Disabling a power jet...?

[fowler175]

I've got a Keihin PWM38 with the power jet plugged/blocked off and I am just wondering if anyone has a rough idea of how much influence the power jet typically has on the rest of the jetting?  In my manual ( it's a 99 yz250), it states that the power jet only opens at 1/2 to full throttle so I would assume that by blocking it off, I would then need to richen the circuit in that range...  I'm just wondering if it will be a large difference in final jetting?  Just trying to ballpark it to start.

[FORMA MOTO]

https://www.thoughtco.com/power-jet-carbs-743539

 

[ss-racing66]

Understand that there’s no fuel pump pushing fuel out of the power jet. It’s normally open, and high velocity air sucks the fuel out, generally during mid to high mid acceleration. ....and then at 8500 rpm, the electric solenoid is closed by the ignition system. 

Think of it as a way to provide a properly jetted carb (good power, no detonation), while also getting good over-rev. 

[fowler175]

Understand that there’s no fuel pump pushing fuel out of the power jet. It’s normally open, and high velocity air sucks the fuel out, generally during mid to high mid acceleration. ....and then at 8500 rpm, the electric solenoid is closed by the ignition system. 
Think of it as a way to provide a properly jetted carb (good power, no detonation), while also getting good over-rev. 

So am I correct then, that by removing the power jet completely I will now need to richen the mid to top? Raise needle & larger main? Or does the power jet have only a very subtle effect on jetting?

[intimdatr]

Why not just clean and service your carb so it functions properly...? Disabling the PJ seems like a step in the wrong direction.

Edited August 15, 2018 by intimdatr

[fowler175]

Why not just clean and service your carb so it functions properly...? Disabling the PJ seems like a step in the wrong direction.

Fair question... the wiring harness and solenoid were already damaged, and I like the simplicity of a carb w/o the electronics so rather than getting a new solenoid I just blocked it off. Now I've just got to get my jetting figured out [emoji848]

[turbo dan]

If the power jet is plugged it will subtract fuel from the mixture. 

They are open until the solenoid is energized which shuts off the jet and leans the mixture.

[ss-racing66]

2 hours ago, fowler175 said:

So am I correct then, that by removing the power jet completely I will now need to richen the mid to top? Raise needle & larger main? Or does the power jet have only a very subtle effect on jetting?

Assuming the jetting was spot-on before the power jet was broken, sure, you would prolly want to richen it in the mid, and the top. 

Personally, I would keep an eye out on eBay, and fix it 

[kxrob]

On 8/15/2018 at 4:57 PM, fowler175 said:

I've got a Keihin PWM38 with the power jet plugged/blocked off and I am just wondering if anyone has a rough idea of how much influence the power jet typically has on the rest of the jetting?  In my manual ( it's a 99 yz250), it states that the power jet only opens at 1/2 to full throttle so I would assume that by blocking it off, I would then need to richen the circuit in that range...  I'm just wondering if it will be a large difference in final jetting?  Just trying to ballpark it to start.

You can block it off and jet around it 

ktm keihins doesn’t have it

ypu are correct in having to riches up the jetting 

[Orange Crush 500]

Sounda like it gona run really lean, better not  run it until u richen things. Always better to be on the rich side.

Why wouldnt u run it the way it was designed?

Were these known to be bad carbs?

Edited August 16, 2018 by Orange Crush 500

[fowler175]

Sounda like it gona run really lean, better not  run it until u richen things. Always better to be on the rich side.
Why wouldnt u run it the way it was designed?
Were these known to be bad carbs?

The PWM carbs don't have a great reputation as they can be a little harder to tune, so they are often replaced with the PWK. I figured I'd play with the PWM jetting & do a couple minor mods to see if I can make it work well, but if I have to spend $ on it for anything other than jets I'll likely buy a used PWK.

[hallsy]

On 8/15/2018 at 8:22 AM, fowler175 said:

Fair question... the wiring harness and solenoid were already damaged, and I like the simplicity of a carb w/o the electronics so rather than getting a new solenoid I just blocked it off. Now I've just got to get my jetting figured out

So how did you block it off? At the jet hole or at the solenoid hole? 

