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 - Tuning SU Carbs

1500 midget 1977 SU Carbs
After having some issues I have attempted to tune my carburetters. Firstly by following John Twist Videos. After two or three runs it still wasn't right. Had a look in the archives and found a link to Tuning SU Carburetters by by "Speed Sport Motors" So far I have followed the procedure including slackening off the throttle links to the two carbs.
The book says unscrew the throttle adjustment screw to the stop or almost out, then turn down 3/4 of a turn.
It also says screw the jet adjusting nut flush with the bridge of the carb or fully up. Then turn the jet adjusting nut down 2 full turns.
It then says start the car and adjust the throttle screws.
Problem, the car wouldn't start.
I got my daughter to turn the engine whilst I pushed the throttles - it started but as soon as I eased off the the throttles it died.
The question is what do I adjust to keep it running to continue the tuning
Thanks
Nigel
Nigel Axtell

Try winding in the throttle adjustment screws the same amount each. They only control the idle speed.

Assume you already have the engine up to temperature and the choke screws (fast idle screws) are not contacting the throttle.

R.
richard boobier

Thanks Richard
Yes your assumptions are correct
Nigel Axtell

I'd suggest taking some time away from the car and recharging 'your batteries' you also take the opportunity to fully charge the car battery as you don't want that to hold you back when you return to the problem

you had the car running before all be it not quite right so I'd go back to those settings to get the car running and fully warmed up for further adjustments if required

adjusting the carbs is best done after a full service and/or in the order of - tappets, CB points/plugs, timing and then after these mixture at carbs

sorry I can't remember the previous issues so could it be that setting the mixture isn't what's causing the problem or that you wont get things exact or constant because of wear in say something like carbs or dissy
Nigel Atkins

Hi Nigel,

I have just done mine twice as acomplete novice. Not perfect but I recon good. They need doing again and I recon that will be the best of the three attempts (mainly because I know I had them better than they are now and fluffed it at some point).

As a novice I can say that the 'Tuning SU Carburetters' was the best thing to follow. For understanding what is happening the John Twist videos are great and Nigel (Atkins) really helped me with a list of Twist videos to watch and I recommend that you watch the first ones in the series that Nigel recommended cos they explained how the carbs work (Only the Brits could dream it up I reccon - much appreciated again Nigel). However once I knew what was happening and in essence what to do the book was excellent to follow.

It desribes how to set the carbs for scratch settings clearly and goes on step by step from there to get them working together. I was really not confident and I was offered help but this book made it all simple step by step for me.

Mine were really bad (Like not touched for 10 years lay up rough and each carb was singing completely differently) and so my first attempt meant that all was cold. Even like this I made a huge difference to the 'tune'. Being cold affects the underlying mixture setting and so the eventual tick over mixture which ends up too rich when at operating temperatures. The second time I let it run up to operating temps all the time and that is much better.

If you need help with the notes I am sure they are on here somewher or drop me an e-mail.
Dave Squire

now the very important first broadcast of the old-new Clarrie is over -

I'm agreeing with (most) of what Dave's put and think it's a great idea to converse with him for the reasons he's put

I would add that the two JT vids on setting the mixture are, IIRC, on setting them as part of a service rather than a new or reinstallation as such

good luck
Nigel Atkins

Thanks for the replies.
I did get it running yesterday as per the book & JT vids and was pleased at the end. I couldn't take it out for a test due to the appalling weather. So i'll try next weekend and check the settings etc when I get back. If I need any further help I'll be in touch.
Regards
Nigel
Nigel Axtell

Great stuff Nigel. Glad you kept going.
I know the performance fellows on here would be able to 'tweek' their carbs but I can only start from scratch and work my way through at the moment but that does work for me.
As always understanding is no substitute in the end for doing so its practice, practice, ........ and just more practice methinks.
Dave Squire

well done Nigel, now what you is next-to-no-more carb fiddling and lots more frequent driving as this will improve the running of the car, just ask Dave

the secret is to get the car full serviced then set up timing and mixture then leave the carbs alone as very much as possible
Nigel Atkins

id be interested to see what happens to this carb tune in late april

im assuming you did this tune when it cold as a frozen lopcicle fart

my thinking is A/F mixtures are very dense in the winter months...if the carbs are adjusted with the dense fuel mixture, wouldnt that mean that in warmer seasons the fuel mixure be much more lean

just prop thought thats been rattling around inthe ol head noodle for a while

prop
Prop and the Blackhole Midget

This thread was discussed between 10/02/2013 and 11/02/2013

MG Midget and Sprite Technical index

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