Power Jet

[eagles22793]

I bought a 2000 yz250 and I looked at the jets in the bike and saw it has a power jet in it but I dont know what this jet does to the bike. I am going to change the main and pilot just like any other bike and I want to know if I need to change the power jet and how do I know when to change it?

 

[BSWIFT]

Got a picture? Not sure what you are calling a power jet. By chance are you referring to the "power valve"?

 

[whenfoxforks-ruled]

The power jet is the same jet as the leak jet. But, I think it has function higher in the throttle range than the thumpers. From the pilot to the main? It should be in the manual.

 

[RM85rider123]

I thought he was talking about the main jet.

 

[eagles22793]

It is in the manual but is does not tell you what it controlls

 

[mathd]

We are talking carb part from the manual right?
i guess they are refering to the needle jet(main nozzle).
without picture and manual its hard to tell.

 

[eagles22793]

No the power jet I know what I am talking about This 2000 yz250 has three jets in the carb a main pilot and power jet
I am wondering what it does and how I know when to change it

 

[mathd]

ok i found it.
check the yz250 2001 manual here it have some info about that power jet.: http://ymenvom.nl/manuals/M5MW-30.PDF

The power jet closes at 8,500 rpm of the engine,
after which only the main jet dominates.
(Powet jet adjustment
The richness of air-fuel mixture under 8,500 rpm
to the extent of 1/2 to full opened throttle can be
set by changing the power jet 1. A larger size jet
results in a richer mixture, and a smaller size in a
leaner mixture.Standard Power jet #50)
There is also a chart displaying its effect on the throttle

Now you know wich part of the circuit it affect.
To know when to change it you should check the jetting link on the forum it have great jetting info.

 

[jsantapau]

I believe that the power jet closes off when the ignition reaches a certain rpm.
at the very top of the rpm ceiling ,when the engine is past its peak power,the power jet closes to lean out the mixture a smidgen to give you an extra bit of over rev.

I have heard of some people disconnecting it and tuning the carb in the standard old fashioned way.Not something I would do just because I hate things that aren't the way they are supposed to be

Never having to try and tune one myself,my GUESS is that You tune it with the pilot,needle and main like the old style and if you run into problems like bike runs well at extreme rpms but pings right before you hit the usable rpm cieling you would fatten up the powerjet.
It would be something that you would tune at extreme high rpm but I doubt that I could tell the difference since I rarely ever really get the throttle pinned all the way open anyway so I would more than likely just tune with the pilot,needle and main and not worry about it.

 

[jsantapau]

oops a bit late

 

[whenfoxforks-ruled]

So it effects 1/2 to 3/4 throttle. I do not recall any sensors directly connected to the jet.

 

[eagles22793]

Ok now should I change in like I do with the main and pilot to lean the bike out or hardly mess with it

 

[jsantapau]

So it effects 1/2 to 3/4 throttle. I do not recall any sensors directly connected to the jet.

http://www.yamaha-motor.com/sport/parts/home.aspx



If this is the same carb that he has on the bike part #27 is the power jet and part #30 is an electric solenoid that opens up the passage way for the powerjet which is rpm controlled

"Ok now should I change in like I do with the main and pilot to lean the bike out or hardly mess with it"

well you have the area it affects thanks to Mathd and there is an assortment of jets so it is tuneable,but having never owned one,jetted one or even cleaned one, I can't answer your question with any knowledge or experience to back it up

 

[BSWIFT]

Interesting, the #50 jet is for the 2005 and the same part# shows it as a #55 on the 2000 model. Didn't even know it existed.

 

[whenfoxforks-ruled]

Frigging old guys? So does it effect 1/2 to 3/4 throttle range? Thats into the main already, and towards the end of the slide/needle? It sure seems that would be a tough one to nail down. I like drastic changes in the jets to see exactly when and what it effects. And when my son and I ventured into the thumper carbs with leak jets, no one had a listing in the Honda book, but Yamaha had all kinds of leak jets/power jets! Wonder if JD recommends changing these, and to what? And, oh boy, another sensor to check with an ohm meter!