MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - SU Tuning

midget 1275, I have just replaced the spindles on my cars old carbs. and have been trying to set them up this morning. Ignition is ok, car starts and runs fine.
I started with the jets down one and a halve turns from the top, set the throttles to suck equally and the idle to 1000. All the indications were that although the engine was running ok, it was too lean, and lifting either piston caused it to stop. When I screwed the jets down a couple of flats, the revs rose too high, and could not be brought down on the throttle screws, screwing the jets up reduced the revs. This is totally opposite to what I expected, as richening the mixture should result in a drop in revs. not the opposite.

Anyone any thoughts.

Dave
Dave Barrow

Dave

If you get to a point where you can’t bring the engine speed down with the idle screws it means you’ve either got an air leak somewhere or the butterflies aren't fully closing. I take it you centralised the butterflies before tightening the screws?

Bob
R.A Davis

Dave,
did you check that you assembled the gaskets between the carb body and the air filters the right way up?
Guyw

Sounds like an air leak, try spraying some carb cleaner around the outside of the carbs, if the revs go up there is a leak. I assume that as you have just rebuilt them that the dash pots have oil?
Peter Ottewell

What guy said

theres that hole at 10 oclock on the front and 2 oclock on the rear... its easy to block the holes with the gasket

did you put oil back into the carbs...20w50

did you use new butterfly disk... the new kits are not bevealed...there just like a coin... and wont seal very well.. if thats the cas reuse the 9ld butterfly disk
prop

Good point by Prop on the butterfly shape
Also make sure your idle speed screws are backed right off when centering up and give the butterfly a light tap or two and same on the ends of the shaft to get the butterfly completely seated and centered in the bore
Cheers
willy
William Revit

Thanks for all the suggestions guys. Yes I did centre the discs in the spindles, all new gaskets, and tuning done without the air filters fitted. (K&Ns) Dashpots topped up with 20/50.
Had another go this morning. Jets raised a couple of flats, now about 7 flats from the top. Could not detect any air leaks. Engine runs reasonably well, idle about 1100, and car drives really well. A test with a gunson Gas Tester registered 3.4. A vacuum guage test showed a good high reading, but erratic.
readings.
The engine still shows signs of running too weak at idle, but there does not seem to be any way to make it richer.

Thoughts are at the moment to drive it a bit, and check the plugs.

Dave
Dave Barrow

IIRC starting point for mixture is 11 flats or two turns. If dropping the jet is increasing the revs and throttle screw is having no effect might the problem be with the fast idle screw?

Bill
W Dunsmore

Did you centre the jets?
Guyw

To follow guy

when you raise the pistons and let them fall do they fall without effort and wjth a ( thunk ) sound

Have you tried srayjng down the intake assembly with carb cleaner or wd40 with the engine running to test for any air leaks

prop
prop

Yes I was careful to check the jet centering,they both bottom out with a nice metallic click.
Good point about the fast idle stops Bill, I think they are clear, but will check.

Dave
Dave Barrow

"The engine still shows signs of running too weak at idle, but there does not seem to be any way to make it richer."
As per Bill's comment, at 7 flats you are well short of the two turns default position. Unless the carb is worn, the default position is very close to correct and would expect it to be weak at 7 flats.
If lowering it makes it idle faster without doing anything else, it indicates that the mixture is getting closer to correct, ie richer. If you get to 12 flats and it is definitely lean, there is an air leak, most commonly from the dreaded thin manifold gasket above and below the port at the head face.

I suspect the problem will be in the centring of the throttle discs. I find that even though I stress to each new young fella working for me the need to be meticulous about this, the first couple of such jobs they do inevitably show the same won't-idle-slow symptoms and have to removed and re-set (which sadly is un-chargeable time ... ). They finally get it that the only way to be sure it is centred is to hold it up to a bright light so any gap round the edge can be seen. Looking at it any other way is nowhere near as effective.
Paul Walbran

Thanks Paul, I am ordering Two new discs today, and will have another go at centring them, more carefully this time.

Dave
Dave Barrow

Are your disc beveled?

If not the new ones you just ordered may not be ither..if the old disc are beveled and dont have tbe pop up valve id reuse those
prop

Prop

No, the old discs were just standard, and that is what I have ordered.

Dave
Dave Barrow

Then they should be good to reuse as they will have the bevel ground onto the edge...
prop

hi dave
My midge 1500 had 2 shot sus when i bought it.
Have to confess went the weber way .Once sorted it stays that way. That is my own experience. Reason for my comments where abouts in west sussex...i am near burgess hill ???
rgdsw tony
a boyle

Update.

Good result, I fitted two new discs, and carefully centralised, slow running is now spot on, and car drives better than it ever has since I got it.
Thanks everyone who helped.

Dave
Dave Barrow

This thread was discussed between 29/06/2016 and 06/07/2016

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.