and lack of a top later.
Mike Kaylor

Yes you can just remove the three screws and take out the whole assembly. There is no seal to worry about. Soak the assembly in degreaser/carb cleaner and it should come apart easily. Be very carefull not to bend the needle!
Can you reach into the carb and lift the piston? There is oil in the dashpots, right?
Jeff Schlemmer

Thanks, Jeff,

The dashpots have Marvel Mystery Oil... Taking out the dampers and pressing up the lifting pin, I can see the piston rising in the back carb, but doing the same in the front carb, the piston never moves. Seems to be stuck in the top position.

Later in the morning, I'll clean the assembly. Wish me luck.

Could this be my reason for the slow starting? I have to crank the starter over and over, but eventually engine does fire.
Mike Kaylor

Mike, To answer youe question about the hard start with a sticking carb piston. Yes, it will make it hard to start. You may have to use a small screwdriver or pry bar to free the piston. Be gentle and be sure to clean the piston as well as the inside of the dashpot with spray carb cleaner, being careful NOT to bend the needle. You also might want to look inside the dashpot to see if it's dented, keeping the piston from moving or returning to the bottom.
As for the oil in the dashpot, I only use 20w50 motor oil as the lighter weight oil lets the piston come up too easily, causing a stall, on acceleration. There are all kinds of opinions about what oil to use, It's kind of like "which is better, a Ford or Chevy"? However, I only use 20w50 in all of the cars that I work on and it works very well.
Good luck.
The MGDOC
mgdoc

Try loosening up the bell and moving it around to see if that helps, sometimes it's slightly off center. The next thing to do is to look at the inside of the bell and use scothbrite clean it out and smooth any burrs, same with the piston. Next check the needle and see if it's worn as the jet assembly may have to be recentered.

That's the whole show fixing a stuck piston.
Mike MaGee

Much thanks, mi amigos,

Old Dan, the 69B, is purring like a ... well.... With your encouragement, I took the bell housing, chamber or whatever you call the cover off the front SU and found a sleeve from the air cleaner bolt had slipped into the carb and was lodged beneath the piston. An easy fix (10 minutes) for which Jack's Toy Shop was going to charge me $75. He said he would spend an hour chcking out the cargs. You guys are life savers.

We'll see tomorrow whether it affects the cold start problem.

Mike Kaylor

69B started on first cranking with both SU carbs operating. Thanks again.
Mike Kaylor

This thread was discussed between 31/08/2005 and 01/09/2005

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