idle mixture screws

A couple months ago I recieved a box of parts and a frame that i am building an XJ650G from. However, when I got to the carbs, I had a set of carbs that had a float post broken off, and another set that had siezed butterflies. I decided to make one good bank between the two, and was able to get one of the seized up carbs apart without scoring any of the wear surfaces. I placed it in the good bank in place of the one with the broken post, and swaped all the jets, rods etc. Everything worked fine except the idle mixture screw. The two sets of carbs have two different thread pitches for these screws. One is fine threaded, and the other is course. The tech manual doesn't give any procedure for adjusting these. Most of what I've read says 2 3/4 turns to 3 turns out, but is that for the fine or course thread version? These carbs, the best I can tell, are identical except for this threading. Both are Hitachi HSC 32. I believe one set came from a 1980 XJ650 Maxim, and the other from a 1983 XJ650 Seca.

 

The 1980-81 XJ650 Maxim & Midnight Maxim carbs used coarse pitch idle mixture screws; all other versions of the Hitachi HSC32 and HSC33 series Hitachi carbs used on the XJ series bikes used the more common fine-pitched screws.

Your question is a good one, as "2-3 turns" results in a different needle position within the orifice depending on whether the screw is coarse or fine threaded. I suppose someone who is good with the math could come up with a comparable guide to determine vertical movements of a fine pitch (.50) versus a coarse pitch (.75) screw; my first thought would be that a coarse pitch screw moves 1.5 times as much vetically for each full turn as does a fine pitch screw. Thus 2-3/4-to-3 turns on a fine thread screw would be 1.8-to-2 turns on a coarse pitch screw, assuming thread pitches have a 1-to-1 correspondence in their screw-movement capabilities.

Sounds like a Colortune tuning plug is in your future..........!

 

I pulled the mixture screw out of a virgin 81 rack to measure the thread pitch for chacal. It was about 1.75 turns from bottom.

Good job on the math chacal!

 

The number of turns doesn't mean anything except as a starting point since you still need to adjust for the correct mixture no matter how many turns it works out to be. When you use a Colortune to set the mixture correctly you will be setting it for the proper color and won't be counting the turns anyway.