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Jetting question

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Okay to start this is a Yz250f for dirt oval Kart racing... 290cc big bore, open K&N filter, powerbomb exhaust right into a free flow muffler...

got the engine running okay-good with a Mikuni SBN 38mm carb...

just acquired a 2008 Yz FCR carb... 

got the engine running okay... but I have to keep the pilot jet on the large size... #55 with Mixture screw 3.5 turns out...

other than that it is stock jetting... no bog or hesitation... great on low and high rpm... crisp throttle response...

 

anyone think it it would be okay to go up one more size on the pilot jet to jet the mixture screw at 1.5-2turns out???

also I never had the need to start it with the choke or hot start... we start it with a hand held electric starter...

 

also I have to keep idle at 2300-2400rpm or it will die and not idle... at said rpms it idles great and responded well...

 

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At 3.5 turns out on the fuel screw, you risk loosing it.  I'd go up one more pilot jet size to try to come back down to 2-2.5 turns out or less on the fuel screw.

The engine is supposed to idle at around 1800 rpm, but with the bore kit, pod filter, and open exhaust, I'm not sure what you'll be able to achieve.  You're on your own..... good luck.

PS Had another thought on idle.  Have you tried a GYTR flywheel to help control idle?  A little more flywheel might help it.

Edited by ronbuell

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Went through the entire carb in and out again... cleaned , washed blew everything out...

stayed with the #55 pilot jet, and have it running with 2 turns out on the mixture screw...

still has to idle at 2200-2400rpm

the valve clearances are good, valves are sealing good also...

the engine has a NOS head 05 with new SS valves and dual springs and S2 hot cams...

wondering if that would contribute at all to the higher idle setting???

 

thanx

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I would think not.  What method are you using to determine idle RPM?

OK, 2 turns out is right in the sweet spot.  SS valves are heavy, so watch your RPM and stay off the rev limiter.

S2 Hot Cam is a high RPM power maker, but I know its a good match to the bore kit.

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It's called a Mychron 5 gauge.... more kart racing oriented

displays temp, rpm, lap times and data logs...gps

Everything I have read about the big bore kits always recommend leaner jetting...

I thought it would balance out big bore leaner jetting, then open intake and exhaust richer...

i also have a AFR meter hooked up, to monitor that side of things...

idles at 2200-2400 with 13.5 afr... 

The motor doesn't really settle down till hit hits 150-170*f, before that idle goes up and down enough to,set the idle at 2200-2400rpm

when the motor reaches 150-170*f it usually idles high 2500-2600rpm but steady... then turn the idle back to 2200-2400rpm...

 

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Ok, if you have a sniffer on it and its reading 13.5, that's pretty good. Stochoimetric is 14.7, which perfect burn but too lean for good power.  I can't remember the exact number to hit, but if its got good power, cranks good hot and cold, you're GTG.

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