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MG Midget and Sprite Technical - Sticking Carb Piston

Hi,

Having started the engine for the first time recently all seems fine, other than the revs are a little high.

Found the problem - on both carb pistons, they are getting stuck and won't seat properly on top of the jet. About 1mm on the right and about 2mm on the left. Up to that point the piston is rising and falling smoothly.

I have replaced the jets, bearings and needles. Old ones where badly worn or pitted.

If I drop the jet by applying full choke then they drop all the way down smoothly.

If I remove the suction chamber with the jet flush with the top of the bearing the pistons drop all the way down.

I think therefore this is more about alignment issues. i.e. when you tighten everything up the needle does eventually touch the jet.

So does anyone know what adjustments I can make that will stop this from happening please?



James Paul

James

Have you got fixed or sprung needles? With fixed needles it may be necessary to tighten the jet with the piston fully down.
Also try rotating the chambers or swapping them over as they can distort
Bob Beaumont

Hi Bob,

They are fixed needles.

When you say tighten the jet. Can you be more specific please?

Do you mean with the adjusted nut for the jet bearing or the choke arm?

Thanks


James Paul

James,
I guess you have HS carbs (flexible pipe between the float chamber and carb base).
Therefore I would as Bob suggests rotate the chambers before tightening the 3 fixing screws.

Basically fit chamber and screws loose, press down on top of chamber to locate and check the piston drops down - if not slightly rotate it and try again - hopefully you will find the ‘sweet’ spot where the piston drops with a nice clunk - then tighten the screws and test again.

R.
richard b

Hi James

basically you are centering the jet to the needle.

1)the choke lever should be disconnected.
2)remove the jet it may be necessary to remove the fuel feed pipe from the float chamber.
3)remove jet locking spring and adjusting nut
4)replace the jet and insert fuel feed back into float chamber
5)slacken the jet locking nut (the large brass nut the jet goes through) Make sure the assembly is free to rotate
6)Remove the dashpot and gently press down the piston
7) nip up the jet locking nut keeping the jet hard up against the jet bearing.
8) check the piston drops with a 'click' sound on the jet bridge.
9) if not repeat 6 and 7 above. When all is goos remove the jet feed from the float chamber, remove the jet. Replace the spring and jet adjustment nut. Refit the jet and feed pipe to float chamber. refit the choke control lever.
10) I then turn the jet adjusting nut up as far as it will go then back off 12 nut faces. This gives a good starting position.It will be necessary to adjust again for the best position once the engine is warmed up.
Bob Beaumont

Hi Bob,

They are fixed needles.

When you say tighten the jet. Can you be more specific please?

Do you mean with the adjusted nut /screw or the choke arm?

Thanks


James Paul

James, I had exactly the problem you've described. I also spent ages fiddling with the dashpots.

Centring the needles by the book, i.e. as Bob describes, solved it and both pistons and needles move smoothly and make a feint click when they drop onto the jet bridge.

Colin
C Mee

I’ve never needed to centre HS carbs, only the older H types - maybe I’ve been lucky !

I think the procedure is on Burlin’s site but I have a copy you can pick up if passing.
richard b

Thanks all!

I've give it a go and report back.
James Paul

Hi,

Success!

I didn't know that you could align the jet to the needle by rotating the jet bearing before locking it into position with the top nut.

Once I had done that and fitted the jet adjuster nut, it was all ok.

I noticed that one of my dampers wasn't dead centre in the piston so corrected that as well.

Many thanks

James

James Paul

Great stuff. Glad your sorted!
Bob Beaumont

One further question.

Assuming the oil was the same type and level in the pot and the jet was removed, what else would cause one piston to fall quicker than the other?
James Paul

James

the test for a piston/dashpot is to seal the transfer hole in the piston (the one on the side near the needle locking screw) with plasticine or similar. replace the piston into the dashpot with the damper in place. Make sure the piston is at the top of its travel. Invert the assembly and allow the dashpot to drop off the piston. It should take about 3 to 5 seconds.

if its outside this time then the cause could be:

1) Dirty piston or dashpot or oil on the piston stem

2) different or weak carb dashpot springs

3) worn or distorted dashpots.




Bob Beaumont

Thanks Bob!
James Paul

With Bob's last test you can also check you have not mixed up the pairing of suction chamber to piston rod as they're a machined matching pair.

See John Twist's video for more details. - 41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA

If you've cleaned and lubricated the piston and inside of suction chamber then note only the piston rod is lubricated, as excerpt from SU booklet below.


Nigel Atkins

Well that's one argument in favour of a larger single carb then.😉.
anamnesis

I like John's videos. Usually very easy to understand, although often hard to see what's going on with the poor video quality in the older ones.

Did his test and it worked and made sure mine are matched.

Thanks Nigel.
James Paul

James,
interesting as looking back at the older JT vids the quality of image stands out but the actual camera work seems fine to me, I think it's best viewed on a larger monitor screen than a phone or tablet perhaps but wouldn't know as such small screens are no good for me and viewing outside is a waste of time and annoyance to me. I supposed if the vids are viewed on too larger screen then the older quality of the vids will stand out more and even perhaps be more difficult to see, but again I'd not know as I've not got the facilities.

If you've not got it the SU booklet is another source of info (first page of pdf is blank). - https://mgaguru.com/mgtech/books/pdf/Tuning_SU_Carburetors.pdf
Nigel Atkins

Thanks Nigel,

Appreciate the help as always.

Just finished with the timing strobe and I was at plus 5 so turned the dizzy a little anticlockwise and now she’s running really smooth.

Yet to drive her to see what it’s like through the range but we’re getting there.

Seats and interior back in two weeks so hopefully take her out for first spin, if the weather holds up!

Thanks
James Paul

Sounds good James. Have you tried moving up and down the drive yet? I remember doing that early last year and it's a great relief when the clutch and at least 1st and reverse work!
Bill Bretherton

Hi Bill,

I have and yes it was a real buzz to feel it moving without me pushing it!

James Paul

This thread was discussed between 12/10/2022 and 13/10/2022

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