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MG MGB Technical - SU carb problem

Attempting to improve the tick over on 71 roadster have always used the rubber hose to the ear method with varying success to get the balance of the tickover. Having acquired a crypton synchro gauge I am getting some strange readings.Ajusted the tick over to 700 revs, the gauge was reading 2 on the scale on both carbs. when the revs were raised to 2000 revs the front carb gave a reading of 6/7 the back one still read 2. The piston is not lifting with the increase of revs. When looking into the air intake it is obvious it is not lifting. The dash pots have been topped, it is not stuck as it lifts easy enough with the finger so I am baffled. Any suggestions please. Trev
Trevor Harvey

If it lifts as easily by hand as the other carb then the implication is it is not sticking. However you really need to do a drop-test to confirm that http://www.mgb-stuff.org.uk/fueltext.htm#droptest

However before the drop-test check that both idle screws are lifting off the stops at the same instant, i.e. check the linkages are opening both butterflies together.

If so then possibly one piston spring is stronger than the other. They should have paint marks so be the same, but you could swap them over and see what happens.

Also unscrew both damper caps and open the throttle a little and see whether both pistons are rising together or not. If they are now then there is a problem with one of the dampers or oil chambers.

You can swap the pistons and covers over between carbs in case there is some issue between piston and carb body, but at this stage make sure you keep each piston with its cover as they are selected to be a matched pair.

Another possibility is one jet not centred so the needle is wedged in the jet until sufficient suction occurs above the piston skirt to release it. For that lift each piston up in turn and let it drop, it should drop smartly with a metallic click. A softer noise indicates the needle is binding in the jet - it could be bent as well as the jet non centred. This should be done with the jets wound up to flush with the bridge to be sure they are free when in the normal running position.

If all else fails do the drop-test, and if one or other are out (takes too long) check the skirt and inside of the cover for rubbing or dirt, clean them. If still out or if one is too quick and the other too slow try swapping pistons between covers, just in case they had been swapped previously. If both drop-tests are in spec then keep the pistons with those covers.
paulh4

There is also a possibility that the pistons and covers may have been swapped over/mixed up in the past, and one carb is sealing better than the other, causing more lift. But how would you know?
Dave O'Neill 2

See my last para.
paulh4

Thanks for the reply’s, have improved the balance, not sure how done a more thorough clean of things and replaced the oil in the dash pots but I think the main problem was the linkage making sure that both throttle links were opening at the same time, question. The clamps either end of the connecting bar fit into a slot that allows a lot of movement, where should the movement be should the throttle linkage open as soon as the link bar moves or after the pace has been taken up. Hope you can understand my question.
Trevor Harvey

Sorry, Paul...missed that bit.
Dave O'Neill 2

Trevor, Is this the device you are talking about. If so, I have never been quite sure how to set it up myself either!

Andrew Dear

Trevor
The slots are there to allow a little movement at idle so that idle speed adjustments can be made without needing to adjust the linkages--
The way I do it (which is probably wrong but it works is--

Let both idle speed screws right off so that the butterflies are fully closed on both carburettors
Adjust the linkages up so both the tangs are at the end of the slot and the butterflies move instantly and together equally as soon as you move the throttle cable--be really fussy getting it equal and instant
Screw the speed adjusters down till the butterfly 'just' starts to move on both carbs and then another full turn
Start it up and reset your idle speed and idle speed balance with your synchro tool (in my case still a hose to the ear)
Then go drive it
willy
William Revit

Andrew No, different carb, but maybe a similar problem.

Willy, You seem to have the same thought that I do, that the throttles should open instantly but I tighten the clamps after setting the tick over speed.
Another thing I am not sure of is the position of the needles in the piston blocks, should the flat on the needle be flush with the base of the block or pushed fully into the hole?
Trevor Harvey

The shoulder of the needle should be flush with the bottom of the piston. If a floating needle (which it sounds like you have) then still focus on aligning with the piston rather than with the surrounding securing boss.
Paul Walbran

It was rather long, Dave.

"the butterflies move instantly and together equally as soon as you move the throttle cable"

Not quite instantly, there is supposed to be a little free play in the cable so the butterflies can't hang open on the cable. The free play is between the finger on the throttle interconnecting shaft and the choke shaft on both HS and HIF.

The important thing is that both butterflies _start_ moving at the same instant, and is tricky to set up. When you have the air-flow balanced at idle i.e. when the idle screws are on the stops you then need to check the balance just off idle i.e. cable pulled slightly, and it can take several small changes in one or other clamp to get this right.
paulh4

Paul H


In the method I described above--
if you set it all up with 0 clearance with the butterflies right back, then when the idle screws are adjusted in to get correct speed, this gives a little clearance in the slots and allows the butterflies to return to idle position unhindered
The idle speed can then be adjusted to any speed without effecting the previously adjusted throttle linkage settings
Trevor appears to get the gist of what he has to do so I'm sure he'll sort it out
It is important to have just that tiny bit of clearance between the tang and the end of the slot when the speed screws have been adjusted in, which happens automatically as a part of using this method
I took it as a given that Trevor would understand that the cable needs a little end play as well so that it returns freely- my mistake leaving that out but we were talking about balance and slots
Cheers
willy
William Revit

Well, the WSM says to put the appropriate feeler gauge between the finger and the choke shaft, then tighten the clamps with the lever pin resting lightly on the lower arm of the fork in the carburettor throttle lever. But the effect is the same i.e. once the feeler gauge has been removed there will be the correct free play, and in theory it should ensure that the gaps in the forks should be the same and both carbs will open together. But that's not always the case, it can take careful tweaking of one or the other to get both carbs pulling the same amount of air just off idle as well as at idle, and have some free play in the cable and linkage.
paulh4

Well-!!
I don't want to get into a backwards and forwards thing about this, just posted the method I use - trying to help
I had noticed in the first few posts that no-one had mentioned the obvious throttle linkage balance issue so just threw my bit in to help Trevor -- out
William Revit

Thanks willy, your imput was appreciated. We ca all learn from people’s experiences. Trev
Trevor Harvey

This thread was discussed between 10/05/2019 and 12/05/2019

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