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MG TD TF 1500 - spark plug color

The TF has been running enevenly and the idle, good when cold, dropped to almost stalling when hot. I removed the plugs and as you can see from the attached picture, after 50 miles of driving after the previous cleaning, the two at the bottom right, rear ones, are very black (in real life they look even blacker) whereas the other two are reasonable. The jet heads were tight up against their adjusting nuts on both carbs for the 50 miles. I adjusted the mixture on both carbs a few miles ago six flats down and then weakened both of them a little by the same amount but could not detect much of a change in revs. Does anyone have any suggestions besides weakening the rear carb some more?



Ron Coates

Check your float level.
Jim B.
JA Benjamin

Dave Braun put out this helpful detailed method of getting the fuel level the same in each of the carb bridge points. You will need a caliper with a depth gauge. It is a little fiddly but can be done and it does work.
You may have to copy and past the link?

http://www.dbraun99.com/SU%20Carburetor%20tuning%20gen%20rev%20d%20161127.pdf

Adjusting the bend in the forks in the float chambers moves the level up or down. Getting them the same is the trick and have the fuel at the right depth.
Rod Jones

Good idea to check the suggestions above first. If you can not get the right colour and smooth idle after checking the float level and/or winding the adjuster up further, you might have a worn jet (and needle). They wear when they are not perfectly centralised because the needle rubs against one side. Sometimes it is even visible to the naked eye that they are slightly oval when viewed from above with the suction chamber and piston removed. If this is the problem, a close look at the needle (perhaps with magnifying glass) might show longitudinal scratches or a wear ridge on one side. The needle should only have about one thou' radial clearance in the jet, when down at its largest diameter.

Winding up the adjusting nuts weakens the mixture but really only at idle and just above. Mixture at higher revs is mainly set by the type of needles.

I assume your rear air cleaner is not restricted? Because that will also richen the mixture.

Bob
R L Schapel

Thank you for your suggestions. The air cleaner is fine, so I will look at the float level and if OK, try weakening the mixture on the one carb further and if it doesn't work, I will remove the jet and needle. Thanks again.
Ron Coates

You may also want to verify that both needles are the same. Sandy
smb brainsky

Don't forget the damper springs, make sure they are the same and are the Blue springs.
B W Wood

Just this morning, coming home from our morning walk around the park, the TD started running badly. Missing on one or more cylinders. Rough idle, low power, intermittent symptoms, one backfire.

Noted a fluid leak on the passenger side (right) backing out of the parking space.

Made it home OK about 2 miles.

Noted that the rule pump kept ticking without the engine running. Then saw that fuel was flowing out of the rear carburetor fuel bowel overflow pipe. (Hence the fluid leak in the parking space. )

Found that the brass hex part that forms the seat for the float valve had come loose. Even though the valve and float were working, fuel was getting around the valve.

Tightened it, checked the float level, checked the front one also and it seems we are back to a good running condition. fuel pump has minimal long term ticking in the non-running condition.

I remembered this thread, Just another thing to look for.

Jim B.
JA Benjamin

I did check the float level and it was too high. I adjusted it and ran the car. It ran much better and the idle was the same cold and hot. I am pleased and thank you all for your suggestions.
Ron Coates

This thread was discussed between 27/07/2019 and 13/08/2019

MG TD TF 1500 index

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