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Power Jet #40

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The best way to bring the stock YZ250 carb into balance on pump gas is by reducing the power jet and increasing the needle. I've been doing this for a long time and have mentioned it here once in a while but now the PM's are coming in and topics are starting to get hijacked so here's a new topic.

Think of the power jet and needle as a pair working together, kind of like the pilot jet and air screw.

When the slide moves up about half way it exposes the power jet spray bar. When that thing is exposed and rev's build it starts to spray, a LOT! The Thing looks like a mosquito sprayer for those who have never removed the subframe and watched it.

The YZ250 is known for its lean spot at 3/8 throttle which is the needle. If you tune away the 3/8 lean spot and all its erratic surging etc. everything after that gets to rich and spoogie. The bike is either erratic and then runs clean up top, or pulls hard and clean the first half then dirty up top, or a little of both.

I need a bike that pulls hard and clean for the first half and I like it to run clean up top of course so years ago I decided to raise the stock needle for the first half and tone down that mosquito sprayer for the Second half. The needle was easy, I just raised the clip. Then I spent $7 for a #40 power jet for the mosquito sprayer.

GOLD, I have a bike that pulls hard and runs clean bottom to top, I have not touched either ever since. It really brought the middle meat of the jetting into a perfect balance for me. Perfecting the pilot and main jets became very easy after that because they were so isolated.

Another great thing about it is I don't touch it. I ride 500-1500 feet, 35-85°, 40-100% humidity. Once in a blue moon I turn the air screw 1/8 or 1/4, that's it. Constantly changing jets and needles is just not my thing so I love it. I would think that people at high elevations would especially benefit from the #40.

This has nothing to do with what some call the 1/8 burble, that's a whole other topic that I'm not concerned with because I don't ride in that range much and I actually like a little of that burble, it helps with grip.

I am sure it's not for everyone but that's my story and I'm sticking to it! Something to consider, hope it helps some of you.

Kinger

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My particular settings for northeast woods at 1000 feet:
45 pilot jet
N3EW-3
40 Power jet
175 Main jet
1 turn out on their screw

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Forgot to say a couple winters when I knew I was going to ride a lot below 35° I did put in a 178 stock main jet

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I think the Suzuki needle is an effort to make it work with the large stock #50 power jet. I don't have any experience with Suzuki needles LOL there is many topics on it though.

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Well, as I told you, I'm giving it a shot. Just placed an order with RM-ATV and threw a #40 in with it. Thanks for the write up.:thumbsup:

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I'm watching this thread with interest.  

I've been running the zuk needles and I'm at high elevation.  Summer riding for us is 9000-12,500 ft.  Winter is from 2500' - 7000'.  My bike actually runs very well, far more crisp than the KTM 2T xc's feel, but I don't know what they are doing for jetting.  

You say go leaner on the Power Jet and then richer on the needle.  How much worked for you?  Half clip, whole clip?  Maybe I'll pioneer the zuk needle testing with 40 PJ.  

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Kinger I have a couple questions if you don't mind..

From what I read it sounds as though your working with the parts Yamaha offers for the YZ. With that in mind I assume your using the stock #7 slide, is that correct?

Prior to making the change did you ever question what seems like a rather large jump from a #50 to a #40 powerjet. If we were talking pilot jets I doubt most would be willing to make that change without trying something in the middle first. I know Yamaha only offers optional power jets in size 40 and 60 in the YZ fiche but they are available in the same increments as pilot jets (48, 45, 42 and so on). Those sizes are available from Yamaha in the parts fiche for some of their carb equipped 4 strokes (YZ 450F). Any thoughts as to why Yamaha doesn't offer them in the YZ fiche?   

