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MG TD TF 1500 - Too rich - too lean - Jack Sprat's carb solution?

Ok, in the recent past Lonnie and I have been having similar problems... carburetion issues which have the following symptoms:
1) With air filters removed, engine runs
2) Air filters on, tremendous difference, and hard to keep the engine running.
3) Both Lonnie and I initially had black smoke (Lonnie had different issues later)
4) adjusting the fuel mix via the bottommost carb nut had little effect (at least for me, and I believe he also was not seeing much difference there).

Here is the solution. At least, it worked for me, and it works perfectly.

And here's the problem:
The Moss/repro SU carb rebuild kit contains a washer which is TOO THICK. The result is it forces the jet to sit too far below the bridge, resulting in chronic rich fuel flooding.

Removing that washer instantly solved the problem.

It's washer No. 33 in the Moss diagram, the very topmost washer in the jet assembly.

I did not discover this myself. I set up my carbs on the intake manifold and went down to Ernie's British Cars in Tucson AZ, and we spent some time looking over the carbs. About six seconds, before he pointed to the jet and said it was sitting too low, and told me if I had installed the replacement washer, that was the problem.

Fix cost: $0.00... remove washer.

All symptoms instantly gone.
1) Nice middle color to the exhaust, no black smoke; mostly invisible.
2) - here's the key element - the engine now runs smoothly with or without the air filter. Prior to this, with the jet too low and fuel rushing in, the engine needed a LOT of air to combust it, or it would die. So without the filter, it would run more or less ok - with lots of black smoke - but with the filter, it would lug down and die in a few minutes unless I set the throttle up so high that she was revving like crazy (and thus combusting better).

I've read threads in the past about the height of the "bridge" in the carb and how to set the jet to the right height. With the washer in place, that was impossible, so I assumed that my setup was a little different but still correct.

Now I understand it better.

Problem solved.

LONNIE... please remove washer #33 and report on whether it works for you!

It's always nice to find a FREE fix... even if it cost you 1000 manhours....

Geoffrey M Baker

Geoff - any chance that there was a washer stuck up in the housing? Bud
Bud Krueger

Jeez - I don't wanna tear my carbs apart AGAIN!!

The copper washer (#33) that I replaced was the same thickness as the one I removed. (Of course the car was running terrible before I rebuilt the carbs, so I can't say that it was the right size.)

> Anyone else have an opinion on the washer?

I'll try one more time to balance and set the mixture on my carbs before disassembling them.

My car runs pretty well, but the carbs show symptoms of being too lean when I lift the suction piston. And like Geoffrey, the idle RPM drops drastically when I install the stock Volks air cleaners.

Still trying to rig up a way to install two Rochester B carbs on my MG. The carb works well on my '54 Chevy truck!

- Lonnie
LM Cook

I always use Joe Curto 1 718 762 SUSU !
He has the best SU parts & always helps with any questions,
Len Fanelli
Len Fanelli

Len -

Joe added bushings to my throttle bodies and installed new throttle shafts. He answered all of my questions. Nice guy.

I'm sure that he would have done a perfect job rebuilding my carbs and prevented my current problem. But doing it myself was the only way for me to learn about SUs.

Lonnie
TF7212
LM Cook

Geoff -

I showed my truck at an AACA club show this weekend and talked to a TR-3 owner about SU carbs. Guess what - he also recommended removing the copper washer for the upper jet bearing.

I still want to do some more tests on my carbs before removing the washers or measuring them for correct thickness.

I found information about the copper washers, plus some other related information on the Jaguar Clubs of North America web site. "SU Carburetor Tips" By Jim Taylor.
http://www.jcna.com/library/tech/tech0006.html

Selected portions of the article. Read the complete section in the link for details.

> Section XV. AFTER MARKET PARTS -SU CARBS
On the H series carbs the upper jet bearing copper washer is supposed to be 0.016" thick. Many after market kits have this washer 0.025" or even 0.033" thick. This holds the complete jet assembly down farther away from the needle than intended and defeats about 1/2 turn of the mixture adjustment nut (ability to lean out).

> Section XXXI. BRASS JET LEVERS
The "H" series carbs use brass jet levers as a means to move the jet when the choke is pulled. The main pivot hole in these brass levers is 5/16" in diameter with a 3/16" clevis pin as a pivot. The ensuing lost motion allows the fast idle cam to be actuated before enrichment occurs.

> Section XXXIII. FLOAT LEVEL SETTINGS
All "H" and HD series use a 7/16" diameter test bar.
(* note I used 3/8" as specified in the WSM and in the instructions in my SU Master Rebuild Kit. - Lonnie)

> Section XXXVI. FAULTY HD JET ASSEMBLIES
The exposed length of the jet should be the same as that of the jet bearing.

By the way, congrats on spotting the missing return spring as your problem in another thread. I have done the same thing.

Lonnie
TF7211
LM Cook

Take that washer out! I'll bet you will be amazed at the difference...
Geoffrey M Baker

Could anyone confirm the .016 thickness for the upper jet bearing washer? I wonder what Jim Taylor's source was for the measurement he mentions in his article.

I would not think that removal of the washer is the proper solution. It may solve the problem - but I would like to have that upper washer in place. Even if one needed to be machined to the .016 thickness.

Any comments from the group?
DLD

This thread was discussed between 26/04/2014 and 26/04/2015

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