whats the needle for? is it mandatory to change it when i change the main jet? oh and whats the pilot? does this need adjusting also? thnx~

 

I believe that on most carbs... the tip of the needle closes off .. or nearly so... the main jet when the
throttle slide is mostly closed. When the throttle slide is opened... the needle pulls up out of the
jet. The needle is tappered, and shimmed to changed the point at which it lifts out of the jet.
( you could say that the needle is a main jet plug at low throttle possitions ??)

So ... it is mostly a slow/mid range adjustment ?? once it is out of the jet hole.... the jet hole
determines the amount of fuel that will be induced into the engine.

Only needs adjusted if you are getting a "bogging" effect when coming off of the idle ... I have
not changed them on KFX ..... even though I have changed the mains from 135/140 to 142/144 jets.

Hope I got this right .... if not ... I trust someone will help us out [img]i/expressions/face-icon-small-smile.gif[/img]

 

No it's not mandatory, matter of fact it's best to make one change at a time. The needle length and taper effect mixture at one quarter to three quarter throttle settings. You can adjust it by moving the e-clip to different slots on the needle, the top slot (#1) is the leanest. Caution IS advised, you can easily stick your piston at half throttle if your needle is set too lean. The basic rule is:
pilot jet, mix screw and slide cutaway - idle to one quarter throttle
needle and needle jet - one quarter to three quarter throttle
main jet - three quarter to full
In reality, there isn't a solid black line between these circuits, they overlap slightly. But it helps to think of them as being separate. Also changing one circuit could have a slight secondary effect on the others, how much varies from carb type to carb type. So... you figure out at what throttle setting the poor running occurs, and try to make a determination as to whether it's rich or lean, and address that circuit. How many times you accept or reject changes is the process of fine tuning. I see from your previous posts you have a 250R. Jetting is most sensitive to temperature\elevation and crucial to performance and reliability on a hi perf two stroke like you have, especially main jetting, and lean IS mean, but also dangerous for the piston. For a number of years I was involved in 1000 foot snowmobile speed runs on our frozen lakes in the winter. 150HP sleds with needle tipped ice picks running over 100MPH are great fun. Anyway, we were interested in main jets, 'cause the only throttle setting is fully open. The trick was to run as lean as possible without detonating the piston into oblivion. The best way to do this is to run a hi octane racing fuel, certainly above 100 octane (to avoid detonation), and lean the main jetting as much as caution would allow, as much as 8 or 10 sizes from stock in some cases. You could see speeds go up as the jetting went down. If you then put regular 92 octane premium in with that jetting, instant meltdown!! I guess my point is, you need to put a safety margin into your jetting (rich enough) for trail riding, different temps, fuel octane availibility, etc, yet it has to be lean enough to run good and not foul plugs constantly.

 

Very Well Said Dzzy . I printed this to put in my KFX file. I will be able to use
this as I try to fine tune my KFX this week.


Question ..... about the "lean is mean" thing ...... Is that true for 4 strokes also ??

 

vforcer, in a way it is but not as much as with two strokes IMO. The stakes are higher in the two stroke lean game, as often they run into serious detonation problems before they actually start running worse, which holes a piston, or begins to melt the area around the steel locating pin for the rings. Unless you are running some radical compression and\or timing advance, this is a failure I have almost never seen in a production four stroke engine, but I routinely see it on two stroke sled engines. I also think temp and\or elevation effect production two strokes more than production four strokes. Four strokes can have their finicky times too tho. I have seen cases where one jet size up turns a pooch into a monster, and one more turns it back into a pooch. Other times a jet size either way doesn't seem to make much difference. I guess every machine and situation is a little different. Good luck jetting yours. Have you had the dial a jet long? and do you like it?

 

DZZY

Thanks for the info !

As for the Dial a Jet .... this is my first attempt.

The thought of being able to adjust the fuel induction a bit
without taking the carb apart every time appealed to me.

As it turns out ... the KFX carbs are very easy to change jets
on.... It can be done without removing any thing except the bowls.

Anyway ..... I am pleased with the results I have received so far.

I will be installing dual exhaust next week ... have already put
in larger jets ... and even so... it doesn't seem to be too rich yet ??

After the pipe is on... the intent of course is to leave it a little lean
and then use the D-A-J to make up the difference. I also have the
sliding- adjustable opening in the top of the air box.