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MG TD TF 1500 - Advice needed on carb settings for Judson/XPAG

Hi everyone:

Three trips in the TD now, all less than 5 miles but with the engine warmed before pulling out of the garage. Today's drive was to bring the car back from the upholsterer - the soft top and sidecurtains are done [see the picture, they did a good job]

But it feels pretty rich - bogs under 2000 but lights right up above 3000. I'm not sure if it's the fast road cam or the porting or the header or the carb tune, but I've got to start somewhere:

Anybody got any good starting carb settings for the big SU? We're at 2200 feet above sea level. The drive this afternoon was at -8º C.


Dave Jorgensen

Dave you could pull the damper spring out and see what happens. I know I'm running rich and one of the days I'm going to get a wide band sensor so I can play with some tunning. You can see what happens to the A/F with the spring out on this chart in the black. Red line is with the spring in. Would be nice to have a shop with a sensor and a dyno so you could set it up under load.



You say 'bogs down'. Is that a result of over carburation or being too lean?

Assuming its likely too lean, I would be inclined to look for a needle with a more slender bottom part.

I went through a similar excercise with the M-N on my TD. It took a while and about a half-dozen needles, but once I got the right one, it went like a rocket.

If you don't have access to a needle chart, let me know. I have one somewhere in my archives.

Gord Clark
Rockburn, Qué.
Gordon A Clark


Your phrase "bogs under 2000" could indicate a rich mixture at a low rpm. If it were a lean mixture, I would expect some spitting back throught the carb. What needle is in the carb now, as this would be your starting point. How many "flats" are you down from the top? It could indicate too many "flats" down. What do the plugs look like? There are many ways of analizing the problem. I have some contours on the 1 1/2" carb needles and could compare them with what you have now.

The top and side curtains look good and I'm sure your next trips in the car will be more comfortable.

George Raham
G. L. Raham

In the cold weather, you might try switching to kerosene or diesel in the carb slide damper.

If the air temp was -8C, it'd be interesting to use a thermocouple in the venturi to find out how much colder it is with the vaporizing gasoline dropping the temp further. Wonder what an infrared temp sensor would register?

I had installed a set of 4 Honda motorcycle "constant velocity" carbs onto our Honda CRX 2 car, and we ran it in our Michigan winters. I was pleasantly surprised how well those aluminum slides worked in subfreezing temps. When it finally rusted away and was retired, my wife cried.

While in the process of replacing & adding instruments in the cluster, I'm planning to hide an O2 sensor behind exhaust manifold and adding an air/fuel ratio gauge. Since having to provide my own stock 1 1/4" SU onto a Marshall, there's sure to be a lot of tweaking (or maybe just rob the 1 1/2" SU off the Magnacharger since it has performed flawlessly). There were numerous occasions when our '51 TD & Eaton blower had rust clogged carb, strainers, line or fittings and ran lean. A gauge may give a heads up on transients.

jrn Northrup

One other thing Dave. You might try advancing your base timing a few degrees.

The link to my favorite needle selector site.

Dave Braun

I don't think it can be fixed!
Needs to be installed on a TF.
Best thing to do is pack it up and send to me.
I'll send you back a pair of 1/2" SU's to put on the TD.
That should solve the problem.
...don't forget the correct pully's.ect ;-)!

Hey was worh a shot! LOL
David Sheward

Thanks all of you for your help - Dave B. thanks for the needle selector site, and George, if I can't get the little monster as Canmore, then maybe we can do a side-by-side comparison.

David S: Yours is a pretty good offer, but I had a better one last week. A fellow wanted to trade me his 1979 E350 Econoline with a 460/4-barrel/automatic. He thought he'd do me a favor by taking this old piece of British Iron off my hands and giving me a modern piece of American technology.

I reluctantly said no to that one, too.

Dave Jorgensen


You did not say what size jet and which needle you are using.


I am running a .125" jet ( SU # 4185 )with a VE needle in a single H4 carb for Calgary , but if I run it at the coast I change to a VG needle. I set my timing at 4 degrees ATDC static but this will depend on your cam. I am running a Crane Sport cam F-222/280-2-10. This the 9.3-1 compression and 6 psi boast with a Shorrock 75. I am also running two fuel pumps. Hope this helps.
Hope to see you toy soon
James Herbert

Jim H-
Out of curiosity, why are you setting timing ATDC, and how did you determine that? Seems decidedly odd to me.

FR Millmore

That is the static setting to give me 7-8 degrees BTDC on fire up and 28 degrees BTDC at 4200 rpm.
James Herbert

Jim -
OK, a sort of rough and ready setting of a stockish dizzy? 7-7 on fire up when set at 4ATDC sounds like a worn out advance system. Might gain a good bit of low end efficiency by a good distributor setup session.

FR Millmore


On the various T-types I have supercharged over the years with a H4 carb, I have found the magic combination is a .100" jet with a RA or RH needle. both these parts are available from Joe Curto. Start 6 flats down on the jet and set the timing at TDC static or 4 degrees dynamic. Put in a fresh set of plugs and keep an eye on the insulator color. You will be looking for a dark brown color, like a good expresso.


Ben Travato
Benito Travato

Dave, what camshaft are you using, & what are the timing specifications?
I have new Roller lifter camshaft kits in stock designed specifically for blower applications, as well as all other XPAG applications.
Again as Ben stated call Joe Curto 1 718 762 SUSU.
Len Fanelli

This thread was discussed between 06/02/2010 and 10/02/2010

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