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MG TD TF 1500 - TD Running Rich?

got the car back on the road today. for the first mile it wanted to backfire at about 3k RPM. then it seemed to smooth out. i do not seem to have a lot of acceleration from 3600 above. the car seems to ease on up.
i drove about 10 miles and stopped to talked to someone for about 15 min. when i went to start it back up it backfired at idle and when trying to accel in first gear. then again it smoothed back out. the car idles fine. i do not see black smoke but it does not need much choke when it is cold.
i pulled the plugs when i got home and they were all very black. the are autolite plugs #4123.
i have not checked the timing yet or the valves.
but all indications to me seem to make me think it is running too rich.
i read in the archive about not messing with these carbs until gas starts to run out of them so i was looking for areas to check before i went that route. i did balance the carbs the other day.
i did not do the lift test on the carb cylinders. i may try that tomorrow.
TLW Wright

Sorry all. just read the post about the engine numbers and 3/4 plugs. it appears the 4123 plugs are 1/2" and should be 3/4. i will check them tomorrow. first thing is to get the right plugs...
TLW Wright

You're sure you need long reach plugs?
Gene Gillam

several posible reasons for the performance

a) backfire could be over retarded ignition, so the fuel is still burniong when the exhaust valve opens

b) similarly with too rich mixture, with the unburnt fuel still burning in the exhaust.

c) the backfire after being stopped is probably excess fuel leaking from the carburettor during the wait

Overall I would check the timing, and then check whether the floats in the carbs are flooded, or the jet far too low in its setting.
Ian Bowers

How can I know if I need long reach plugs? Measure the threads there?
If it was timing, would it not continue to backfire? Once I get it thru the first bit off backfire, it does not backfire anymore.
I will check the timing today. I believe I read it should be a little before TDC with a timing light. 4mm or so.
TLW Wright

How can I know if I need long reach plugs? Measure the threads there?

It depends on the head. I don't know if the XPAG used long reach plugs. Possibly the mark II? There's a chart I saw indicating what plugs certain heads use, but I can't find it. The XPEG, mine anyway, uses long reach plugs. PJ
Paul sr

I had a similar question on plugs with my TD8986 which with its original engine should take short reach plugs. However, I measured with a large nail and a plastic tube over the nail, the depth of the threads in the head it turned out that I have a long reach plug head. So sometime before my ownership, over 35 years ago, a previous owner had changed the head.
John Quilter (TD8986)


See the thread from Sept 6 "TD Spark Plugs" for info on the correct plug based on engine number.

Jim Neel 53TD28423

I did read that thread. My car is a July 1952 car. But the engine block has a vin in the 24000 range. After reading that thread I thought I probably needed the longer plugs. I pulled one out today and it does not look that deep. But who can tell by eye really. I tried the nail and tube but was not convinced it measured 3/4. I thought of taking a bent pick and counting the threads as I slid the pick out. I thought I read somewhere about the number of threads and the depth. But that could be something else.
Part of my issue here may be that I have started the car and run it in the shop several timeswhile working on things and people stopping over to have a look. If I do have some gas leaking then I have not cleaned the plugs up at all in the garage.
Was just hoping the issue was something simple like the plugs.
TLW Wright


Take a paper clip and bend the end over, insert it into the spark plug hole, hook it on the inside edge, mark it on the outside, then remove and measure...simple.

Gene Gillam

Yes I am an idiot...
Well just took the car out and had an issue with it spitting and backfiring... Then I pulled the choke for the heck of it.... And it took right off. Then I realized I am probably not giving it enough time to warm up before I take off. It idles so well I just assume it is warmed up. Duhhhhhh. Then after it gets going it is fine.
Having never driven one of these before the only thing is it seems to have no power above 3000 rpm. I can accel from there but it is pretty slow. And maybe that is normal.
I will check the plugs though. They certainly look like the car is running rich. And I will check the depth. Thanks for the tip
TLW Wright


More info on the head:

At the front upper outer edge (right beside the valve cover) on the carburetor side of the engine you'll either see 22952 stamped in the head indicating short reach plugs (1/2") or 168422 indicating long reach plugs.

