My Mk III is back on the road after swapping the 20 tooth countershaft sprocket for a 22 tooth sprocket - a truly great improvement! I still have a bit of carb tuning I need to accomplish and all will be well, at least for a while.
I had the stock 932s refurbished by AMR, which involves truing the slide bore and replacement of the original slide with a hard chrome plated bronze slide sourced from a Mikuni carb. The mod does away with the choke slides, which really isn't a problem. I had this mod done to a set of 930s and have run them over 20K miles on my 750 with no issues. But on the MK III, I have a problem with lean running at small throttle openings. The bike idles perfectly once warmed up a bit, but from just off idle (the most lean) to about 1/4 throttle it's seems a little lean. The plugs' porcelain insulators are clean/white but you can see a light tan circle deep in the plug where the insulator meets the shell. The pipes have blued a little bit, indicating a hot exhaust. And just off idle the engine will stumble a some, even spit back through one carb when cold.
The bike is equipped with a K&N filter, swept back 1 1/2" header pipes, and Dunstall replica mufflers. Ignition is Boyer Brandson. The camshaft may be stock, or it may be a 70's vintage Norris street grind, as that is what the local dealer often used for warranty work. The carbs are presently equipped with 106 needle jets, standard slide needles with circlip in the bottom/richest slot, and the slides are marked as being 3 1/2 cut-away. The spray tubes are the stepped units as was standard in a Mk III.
My first thought is that the cut-away is too big, but maybe there is some problem with fuel flow through the transition port in the floor of the venturi. My understanding is that port bleeds air into the idle circuit when the throttle is closed, but then will flow fuel during the transition from idle to needle jet circuits. I've read a number of years ago something about the size & height of the cross drilled hole in the needle jet affecting mixture at small throttle openings, but I've forgotten the details. I also found in my parts stash a set of 107 needle jets, but I've been warned that is a big step in enrichening. I still plan on trying the 107s first since it doesn't require removing metal, but I sure would appreciate some opinions on a good process to follow beyond that.
Thanks for your time,
I had the stock 932s refurbished by AMR, which involves truing the slide bore and replacement of the original slide with a hard chrome plated bronze slide sourced from a Mikuni carb. The mod does away with the choke slides, which really isn't a problem. I had this mod done to a set of 930s and have run them over 20K miles on my 750 with no issues. But on the MK III, I have a problem with lean running at small throttle openings. The bike idles perfectly once warmed up a bit, but from just off idle (the most lean) to about 1/4 throttle it's seems a little lean. The plugs' porcelain insulators are clean/white but you can see a light tan circle deep in the plug where the insulator meets the shell. The pipes have blued a little bit, indicating a hot exhaust. And just off idle the engine will stumble a some, even spit back through one carb when cold.
The bike is equipped with a K&N filter, swept back 1 1/2" header pipes, and Dunstall replica mufflers. Ignition is Boyer Brandson. The camshaft may be stock, or it may be a 70's vintage Norris street grind, as that is what the local dealer often used for warranty work. The carbs are presently equipped with 106 needle jets, standard slide needles with circlip in the bottom/richest slot, and the slides are marked as being 3 1/2 cut-away. The spray tubes are the stepped units as was standard in a Mk III.
My first thought is that the cut-away is too big, but maybe there is some problem with fuel flow through the transition port in the floor of the venturi. My understanding is that port bleeds air into the idle circuit when the throttle is closed, but then will flow fuel during the transition from idle to needle jet circuits. I've read a number of years ago something about the size & height of the cross drilled hole in the needle jet affecting mixture at small throttle openings, but I've forgotten the details. I also found in my parts stash a set of 107 needle jets, but I've been warned that is a big step in enrichening. I still plan on trying the 107s first since it doesn't require removing metal, but I sure would appreciate some opinions on a good process to follow beyond that.
Thanks for your time,