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MG Midget and Sprite Technical - carb pistons

I have just dug out a pair of 1 1/4 Su carbs and put them on the car. I have noticed one piston is harder to push up than the other from the air filter side.The harder one sometimes sticks before it drops as well. I have put new oil in the dash pots. What else controls the speed it drops. The springs?
m fairclough

if they have unbiased needles you may need to re-centre the jets.

If they are biased (sprung) needles, try polishing the inside of the dashpot / exterior of the pistons with something like 'Solvo Autosol'.
Deborah Evans

Clean, them and the bell, up with white spirit (cautious). Never use an abrasive. Try to change the pistons, centralise the jet. Look at You Tube; search for: twist Su carburettor

Flip
Flip Brühl 948 frog 59

Make sure the pistons and inside of the dashpots are scrupulously clean.

Also, unless it's too late, don't swap parts between the carbs...although you can never tell what has been done to them in the past.

If the piston is still sticking, try slackeming the dashpot retaining screws and try moving the piston up and down whilst 'wiggling' the dashpot and see if it frees off.
Dave O'Neill 2

Thanks for your advice I will have a go at taking the dashpots off and cleaning them tomorrow. I will let you know how I get on
m fairclough

Dave makes a good point about 'recentreing' the dashpots. However, when I was consulting (for a major restoration company) last year, I saw supposedly re-conned carbs that still presented a problem, despite all the 'ackling' in the world.




"Never use an abrasive."

Fuel is hygroscopic, and, over time, you get a build up of Aluminiun Oxides / Hydroxides.

A VERY gentle polish with a chrome cleaner (such as Solvol) will remove this and cause no issues.

It's something I've done off and on for in excess of 25 years with NO issues (just make sure you remove ALL deposits of the polish!).

I have also used 1400 grit wet 'n' dry at a push.

While a solvent will remove any fuel residues/varnish, it will do bugger all to the oxidation.
Deborah Evans

>>> Also, unless it's too late, don't swap parts between the carbs...although you can never tell what has been done to them in the past. <<<

Yeah, that. I recall seeing a video from John Twist where he was working on a set of MGB carbs, and figured out that the pistons had been swapped. Switching them back restored original operation.

-:G:-
Gryf Ketcherside

I didn't have a lot of time but I took the dashpot off and there were fine vertical score marks on the piston over the three grooves and some muck in the grooves. I wiped it off as best as I could with some tissue and did the same in the dashpot.I put it back together without the dash pot screws but it felt stickier. I then turned the dashpot 180deg and the piston goes up and down nicely. I think I should take them apart when I have more time and give them a thorough clean as suggested. thanks for the advice. I am sure Dave they have been swapped around a lot over the years when I was young and foolish and didn't know better!
m fairclough


It starts and runs. A bit of T cut and it wll be fine.
http://www.youtube.com/watch?v=PaRmZmzR09w
m fairclough

Good news, and thanks for the YouTube link! Nothing sounds quite like a serious A-series engine...

-:G:-
Gryf Ketcherside

The engine isn't very modified, but your right its great to hear that sound again from it.
m fairclough

This thread was discussed between 11/05/2010 and 23/05/2010

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.