Mikuni VM34-168-a bit of wise knowledge.
Apr 5, 2024 13:30:49 GMT -5
neal and Petes2Wheels like this
Post by andy on Apr 5, 2024 13:30:49 GMT -5
I've possibly just read one of the most interesting pieces on carb jetting and inturn increased my knowledge on the aftermarket mikuni roundslide carbs [ in mine and a lot of cases the 34-168 ]. The overall thing to remember is so many area's effecting the different throttle positions. Read below,I found it simply brilliant,especially for my limited brain power ! The actual piece was from a chap on the XS650 site but same rules apply.
Heres the main part to digest :-
First: note the inverse relationship between NJ size and MJ size. Second: note the inverse relationship between slide cutaway and PJ size (smaller number=richer slide, richer slide forces leaner PJ, etc). And finally, understand that needle position directly impacts pilot circuit tuning: A change of 1 step will always force mix screw adjustment and will often force a PJ change. The pilot circuit never turns off, and the main circuit impacts the mix even at idle, through the slide cutaway.
Carburetors have to maintain a usable fuel/air mix across a wide and shifting range of demands, and they have to do it with a few simple components. The most accurate jetting involves compromise. Bottom line: there are many combinations that work, there are some that work better, and there are some that don't work at all.
As far as jet lists go, treat them as possible starting points. Some guys take to tuning aftermarket carbs like fish to water, some guys get lucky, and some guys wind up wishing they'd never been tempted to fool with old motorcycles at all, let alone modify one. The best advice I can offer the beginner who wants to hang aftermarket carbies on his bike is first: Read some threads by others who've done the same, asking yourself if you have enough patience and frustration tolerance to enjoy the tuning job as a project. And second: Buy a responsibly baselined set of carbs from a reputable vendor who owns and knows the XS650 and offers tech support after the sale. In the USA, I can recommend Michael Morse of 650 Central and Gary Hoos of Hoos Racing.
Heres the main part to digest :-
First: note the inverse relationship between NJ size and MJ size. Second: note the inverse relationship between slide cutaway and PJ size (smaller number=richer slide, richer slide forces leaner PJ, etc). And finally, understand that needle position directly impacts pilot circuit tuning: A change of 1 step will always force mix screw adjustment and will often force a PJ change. The pilot circuit never turns off, and the main circuit impacts the mix even at idle, through the slide cutaway.
Carburetors have to maintain a usable fuel/air mix across a wide and shifting range of demands, and they have to do it with a few simple components. The most accurate jetting involves compromise. Bottom line: there are many combinations that work, there are some that work better, and there are some that don't work at all.
As far as jet lists go, treat them as possible starting points. Some guys take to tuning aftermarket carbs like fish to water, some guys get lucky, and some guys wind up wishing they'd never been tempted to fool with old motorcycles at all, let alone modify one. The best advice I can offer the beginner who wants to hang aftermarket carbies on his bike is first: Read some threads by others who've done the same, asking yourself if you have enough patience and frustration tolerance to enjoy the tuning job as a project. And second: Buy a responsibly baselined set of carbs from a reputable vendor who owns and knows the XS650 and offers tech support after the sale. In the USA, I can recommend Michael Morse of 650 Central and Gary Hoos of Hoos Racing.
