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MG MGA - Carburetter dampers

I have been giving my SUs the once over recently and I've noticed that the small brass pistons at the end of the dampers are of different length for each carb. (see pic.)

One of the pistons is 0.306" long and the other is 0.376" whereas both diameters are exactly the same at 0.336".

Has anyone else noticed this? To my mind there would be more resistance to upwards movement of the jet needle piston with a longer damper piston length and this would cause an imbalance in the mixture richness for the two carbs. during acceleration. There doesn't seem to be anything in the SU Manual about different length damper pistons so I guess there's no technical reason why they should be different; I can't think of one.

There was a change in the damper assembly part number for the 1600 (AUC8114) compared to the 1500 (AUC8103) so I wonder if these two have different piston lengths and for some reason I have one of each.
I suppose I could carefully file down the longer piston to the length of the other but then I might have two 1500 dampers!

Would some kind person with a 1600 and a micrometer check the length of their pistons (please no jokes!) and I could then just order one replacement damper.
Thanks .................mike



Mike Moore

I think the factory mod was to make the pistons short. All you need to do is turn the long one down to 0.306 to match the other. The info is on Barneys site somewhere. It makes the piston drop quicker when you close the throttle.
A J Dee

Thanks AJ I found the article. Sorry now I sold my lathe last year. I'll have to file it down carefully. ............... mike
Mike Moore

Just for interest, before you modify them time the drop on both of the pistons. Then swap them over and see if it is different.

It would be most interesting to see the results.
A Bowie

For those that cannot access Barney's website I have attached the Service Memorandum.
Although it says mga twin cam it applies to all MGA's

Mick

M F Anderson

AB I tried the piston drop check yesterday but without holding the damper during fall it tilts to one side and affects the timing. But I did a (very) subjective test of lifting the jet piston with my thumb and in a ten second total lift time the short piston carb. was 2 seconds faster.

Referring to the BMC Memorandum I can't see why the short damper piston would give uneven running. Does that mean that the 1500s out there, and there must be hundreds, with original carbs. are running unevenly?

You don't hear many 1500 owners changing their dampers, at least not on this forum.....................Mike
Mike Moore

This thread got me out under the bonnet to measure mine. I have a 1500 spec car but had a refurbished pair of carbs when I did the rebuild in 1997. They are the shorter damper pistons.

I noticed that the lower edges of the damper pistons on my carbs are shamfered so I am wondering why the memorandum says you can shorten them by machining the LOWER face? I would have thought that machining the top (squared-off) face would have been easier. Seems illogical.

Steve
Steve Gyles

Steve, The jet piston has to experience resistance on the way up but not on the way down so there needs to be a seal to prevent the flow of damper oil through the piston hole. There is no seal on the bottom face so the jet piston can drop quickly.

I have dismantled the long piston damper and there is a thickish brass washer above the piston and the top face of this washer is noticeably dished (concave) to mate and seal with the convex face of a brass sleeve fixed to the damper rod. But less noticeable the bottom face of this washer is very slightly convex and the top piston face is ever so slightly dished. This is why they don't want anyone messing around with this top face.

The intricate construction of these parts seems to me to be the result of many years of experience at SU and I've decided to leave well alone and not to reduce the length of my long piston. Instead I will fork out the 40 or so for a new pair from Burlen!

I'm not sure why there is the chamfer on the bottom end of the piston but maybe it's to achieve a required design length of piston unaffected by any bottom wear on the piston by it banging on the c-clip. Alternatively as the piston is not symmetrical it may just be to identify the bottom end ............Mike
Mike Moore

Thanks Mike. Just had another look and that tallies with my set-up. I still have my lathe. Other than it being such a small part to set-up in the chuck with the resultant small amount protruding for machining, it is a straight forward job.

Steve
Steve Gyles

Just found out from Burlen that the correct damper pistons are available separately for 88p each plus PP!

Part number is JZX 1779 (for the AUC 8114 damper) and is not on the website..................Mike
Mike Moore

This thread was discussed between 20/04/2016 and 22/04/2016

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