MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

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 TD TF 1500 - SU JET ADJUSTMENT

I'm trying to set up my newly Professionally rebuilt
1 1/4 SU's. I have to turn the Jet Adjusting nuts down 14 flats to to have the engine run smooth and start to have some color on the plugs. 14 flats seems like a lot. Has any one else has this issue? I think the neddles are ES
Steve
Steve Wincze

Just over two turns, which may be slightly more than usual, but within the acceptable range I think. Turning the nut only really has a significant effect on idle, what matters most is that you have the right needle and that you get the correct mixture with a wider throttle. Take it for a run and stop without significat idling, then check the plugs. A professional carb rebuilder should have fitted the correct needles for your set-up, assuming he was told what it is, so I would expect them to be OK.
Dave H
Dave Hill

What about the fuel level in the float bowls ? This has an effect on the mixture.


Laurent.
LC Laurent31

Has anyone got tips on how to set the fuel level in the bowls. Burlen's web pages say you should just be able to slide a 7/16" bar under the fork with the flat part of the fork resting on the needle valve. But I find in extremely difficult to push the bar in squarely and find I have to lift it slightly to clear the far side of the lid and am not certain if the fork is still resting on the valve. I had thought of testing the clearance between the fork and pin with a one thou feeler but think the weight of the fork actually presses the pin in slightly so there will be a negative clearance.

Cheers

Jan T
J Targosz

Jan,
This is the write up by Dave Braun (Thanks Dave) which I used with success. The fuel height adjustment is fiddly but can be done with patience. (item 6 & 7) It is important to get then both at the same height.
Hope this helps.
Rod

http://www.dbraun99.com/Setting_SU_Carburetors­_Rev_C.pdf
R D Jones - Ex Pat

Jan. The method written up by Dave Braun is easily the best. Trying to do it with a round bar is much less effective. Even just doing it visually, without measuring, works very well.
Dave H
Dave Hill

Hi Dave,

I have tried Dave Braun's method but found it impossible to determine the fuel level in the jet tubes. The problem was the meniscus. I could measure to the top of it with vernier callipers but as soon as it was touched the surface tension disappeared and the level fell. Also if the jet tube was wet with fuel the level was different from after being left for fuel to evaporate.

Cheers


Jan
J Targosz

Yes it is tricky. If you just do it visually and keep it simple it works. Take the tops, pistons, etc., off the carbs. Switch on the ignition so that the pump fills the float bowls, and take a look down the jet tubes with a torch. If both 3 to 5mm (1/8" to 3/16") below, then do nothing and reassemble. If one is too high or too low, adjust the fork as required - remember to draw out a little fuel if you are lowering it. Switch on the ignition again and recheck the level. Don't at any stage lower the jet tube.
Dave H
Dave Hill

Dave, be careful with terminology. In the US a "torch" would be something with a flame (propane, etc), not a good idea over gasoline! We would call it a flashlight over here. George
George Butz

Perhaps a stupid question on my part, but why does it matter if the the adjusting nuts are 6, 12, or even 20 flats down? I would thing whatever setting gives the right amount of fuel should be OK. If not, why not? Just curious.
L Karpman

Larry,
I totally agree with you, and it's not a stupid question at all, but as the nuts are screwed down,(actually being unscrewed) they are approaching the end of the threads. How close to the end of the threads is too close? ie Too close to falling off. 18 flats is a full 3 turns of the nut.

Steve
Steve Wincze

Thanks Steve. So, how many turns of the nut before it falls off? Anyone measured the number of turns?
L Karpman

I'm going back at the set up tonight,, I'll let you know,, I've got to get this running properly so we can drive to the GOF
Steve Wincze

Steve: I can assure you that even at 18 flats (3 turns) it won't fall off. Mine are in that vicinity.
L Karpman

There are definitely more than three turns on that part. If the nut is turned down a long way its because you are attempting to compensate for the fuel being too low in the jet. Better to get the correct level in the jet first - it should be 1/8 to 3/16" (3 - 5mm) below the top.
OK, its flashlight for a little direct illumination, rather than torch.
Dave H
Dave Hill

Dave: That was my whole point. What is the difference between getting the level correct in the jet by bending the fork(s), or by turning the adjusting nut? Seems to me whatever accomplishes the job should be fine. But I'm far from an expert, so that's why I asked.
L Karpman

Have you talked to the rebuilder? I have used the 7/16" method for float/fuel level and the NEMGTR T Series Handbook for tuning instructions for many years with total success. If the car runs great at 14 flats (and the above directions were followed), I would just drive and enjoy. George
George Butz

I guess it depends how pernickety you are. The ideal is to have the fuel in the jet correct as a starting point. I don't claim to be an expert either, though I have fiddled with a number of them over many years. I follow my reference book on SUs. The ideal would be for the fuel to be level with the top of the jet for vaporisation, but it would also spill out on corners, etc., and that is the reason why instead it is at a lower level.
Dave H
Dave Hill

"What is the difference between getting the level correct in the jet by bending the fork(s), or by turning the adjusting nut?" The difference is that by bending the forks you are not changing the needle depth in the jet. If you adjust the jet down you are enriching the fuel mixture by effectively reducing the diameter of the needle in the jet tube.

Tim
TD12524
TW Burchfield

This thread was discussed between 24/08/2016 and 25/08/2016

MG TD TF 1500 index

This thread is from the archives. Join the live MG TD TF 1500 BBS now