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MG Midget and Sprite Technical - Carb piston movement


I have recently refurbed my carbs on my 1973 1275 midget using one of the standard refurbishment kits. On reassembly the pistons moved freely up and down, giving that metallic click that Haynes describes. However, after filling the dashpot with engine oil, there is a clear resistance in the up and down movement, and there is no metallic click with the pistons falling rather slowly. Is this the dampers doing their job properly, or should the pistons still fall freely?
Any advice is greatly appreaciated.

A Damm

what grade engine oil? maybe too thick in cold weather, might want something thinner...
David Smith

With oil in the damper the action is slow and steady, and it won't click as it falls down, because (surprise, surprise) it's damped! It will work fine with the engine sucking away. If it moved as freely when working as it does undamped, the piston would tend to flutter and give all sorts of problems.
Mike Howlett

Also, the valve in the damper is designed to provide resistance during upward movement of the piston. This provides an enriched mixture during acceleration. So if the movement feels rather stiff when you lift the piston, that's intentional.

Are you sure you put the pistons back in their respective carbs when you rebuilt them? Pistons are matched to carb bodies during manufacture, and swapping them around can cause problems with tolerances.


Gryf Ketcherside

Yes it is dampened however you should still get a decent click when the piston is allowed to fall, whilst it will fall slower than without oil. Pushing the piston up should be considerably harder than falling as the damper is designed primarily to restrict movement up rather than down.
Bob Turbo Midget England

I think hearing the click depends on what grade oil you're using and background noise - I've never been a fan of Haynes

you can get the purpose sold oil for the dampers, Penrite or SU Zenith labelled from the usual suppliers, I think it might be 20 grade but I could be wrong about this

as said you need to keep the parts with their matches, here a John Twist video that might help you with that -

plenty of other John Twist videos on carbs and other useful stuff too
Nigel Atkins

Thanks for all the comments. From a logical point of view, the damping made good sense. But after all the reading about tuning carbs in Haynes and on the net, I was a bit unsure because so much emphasis was put on the metallic click and 'easy/free' up and down movement.

A Damm

This thread was discussed on 07/06/2012

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