As for your having no acceleration over 3000 rpm...something is wrong. Your car should be pulling reasonably hard thru 4000 rpm. If your car is idling fine then it shouldn't need to be given much, if any, choke. Remove the two caps from the top of your carburetors and make sure there's oil in there...if not you will find that your car will take it's own sweet time to accelerate.
Gene Gillam

Is your ignition advance working? Do you have a timing light? Shine it on the crankshaft timing mark and rev the engine. You should see the timing advance. Bud
Bud Krueger

Ok. I will check it out tomorrow. Thanks.
I looked yesterday for the number on the head but did not see it. I will look closer for that tomorrow as well.
I know there is oil in the dampers but maybe it is lower than it should be.
It pulls pretty well up to that 3000 in either gear but feels like it peaks there.
TLW Wright


Before you drive yourself "nuts" with carb adjustment. Start at the beginning. Since your car runs, take it out for a drive and warm it right up. Then set the valves to their appropriate settings .019 or .012 whatever is correct for your engine. Then "static time the distributor to TDC. After all that is done remove the intake plenhm (if you have one) or the air filters ( if you have those instead ). Balance the carbs, remembering to tighten the throttle rod fittings when complete, so the rods move as one. Then you can play with the carbs. When you have it all right....DON'T TOUCH. You will have lots of trouble free miles before further adjustment would be warranted.

Lots of good info on the net of how to tune the TD. Just take your time.

Good luck.
C.R. Tyrell

DITTO what C.R.Tyrell indicates,,, start from the beginning and gived it a good tune up,,take the time to DO IT RIGHT!!!!


The 1/2" reach plugs won't affect the running (other than a teeny bit lower compression), even if the head takes 3/4" reach.

Since you're pulling your plugs out, check the color of the ceramic. White, too lean; black,too rich; light brown, good.

Check to see if plugs from front two cylinders are one color and back two are much different. That can be clue to one problem carb.

Pulling out choke could indicate it isn't warmed up enough, as you said, but, it sound like you're running lean, as opposed to rich. So warm it up, and if backfiring persists, ease the enrichener out to see if it clears up while out.

Be sure to clean/set points & timing, and make sure your voltage is up to snuff.

...and please report back your findings.

Forgot to mention. Set the points before static timing the distributor. Then check them with a dwell meter if you have one. If you don't don't worry about it.
C.R. Tyrell

Some very good advise here, however, one thing I would have a look at would be the advance springs/weights in the dizzy before re-tuning the carbs.
The problem of "no power over 3k RPM" was one I had years ago. One of my springs on the advance was broken and wedged in to where it was causing the condition you describe.

As far as checking your plugs:
Run/drive the car up to about 3k RPM's and shut down "under-power". Coast to a stop, and then check the plugs. You will get a much better idea of how the car is running that way than shutting down from an idle. Pretty sure it was either Gene or Jim that gave me that tip on here years ago.

Check the advance springs.
Proper points gap, good condenser, rotor, dizzy cap, wires. (NOT carbon fiber plug wires ...IMHO those are junk on these cars.
Check for good spark @ plug wires.
Run till warm...set tappets.
Static time.
Using a adjustable timing light, set @ 30 degrees BTDC @ 3000 RPM.
Shut down "at speed" and inspect plugs.
At this point you should be ready to make any adjustments to Carbs if needed.

If all is well, you should be able to set the idle on one of these cars around 500 RPMS if everything is in good order.
"T" cars have a "built in timing alert" (I have found anyway). In 4th gear, under light acceleration, the gearshift will rattle! (Mine does anyhow)

That's my 2 cents...those with more/better experience may have better ideas...but this has worked for me over the years.
David Sheward

"built in timing alert"
make that ""built in timing error alert"
...and a lack of power very noticeable going up a grade.
David Sheward

