jetting 101. top to bottom, or vice versa?

[mtothef]

i recently purchased a dyno-jet kit for an old xt600 i have taking up space in the garage, and in the notes was the following advice: "determine main jet size first, before any other jetting adjustments. then once that is satisfactory, move down to needle, then slide, then pilot, then air screw adjustments."

aside from being a monumental pain in the ass to do on a dual carb bike where the carb seems to have been put in place before having a motorcycle built tightly around it, this is also the first time i have ever seen instructions specifically stating to jet from the top end down.

so, since there appears to be a good population of people who really know their stuff on this forum, and since i am heading out this afternoon to get my 2010 300 a bit more sorted out, i thought i'd throw this one out to the sage minds here.

do you jet from the main first, and work your way down? from the pilot, and work your way up? start at the needle? does it really matter?

thanks!

 

[Krieg]

I've tried it every way imaginable. I prefer Pilot jet first, Main jet second, Needle third, and tweaking the screw anywhere in between as necessary.

 

[iajim]

Not much difference on the order of jetting. "Old school" idea was that you'd start excessively rich and be more likely to suffer a piston seizure at full throttle(main) than at idling(pilot/ needle). With metallurgy improvements, better carbs and better oil you can be off a size or two while checking the jetting and not have too much to worry about. Check on Thumper talk, they may have some info to get you pretty close so you don't spend all your time changing jets and spark plugs. As I recall the dual carb XT 550's were notorious for leaks but the 600's were much better. White Bros. had some 29mm smooth bore road-race carbs that were considered the "hot set up", but that was in 1984!

 

[mtothef]

d'oh!

As I recall the dual carb XT 550's were notorious for leaks but the 600's were much better. White Bros. had some 29mm smooth bore road-race carbs that were considered the "hot set up", but that was in 1984!

noted and aware on the carb issues already. thanks for the input though. i was actually wondering since i was taking my new 300 out today to do some tweaking with it, and the recent work on the xt had me thinking. seems most people (and generally respect as folks who know their stuff) i talk to have some sort of pattern to their jetting, but they are all over the map in direction. mostly curiosity on my part...

 

[mtothef]

now, to muddy the waters...

okay, i've gone through the jetting database, and done my messing around with plug chops. and here's where i am at:

sea level to 1500'
60-ish degrees
40 pilot
n1ef needle 1st clip (as in all the way lean)
175 pilot

i am really happy with the way the bike works here and now. it is a teeny bit rich down low and into the needle, but runs well, and the plug is still dry and chocolatey when doing chops between 1/4 and 3/4 throttle. main may be a hair lean, but i need to do some more chops to be sure. it is very close. plug was still more brown than tan when i checked. overall i am very happy with the power and the characteristics of the bike.

BUT, i am at the rich end of the spectrum for most of my riding. weather is only going to get warmer, and as the snow melts, i had for the sierras to ride. so i am only going to be going to hotter and higher places. normally, i'd drop the needle a clip or so for most on the fly issues, and when going up above 5000' would downsize main and pilot as well. but since my needle is all the way dropped, i am unsure what to do.

the bike came with a chn jet as well. i have no idea if this is richer or leaner or just "different" than the n1ef in the bike right now. and i haven't found any sort of "this needle is leaner here and fatter there, while that needle is leaner everywhere" kind of explanation to how needles are gauged. the sudco needle guide might as well be written in sanskrit for how much it tries to tell me.

anyone here got any solid, "i use this needle in the 3rd clip where you are riding now and then drop it a notch for few thousand feet or 20 degree change upward in temp" rule of thumb for what works?

sorry for the long wind. thanks for listening, and for any advice...

(and, totally unrelated, how rad was it today to be poaching a monday afternoon at hollister, and unload next to the guy who bought the 2010 300 from moore and sons, which was the bike that made me slap a deposit on my bike? small world, happy gasgas riders!)