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MG MGB Technical - SU Piston Travel

Having finally rebuilt the petrol pump, we attempted to restart at the weekend. After having to clean the float chambers again, we eventually coaxed it to life. However the idle is poor which I can live with for now, but I noticed that the SU piston travel was markedly different between both carbs. If I lift by hand there is no noticeable difference and neither has any notchiness, both will return to home at 'roughly' the same rate, (no I didn't time it) When running, there is a noticeable lapse before the front piston rises and a similar lapse before it returns. Throttles are set up as per SU/Haynes, as are the linkages. They 'appear' to be a pair, as both have poppet valves unlike my roadster. So any ideas? Both dashpots were topped up with oil to the same level.
P M Gregory

On first looking at this, I would say it's a balance issue. I the two are out of sync, the one allowing more air through will rise first. You didn't say you had balanced them, so do that first.
Allan Reeling

also have you matched the dashpot pistons to the dashpot as if you mix the two you will get variation

there's a few John Twist videos that will help you, here's two -

41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA

123 MG Carburetors & Doors (first part needles and seats, springs and carb oil) –
http://www.youtube.com/watch?v=v9QI3NlvwiY&feature=plcp
Nigel Atkins

Balancing the cards is easy, if you use two pieces of wire, thus.

Herb

Herb Adler

"both will return to home at 'roughly' the same rate, (no I didn't time it)!"

They should drop smartly, no damping as there is on the rise.

Remove the dampers and see if they rise at the same rate then. If they do then there is a problem with one or other damper, or they are of different types. If they still rise at different rates then there is some other problem, but you must be sure they are giving an equal 'suck' at various steady throttle openings first.

I'm not so sure about Herb's method of balancing, there can be all sorts of reasons why one might rise more than the other. You might be able to compensate for that by having the butterflies opening by different amounts to give an equal rise, but then you would have them sucking different amounts which would not be good.
Paul Hunt

The beauty of using wire, like I showed, is that you can check the balance across the whole rev range. They should remain level, always. All the ones I've tuned work like a dream. If they don't stay level through the revs, then there is some problem that needs to be sorted.

Herb
Herb Adler

If they rise at different rates then the carbs (or parts thereof) are mismatched and the problem should be rectified. If they're not as close to identical as possible then the engine will never run quite right.
Rob Edwards

I'd start with the easy stuff (mainly because I don't know any further) and check the pistons are matched as per the video I posted earlier
Nigel Atkins

Have you checked tappet clearances and cylinder compression pressures? If there is a valve not seating or burnt or piston ring problems, you will get variations in carb piston lift.
jim soutar

I checked the pistons as per the video, swapped them over & they are now travelling correctly. many thanks for the posts.
P M Gregory

always best to start with basic checks before moving on to clever complicated stuff
Nigel Atkins

This thread was discussed between 16/09/2014 and 04/10/2014

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