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MG MGA - Carburetor piston drop speed

Iv'e done a rebuild on my SU carburators but have been unable to get them adjusted properly on the car. The needles were centered. Synchronizing was a bit of a problem. I removed them from the car and while examining them I turned the pioton and chamber upside down to see if the pistons droped at the same speed. There was a noticeable difference in the drop rate of speed. Visual inspection did not reveal any noticeable difference in piston or chamber. Both pistons have the same ID number (4628). I'm wondering what the fix for this might be.
J. Hudrlik

I assume you have replaced the damper oil with new stuff in both carburettors. I have seen a pair of carbs where the damper oil in one carb was different to that in the other which will result in differing rates and poor running.
Assuming that is correct then it should be possible to remove the air filters and look into the rear of the carbs and rev the engine hard. Both dampers should rise to the sam point at about the same speed if they do then you do not have a problem, if they do not then that requires investigation and cure.
Why was it difficult to synchronise the carbs? that ought to be very easy?
Robert (Bob) Midget Turbo

From my limited knowlege on this subject I believe you may try polishing the inside of the dashpot of the slowest moving piston to reduce friction so as to match up the speed. Or it may be best to swap for a pair of recon or new pistons / dashpots.
I think so long as there are no tight spots on the piston travel, the mixture will probably adjust ok but I think the difference in piston speed will probably affect the throttle response and the car wont drive as well as it should.
colyn firth

I am sure you didn't get the pistons swapped during the rebuild, but it is possible that some DPO did. You might try the drop test with the pistons swapped.
Jeff Schultz

When you rebuilt the carbs did you keep the pistons/dashpots matched up? I believe it is recommended. If not, you might also consider swapping pistons.
G T Foster

Here is a good page with info about the drop test and times. Also has some suggestions for fixes.
Jeff Schultz

If you go into the Jaguar clubs of North America and search 'SU carburetor Tips'by Jim Taylor you will find some useful advice on carburetor drop times and balancing. This info can probably be found in other areas even in archives but I've found this article very good.
J H Cole

Take out the needle and repeat test - see if the needle is dragging. The piston does not touch the bore of the case, only friction should be in the centre guide. If necessary, strip it down and check it dry.
Art Pearse

Great video on you tube, from John at University Motors, that may help.

Really to bad John had to shut down.
B Suelzle

Thank you all. Jeff, the Jim Taylor link had some very useful information - also recommended by J H Cole. B Sulzle, thanks for the link to the video by John at University Motors. I'm going to try the test as he suggested with the pistons switched. These are the best suggestions and I appreciate all your comments.
J. Hudrlik

This thread was discussed between 24/11/2009 and 26/11/2009

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