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MG TD TF 1500 - Can't get my su's to lean out?

Good day all
I can't get my Su carbs to lean out. The adjustments nuts are all the way up. I'm getting a lot of black smoke and soot out of the exhaust. When I restored the TD 10 years ago the mechanic, who was very knowledgeable in su rebuilds, told me they wouldn’t stand another rebuild. Can I install larger needles and take care of this. I was also told that I could try setting the needles about 1/64th inch high,
(Not bottomed out)although I have not tryed this. Any thoughts on this? Should I just bite the bullet and purchase new su’s. Thanks John
John Hambleton

John, drop me a email,

Gerard Hengeveld

John, what sort of air cleaner are you using? Have you tried running the engine without the aircleaners? I don't see how a rebuild (if feasible) could solve your problem.
Cheers, Hugh
H.D. Pite


Keep in mind that short of fire, a wreck, or a wife with a big hammer there is not much that can render and SU beyond rebuild. There are people here on the board that can return them to nearly new condition. One of them will undoubtedly be along shortly to offer help.

Best regards,

Jim H.

1953 TD
J. M. Haskins

Why would an SU not stand another rebuild?

Apart from Hughs' remark, and using old fuel, I guess the not leaning out can only be caused by worn jets and needles as a result of bad centering in the past.

Those parts should be able to be renewed in any old SU?
Willem vd Veer

John, Sometimes excess fuel pressure can contribute to a rich mixture due to the pump pushing fuel past the float valve. Doesn't happen often unless a pump with higher pressure is installed or a float valve is bypassing fuel, but it is a possibility. I think 2 1/2 to 3 lbs is as high as it should be. I would also check, as said before, that the needles are centered properly. I agree with the rest that unless the carburetor housing is cracked, there is no reason that an SU can not be rebuilt back to like new condition. We are very fortunate on this board to have experts who do this. Besides, I understand the new ones aren't a perfect match for the looks of the originals if this is a concern and they cost a fortune. I had a pair rebuilt that looked terrible and when they came back they looked and worked like new! PJ
Paul sr

"short of fire"....I have had SU's rebuilt even after that and they worked! Check the pressure of your fuel pump. Sounds like it is too high or nedles are not seated correctly.
David Sheward 55 TF1500 # 7427

Above ideas right on. Also check the fuel level in the float bowls/jet bridge areas (volumes somewhere in the archives)- very important. If a TD, the air cleaner top can cave in and cause rich running. The top jet gland seals (corks) can crack and leak also, and the pistons could be sticking. The guys above are right about rebuilds, I think the mechanic was nuts, as eveything but a massively destroyed body/piston/chamber can be readily rebuilt. Dave Braun on the site does beautiful rebuilds, John Twist (University Motors) and many other places can rebuild them for you. George
George Butz

I have had very good luck with Dana Britton in Wichita, KS. Contact him at e-Mail:
Also check his website

He did wonders for the SU's on my 1936 MG-TA.
John Masters

Also check the floats, and float needle settings. Take the floats out and give them a "shake" if you hear a slosh , they may be leaking and they will never float to a level to shut off fuel through the needle seats. The floats do sometimes get small hairline fractures, especially if they have been tumbled, and will leak fuel inside. Not usually the first thing one looks for, but results in fuel overflowing the "bridge" causing excessive rich running.
C.R. Tyrell

Fit the "Lean" needles in the SU's and go one step hotter on the spark plugs - welcome to 2012 pump fuel.

It's a tad different to mid 20th Century stuff !.

Don't ask me how I know this, but it works for me in MG's from 1936 - 1969 !.


If you need help, try Joe Curto. I am a very satisfied customer.

Jim Neel TD28423

John -- Go to my web-site and look under posts, "Engine Talk" found in 2012 August posts, and Engine Talk #2. which is my current post. I had the same problem and discussions and solutions are presented in those two posts.

John Brickell

Thanks for the good advice, I'm going to have one of our clubs experts look at it. Phil the air cleaners are the pancake type Moss sells.
Thanks for all your help. John
John Hambleton

I would be interested to know whether the needles and jets were replaced. Both can wear and cause rich mixture. Neither are expensive.
Barry Bahnisch

John ,
If you mean these (as shown) S&H type...
IMHO : scrap them! I ran these for 10 years on my car.
Finally took them off and went with stock Vokes & custom made K&H filters. World of differance. Why I waited so long I will never know...but my car ran rich for the whole time before getting ride of these.

David Sheward 55 TF1500 # 7427

Hi David
Nice to hear from you, haven't talked to you since you and your dog built the garage. Those pictured are what I have. Can you tell me where you purchased your new ones? Thanks John
John Hambleton

Actually the S&H were on the car when I purchased it and the original Vokes were in the boot!
As "tight" a fit as the thin S&H were, I never tried the Vokes thinking no way would they fit. Stupid in that when I took the time to look at them I realalized the stock air cleaners were angled and recessed so they fit better.
From reading the BBS here I heard the S&H were considered "restrictive" to air flow. They were 100% right!
Think they make "re-pros" of the Vokes now.
The K&N filters I "custom made". If I can find the thread I will try to reacatavate or post a link to it.
Shows how I made the filters ...pretty easy & cheap.
David Sheward 55 TF1500 # 7427

Black smoke, not blue smoke, right?

Before spending a great deal of time and money swapping parts or carbs...

If you're running really rich, the plugs will tell the story. Pull your plugs and see if just front 2 or rear 2 plugs are running sooty- you'll probably narrowed down your search by 50%.

Remove bells & slides, then drain float bowls by loosening the float bowl bolt under carb body. Then power up the fuel pump and watch the fuel level rise and stop in the carbs' jets. You may find one carb continues to rise and run over the bridge.

Before buttoning up, DO Dave Braun's procedure for setting the float level. It beats the workshop manual's procedure.

And, whatever you do, PLEASE report back here to tell us what you find. It's a real pain troubleshooting a problem and then not finding out the root cause.


This thread was discussed between 04/12/2012 and 06/12/2012

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