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MG TD TF 1500 - SU jet bearing moves inside assembly

How can I stop the jet and the bottom jet bearing from shifting in the jet assembly of one of my H4 carbs?

The jet and bottom jet bearing move back and forth when I pull the choke. This causes fuel to gush out between the jet and the jet adjusting nut. The jet becomes un-centered with the needle and the jet binds inside the bottom jet bearing. I assume that the collar on the bottom jet bearing loses contact with the recess in the jet screw as the jet bearing tilts position. Yes, the small copper jet washer is in place.

There is some play between the clevis pins and the choke mechanism. Perhaps precision clevis pins from McMaster-Carr could help. Don't feel that this is my problem, though.

The jet screw, jet sealing washer, and sealing washer are as tight to the throttle body as I can get them.

I rebuilt the jet assembly this month after Joe Curto installed new bushes and throttle shafts in the carbs. I had replaced the jet bearings and jet about two years ago and they sealed well.

I've replaced the glands and sealing washer twice after the rebuild, with no improvement. (I use Teflon and Viton glands and seals, as I have done for years.) I annealed the upper and lower copper jet washers the second time that I replaced the glands and sealing washer so that they would conform to their mating parts.

It looks like the jet gland spring and the jet lock spring are the only things that hold the collar of the bottom jet bearing against the jet screw.

Does the sealing washer squeeze the bottom jet bearing to lock it in place as it is tightened in the jet-centering process? Perhaps my Viton O-ring is not expanding enough(?) The jet bearing turns as I tighten the jet screw until just before the jet screw and jets sealing washer seat tightly against the throttle body.

What am I missing? The problem is new to me.

(Bruce, I believe that you recently had a problem with the bottom jet bearing turning as you adjusted the jet adjusting nut. Did you find a solution?)

Thanks,

Lonnie
TF681
TF7211


LM Cook

Lonnie
I had a similar problem with my H2 carbs using cork seals. I installed new cork seals and the carbs still leaked, so I ordered 2 sets of 'Super dry Jet Bearing kits' hoping the rubber seals will solve the problem.

Fitted them this afternoon, so far no leaks out from around the jet at the Jet Screw and the jets appear to move more freely.

Jet heights set as per Dave Braun's paper, now to set the correct fuel levels in the bowls.

Stuart
Stuart Duncan

If your jet bearing turns when adjusting then the jet screw is not tight enough,Either the Viton ring is too thick or the copper washers between between them are too thin/missing. the jet screw needs to be tight to clamp the two halves of the jet bearing and washers to the body.
This will also allow the jet assembly to tilt.
See the WSM 8-13 for the washer positions.
I believe some overhaul kits have had wrong thickness washers.
Ray TF 2884

PS I have never had a problem in 70 years with the original cork composition washer.
Ray Lee

I have had same problem a couple of times, and it was the jet washers were too thick. I recently redid a couple carbs with SU kits. The new jet washers were a bit thicker than the old ones, so I used the old ones. O-rings also, completely dry. George
George Butz III

Here is some more information about my "shifting" bottom jet bearing.

Ray - Thanks for telling me to look closer at the carb diagram on page B13. It helped me understand the relation between the washers, bearings, and jet screw.

So - it looks like a thicker upper jet washer might squeeze the upper and lower jet bearings together more tightly. Currently using 0.015". I have 0.015" and 0.030" upper washers.

I read an old manual (somewhere) that said the bottom jet washer should be shimmed to be level with the top of the jet screw. I guess that means to use two. My bottom washer is below the "lip" on the jet screw.

I have used "-210" Viton O-rings for years with good results. .139" ± .004 cross section. Even so, maybe they are small(?)

George is right. The copper jet washers are not uniform in size. I measured new and old ones and found that they differ in size, especially the upper jet washer. The outside diameter of some of them almost fill the cavity in the throttle body. Others are significantly smaller o.d. The inside diameters vary also. As I noted, the thickness also differs.

And finally -
Here is a video of the jet bearing and the leak. Watch the jet adjusting nut in the second clip to see it and the jet move when the choke is pulled.

You can see movement in entire choke mechanism, possibly because of worn, oversize holes. Or undersized clevis pins.

I believe that the noise and rough movement are caused by the jet sticking in the jet bearing and the jet return spring moving it back. The mic on the phone is very close to the carb.

>> VIDEO LINK:
https://www.youtube.com/watch?v=WlwCg7WuXgk

FYI - The carb is SU H4. Front carb. Casting 4468. A few minor differences vs 6020. Same rebuild kit as 6020. This carb was used on early TFs. My TF681 was built on the fifth day of production, one of the first 200.

Stuart - good luck on setting the fuel level. It is a real bear. BTW - Recently the BBS discussed whether Dave's fuel measurement should be 0.160" ± 0.040 below the BRIDGE as Dave recommends, or if it should be 0.160" ± 0.040 below the JET, which would put the fuel level about 0.23" below the bridge when jets are 0.070" below the bridge.

Thanks,

Lonnie
TF681
TF7211
LM Cook

Lonnie,
check the movement has not bent the needle,that is a fair amount of shift,
It is a pig working on TF carbs, I use a tube spanner up through the side vent to tighten the jet assembly
Ray TF 2884
Ray Lee

I would not be surprised if the needle is bent. The jet is scuffed and dented from contact with the jet screw.

I have an old jet that appears to have a taller collar than the new one that I installed in 2022. That might help the upper and lower bearings fit tighter together. I'll check when I disassemble the carb.

Looks like the play in the jet lever link pulls the jet at an angle instead of straight down when the choke is pulled. Is that a problem? How to fix?

Never had the nerve to tighten the jet screw while carb is on the car. Easier for me to remove the carb.

Lonnie
TF681
TF7211


LM Cook

play in the pivots will not have any effect on the
side loading, A 3 pivot link will always side load. When the jet bearings are clamped tight the jet itself takes all the side load.
The main thing is to ensure the choke cable outer
is clamped this ensures a scissor action on the rear carb. The front carb still has some side load
Ray TF 2884
Ray Lee

This thread was discussed between 11/01/2024 and 12/01/2024

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