which jet to adjust first

[Skyman]

I have always been under the assumption that when jetting a PWK Keihin carb you should do the main jet first , Pilot jet second, and then the needle jet third, air fuel fourth, and then Idle screw when fine tuning. I have been all over internet and no one gives the order of which they should be done. Just what they do and I understand that.


IS THIS THE CORRECT ORDER???

If it is not can someone give me the correct order and why?

[DnB_racing]

on a complete unknown quad I allways start with the main, then the needle then the pilot then a/f screw, along with idle when needed,

the only reason i can give for this order is how each cuicuit affects the next

I start full throttle then go down in order

The air screw is most effective between idle through 1/8 throttle.

The pilot jet is most effective between 1/8 through 1/4 throttle.

The slide valve is most effective between 1/8 through 1/2 throttle

The jet needle is most effective between 1/4 through 3/4 throttle.

The main jet is most effective between 3/4 through wide-open throttle.

[Meat]

I'd make sure I was in the ballpark with the main and then work on the air-screw and pilot first, most of my riding is done on the pilot/air screw and very little of my riding is done on the main jet. We're east coast woods riders.

If you work on your main jet first, you'll have to ride through the pilot and needle jets circuits before you're on the main, so if you're way lean on that pilot or the needle clip is in the wrong/lean position, then you're running dangerously lean on possibly 3/4 of your throttle before you finally get on that main jet, which controls from 3/4 to WOT.

Get that pilot jet figured out first, you want your pilot jet sized so it runs best when your air screw is 1-1/2 turns out from seated.

75% of all needles on 2 stroke motors should bet set in the middle position. Moving the clip represents a very big move. This is the reason that aftermarket jet kits come with little washer that you can put under or above the clip, effectively giving you smaller moves of the clip which in turn allows a smaller, less drastic move in jetting.

Since you made sure you're 'in the ballpark' on the main, when its time to test the main jet sizes keep going bigger and bigger till you get it to bog, then work your way smaller till it cleans up.

F-uck reading spark plugs, IMO the new synthetic oils make plug reading less reliable than when burning mineral based oils. Learn to jet by the old "seat of the pants" method and just keep it on the rich side till you feel confident of your jetting skills. Pulling that spark plug after every run is a royal pain in the asss, the motor is hot, the air temp is hot, and then you're looking at spark plugs that will prolly look the same at 3 different main jet sizes. I would recommend that after you get everything dialed in pretty close and you get a few miles on the new jetting specs, you should prolly pull the spark plug just to be sure that electrode is not too white or too tan or black.

[sand_blaster]

My 250r is blobbery from mid to top that mean is it to rich? My plug is black.

[fearlessfred]

Originally posted by Meat
I'd make sure I was in the ballpark with the main and then work on the air-screw and pilot first, most of my riding is done on the pilot/air screw and very little of my riding is done on the main jet. We're east coast woods riders.

If you work on your main jet first, you'll have to ride through the pilot and needle jets circuits before you're on the main, so if you're way lean on that pilot or the needle clip is in the wrong/lean position, then you're running dangerously lean on possibly 3/4 of your throttle before you finally get on that main jet, which controls from 3/4 to WOT.

Get that pilot jet figured out first, you want your pilot jet sized so it runs best when your air screw is 1-1/2 turns out from seated.

75% of all needles on 2 stroke motors should bet set in the middle position. Moving the clip represents a very big move. This is the reason that aftermarket jet kits come with little washer that you can put under or above the clip, effectively giving you smaller moves of the clip which in turn allows a smaller, less drastic move in jetting.

Since you made sure you're 'in the ballpark' on the main, when its time to test the main jet sizes keep going bigger and bigger till you get it to bog, then work your way smaller till it cleans up.

F-uck reading spark plugs, IMO the new synthetic oils make plug reading less reliable than when burning mineral based oils. Learn to jet by the old "seat of the pants" method and just keep it on the rich side till you feel confident of your jetting skills. Pulling that spark plug after every run is a royal pain in the asss, the motor is hot, the air temp is hot, and then you're looking at spark plugs that will prolly look the same at 3 different main jet sizes. I would recommend that after you get everything dialed in pretty close and you get a few miles on the new jetting specs, you should prolly pull the spark plug just to be sure that electrode is not too white or too tan or black. if a person holds the throttle wide open you are not running thru any other circuts but the main ,just because the motor has to build revs doesnt mean that you are running thru any other circuits.if the throttle is wide open you are on the main jet .if the throttle is halve open you are on the needle. i beleive reading the plug is the best way to jet and would not do it any other way. read up on the net or even on this site on how to read plugs correctly

[Meat]

Right on fred.

How do you get to full throttle/main jet without going through the other jets ? You would have to hard launch off the line and pin it WOT immediately after dumping the clutch and still technically you'd run through all the jets for just a split second. Maybe I'm confused on your technique. Could you explain some more...

