Mixture Adjustment on TC with Twin SU 1 1/4 Carbs

[Paul Mascuch]

I just finished rebuilding the XPAG engine on my 1948 MGTC. After running break-in oil and putting a number of heat cycles on the engine I switched to my standard oil. The engine had been running hotter than it had been before so I started checking for a reason.

The radiator, thermostat, proper positioning of the head gasket and timing all checked out so next I checked mixture. When I pulled the plugs they were black and full of soot. I installed my color tune which lets me see the flame color in the combustion chamber. The flame was very yellow so I started moving the jet up one flat at a time. When I finally had the jet cranked as high as it would go the combustion chamber flame color was better, but still showing too rich.

I checked the float level in the carbs and they were very low. This should tend to cause a lean mixture. I reset the float level to 3/8". I checked the needles on the pistons. They are both stamped 156. I made sure the shoulders on both needles were flush with the bottom of the carb piston. Both needles are centered in the jet. I balanced both carbs for equal draw.

I cranked both jet adjusting nuts down 12 flats again as a starting point and checked mixture by very slightly lifting the piston in the carb. The idle speed went up and stayed up indicating a rich mixture. I then repeated this test after turning the adjusting jet nut one flat at a time. I saw no real change in how the engine responded all the way up to the jet nut being cranked all the way in. Over this entire range the engine runs smoothly and has good power.

At this point I'm scratching my head trying to figure out what else to try. Does anyone have any suggestions for me.

Thanks,

Paul Mascuch
1948 MGTC

[Marv]

Weigh in Joe Curto, what say you! I've experienced this before on a MGB. Changing the disks and one shaft out was the solution there.

Thanks, Marv

[frenchblatter]

You may try dropping the needles 0.015"-0.025". I've had to do this, as I had the same problem of the jet nut being fully up and the carbs still running rich. I think it's brcause the brass insert has been pushed into the ali piston. Whatever the reason it worked.

[John Cockrem]

Hi Paul, For what its worth, I had a problem which stumped me for a long while with the front carb running too rich.Turned out that the float bowl was not breathing or venting properly.Sorted that and problem solved. Those little copper pipes really are vents more than overflow I recon.
I agree with Norman re dropping the needle. If that dosent help, and the jet is not worn (check it with a magnifying glass) beg borrow or steal (only kidding) some leaner needles as all the screwing up of the nuts won't always help.eg I am running needles out of Phil Cassie`s TA and they are doing the trick...Thanks Phil! Hope he dosent want em back!
Good luck John Cockrem TC 2268

[Richard Michell]

Others will be more expert but your needles are almost certainly not 156's. I think that is the diameter of the part that goes into the piston (0.156 inches). The needle is identified by a pair of capital letters. From memory, ES is standard for the TC. It is stamped on that same part that is lodged in the piston but can be hard to see.

[Duncan M]

One thing that can keep the jet height wrong is #26 in the Moss catalog not getting compressed enough. This cork needs to soak in oil overnight and starts out something like 3/16" thick that is visible, but typically gets tightened down to around 1/16." Not getting that cork squished enough will also make it impossible to tune-- other than too rich.