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Setting the SU carbs

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Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
Just to clarify this, I did get a what, where, when, type of answer from Burlen, which is to their credit. I took one of my carbs apart again and measured the new jets and compared with the old ones (which I assume are original as they were made by a very different manufacturing method). They both measured 2" exactly. So I assume the level of the top of the jet being below the bridge is a matter of design.
I have ordered a pair of Stay-Up floats from them to resolve an issue of setting the fuel level in the jet. I reported many months ago that I bought a pair of replacement brass floats elsewhere and they weighed 32g each compared to 21g each for the originals. This meant that the fuel level was too high when set by the method in the WSM, so I set it by the meniscus method instead. This worked fine, but meant bending the forks in the float chamber much lower than usual. However, for the sake of simplicity, I wish to return to using the method in the WSM, hence the new floats. Beware overweight floats sold on online auction sites!
Dave H

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Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
The Stay-Up floats weigh 22g each and the engine now runs well with them.
Dave H

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AZTD Silver Member Mike Grogan
Glendale, Arizona, USA   USA
1953 MG TD
Have a pic of the Stay-Up floats ?
Just can not see it.



1953 MG TD TD23816

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Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
No sorry I didn't take any. Same shape as the brass floats, but smooth black plastic moulded with a brass disc top and bottom. Slight whiff of sulphur about them, so I imagine they are a sulphur containing engineering plastic foam.
Dave H

MG Cobbler Fred Horner
Westford, MA, USA   USA
Hi Dave,

Did not know you had a separate thread going re: jet seat not able to come up high enough. Have you reached out to Joe Curto? He is an expert on SUs. www.joecurto.com.

Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
I get all my SU parts directly from Burlen here in the UK. I asked them on this issue and they responded, but on further probing I had no satisfactory answer. All new parts, except for the carb body and the jet is never going to be level with the bridge.
I also asked them why the copper washer they supplied to go under the rim of the lower jet bearing was so thin that the rim would sit below the jet locking nut top surface, allowing the lower jet bearing to turn rather too easily with the jet adjusting nut, but they have gone quiet on that too. Either they don't know, or they are too embarrassed to say, or they don't like customers asking difficult questions.
I'm very tempted to try machining some parts myself to get the jet where I think it should be, but was beginning to think this particular SU carb was intended to be like that. It seems not.
Don't want to ask Joe Curto because I won't ever be buying anything from him and wouldn't want to waste his time.
Dave H

MG Cobbler Fred Horner
Westford, MA, USA   USA
OK, Dave. Do you happen to have a measurement of the jet length, Say, from the top side of the "U" shaped pivot point to the top (since the "U" will bottom out on the adjusting nut)? I can compare with the many that I have and see if there is a difference. So, are you using the 1 1/4" H2 SUs or are you using the 1 1/2" H4 SUs?

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Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
Yes, I thought of that one too (mentioned in the messages above) - they measure exactly 2", both the new ones and the originals that I replaced. 11/2" H4s in a TF. Has all new Burlen Superdry seals, which includes new upper and lower jet bearings, jet nut, locking nut, spring and gaskets. However the jets were in the same place below the bridge with the old cork seal set up.
Dave H

MG Cobbler Fred Horner
Westford, MA, USA   USA
Ok, Dave, Sorry to belabor the point. I measured several that I have of various manufacture and they all measure 2". so, I assume that is the standard for H4 carbs. I don't know why you cannot achieve a flush fit of the jet seat on the bridge unless there is some other part interfering. You should be able to take all the lower jet parts off the carb body and visually achieve a flush comparison by pushing the spring loaded assembly together and seeing that the jet seat comes flush with the upper bearing. If this cannot be accomplished you should be able to determine the offending part in the assembly. The only two parts I can see that could cause a problem is the large cork gasket and the thin copper washer to the carb body. With these removed all the parts should come up flush as far as I can see. I wonder if the "lower jet bearing" is too long? I don't think this is a part of any kit, so, it should be original to the carb?

it would be interesting to determine the root cause of this issue as I have not seen this one before.