ok, no number on the head that I could find. but the plugs look to be right. with a bent wire and a sharpie I came out to 1/2 on the threads.
I am in no hurry on this so I will do like you all suggested.
the plugs are black and plenty black which made me think rich.
but the advice is very sound. start over and go from the beginning. when in doubt...
hopefully I get some time Friday to get on this.
really the last thing I want is to mess with the carbs. the car idles fine and I could probably idle it down to pretty close to 500 RPM. it easily idles to 800 now. Idle has always been good. No miss or hesitation at any point while driving once it is warmed up. even at higher RPM, it does not miss. it just seems to flatten out just over 3000 rpm. it will accel past there but slowly. again it may be normal and I just expect more. but up until there it pulls right along as I would expect.
Also the car has been run very little over the last number of years. probably just running it and cycling some gas will help a lot.
good story though.... I was out riding Saturday. decided to turn right at the next road and slowed to do that. turned the corner and was driving on when the car behind me starting blowing the horn. did not think much about it at first so I kept on driving but they kept blowing the horn. So, thinking something was wrong I pulled off the road. Two young guys pulled up beside me and asked me what the deal was... (won't say what the car looked like in this politically correct world) I was a little surprised by the question and asked what the heck they were talking about. then they asked why I gave them the finger. the questions was "Bro, what the h*LL did we do?" then I realized, as they were showing me the gesture, and I started laughing. I said guys these cars do not have turn signals. I was signaling a right turn. they also started laughing but it did make me think I have to watch how I signal from now on. who knows how many people I have insulted lately??? when you think about it younger people have never driven or been in a vehicle that does not have turn signals. I wonder if you even have to know hand signals to get a license anymore? Not sure if I had one finger up or two but I can see where they might think it was the wrong one.
TLW Wright

Head casting #- look on the top of the right side between the front and next headbolt back. Do you have the stock oil bath air cleaner? If you do, take the top part off and try driving like that. The support legs can bend or the top caves in and at higher RPM starves for air causing rich mixture. Sis almost got in trouble same way with hand signal bike riding. Have to make sure you hold the fingers together! George
George Butz

That is funny on the signals!

Do try the "shut down under power" to look at your plugs.
(Mine look sooty if if pulled after idle ...but much different story when looked at this way.)

I'm still putting my money on problem with the advance springs as it sounds so much like what mine was doing.
There are 2 counter weights under there...only one of mine was moving.

Keep us posted & best luck.
David Sheward

Tow, Delaware might be unique..but there is no finger pointing in hand turn signals for my state, illinois, or in any google search result I could find. Regards, Tom
tom peterson

haha, I did not realize I was doing it either. old motorcycle habits maybe...
TLW Wright

Checked the springs on the advance weights and they were fine.
next I checked the point gap. they were closed a bit. the book says .010 to .012 so I set them at the .012. then remembering I had pulled the caps out of the top of the carbs when balancing them, I decided to add a little bit of oil back in. just about what I saw left in the pan when I had pulled those out. maybe a 1/4 of a teaspoon.
I thought it would be a good idea to test drive in between each change to see if anything I did made it worse. after this part the car did seem to be a little bit better, but more at the lower rpm.
next I went to check the timing. I had read an article about getting it close and then advancing the timing until it sputters, then retard it until it smooths out. (assumption is it allows for wear in the timing chain, cam, i guess) either way I could not move the dist to adjust it. I was a little pressed for time so I left it alone for now. I will work on that tonight and take some time. I want to make sure I know where it is before I move it... that car starts, idles really nicely so it can not be too far off.
then I got to the valves. i watched a video with John twist and he said to set the valves at .015. SO first i tightened the valves at .015. took a ride and the engine did not seem to have much more punch but it did feel better. SO i went back and set the valves again at .012. (engine num 24763) this time on the test drive i could feel the engine was smoother. valves are still noisy so i guess they are still happy. i can not say it had a lot more pick up but the car seemed like it wanted to cruise at 50 as easy as it wanted to cruise at 40 before.
i need to get the timing checked before i do anything else. but the car is definitely feeling better.
after the timing i want to check the carb balance again and also make sure the cylinders are sliding easily.
TLW Wright

I had the same issue with the valve clearances. I have them set at .012 now as well, and it did affect the timing a little. Static timed it at TDC, and what an improvement. So I think you are on the right track.
C.R. Tyrell

Checked the timing tonight. When set to TDC timing mark, If I wiggle the rotor button clockwise even a little bit, my test light goes out.
So I think I am good.
TLW Wright

Sounds like you have it! Let us know how it runs.
C.R. Tyrell

Well mystery solved...
when in doubt always check the simplest things first...
I was resetting the idle yesterday and noticed that the linkage between the carbs was not moving the front carb. So essentially I was running on one carb at speed. that would certainly cut back on the power! I tightened up bolt that had come loose and wow it runs a lot better... imagine that.
but hey, at least I know that everything else is set correctly. I may still be running a little rich but I am going to run it this way for a while to see how the plugs look. in fact I believe I will buy a new set and run them to see how they look before doing any more adjustments.
TLW Wright

Willem van der Veer

This thread was discussed between 07/09/2013 and 30/09/2013

MG TD TF 1500 index

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