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MG TD TF 1500 - 'Spongy' Needle in Carbs

I've owned my TF-1500 for almost two years and I have had my carbs apart a number of times. But my carbs are the only SU carbs that I have ever touched, so I have no way to know how a perfect SU should look and feel.

My carbs have always run rich - adjusting the jet adjusting nuts all the way up does not lean them enough. In fact, adjusting the nuts doesn't make a whole lot of difference. My next step is to lower the needles 1/16" - 1/8". But before doing so, I played with them again today. Here's what I found:

1) The piston has never landed with a metalic "clank" onto the bridge. It lands with a solid, but mushy "thud." Even with the suction chamber removed, it still lands with a thud, and only ocationally with a "clank." I believe that the needles are centered in the jet.

2) I removed the damper and inserted the aluminum tube from the SU carb adjusting kit into the damper reservoir of the piston. I found that I could push down on the piston and it would move down about a millimeter. It moved back up the same distance when I released the pressure. I guess that I could say that the needle is "spongy." The same "spongy" situation exists with the chambers removed. Obviously, the piston is not contacting the bridge.

So - it seems to me that the tip of the needle is hitting something that is keeping it and the piston from dropping any farther. If that is so, then lowering the needles will not help. Don't want to take the carb apart (again) until I get some input from the forum.

>>> Is it common for the piston to be able to be moved as I described?

>>> Does your piston land with a metallic "clank"?

>>> What is my next step?

Thanks for your help,

Lonnie
TF7211
LM Cook

Lonnie. When the piston is down it should be all the way down. Sounds to me like it may be hanging up on the first step of the needle into the jet. If that is the case then it would always act like it was fat at idle moving the jet up or down likely didn't change anything as the needle would still hang on the jet and thus the piston would move up or down accordingly That would be my interpretation anyway. Look carefully at the the top step on the needle for any rubs. The Pistons should drop with a thud/ clank and stay at the bottom this is checked with the spring removed chamber on and snugged and the caps off so as not to limit air movement

Also if you are running with the flats all the way in you may have the needles set to deep on the Pistons. The diameter step for the set screw should sit flush with the bottom of the piston. And not seated all the way into the mounting hole

Hope this helps

Bill Chasser Jr
TD4834
W. A. Chasser Jr

Bill -

Looks like I'll be pulling them again and try to do a better job of centering the jets this time. I saw a worn spot on one of the needles near the piston today. Looks like it may be rubbing on the jet. It looks like the piston would have remained fully down when I pushed it. But it moved back up when I released the pressure. I won't know what is holding it up until I take the carbs apart.

Thanks,

Lonnie
TF7211
LM Cook

If it's running rich your first step should be to check the float bowl level. Too high and it will always be rich. I believe it is 1/8 inch. Then check and re center the needles and jets.

Hope this helps

Stuart
S Grimm

Lonnie

If you want to see and hear how it should sound -
Just remove the Jet from the jet bearings. (Pull it all the way out to the bottom) Then try the drop test without it and see if you hear the Clank on the bridge.
If you do?
Then you are definitely hanging up on the jet and the needle is not centered correctly.
You will have to undo the linkage to the jets to do this. So check all the clevis pins for wear while you are at it :) and the holes for oval.

To help with the centering of the jets - try new copper washers top and bottom on the jet bearings and the bottom Jet Centering clamp nut washer {Cork or Teflon} so it won't slid back into a memory spot on the washers. I normally lower the needle in the piston and clamp it a little proud on the shoulder. To ensure the jet is right to the top of the needle and therefor centered. Then lock it gently but firmly with the large jet centering nut. Now move the needle back up to the correct shoulder depth position and re-clamp it. Try the drop test again - you should here a good metallic clank as the piston hits the bridge if everything is centered.

Rod
R. D. Jones

An update to the "spongy" jet-needles and overly rich carbs in my TF-1500 ...

SPONGY JET NEEDLES:
I found the source of my "spongy" sounding carb pistons and jet-needles. I was concerned because the piston didn't hit the bridge with a "clank" when I raised and released it. Instead, had a soft "thud" sound. Also, I could push the piston down farther after it seated, and it would return back up when pressure was released.

I looked at the bottom of the piston. It has a small spring-loaded brass plunger that is a little proud of the bottom surface of the piston. The bridge contacts the plunger, not the piston. The spring allows it to drop a little farther and return when pushed. I don't remember seeing any reference to the plunger in SU docs or WSM. What is it? What carbs have it? Another bit of knowledge on my newbee MG journey. See photo below.

RICH MIXTURE:
I could never adjust the carbs lean enough, even with the jet adjusting nuts screwed all the way up.

I found the old jet needles that had I removed when I rebuilt the carbs after I bought the car. They were extremly lean needles -- leaner than standard "lean" needles. So it looks like the previous owner struggled with rich carbs also. I replaced them with standard needles when I did the rebuild.

Some club members came over yesterday to help. We lowered the jet needles 0.035". Centered the jets. Adjusted the float levels to specs (I'm still not good enough to set them using the fuel depth in the jet below the bridge.) A club member adjusted the mixture, so I don't know how high the jet adjusting nuts are after we lowered the needles. They are running great.

Lonnie
TF7211

> image: Brass plunger on bottom of carb piston. Needle flush with piston.

LM Cook

Lonnie. Glad to hear you have them sorted out and it is running smoothly. My piston have this also I never realized that it was spring loaded.

Cheers

Bill Chasser jr
TD4834
W. A. Chasser Jr

Is it possible that is an Hitachi/SU clone piston. I think all SU pistons are fully machined, I don't see casting marks on mine.
Ray TF 2884
Ray Lee

Ray -

I don't know anything about non-SU parts. Who knows what parts have been replaced on my TF over the last 60 years. Both pistons are the same. I don't want to remove the suction chamber to take another look at the piston ... I may put a hex on my carbs!!

Lonnie
TF7211
LM Cook

Hi Lonnie,
If it is working then don't worry about it, at least that is my attitude.
Hitachi made SU carbs under licence but did some modifications and I wasn't sure of any parts could be interchanged.
I have seen a TF with Hitachi 38mm (1 1/2") carbs with 2 left hand float chambers.
Ray TF 2884
Ray Lee

Sounds like an 'improved' piston stop to me. I also suspect Hitachi.

In the regular SU carb, the piston stop holds the piston just proud of the bridge so the inrush of air won't momentarily pull down the piston and shut off airflow.
Dave Braun

Anybody out there with a 240Z or Datsun 1200 please have a look and see what your piston stop looks like. (Just being nosey)
Ray TF 2884
Ray Lee

Thanks for the info on the piston stop, Dave. I assumed that the piston should sit flush onto the bridge. The extended stop destroys my theory that the stop caused the piston and therefore the jet needle to be too high. I feel that this newbee (me) is getting "newbee-er" instead of advancing in my knowledge about MGs.

I believe that most of the parts on my TF-1500 are correct. Is there any advantage to using Hitachi suction pistons - - other than to replace the originals that were damaged? Anyway to confirm that they are Hitachi and not the original SU pistons that came in the carbs?

Here is one of my throttle bodies. Is it the correct one.

Lonnie
TF7211

[image] SU carb throttle body

LM Cook

This thread was discussed between 28/08/2015 and 08/09/2015

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