Big SU on GS850

[tirebiter]

and while I'm still of the opinion that a 1.75" is too large for this engine

The only real problem with the 44 on this is the utter buggeration in trying to achieve a decent tickover and pickup - idle was always like a bag of spanners.

I've been around many British cars with SUs and Stombergs. I always figured with the air piston all the way down (like at idle, there is about the same amount of venturie opening on all of them, regardless of carb bore diameter. Almost closed ! I have a feeling you needed a drastically different needle diameter at the extreme top (1 or 2 eighths of an inch) of the profile, than the needles you tried in the 44.

With one carb you COULD cheat and do the sandpaper routine to make different needle profiles for experimentation.

Does the bimetal mainjet holder in the HIF do a good job with the amount of temperature fluctuations encountered with an air-cooled engine ? That function was "iffy" at best on the 1974 (and 1/2) MG-Bs that came into the USA with them as stock.

 

[Grimly]

I'd decided to try the smaller carb to keep the gas speed up at idle and it had an immediate effect, falling right into a zone of acceptable idle behaviour the 44 never showed. There were variations between needles in the 38, but all of them I've tried so far idle much better and one or two are near enough perfect out of the box, which I found useful, as I could lay that bit to rest for a while and pay attention to mid-range and other fuelling.
The entire thing is a series of compromises and happy accident - the naked manifold might have led to fuel puddling problems when cold, but it didn't, as it heats up quickly from cooling air, and after the first mile is warm enough to keep the fuel vapourised within it. The thermostatic jet control might or might not be working properly, but I won't really know until fit a heat shield to the manifold that will protect the float chamber from engine heat - although that shield was originally designed to shield the carb from exhaust manifold heat on the Mini, it might have a beneficial effect here.
I did an IR scan on the carb and manifold last year, but don't have the figures to hand. I don't recall they were anything to be alarmed about, else I'd have done something by now. There is the normal warm-up period, and once it's warmed up, it runs quite well, but even during the warm-up it behaves like the normally carburetted engine. It doesn't fart or pop, or splutter or die, or do any of the things that a bad carb setup would do. The SU is really quite a well behaved carb once it's in the ball park.
All I really have to chase now is that missing top end, and while the airflow charts say the 38 should comfortably flow enough to meet the demand of the 850, and the manifold has already shown that it can feed the engine in top gear at a higher flow, I want to see what I can get out of the 38, before I move back to the 44.

 

[tirebiter]

OK, so it's the 38 for now. I undersatnd you wanting to go at this in a logical sequence. You mentioned the "italian tune-up" That usually means somewhat extra rich running at low-moderate RPM and throttle openings.

I have a couple of questions ;

Do you have any evidence of what the mixture is at higher RPMs ? Plug chop or ... ?

Also ... if yo uwere to unscrew the carb damper, does the air piston lift all the way when the bike is stationary and you crack the throttle open fully for a brief moment ?