[fowler175]

So how did you block it off? At the jet hole or at the solenoid hole? 

Blocked off the jet... I just put the solenoid back in the hole & removed the wires

[hallsy]

On 8/16/2018 at 9:57 AM, fowler175 said:

On 8/16/2018 at 1:54 AM, Orange Crush 500 said:

Sounda like it gona run really lean, better not  run it until u richen things. Always better to be on the rich side.
Why wouldnt u run it the way it was designed?
Were these known to be bad carbs?

The PWM carbs don't have a great reputation as they can be a little harder to tune, so they are often replaced with the PWK. I figured I'd play with the PWM jetting & do a couple minor mods to see if I can make it work well, but if I have to spend $ on it for anything other than jets I'll likely buy a used PWK.

What are the minor mods your planning on doing?

[fowler175]

What are the minor mods your planning on doing?

Filing notches in the slide and the needle "shroud" on the carb body. Apparently it helps with tuning at idle & just off idle...

[hallsy]

12 hours ago, fowler175 said:

14 hours ago, hallsy said:

What are the minor mods your planning on doing?

Filing notches in the slide and the needle "shroud" on the carb body. Apparently it helps with tuning at idle & just off idle...

Sounds like you made these changes but did not run it this way with the same jetting it had prior to the changes, instead you went leaner on the pilot/main/needle, is that correct?  

[fowler175]

Sounds like you made these changes but did not run it this way with the same jetting it had prior to the changes, instead you went leaner on the pilot/main/needle, is that correct?  

Yes, just basing my "guesses" on the 98 yz250 pwm jetting which was not a power jet... and the bike is a 265, so it would typically require slightly leaner jetting. I've installed a 170 main, N3CW needle and 45 pilot. It seems pretty decent, but my governor came apart so I'm waiting on parts to do some more testing [emoji106]

[MCRIPPPer]

is there such a thing as a power jet PWM? i thought only some versions of pwk had a power jet? i have a PWK with power jet delete on my 98 yz250. i have it running almost perfect with 50 pilot, 168 main, 3rd clip N3EJ (98 yz needle) #8 slide. only problem is that i think it runs lean at 1/4 throttle just slightly. not sure if it needs a #7 slide or a half clip shim under the needle. 

other than that it is so smooth its unbelievable compared to the PWM. 

Edited August 25, 2018 by MCRIPPPer

[hallsy]

4 hours ago, fowler175 said:

Yes, just basing my "guesses" on the 98 yz250 pwm jetting which was not a power jet... and the bike is a 265, so it would typically require slightly leaner jetting. I've installed a 170 main, N3CW needle and 45 pilot. It seems pretty decent, but my governor came apart so I'm waiting on parts to do some more testing

Ok but the 98 motor is different, at the very least you can see from the parts fiche that the powervalve is completely different, don't know about anything else, could differences in those two motors require different jetting aside from just the powerjet? Also sounds as though you changed needles from the stock 98'/99' needle (N3VF) to the stock 00' needle (N3CW). Thought I read where you are running the N3CW in clip 2, don't know what clip you ran the N3VF in? There are two somewhat significant differences between those two needles, do you understand what those differences are?

[fowler175]

Ok but the 98 motor is different, at the very least you can see from the parts fiche that the powervalve is completely different, don't know about anything else, could differences in those two motors require different jetting aside from just the powerjet? Also sounds as though you changed needles from the stock 98'/99' needle (N3VF) to the stock 00' needle (N3CW). Thought I read where you are running the N3CW in clip 2, don't know what clip you ran the N3VF in? There are two somewhat significant differences between those two needles, do you understand what those differences are?

Yes, I have the kiehin jetting guide from jd jetting and have done a lot of comparisons... I realize the 98 engine is much different than the 99, but I don't think the jetting would be drastically different between the engines with the same carb.