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MG TD TF 1500 - Tuning my SU 1 1/4'

Hi,

Have been spending part of the evening sitting in front of my two SU H2 1 1/4" trying to adjust them.

Synchronization wasn't any problem but when adjusting the mixture i became a little puzzled!

I started as usual with six flats down on both carbs. and then continued one flat at a time for both of them trying to find the point where the idle doesn't increase anymore.

I believe that i found it around ten to twelve flats down.

Then starting to lift the 1/32" to see that the engine speed up for a moment and then settle back to only slightly above the original idling speed!

I have also learnt that if lifting 1/4" and the Engine is still running the mixture is way to rich!

My problem is that the rear SU is stalling when lifting 1/32" at ten flats,and the front SU isn't stalling at 1/4" when I screw the nut up tight to the top, that is "No flats down at all!".

The reason for starting to adjust the carburettors was that I had black smoke out of the exhaust. That would say that something is to rich!

Probably have to take the SU's apart but would be happy for some thoughts!

Best regards

Anders in Sweden


T Dahlberg

Check the float levels. They must be correct and the same. Great picture !! George
George Butz

Hi, how can a difference in the floatlevel give a richer mix in one of the carbs?
Thanks :-)



T Dahlberg

The level of fuel in the jet is set by the level in the float bowl. The level of fuel in the float bowl is controlled by the fork that opens and closes the inlet valve. Bending the fork adjusts the fuel level. Ideally the fuel level needs to be about 1/8" below the top of the jet.
The nut at the base of the carb only really makes a small difference to the mixture at idle, not at higher throttle openings.
Dave H

Dave Hill

Hi Dave,

Thanks for your comment. Hadn't really thought of the fact that the fuel level could affect the adjustment in that way. But, I understand that now.
I will check both of the forks this evening and see if they differ much from each other.
Best regards
/Anders
T Dahlberg

Good article here:
http://www.mg-cars.org.uk/imgytr/hints&tips/carbtune.pdf

Decent video here:
https://www.youtube.com/watch?v=BSbZeU2eLwk

Dave Braun's article here:
http://www.dbraun99.com/Setting_SU_Carburetors%C2%AD_Rev_C.pdf

Good Luck!

Stuart
S Grimm

Hello again,
Now all is as good as could be.
I adjusted both forks and lowered the fuel level.

Went by the book and synchronized both carbs, started with sixflats down and so on.

Now I have an even and wonderful idle at 750 rpm. :-)

And no black smoke from the exhaust.

Next week I will change for an 123-distributor and see how that works!

Next after that starting to fiddle with dad's single S.U.HV8 / Volumex on the 1938 TA-Special. That might be a little trickier than the double H2's on the TC.

Best regards

/Anders
T Dahlberg

This thread was discussed between 19/06/2016 and 28/06/2016

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