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Yes, all Yamaha stuff, and the N3EW In particular. I am using the stock #7 slide which I like most of the time but is a little rich even for me in the heat of the summer. Someday I might try a #8 for those dog days.
At the time it did seem like a big jump #50 to #40 but I figured it had to be a different effect or scale and it must be OK if that's how Yamaha offered it. When I first tried it I was expecting a more noticeable change but it was pretty subtle. I still had that weird surging 3/8 lean spot of course on clip position 2 so I tried clip position 3 again and that's when I struck gold. It ran great everywhere. Shortly there after I saw various sizes available for different applications but I felt like I had hit it perfect and didn't want to change anything.
Not sure why Yamaha offers it that way but I must say, for me it works.

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On 8/9/2017 at 8:48 PM, Kinger317 said:

Yes, all Yamaha stuff, and the N3EW In particular. I am using the stock #7 slide which I like most of the time but is a little rich even for me in the heat of the summer. Someday I might try a #8 for those dog days.
At the time it did seem like a big jump #50 to #40 but I figured it had to be a different effect or scale and it must be OK if that's how Yamaha offered it. When I first tried it I was expecting a more noticeable change but it was pretty subtle. I still had that weird surging 3/8 lean spot of course on clip position 2 so I tried clip position 3 again and that's when I struck gold. It ran great everywhere. Shortly there after I saw various sizes available for different applications but I felt like I had hit it perfect and didn't want to change anything.
Not sure why Yamaha offers it that way but I must say, for me it works.

Yes it must be a different effect or scale. As you said its working in conjunction with the main circuit to some degree. In A. Graham Bell's 2t performance tuning he indicates that when the Keihin powerjet is closed it leans the mixture by @ 10%. Seems to make sense then that the powerjet would require a much larger change in size to make a noticeable ("subtle") difference in the total amount of fuel delivered.      

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On 8/8/2017 at 4:24 PM, Kinger317 said:

My particular settings for northeast woods at 1000 feet:
45 pilot jet
N3EW-3
40 Power jet
175 Main jet
1 turn out on their screw

Put the 40 power jet didnt ride it yet but I have a hanging idle after i rev it. Should i move the clip to the 3rd position? its at the second.

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Put the 40 power jet didnt ride it yet but I have a hanging idle after i rev it. Should i move the clip to the 3rd position? its at the second.

It shouldn't give you a hanging idle especially on the stand. What is your elevation? Try the richer needle setting you liked earlier but said gave you a little spooge. You have to just ride it and see but the 40 power jet should let you go richer on the needle.

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Yes it must be a different effect or scale. As you said its working in conjunction with the main circuit to some degree. In A. Graham Bell's 2t performance tuning he indicates that when the Keihin powerjet is closed it leans the mixture by @ 10%. Seems to make sense then that the powerjet would require a much larger change in size to make a noticeable ("subtle") difference in the total amount of fuel delivered.      

Yeah I remember reading something from Yamaha I think saying that at 8500 RPMs when the solenoid shuts it the mixture is leaned by 10%. The power jet is already tapering off at that point and the main jet is full open with max affect. I think at like 5/8 throttle the power jet is really cooking and the main jet is just starting to get going. The power jet must be contributing a lot more than 10% at 5/8 and even 3/4 throttle. The picture I attached from the Yamaha manual sort of shows that.
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After extensive tuning with the suzuki needle setups (the NECJ and NEDJ needles)  I decided to give the 40 power jet a try. As of right now I can honestly say I like it better. The suzuki needle had great response and a slightly better LUG really down low but I could never get it to quite pull threw the 1/2-wot like the stock needle. As of right now I'm at 178/3/48 with the 40 pilot.  Figured it would be a good place to start. 

I've noticed your post several times kringer in the suzuki needle forums and such. Awhile back I bought a 40 powerjet and just today decided to toss it in. I'm at the same 1000ft elevation as you but in MI. 

Edited by Bren Ledger
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After extensive tuning with the suzuki needle setups (the NECJ and NEDJ needles)  I decided to give the 40 power jet a try. As of right now I can honestly say I like it better. The suzuki needle had great response and a slightly better LUG really down low but I could never get it to quite pull threw the 1/2-wot like the stock needle. As of right now I'm at 178/3/48 with the 40 pilot.  Figured it would be a good place to start. 

