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MG TD TF 1500 - SU jets

I am looking in the archives but thought I would ask about this one. Carbs on my 50 TD are adjusted so there is bairly (if any) upward adjustment left. Using a color tune and they are still a bit rich. Still getting some soot at the exhause. New engine, carbs were rebuilt by Twist about 10 years ago and only have about 6 thousand miled on them. I know this could be worn needle or metering tube as the engine has required an adjustment upwards on the nut about every time I tune it up (Checked at the start of each year) Thinking that there may be play in the piston or the metering tube was not absolutly centered. Would the recommendation be to just renew both the metering tube and the needles?
Bruce-C

Bruce,

The "metering tube" as you refer it to, is in effect, the jet. The jet orifice is .090 for all Ts. However the jet is supported by centering bearings - one at the bottom, and another at the top.

In turn, these are supported by flexible (cork) mountings. Look at any H4 breakdown diagram and it is shown. This arrangement, over a period of time, can go off-centre causing wear on both the needle and on the jet. In the latter case, it can become oval and will allow for an increase in fuel flow.

Just replacing the jet and/or the needle will not necessarily resolve your problem.

But having your carbs rebuilt by a reputable re-builder like University Motors should allow for a good set-up, however its entirely possible that the cork glands that centre these 'bearings' have worn or gone off-centre over the last 10 years.

When this happens, one tends to tighten-up on the jet to compensate for a worn orifice, moving it perhaps, to its limit.

With the carb(s) in place, remove the bell assembly and closely examine the jet using a jeweller's loup. If it is badly worn, it should be obvious and you will have to rebuild the entire jet assembly. (Burlen make complete kits).

Failing that, remove the carbs and check the jet assemembly and be prepared to replace the critical parts as noted above.

Another possible source of your problem can come if there is any off-set force applied to the bottom of the jet where it is fastened to the choke linkage. If it is pulled off-centre, it may pull the jet off-centre as well.

Gord Clark
Rockburn, Qué.
Gordon A. Clark

hey, I was typing an answer, but lost it. I concur with everything Gord says, and would add that buying new needles and jets gets you about halfway to the rebuild kit price, which you will need to re-center the jet. One kit does two carbs and includes the aforementioned jet and needle. Dave DuBois prefers the teflon rings instead of the cork gland seals, but I've never had trouble making the cork seals work.

warmly,
dave
Dave Braun

With only 6K miles, I kind of doubt wear on the jet or needle. If the upper cork jet seal starts to leak it will enrich the mixture. Check the float levels also to make sure gas not too high in the float chamber. A "free" fix is to lower the needle in the piston a touch, which will give you more adjustment toward lean. George
George Butz

With only 6K miles, I kind of doubt wear on the jet or needle. If the upper cork jet seal starts to leak it will enrich the mixture. Check the float levels also to make sure gas not too high in the float chamber. A "free" fix is to lower the needle in the piston a touch, which will give you more adjustment toward lean. George
George Butz

George,

I thought about the float level, but he said that he had to modify the mixture at each tune up, which indicated to me that if it were float level, it was changing (going higher) each year. For those who want to know, I use about 1/8 inch below the bridge level to double check the float setting, but it is hard to see. Almost easier to simply get the drill rod out remove the float bowl lid and check.

I think that if the jets were poorly aligned in the first place, so poorly that they continued to wear and not stabilize, he would be constantly needing to lean the mixture each year. That implies some damage which Gord described, shipping damage or a poor rebuild.

