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MG Midget and Sprite Technical - carb pistons

hi all.
i bought a frogeye sprite about 6 wks ago and could not get the tickover right and kept cutting out when stopping at lights etc.stripped the carbs and noticed some scores in the suction chambers, cleaned them all up and got the pistons moving nice and free. set the carbs up regarding mixture and syncing both carbs and checked the linkage to make sure both open at same time and car now runs quite well and ticks over ok. 1 thing i noticed though is when you just blip the throttle the rear piston comes up about a quarter of an inch and drops again but the front one doesn't unless you give it more throttle. any ideas what could be wrong. both pistons definitely moving free.

bob taylor

Jet centring?
If you haven't already, check the nut holding the jet assembly is secure; mine was fractionally loose which caused the needle to stick just enough to stop it moving freely.
Jeremy MkIII

Hi, couple of quick thoughts-

. did you clean the pistons and inside of dashpots and only lightly oil the piston rods?

. have the dashpots to pistons been mixed up and no longer matched, see - 41 Matching SU Air Pistons -

And, of course, have you got a relevant copy of the Driver's Handbook to guide you.
Nigel Atkins

jeremy, yes will check that but think ok.
nigel, don't know if been mixed up in the past by previous owner, could change them and see. and YES i do have the handbook to guide me. how could anyone on this forum not have when we have been reminded by your good self a numerous amount of times. :}
thanks bob.
bob taylor

have a look at that John Twist video to see how to check.

Only second plug of the day for the good book, some still resist going down the right path. :)
Nigel Atkins

I'm guessing the jet need centering. Should be easy enough to test with engine off, by lifting the piston just a little (not too much) and making sure it drops cleanly with a click when released.
Handbook is really useful for covering your head if its raining :)
Graham V

yes it does that graham
bob taylor

Nice to know you lads have your copies laminated, my master copy lives in a velvet lined box in a darkened, climate controlled room, viewing can be arranged on certain days of the year.
Nigel Atkins

When you say "yes it does" I'm not sure if you are referring to keeping the rain off, or the piston clunking.
Assuming you are ignoring my bad sense of humour, and its the latter, it really is quite easy to fit the rear air filter (or just the filter gasket) upside down. That will blank off the suction chamber vent. Can't happen on the front as it has a vent pipe.
Certainly easy to check for starters.

Failing that I would try to replicate with engine off and slowly try to eliminate things.
Graham V

Unfortunately from experience I can say that having one filter gasket on upside down will affect your idle and when you the throttle open quite a bit but less noticeable at throttle openings between.

I would advise against fitting the gasket or filter when in too much of a rush when rain water is running down your back giving you soggy underpants, again (only this time the liquids cold).

Graham, humour can only be good with the good book

A good spot with gasket. If only I could remember all the problems and solutions (and understand them) I've had on my classics I'd be an expert in that area (only).
Nigel Atkins

This thread was discussed between 27/06/2018 and 30/06/2018

MG Midget and Sprite Technical index

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