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MG MGB Technical - How to adjust height of the bottom-jet bearing?

The jet bearing is not flush with the bridge of the carb body , on my TD....
I can get the jet even with the bearing, but don't know how to get the bearing even with the bridge of the carb....
Do I just add a thicker washer, or are there shims, or does it matter?
Carbs are running rich, and can't seem to correct it.
PO rebuilt both, using new parts...
Jet needles are marked "A" .090.
Thanks for help.
E.B. Wesson

Clarify some stuff.
Why are you asking on the B board - not that it really matters?
The jet bearing that comes to the bridge is the top one; bottom is the one on the bottom, with the adjusting nut.
If the jet & bearing below the bridge? If so, you need a thinner copper washer where the top bearing sits in the body. Known issues with repro washers - wrong thickness, or he put two in. I've also seen fibre washers and all sorts of other crap used.
Both this washer and the thin/narrow washer between the lower bearing and the big retaining nut is often left out, or the retaining nut is not fully tightened. This causes fuel to leak around the outside of the jet/bearing assembly, giving constant uncontrollable rich condition.
Some replacement jets are too short, and the bearings don't always quite reach the bridge. But the top of the jet must be adjustable to somewhat less than the generally correct .060 below the bridge, which should be when adjusted about 10 flats down. And needles must be correctly fit shoulder flush with the piston face.

FR Millmore

Is it even supposed to be flush? Whilst the overall cross-section of the HS in the WSM does appear to show it flush, the detailed cross-section of the HIF clearly shows it below. As long as it isn't sticking above, and as long as you can get the jet to flush then two full turns back down for the starting point, then if the mixture is still rich then there is something else wrong - like a .100 jet where it should be .090 for example (has been known).
PaulH Solihull

As Paul said the position of the jet bearing is of no consequence, the relationship between the jet and needle is the important bit. I presume your carbs have fixed needles If the carbs are running rich and won't lean off, check that the jets are centred properly and the needles aren't bent and that the piston moves freely in the suction chamber (dashpot). Take the dampers out, lift the pistons and allow them to drop. You should here the "click" as the piston buffer hits the bridge. Look through the carb, the gap between the piston and bridge should be very small, i.e., created by the buffer. While doing this you can check that both pistons fall at the same rate.
Allan Reeling

Thanks, guys, and sorry for posting here....(OOPS!)
I often lurk on the "B" forum, since I've recently sold two "B's"...
I was day-dreaming when I posted about the T....
Anyway, the WSM does show that the bearing on the TD carb, should be level with the body of the carb, and then the jet , even with the bearing....
Both of mine are slightly below the body, and I had to put both adjusting nuts in the lathe, and take a few thou off the bottoms, in order to bring the jets even with the bearing...
The needles are slightly tapered, so I wonder if pulling them out of the pistons, slightly , to make up for the lower bearing and jet, would make them work better?
Will have to hold off complete disassembly till this Winter, when I am planning to pull the body and engine/tranny.
I have the old-style SU service tools, and had a centering button for the jets, so I was able to center them. (major pain though)..
I did find that the front piston chamber would not seat properly on the carb body, and tightening the screws would lock the piston....
Checking the base of the chamber, I found it to be uneven, and turned off a few thou to square it up...
It now seats correctly....
I will be more careful of where to post, in the future.
E.B. Wesson


Move over to the TD board and ask Dave Braun. He rebuilds carbs and will know the answer. Dave has a TD and a B .
Bruce Cunha

" the WSM does show that the bearing on the TD carb, should be level with the body of the carb, and then the jet , even with the bearing...."

It might show it but what do the words say? Dave Braun elsewhere states that the jet should be below the bridge, he uses a dial caliper to put them .070 below as a starting point, how that relates to two full turns down I don't know. Flush with the bridge in that case would make it too weak to run. I'd be surprised if SU carbs on a TD had such a different setting to them on other MGs like the MGB.
PaulH Solihull

I already wrote what has to happen, above.

The TD WSM (and all other original SU reference material) does NOT show the jet bearing as flush with the bridge, rather it is "slightly" below. Rather confusingly, the drawing shows the jet and top bearing flush with each other, which can but only sometimes does happen - but that is dependent on final mixture adjustment.

The top bearing must not protrude above the bridge, as that would interfere with the piston travel. Otherwise,
position of the jet bearing is not significant except as an indicator of correct assembly with correct parts. The usual position of the bearing in all SU carbs is below the bridge but above the normal base setting of the jet ("12 flats down". Typically .020-.030, but I've never bothered to measure it.

The jet is always referenced from the flush with bridge position, commonly given as "12 flats" down. The adjusting nut threads are 26TPI, so 12 flats is .0769". In practice, I've found that 12 flats down is usually too rich, so my base adjustment is 10 down = .0648; That would be in the .060-.070 range, as per Dave B and myself. My statement above covers this range, with attention bias to Ed's problem of being too rich: "But the top of the jet must be adjustable to somewhat less than the generally correct .060 below the bridge, which should be when adjusted about 10 flats down. "

There may be several things wrong with "12 flats down":
Some references say to screw the adjusting nuts "all the way up" This is incorrect, as the jet may then be above, flush, or below the bridge. 12 flats down from that is an uncontrolled variable. There are about six parts per assembly subject to variation in manufacture, plus wear, which determine this. So the correct reference is "12 flats down from flush with the bridge".
But, sometimes the jet will not reach the flush position, because of wrong parts or wrong assembly, and you need to correct the "12 flats". Short jets, thick washers, worn adjuster nuts are all common. Well used adjusting nuts develop impressions from the jet over years; these must be faced off flat else you get random variations not consistent with how many lats you turn the nuts. New production lower bearings and nuts apparently use metric threads, which throws everything off - and is annoying as hell.

All of which leads to our procedure of setting the jet height with a depth caliper at the number of choice. I will base set them at .070 in cold weather or with a new and stiff engine, .060 in hot weather, or .065 for a bench setting. Final engine hot adjustment usually turns out to be in the .055-.065 range. As I've been attempting to point out, any large deviation from the expected numbers indicates a part or assembly problem. These can generally be worked around, if they are not gross faults, but you have to understand the principles. Any situation which requires setting the jet needles at other than the specified "shoulder flush with piston face" indicates a fault in the jet assembly, or a serious vacuum chamber/piston/spring error, or a wrong jet/needle basic size.

FR Millmore

Thanks for this info....I understand what you are saying, but , unfortunately, without completely disassembling both carbs, I will not know where the problem really is...
While the PO used new parts, he must not have set them together correctly, since , even fully up, the jet barely gets to the top of the bushing, and anything below that , causes the carbs to run rich, at least at idle....
Even 2 flats down, causes black smoke, at idle.
When I pull the car apart this Winter, I will take the carbs apart, and, hopefully determine the problem.
I have moved my problem back to the "T" forum, and continue posting results there.
E.B. Wesson

Still sounds like a mismatch between needles and jets to me, i.e. the jet too big for the needle.
PaulH Solihull

This thread was discussed between 19/08/2011 and 24/08/2011

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