I've noticed your post several times kringer in the suzuki needle forums and such. Awhile back I bought a 40 powerjet and just today decided to toss it in. I'm at the same 1000ft elevation as you but in MI. 

Yeah, the first testimony! Sounds like it may work for you, you might find the 45 pilot gives you what you liked about your old set up with the other needle.

Are you turning hp revs on a track? Or pulling torque off track?

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1 hour ago, Kinger317 said:


Yeah, the first testimony! Sounds like it may work for you, you might find the 45 pilot gives you what you liked about your old set up with the other needle.
Are you turning hp revs on a track? Or pulling torquing off track?

I'm trail riding, but I'm pretty hard to please. MI trails are tough they often go from black dirt and tree roots to high speed deep sand rollers so I try my best to tune for a overall strong jet setup throughout the range. With the short time I have with the 40 powerjet setup I have noticed a increase in 3/4 to WOT pull like I had originally with the stock needle with just a small decrease in tractor power out of corners, Like you said might be contributed to the 48 pilot jet. 

My favorite suzuki setup (NECJ 3,45, 170) this ran pretty darn good with what i felt was a small decrease in 3/4-WOT pull and was a little rich on the needle. 

(1/2 clip leaner) NEDJ 2, 45, 170 was lean on the needle.. Lost 1/2-3/4 pull. 

I'm hoping, like you had mentioned the 40 powerjet will give me a better balance.. The YZs seem to like fuel to make good power. Definitely in my experience difficult to get them just right (responsive, spooge free and pullin' hard) 

Sorry for being long winded but figured it may help someone. I've had my carb off 30x times. 😂 

Edited by Bren Ledger
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40 minutes ago, Bren Ledger said:

I'm trail riding, but I'm pretty hard to please. MI trails are tough they often go from black dirt and tree roots to high speed deep sand rollers so I try my best to tune for a overall strong jet setup throughout the range. With the short time I have with the 40 powerjet setup I have noticed in a increase in 3/4 to WOT pull like I had before with just a small decrease in tractor power out of corners with like you said might be contributed to the 48 pilot jet. 

My favorite suzuki setup (NECJ 3,45, 170) this ran pretty darn good with what i felt was a small decrease in 3/4-WOT pull and was a little rich on the needle. 

(1/2 clip leaner) NEDJ 2, 45, 170 was lean on the needle.. Lost 1/2-3/4 pull. 

I'm hoping like you had mentioned before this will give me a better balance.. The YZs seem to like fuel to make good power. Definitely in my experience difficult to get them just right (responsive, spooge free and pullin' hard) 

Sorry for being long winded but figured it may help someone. I've had my carb off 30x times. 😂 

Why not try this with bigger main? Bigger main will richen the needle some and also might give you the WOT power you felt you lost? Just a thought though, might not be good at all 😅

Edited by Christoffer Brandel

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4 minutes ago, Christoffer Brandel said:

Why not try this with bigger main? Bigger main will richen the needle some and also might give you the WOT power you felt you lost? Just a thought though, might not be good at all 😅

I think I did but would have to look at my notes to make sure.  If I recall I tried a 168 and a 170 but it very well may need a 172 due to the leaner needle setting. 

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I'm hoping like you had mentioned before this will give me a better balance.. The YZs seem to like fuel to make good power. Definitely in my experience difficult to get them just right (responsive, spooge free and pullin' hard) 
Sorry for being long winded but figured it may help someone. I've had my carb off 30x times.  

I'm not afraid to run fuel through it either, above the pilot. It runs real cool that way and parts last a long long time.
I don't care about spooge because it runs so good, when it starts to spooge I repack the silencer and it's spooge free for another 15–20 hours.
My only comment to you is maybe the 45 pilot. The 178 main it's probably good for you if you're ringing it out in the sand. I like the 175 for 10 months a year but I'm not ringing it out in the sand.
Best thing is when you dial the pilot and main you may not touch it much after that except occasional air screw. Thanks for the feedback, this is interesting.
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