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SU Carburettor Pistons


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I have tried to balance and generally set-up my SU carbs as part of my 4A recommissioning activities. With K&N filters off and engine around 1000rpm, I have set throttle stops and linkage using a Gunson Carbalancer device. Also tried to interpret mixture colours using a Colourtune fitted alternately in cylinders 2 and 3... screwed in the mixture nuts a few flats, (plugs had looked as though the mixture had been over rich). Now will run for a few miles and check plug colour before any further action here.

 

However, I noticed that a gentle blip of the throttle was causing an immediate lift of the piston in the front carb, but scarcely any sign of upward movement of the rear. Both seem to have an equal level of Penrite damper oil, so what could be causing this difference in response? The rear carb is closer to the connection to the PCV valve and servo, but I would have thought that the balance pipe would have evened out the suction.

 

Without the dampers both pistons drop and produce a reasonable metallic click, so I would not have expected the needle to be binding on the jet. The piston springs do not appear to be a problem; as far as I know they are the originals and appear to be undamaged.

 

Any suggestions? Would it be normal to see any lift of the pistons while the engine is at relatively low revs and not on load? The engine has probably done less than 100miles of its running in period, so I have been trying to keep revs down

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Hi Robin,

the only time I have seen one of my pistons not lifting in line with the other one was when the butterfly linkage had worked loose.

This stopped one of the butterflies not working so no pressure effect on the piston.

 

Roger

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Hi Robin,

 

It happened when i had 2 different dampers fitted - one was a small damper from AUC8102 and the other a larger one AUC8103 i believe... once paired and filled with 20w50 engine oil the carbs work a treat!

on the other hand i did spend a few hours making sure they are perfectly matched - both rebuilt and synced using carbalancer and a colortune..

 

also check that the dashpots have no vent holes if the dampers have one already - i made a mistake fitting a dashpot with internal hole and the vented damper which resulted in instant damper oil loss... described here http://mgaguru.com/mgtech/carbs/cb105a.htm

 

Luka

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Robin, Could be bent damper rod binding in the guide. Confirm by blipping and watching pistons with both dampers removed. Or possible the damper valve has gummed up - it should resist oil moving upwards through it , but not downwards.

Less likely is the non-lifting damper has no air vent hole in the hexagonal cap.

Peter

Edited by Peter Cobbold
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My guess is that although you might have good balance at idle, when the throttle linkage operates one butterfly is opening before the other. It's a fiddle to get them both synchronised in opening as there's a clearance between the operating fingers and the throttle plate levers. It needs to be identical. I tried using feelers but ended up doing it by eye.

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My guess is that although you might have good balance at idle, when the throttle linkage operates one butterfly is opening before the other. It's a fiddle to get them both synchronised in opening as there's a clearance between the operating fingers and the throttle plate levers. It needs to be identical. I tried using feelers but ended up doing it by eye.

+1 It's a fiddly job to get them right. Use carbalancer (or a length of hose, & listen to the hiss volume) to get them balanced & at the desired idle RPM. while keeping the linkage clear of the butterfly forks, by adjusting the idle speed screws. Then bring the throttle linkage levers to be close, but not quite touching the butterfly spindle forks, finally open the throttle while watching the spindle forks, & make sure they start moving together.

 

Bob.

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Any SU problems or spares then Andrew Turner is your man. An expert in SU's & a helpful guy also.

Cheers.

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www.su-carbs.co.uk

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Thanks for the suggestions. Update: checked piston fall (as per videos from Peter W) One faster than the other, so swapped over and now the pair now seem closer in time. Throttle idle screws set to give equal suction (using carb balancer). Also looked at the actuating "forks" and confirmed that they respond simultaneously (by eye).

 

Checked springs and found that one was longer than the other; this was a surprise as I had not noticed this despite owning the car for 38 years! Put the longer spring with the faster rise piston and reassembled everything. Result no obvious change... piston in carb No. 1 still starts to rise first when the accelerated is gently depressed. The two pistons may be more synchronised at greater throttle openings and corresponding higher revs, don't like to take it too high at the moment as I am only 100 miles into running in.

 

Will look into getting a new pair of springs and will investigate further...

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