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Mikuni Needle Jet

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Mikuni Needle Jet

Postby MS850 » Tue Apr 18, 2017 7:35 am

I have a question about simple jetting. I hope this makes sense.

Mikuni jet needle O-0 is one step richer than N-8. My question is what do think the jet needle clip position would be to equal the same richness using the different needle jets.

Example-- would clip in the 2nd (O-0) position be the same as 4th (N-8). Jet needle has 5 positions.
Thanks,
Mark
Roseville, Ca

1975 850 Interstate
1974 Yamaha MX250
1973 Honda CB500
'07 650 Vstrom
'63 McLane Lawn Edger
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Re: Mikuni Needle Jet

Postby marinatlas » Tue Apr 18, 2017 8:26 am

Hi, in my humble opinion, I think that moving the clip up and down will change the height (everybody agree ?), changing from one size of needle jet , regarding Mikinu (!), will change the taper of each , so at the end , i t "could" work BUT those change are quite different ..........! correction,I am wrong , changing the needle jet will change the size of the cross sectionnal size from 2.590mm to 2.600 ( N 8 to o0)...........
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Re: Mikuni Needle Jet

Postby MS850 » Tue Apr 18, 2017 8:48 am

Thanks Martin.
This is in my Yamaha DT250 so probably in the wrong thread, my apology.

Johnson Valley about 3000 elv. Coming back from the last ride on a dirt road, 1/4-1/2 throttle. The plug was black, not oily, sooty.
Situation is it runs rich with clip on 2nd position (from top) after installing Boysen Reeds, which they said is normal and leaning mixture may bey required. So was thinking going one size leaner needle jet to get the clip closer to the middle, so to have leeway riding in higher elevations. I live at sea level.
The plug didnt foul, but at times after starting cold sometimes it will foul. Once engine hot, no fouling.
Thanks,
Mark
Roseville, Ca

1975 850 Interstate
1974 Yamaha MX250
1973 Honda CB500
'07 650 Vstrom
'63 McLane Lawn Edger
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Re: Mikuni Needle Jet

Postby edgefinder » Tue Apr 18, 2017 9:24 am

With the needle clip in the middle you have to open the throttle about 1/8 before the taper of the needle starts coming out of the needle jet. Before that point your using the pilot circuit to adjust or override the fuel flow between the top straight part of the needle to needle jet diameter. I think your on the right path, one needle jet size to the next is very small as marinatlas pointed out but your original may have some wear. Cant tell you how many bikes ive seen that wont respond to air screw or pilot jet adjustment because the needle jet was too big.

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Re: Mikuni Needle Jet

Postby marinatlas » Tue Apr 18, 2017 10:18 am

Hi, did you try first to adjust air screw (out , so more air , CCW), or smaller pilot jet , then after only smaller needle jet ........
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Re: Mikuni Needle Jet

Postby 1up3down » Tue Apr 18, 2017 12:30 pm

standard sea level type altitude is close to a 40 pilot jet, assuming that is what your Mikuni came with

instead of first dropping the needle to get a little leaner to cure the plug darkening put in a 35 pilot jet with around two turns out from seated on the air mixture screw

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Re: Mikuni Needle Jet

Postby MS850 » Tue Apr 18, 2017 12:44 pm

The stock pilot is #60, yeah I did open the air screw, I might have a 50 or a 55 pilot I could try.
thanks.
Thanks,
Mark
Roseville, Ca

1975 850 Interstate
1974 Yamaha MX250
1973 Honda CB500
'07 650 Vstrom
'63 McLane Lawn Edger
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