QuestionQUESTION: I have a 1953 MGTD and I can't pull the choke cable out. If I disconnect the cable from the linkage the cable moves back and forth smoothly. With the cable disconnected I can't move the linkage by hand. The car runs smoothly when running but I always have to choke it to get it started. I don't know what I am missing. Can you help? Thanks so much. Sandy
SU Carb type H
ANSWER: Hi Sandy. I would start by removing the clevis pin at the bottom of each choke lever so that the two levers are no longer connected together. Next, try to operate each lever separately by hand. You will probably find that one of them works but the other one is stuck solid.
Have a look at the attached diagram. I refer to parts on the left hand picture as (A1) through A(10) and parts on the right as B(1) through B(10).
When you push the choke lever to the left it rocks on a clevis pin (where the arrow points to A7), stretching the return spring (A1) and pulling the main jet downwards (B1). My guess is that one of your main jets has corroded in its sleeve so it will not move.
To free the jet, unclip the return spring (A1) remove the clevis pin (A3) from the bottom of the jet and swing the lever (A7) out of the way. Insert a small screwdriver in the fork at the bottom of the jet B(1). Rotate the jet all the way round through 360 degrees. That should free it. Now pull it downwards about 10mm (3/8 of an inch) and push it back up again. Do that a few times and it should free up. It might help to drop it about an inch and lubricate it with a few drops of oil. Do not remove it completely.
Re-assemble using new split pins (A2) and the choke mechanism should now work OK.
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QUESTION: Hey Barrie, Your answer was right on!! It took me maybe 15 minutes to fix the choke. HOWEVER, now the car is running extremely rough and if I push the choke all the way in the car will cut off. I ran the car (had to pump the gas to keep it running) for several minutes hoping it was just cold and needed to warm up. Any suggestions? Thanks, Sandy
ANSWER: Hi Sandy. Now that the jet is fully returning to the 'up' position it sounds as though this is causing a weak mixture on that carburetter. You need to richen the mixture on that carburetter until the engine runs smoothly. The adjusting nut is clearly shown on the diagram I sent you, item B2, and it may need a lot of adjustment.
Before you start to adjust the mixture you will need to disconnect the connecting rod that joins the two choke arms together and after you have finished you will probably need to alter the length of the rod before re-fitting it.
Do you have any experience of adjusting a pair of SU carburetters? To do the job properly is quite complex. In my book 'Barrie's Notes' this task fills 3 whole pages.
Let me know how you get on.
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QUESTION: Hey Barrie, Went out this a.m. to try to adjust the carburetter. I started the car before I started and while it isn't running 100% it would idle without me pumping the gas. After letting it run for a while I also was able to push the choke in completely without it cutting off. I might add I was very happy about this development because I was a little hesitant about trying the adjustment! Anyway, the question: Before I try to adjust the carb do you think by chance it could be bad/old gas? I don't drive car that often and the gas is probably 4 months old. Also in the regard would it be better to drain gas out of car over the winter if I'm not driving it. Your thoughts/recommendations? Thanks again, Sandy
AnswerWow. That's quite a big question you are asking, Sandy.
There is some controversy here in the UK over modern gas with added Ethanol. It would appear that older cars with carburetters are suffering from rough running, especially when they get really hot. The MG TD in particular has the float chambers too close to the exhaust manifold so the fuel overheats and boils. I don't think you have this problem. Also, modern gas should not be left to stand around, it needs to be used. However, I don't think you have that problem either, because once gas goes bad the car will not run at all. For long term storage I park the car and seal the fuel filler flap with duct tape so it cannot evaporate or take in moisture from the air. Coming out of storage, I add 2 gallons of fresh fuel BEFORE trying to start the car for the first time.
You must regularly check that the oil dashpots on top of each carb full of oil. That is really important. It is possible that your troubles are due to a lack of oil in the dampers.
Unlike a Holley or a Weber, the SU carburetter does not have an accelerator pump, so pumping the gas' does not help when trying to start a cold engine.
Remove the air filter and alloy inlet manifold so you can see inside the carbs. Can you see the tops of the jets? My guess is the top of the jet on the good carb is about 1mm BELOW the jet bearing #B5#, whereas on the bad carb it is level or even sticking up above the jet bearing. If I am right, disconnect the choke linkage bottom bar and then lower the bad jet #using the adjusting nut# so it looks about the same as the good one. You will probably need to adjust the length of the choke linkage bottom bar after you have done this.
Give that a try. The engine should run better, although you may find that the tickover is now too high.
