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TR4/4A Vacuum connection on SU-H6 carbs

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eyesie

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I am trying to install the vacuum connection on the front carb. The part I have from an old carb, see photo, does not seem to want to screw in. The the ad appears to be too big. Can anyone tell me what is the thread in the manifold? Were ther different sizes? I don't want to have to pull the carbs off to get a better look at this threaded hole, but if I have to!! Moss show P/N 80, and call it a union, which it isn't, but their price is $23 for what appears to be a standard compression fitting.
Any thoughts would be appreciated.
 

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I have a spare set of SU 878's's that I just measured with a digital caliper and get a reading of 3/8 and seems to be fine thread. The part# you refferenced in the Moss catalogue specifically states for 768's but I'm guessing they would be the same since they don't list one specifically for 878..but maybe not..
 
IIRC there were at least two, maybe three different arrangements used by SU (if not necessarily Triumph), with different threads in the carb body. As confirmation, I picked two bodies at random from the parts bin, and confirmed that they have different threads. The body with AUC6040X molded into it still had it's fitting, which appears to be 5/16" BSF (22 tpi and has 5/16 BSF flats on it). The body with AUC6040AC molded into it had a visibly smaller hole and the adapter from the other carb will not screw into it. I suspect the 6040X is from a 786 carb and the 6040AC is from 878; but don't know for sure. If someone else knows how to tell the bare bodies apart, I'd like to hear about it! Oddly enough, they both appear to have 6058 hand-inscribed below the casting number.

Also FWIW, the TR3 SPC and TR4 SPC list different part numbers for that adapter/union.

There is also the possibility that someone has mistakenly rebuilt a rear carb as a front carb. The rear carbs have a blank (unthreaded) hole.
 
I've just been looking at this exact issue. Mine was attached without a fitting. At first I thought this was a mistake, but on looking closer, it appears that the hole in the carb is machined to accept a compression nut directly. I plan to hook it up and see if it's actually holding vacuum.
 
Snowkilts said:
I've just been looking at this exact issue. Mine was attached without a fitting. At first I thought this was a mistake, but on looking closer, it appears that the hole in the carb is machined to accept a compression nut directly. I plan to hook it up and see if it's actually holding vacuum.
Click to expand...

Just verified that the machined-in compression fitting in my H6 carb does in fact hold vacuum. There is no separate union or fitting (Moss #80 in OPs picture). Unfortunately, I also confirmed that my vacuum advance unit does not hold vacuum, so I guess I'll be replacing that (again).
 
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