Welcome to our resource for MG Car Information.



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 - Rear Carb Issue

Yesterday, a neighbor and I adjusted the air flow (synced the carbs) and adjusted the mixture as best we could. Car idled well at 800 RPM and the timing with the Petronix was 12 degrees BTDC at idle and 30 at 3000 RPM. I believe Jeff Schlemmer's recommendation is 12-14 and 30-32. All seemed good.

This morning I changed the plugs and set the gap on the Champion RN5Cs at .025. When I attempted to start, it eventually turned over (never in 8 years has the car done this), ran rough for bit and spit and sputtered and back fired though both carbs several times as I lifted the throttle lever to rev it a bit.

After temp was reached and a smooth-ish (not the best)idle at 800 RPM I decided to check the mixtures again. The air cleaner manifold is OFF. When I lifted the front piston a tad, the RPM changed noticeably, I watched the piston move the 1/32 inch. When I lifted the rear piston, no change lifting at all, in fact I could lift the piton up all the way it seemed, with no noticeable change in RPM. Something is terribly wrong I assume.

I would appreciate any and all input to help me along the way. Thanks.
L Karpman

First things...check you spark pug wire order. You may find that 3 and 4 are reversed.
MG LaVerne


It sounds like you’re not getting enough fuel from the rear carb or maybe reduced flow? Not sure.
Have you had a look at the set up procedure from Dave Braun? He works you through the fuel level set up and jet position etc. which should help you get to the correct position.­_Rev_C.pdf
This might help you check a few things that may point you in the right direction.
Rod Jones

Thanks guys. I should have said lifting the rear piston to at least 1/4 -3/8" had no effect, not "almost all the way."

Checked the plug wires and found a short in #2 (don't ask how I found out :-)and the #1 connector was loose! Repaired the connections. My connectors are 1-3-4-2 counterclockwise.

Also had to adjust the timing a tad afterwards. Runs just fine now, but tomorrow will tell the tale when I try to start her. If she is still hard to start and backfires thru the carbs, then next I'll swap the coil to a spare. I will also review Dave's carb setup page.

Also, I still have the "dieseling" on shutdown, and I'm hoping a long run in the car will resolve that, and it's not due to a hot spot in the engine. I thought changing the plugs might resolve it, but it did not.

With my medical issues I simply did not have the ability to drive the car as I did before, so it sat. I should have at least started her more often, and that I regret.
L Karpman

DID you try lifting the pistons after you corrected the bad wire problems????
The dieseling is a common happening,, get in the habit when you come to a stop, of putting it in first, right foot on the brake, turn off the key, and simultaneously SLOWLEY bring the clutch up as you keep your foot on the brake,,

I'm not sure what you mean by
>>it's not due to a hot spot in the engine<<

Also, if it is hard to start,, try a little choke before you go after the coil,, if some choke works, you know you mixture is not correct,,,

Steve Wincze

Yes Steve I did. Same result. Lifting the rear piston 1/4 to 3/8" before any effect on the engine speed.

As for the "hot spot" I am referring to carbon build up in the cylinder(s) or on the piston.

I have been shutting the car off as you describe since the dieseling suddenly began. Thanks.
L Karpman

Welcome back Larry...long time no see you on the forum.
Gene Gillam

Thanks Gene. Yeah, it's been a while, and during that time away from the MG my brain has dumped lots of valuable info, but I'm slowly getting back on track.

Well after sitting overnight, the TD started easily this AM, but with rough running when cold. No backfiring thru the carbs, but still rough.

I can't believe I can lift the rear piston so much without it changing RPM. I'm was suspecting old bad gas, but I have to believe it would affect the front carb too. I just have to nail down why I can move the rear piston so much without any noticeable change in RPM.
L Karpman

Since the wiring appears to be ok...the next thing I would do is pull the float bowl lid on the rear carb. I suspect you will find no gas in it and that front carb is supplying all of the fuel at the present time. The symptom you describe could be either spark to the two rear cylinders or no fuel.
MG LaVerne

One other thought: recheck the sync. If there is no idle airflow through the back carb, lifting the piston won't change anything. When at idle, quickly place your palm over the carb opening and remove. The suction should feel the same for front and rear. "All seemed good".. then you changed the plugs. Could you have a new plug that fouled or is bad? George
George Butz

LaVerne: I did pull the rear float bowl cover and it was full of fuel.

I believe we (me and my neighbor) found the problem and fixed it. While observing the piston movement I was so fixated on the movement, I failed to note the white-ish buildup on the pistons. We pulled the pistons and they were coated with this white-ish gunk all over, including the surface of the jets, and the groove. We thoroughly cleaned the pistons and carb bodies and reinstalled. Instantly upon starting, We could tell the difference, and SEE the difference. Both pistons now move smoothly in unison, and lifting both the pistons one at a time causes the desired rise and then settling of RPM.

My neighbor, who is a mechanic, helped me discover this and was a great help. The dieseling was also greatly reduced. He also recommended I advance the timing a few more degrees and now the dieseling has ceased (for now) and the engine is smoother.

Only time will tell if the problem is resolved. We are expecting cooler weather soon, and that will give me the opportunity to test it all out, rather than just idling and revving in the garage. Thanks for everyone's suggestions.
L Karpman

If lifting the piston over 1/32 has no affect, first thing I would suspect is it's set too lean! If it speeds up above 1/32, it's too rich! PJ
Paul S Jennings

Hi Larry,

It is essential with oxygenated fuels (alcohol or ethanol at 10%) which are common now to put in Stabil or some other fuel stabilizer if you are laying up the car more than two months, the warmer it is in your storage area, the worse it will be. The alcohol allows the fuel to form all sorts of nasty organisms that can clog, stick, or otherwise affect the behavior of the fuel pump and carburetors. Once you add the proper amount of Stabil (or Sea Foam or any other reputable fuel stabilizer) drive the car for about 10 miles, get it nice and warm, change the oil run the engine a bit and then let it sit, not starting it during storage periods. Most of these kinds of problems will disappear.

The reason for changing the oil is to remove any of the acids from the oil which is sitting on the bearing shells.

Dave Braun

Thanks Dave. Yes, I believe that is the obvious issue I have/had. The TD in the past rarely sat un-driven for more than 2 months, had Seafoam, but while dealing with some health issues, this time the car sat for much longer and mostly in a very hot garage in the Texas heat.

I need to drain my tank as soon as I find a place that will take it for disposal. I'm working on it though.
L Karpman


Drain it into your lawnmower engine...

Gene Gillam

This thread was discussed between 09/09/2014 and 11/09/2014

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.