YZ 250 jetting

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8/15/2018 10:50 AM

Hi guys,
I would like to know if anyone has a jetting spec for a Bill's Pipe and Silencer and boyesen rad valve for a YZ 250, because I can't find Bill's jetting guide anywhere. Also, I run Motul 800 and it says to mix it 50:1. Is the stock jetting only suitable for 32:1 premix? Should you re-jet the bike if you want to ran 50:1?
Thanks

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Heavy Hitter

8/17/2018 6:05 PM

I have a 2017 YZ250 with a vforce reed and fmf silencer and I just run stock jetting at 32:1. The manual says 30:1, but I run 32:1. I only adjust the air screw +/- 1/4 from stock depending if it’s really cold or hot out. The bike runs great all the time with this jetting/premix ratio. I would not run it any more lean than 32:1. I am a decent pace Local A class rider.

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8/20/2018 2:32 AM
Edited Date/Time: 8/20/2018 2:39 AM

Jetting is affected by temp and elevation as well as reeds and pipe.
You really need to experiment to work out where you need to be

Fwiw most of my riding is fairly close to sea level and my jetting was slightly lean on the main jet with the bike completely stock. Went to a 185 main jet and it went maybe slightly rich. I threw a DEP pipe and muffler on it and it was perfect. I'm running silkolene at 40:1.

At 32:1 the air to fuel ratio is leaner than 50:1 meaning there is less fuel to air at 32:1 than 50:1. The idea is to pick a ratio and jet to that. Plug color and exhaust spooge will help determine that

Depending on where you ride I would go up 1 or 2 sizes on the main jet and see how the plug looks and go from there

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8/20/2018 7:09 AM

With the pipe and reed cage....I would go up one size on the main. Try to run at least 50% race gas. I would go with an oil that recommends 32:1-40:1 ot 50:1. I'm sure it would be fine but why fuck with what has been working well for so long.

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8/20/2018 5:37 PM

It will run much cleaner with a NECJ needle that came stock in the RM250.

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21.5 350 XC-F
96' XR600r

8/21/2018 6:28 AM

Very important....what year is the bike? They made some jetting changes along the way and settled on something after a couple of years. Search MXA for the years.....

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8/21/2018 4:34 PM

Thanks for the replies guys! It's a 2018. Obviously it is pretty rich with stock jetting and a 50:1 ratio. It is the EU spec, would changing to the US jetting make any noticeable improvement? I will only have to swap the main and pilot jet and the clip position of the needle. I don't have anything against any premix ratio, I just feel that using more oil that what Motul recommneds is simply unnecessary.

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Heavy Hitter

8/21/2018 4:40 PM

Andy7 wrote:

Thanks for the replies guys! It's a 2018. Obviously it is pretty rich with stock jetting and a 50:1 ratio. It is the EU spec, ...more

Why do like to run 50:1? Why not 40:1

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2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @2HRacing
Thanks to : Factory Effex, N2Dirt, Acerbis, DT1, Fasthouse, Matix, FMF, ASV, 100% & Mika Metals

8/21/2018 4:59 PM

mattyhamz2 wrote:

Why do like to run 50:1? Why not 40:1

I was pretty skeptical at first about running 50:1, I always run 40:1. But my brother has been using 50:1 and there is plenty of oil on the cylinder walls of his bike, so I assume it is enough. Anyway, I wouldn't mind continuing to use 40:1, was just curious about Motul expressing 50:1 as the ideal ratio for the 800.

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Heavy Hitter

8/21/2018 7:05 PM

Andy7 wrote:

Thanks for the replies guys! It's a 2018. Obviously it is pretty rich with stock jetting and a 50:1 ratio. It is the EU spec, ...more

mattyhamz2 wrote:

Why do like to run 50:1? Why not 40:1

Andy7 wrote:

I was pretty skeptical at first about running 50:1, I always run 40:1. But my brother has been using 50:1 and there is plenty ...more

Ahhh gotcha.

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2017 RMZ450
2005 YZ250-sold :,(
1998 YZ250
2005 KX250F

80% of the time it works every time
IG @2HRacing
Thanks to : Factory Effex, N2Dirt, Acerbis, DT1, Fasthouse, Matix, FMF, ASV, 100% & Mika Metals

8/23/2018 7:46 AM

Hey bud,
This "option" is not for everyone, and I know some say its a waste of money and unnecessary if you get the stock carb dialed in which I agree 100%. With that said, I am a vet rider and had the money so I opted for the Lectron carb upgrade. Lectron builds the carb to what mods you have done to your bike, and have a lot of data on file, so it is real close to being perfect when you bolt it on. NO JETS TO MESS WITH! I literally kicked the bike one time, no choke and it idled smooth and the bike sounds so crisp. the only adjustments you would make are a twist of the metering rod and a turn of the jet screw. So simple that's why I went this route. Just an option to think about if you haven't already. The more options the better. Good luck man.

Below were my mods on my 09 YZ125

144 kit
Vforce4 reeds
FMF fatty
FMF shorty
Twin air filter

Sincerely,
Slowoldman

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