[DnB_racing]

Originally posted by fearlessfred
if a person holds the throttle wide open you are not running thru any other circuts but the main ,just because the motor has to build revs doesnt mean that you are running thru any other circuits.if the throttle is wide open you are on the main jet .if the throttle is halve open you are on the needle. i beleive reading the plug is the best way to jet and would not do it any other way. read up on the net or even on this site on how to read plugs correctly i agree that the circuits are separate and to start with main,and the circuits do affect others,

however the synth oils and race fuels, do make plug reading much more difficult, it takes much more running time just to get any color on the plug,

on a break in with good dino oil and regular fuel a plug chop is a good way to go, but for a tuning type of jetting its got to be by feel not as much plug chop,

it just would take too much run time, for the plugs to show any change, unless its extremely rich or lean,

and if you cant feel the performance and response or lack of it you shouldn't be jetting anyways..

most jetting is done by feeling how the quad runs,and a plug chop is done for the first run on a rebuild,
that being said doesn't mean to ignore the plug, but it just takes more running time to get a good color response, yes ALLWAYS check plug but also learn to feel the quad, and usually the plug will be just right

[DnB_racing]

Originally posted by Meat
Right on fred.

How do you get to full throttle/main jet without going through the other jets ? You would have to hard launch off the line and pin it WOT immediately after dumping the clutch and still technically you'd run through all the jets for just a split second. Maybe I'm confused on your technique. Could you explain some more... most of the problems causing catastrophic damage is done from the main at wot, most damage isn't from the lower throttle circuits,

if you do the main first, yes you might have to come back and make small adjustments as you change the pilot circuit but its a safer way to go,

if you do the pilot first then it takes more going back and forth to get it turned in through all the circuits

obviously if you cant get WOT without it falling on its face you have to work on things but the main at WOT is only main

so to the Original Poster: as you can see just from the differences in our responses there are too many different variables to have a cut in stone order, a lot depends on the quad and the person doing it,

find what you feel you can do safely without causing damage and which will take you the least time....

[Skyman]

Thanks guys. It pretty much is anyones call. I did learn that the new oils cause the plug to take longer to show burn. It is feel until that plug has some time on it.
I agree the needle changes are huge.
As long as their is not a specific order getting it dialed in is a journey in learning.

Thanks guys, I love this forum.
Real people with real answers.

[Skyman]

@ sandblaster
I would try the main first. If that doesnt help. then go to the needle.
Sounds like its running rich. Especially if the plug is black.

[wilkin250r]

I don't think running through the pilot and needle are a significant problem on your way to the main jet, even if those circuits are dangerously lean. The problem with a lean condition is heat, and you're not going to get a significant heat buildup in the half second it takes you to open the throttle.

But that's besides the point. I always do the pilot and idle first. If you fire up a cold quad, fresh rebuild, and immediately go screaming off at WOT in order to get a main jet reading and adjustment, you're a moron. You're going to seize your engine.

I do the pilot first because I'm still sitting in the garage, and the quad needs to warm up to operating temperature, you can jet a cold engine. So I adjust the idle while it's sitting there idling. That's really the only basis I have for doing the idle first, it's that simple.

After that, I'll do the main, because it has an impact everywhere else. The best explanation for that is to examine HOW the needle works, and why it's tapered the way it is.

Fuel will flow up from the bowl based on engine vacuum and air speed through the carb. It is NOT based on air volume. If you're running 4000rpm, and you go from full throttle to half throttle, you're still trying to pull the same amount of air (250cc at a rate of 4000 times per minute), but since you've just closed the throttle halfway, it's trying to pull that same amount of air through a much smaller opening. This creates much higher airspeed through the carb, which means it wants to pull MORE fuel, not less.

The needle is fatter at the top because you want to effectively "plug" that fuel passage more. You want LESS fuel at low throttle positions, but because you have higher airspeeds, the carb wants to pull more. In order to counteract that, you have to put a bigger restriction in the way. That's why the needle is tapered, to plug up that hole passage and provide that restriction.

BUT, the main jet is sitting at the bottom of that fuel passage, and the fuel will have to flow through that main jet before it can reach the top of that fuel passage and be affected by the needle. So your main jet will indeed have an effect on your needle jetting. It's not a significant effect, but it's definitely there. You can always compensate for it, and at WOT the needle is almost completely out of that fuel passage, so the Main jet does indeed affect the needle, but the needle doesn't affect the main jet.

That's why I do the Main jet before the needle. One or two jet sizes probably won't affect the needle jetting much, because the needle is going to be a much greater restriction than the main while operating at mid-throttle positions. But major changes in the main jet will indeed affect the needle settings.

[fearlessfred]

Originally posted by Meat
Right on fred.

How do you get to full throttle/main jet without going through the other jets ? You would have to hard launch off the line and pin it WOT immediately after dumping the clutch and still technically you'd run through all the jets for just a split second. Maybe I'm confused on your technique. Could you explain some more... im not really saying do one or the other first . if i fired my motor and it started hard and didnt wont to idle well,i would address that first before going to the main.i would than go to the main jet and than to the needle. the length of time it takes to open the throttle is not enough to be concerned with. my point really is dont confuse rpm with throttle opening. if your throttle is wide open and your rpms are at 3500 you are still on the main and not the needle and i was not sugesting taking a cold motor and holding the throttle wide open. i do agree that alot of tuning is done by feel and ear,but not everone can do this. i can tell by listening if you are to fat on the main,but might not be able to hear if its to lean so i would use the plug to know what way to go ,even if coloring is hard to get to get you can read the ground base circle and make sure there is no glazing on the porcelain. i beleive the plug is a window to see inside the motor. i also beleive that doing the main first is the fastest way of jetting