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MG TD TF 1500 - SU carbs

Has any one out there had the same problem as I have, my TD is running very badly, running very rich on tick over but then weak when speeding up. This alters when the air filter is removed but is not choked in any way. The main fault I can see is the pistons in the carbs are not lifting evenly,as I increase engine speed the left piston lifts as expected but the right one only lifts very slightly even if I remove the top damper I have swopped the dashpots over and the pistons but the right one still only lift slightly,I have checked the airflow with a balance meter the air flow is identical,I have removed the damping oil from the r/h dashpot but still does notlift on the run, with the engine stopped the piston lift and falls freely by hand. Please help as I am gettig desperate.
Adrian Wells
TA WELLS

Adrian,
Purely theoretical, it seems that both dashpots and pistons are okay. Nothing "mechanically" wrong with them I mean. But no matter which one you use: only the right carburettor refuses to lift the piston. An equal amount of air flows through both carbs so again nothing wrong with the butterflies (or synchronizing). In theory both should be able to create the same amount of vacuum on top of the pistons. I would guess (but it's a real guess!)that no matter which dashpot you place on the right carb; not enough vacuum is created there which can lift the piston. Is it possible that the right carburettor body is damaged so there is an air leak to the dashpot ??? The fact that one of the pistons isn't funtioning properly, probably causes the lean mixture on high revs ???!!!
Good luck !
Nick, 52TD/63midget.
Nick

Make sure the throttle shaft coupling is not slipping- sounds like the throttle may not be opening on that carb.
George Butz

Or you may have a mismatched set of carb pistons - one heavier than the other.

Lew Palmer
Lew Palmer

Hi, Adrian - a common, but often overlooked, issue is having an air cleaner gasket misplaced, and covering the two holes that are just above the air cleaner mounting holes in the carburettor body.

I believe you'll find that one of these is plumbed into the underside of the piston to let ambient air into the bottom of the dashpot assembly. It's called the "air rectifier hole" and must not be plugged or blocked by the gasket. If this is plugged, you end up with a suction on one side of the piston, and an induced vacuum on the other side, and the piston doesn't move very far, if any.

Sometimes a problem if you make your own gaskets, as I often do. Best of luck - - Alec
Alec Darnall

THANKS TO ALL REPLIES,I have tried what you have suggested to no avail, however I eventually decided to check to diameter of the air pistons and found the r/h was .0030" down on circumference, this was enough to allow the vacuum to escape so the piston would not lift.I have now wrapped a piece of aluminium tape around the piston ,it now lifts as it should as a pair.The car has been transformed as this fault must have been there for some considerable time, the performance you could say is great because itwas like driving with only one carb. thanks again for your input it was very much appreciated.I am going on a 200 mile trip this week-end so that will prove it. Regards Adrian Wells TD845
TA WELLS

Adrian,
Strange that when you swapped the left dashpot/piston on to your right carburettor it also refused to lift...(You swapped piston/dashpot as a unit did you?) The SU manuals have a method to check the amount of tolerable wear between dashpot and piston. In short it goes like this: take the piston and blind off all holes in it, lightly oil the stem of the piston (which goes in the damper), remove the damper from the dashpot, put the dashpot over the piston right to the end then turn the whole unit upside down and count how many seconds it takes before the dashpot drops until it is on equal level with the piston edge. The allowable time depends a bit on the size of the carb. but around 5 sec. is okay!
Normally the piston doesn't make contact with the dashpot wall so it shouldn't wear. If yours is too small for the dashpot I suspect the wrong piston or dashpot is used ???
Good luck,
Nick, 52TD/63Midget
D.G.J. Herwegh

Hope your trip goes well, but I would suggest this fix is only a temporary measure until a matched pair of piston and dashpot are obtained.
Are you going to a classic car show this weekend. I intend going to the MGCC meeting at Groombridge Place, Kent but in a modern not the TD.
Henry

Henry,the MG meeting was at Burly House Ripon Yorkshire, must have been 1000 MGs all models and eras.
You are right about a tempary fix, I am looking for second hand pistons and dashpots. the only problem I had was the plugs kept sooting up as now I am getting a lot more fuel delivered. I purchased a handy book on SU carbs.Your su companion, It seems standard ES needles run Too rich with modern fuels, I have now fitted EU needles and the miss fire has almost gone due to cleaner running. I may even try EW needles as suggested by Donald Jackson (your SU companion) as I have raised the cr. to improve performance. I will keep you informed.
Regards Adrian Wells
TA WELLS

This thread was discussed between 03/08/2005 and 09/08/2005

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