The 74 and 75 850's had split spray tubes. They also need the longer needles ( 4 marks on the top ) to match up. There were flat spot problems on some of the early 73 850's and the split spray tube helped atomize the fuel better and overcome it. The 73 750's and combats with 32 mm carbs used the standard needle ( 2 groove ) and round spray tubes.
Thanks Bruce, I knew the black box Mark 3 came with cut aways and I figured the Mark 2A probably did. What about the ham canned 850's?
I am currently setting up two 850's, 73 and 74. Both have cutaways with 3 1/2 slides and the fat 2 ring needle. I change the 73 to a 3 slide, 4 ring thin needle, K & N in the ham can. Much better, however it seems a little 'heavy' at idle. No flat spots. I am also running Autolite Ap 63's. I don't know which direction to try first, 3 1/2 slide or a hotter plug.
Currently running Boyer at 31.
I am guessing the 3 1/2 with the thin needle is more correct with the cut away tube.
How should a Combat be set up with cutaways? Should I not even bother and change the tubes?
I have run cut-away and non cut-away spray tubes on my ‘75 Commando. My seat-of-the-pants dyno cannot tell any difference in performance. However, a “real” dyno might prove otherwise. I believe the #3 slide is too rich for your 850 bikes, but a #3-1/2 with a K & N and free-flowing exhaust will be a touch lean off idle. However, you can adjust the air/low speed mixture screw IN slightly to get rid of that lean spot. The classic symptom of an overly rich slide is that the bike will flood and die when stopped after running at speed. I do not recommend trying to fix an air/fuel mixture problem with spark plugs.
Jason is on the right track. You may need to do some experimentation. I am only using stock figures. Sometime when I sleeve them the customer says they are spot on and of couse sometimes too rich. The richness sometimes happens after sleeving . In the past the local norton dealer ran into odd problems. I had my 71 roadster back then . The 850 stumble as I said seemed to be corrected by the split tube and needle change.
I wanted to thank you guys in trying to sort out this spray tube thing, even the comment about two strokes.
Rattling around in the bottom of my roll around is a 932 / 431 which came off of a 250 Bultaco Pursang from more years ago than I want to count. I think it was from a 74, maybe a 73. Can't recall. However it was interesting to me how it was set up, so I'll share.
Cut away spray tube, 2.5 slide, 106 needle jet , 310 main, 4.0 pilot jet exposed in bottom of carb body, float seat cut away for better fuel flow. Unfortunately, there is no needle.
This just basically confirms what has been written that there is a difference in 4 stroke and 2 stroke carbs.
It seems the consensus is 750's run straight tubes, 850's are better with cut tubes. When one orders new carbs, one needs to be careful to get what one wants.