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Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
The Burlen Superdry kit, which I have fitted, has all rubber seals instead of cork, including the large seal above the jet lock nut. New brass parts include two brass washers (plain instead of dished to rest against the rubbers seals), upper and lower jet bearings, copper washer, jet adjusting nut, jet lock nut (and the separate aluminium ring is dispensed with as its now integral with the lock nut), plus the spring between the seals and the spring under the jet adjusting nut. The new seals won't fit the existing brass parts, hence all these parts. Only the original needles and jets can be used, but I replaced those when I rebuilt the carbs with cork seals. Neither with the cork seal set-up nor with the Superdry kit can I get the jet flush with the bridge. Its not caused by the lower seal as I can get it as tight as the previous cork seal - only a thin black line visible. I am beginning to think that the problem might be that the thin copper washer under the top rim of the lower jet support is much too thin. It means that there is not enough friction to stop the lower jet support from rotating when the jet nut is screwed up firmly, thus it never screws fully home, and the jet is thus lower than it should be. As it is the engine is running fairly well. There is a slight hesitation in the transition from idle to fast running when I press hard on the accelerator. The plugs look normal after a decent run, but very sooty from idle, and I need very little choke. It all adds up to too rich at idle and the jet too low. I will strip it down and investigate fully before long, but I can live with it for the time being. As I may have said, I have already told Burlen that copper washer appears to be too thin. I'll have to see if I can make some thicker ones (not easy as they are exceptionally narrow).
Dave H

Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
Ok, so I dismantled the carbs again and fitted a second copper washer under the rim of the lower jet support so that it was now slightly proud of the jet lock nut. That did it, it stopped the lower jet support turning when I tightened the jet adjusting nut. I was then able to set both jets at 1.8mm below the bridge. Took a bit longer to centre the jet satisfactorily though. The result is a smooth transition from idle to faster running and normal looking plugs. Only issue now is idle is too fast. I must have disturbed something enough to get an air leak I think.
Looks like I was right advising Burlen those copper washers are too thin. Will they do anything about it?
Dave H

MG Cobbler Fred Horner
Westford, MA, USA   USA
From your last post it sounded like you were on the right track (lower jet bearing turning, not allowing the jet adjusting nut to tighten enough to draw the jet high enough). The washer between the fitting nut (to the base of the carb) and the lower jet bearing is very thin, but I think that if the washer and cork (or rubber gasket in your case) is too thick, you never get enough tension on the lower jet bearing and it spins (as you have seen). It probably does not matter if there are two washers in there or one thicker one, as long as there is tension between the fitting nut and the lower jet bearing.

You mentioned that you had trouble centering the jet? Do you use the special tool (factory tool) designed for this? It works great and is available from Moss Motors (part number 386-500). The tool in the carb tuning kit (in the little blue plastic case) I think does not provide enough depth to properly center the jet.

Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
No, it wasn't the washer you refer to, that (in rubber) is very thin and on tightening only a very thin black line is visible. It was definitely improved by doubling up on those very thin copper gaskets. When two are used the lower jet bearing stands slightly proud of the jet locking ring and presses firmly against the upper jet bearing, the friction being enough to stop the lower jet bearing turning when tightening the jet adjusting nut against the spring. The additional friction makes centring the jet slightly more difficult, maybe because I like to centre the jet just before it nips up tight.
Haven't been aware of a centring tool, so looked up the part number and illustration on the Moss website. Its looks fairly simple and I could make one, though not exactly sure how its used, presumably in some kind of jig that holds it central over the bridge, with the reduced end in the jet. I made my own tools, in bent wire, for balancing the carbs and they work perfectly - remove the damper pistons, push into the top of the pistons and a horiontal wire from each almost meet opposite one another. With the engine running, and when balanced, they rise together in unison on opening the throttle.
Thanks.
Dave H

Buckdendave David Hill
St Neots, Cambridgeshire, UK   GBR
1954 MG TF
Ok, I found the description of how to use the centering tool on the Moss website. One end fits in the jet and the other in the top of the dome / vac chamber. Easy to make from a length of brass rod.
Dave H

MGTF1500 Ardeche France Thierry SUCHIER
TOURNON SUR RHONE, Rhône-Alpes Auvergne, France   FRA
Dave
Can you give us a quick description to make this centering part?
Yours sincerely, Thierry de l'Ardèche

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