Just a thought...

warmly,
dave
Dave Braun

Bruce, I do not think the jets/needles are worn, the pistons would be sticking. Forget the color tune.Does the fuel pump stop ticking when the engine is off, & for how long between ticks? Remove the floats, & shake for them, they may be filling with fuel. Buy a dial type vernier caliper,& set the jets to .072" below the bridge, (with the chanbers removed). Bring the engine up to running temperature, raise the idle speed equally on each carb, to 2,000 RPM connect a DVOM type Tach, turn up the adjusting nuts one flat at a time, again equally, until the highest speed is reached. 1 718 762 SUSU is Joe Curto's Phone # in N.Y. (make sure the jet is returning when the choke is off; I.E, the jet returns all the way up to the bottom of the nut, Joe supplies the jet return springs that work!)
Len Fanelli

Hi Len, I appreciated your comments to Bruce on his SU problem but would like to know what a "DVOM type tach" is. It's something I have not heard of before. I have had a problem with return of the jets after use of the choke, and would like to contact Joe Curto concerning SU rebuild Kits. Would you have his e-mail address? Thanks for your comments to the BBS.
George Raham
TD 4224
G. L. Raham

George, Digital Volt Ohm Meter.
http://www.joecurto.com/
JoeCurto@aol.com
His parts are the best, (they work!)
also an TD owner
1 718 762 SUSU
Len Fanelli

WOW 9 fantastic answers in 12 hours. As usual, the group comes through with what I need.

Carbs have the cork seals. The fuel pump only clicks about 5 times when the key is turned on, so I don't believe they are leaking (car will set a good part of the work week.) Also not seeing any fuel leak issues.

I did check the floats this year and they were sealed. I appreciate the reference for Joe Curto's. Will set the needles per Len's instructions. Think the last time I set the needle, I just put it flush.

Setting the floats is one of those items I have to say that I really am not all that clear about. Dave, can you elaborate on the 1/8". I have one of the SU carb setting kits, with the centering tool, and funny oval adjuster for the floats. I think I have it close.
Bruce-C

Hi Bruce, sure.

Normally, one sets the floats by turning the float bowl lid upside down and placing a 7/16 rod under the cylindrical section of the float fork, and assuring that the lever is contacting the seated needle.

This setting should result in the fuel level visible in the throat of the carburetor about 1/8 inch below the level of the bridge (the flat section in the throat of the carburetor through which the metering needle passes. If you remove the dashpot and piston, you can look down into the jet and see the fuel. The actual measurement would be more like 0.16 inches, but that's only about 1/32 greater than 1/8. This would confirm the correct float height.

The suggestion Len is making is to set the height of the jet below the same bridge at .072 inches using a dial caliper. I happen to use .070, but that is close to proper metering level for a starting point, assuming the needle is properly mounted in the piston. Remember, the carb is designed to use a venturi effect to pick fuel up out of the bridge, with the metering needle restricting flow by blocking the center of the .090 orifice of the jet. Too high of a fuel level caused by allowing a float to rise too high before it shuts off the fuel in the bowl and the venturi effect picks up too much fuel. Hence, a higher jet is needed to move the .090 orifice to a thicker part of the needle.

Trouble is, the stations which our cars use are about station three through seven on the needles, so moving the jet for a high fuel level may cause the car to run poorly. The jet height, the depth the needle is installed in the piston, and the float level all interact to meter the fuel. If you mix in a worn or offcenter needle or jet... well who knows!

warmly,
dave
Dave Braun

Our current blend of gas seems to have a lower viscosity than the earlier blend. This results in more gas flowing thru the same size hole between the needle and the jet. The only solution is to lower the float about 1/16 inch. Also note that the Workshop Manual has the wrong setting for some SU's (H4's I think). In any case, if too rich when the nut is as far as it can go, lower the float!
Don Harmer

Don, you may be on to something about the gas being different. Pretty much all of the gas in Florida is now 10% ethanol . No doubt mileage decresed in my street car when this started. That said, I would have thought you would have to richen the mixture up? George
George Butz

George, My 2 cents on today's fuel, I have burned 2 pistons from being too lean, the needle, not the jet height. I have, 1 1/2" carbs, with GJ needles, I now have MGB needles, which are about a #5 needle. Car runs great.
Len Fanelli

This thread was discussed between 17/08/2008 and 20/08/2008

MG TD TF 1500 index

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