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Pilot or Needle adjustment

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Hey ThumperTalk

I recently had my bike rebuilt and ported with the squish band height optimized. At break in, I checked after 30 minutes of riding slowly and the plug was a beautiful tan color, right in range. Later I started riding more aggressively and checked another new plug and it was more black with almost no tan. Would riding slower at barely any throttle be the pilot jet making it tab, and then riding harder at half throttle to 3/4 throttle be a needle adjustment. The atmospheric temperature did go up 10-15 degrees but I don’t think that’s it. Should I raise the clip or lower it? I still feel the lowend isn’t quite as crisp as it should be. Any advice is appreciated.

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0 throttle is the pilot, once the start to open it the needle comes into play. 10-15 degrees warmer would make it run richer, depends on the bike how much. Ive seen a few bikes blow up at the track when the temp dropped drastically, leaned them way out. It was kind of sad because it was a kids race.
If you want to lean it out raise the clip on the needle which lowers it farther into the emulsion tube.

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Crisp lower throttle is a combo of pilot jet and air screw, try adjusting the air screw, warm it up , ride slowly in 1st and crack it wide open fast, tune out the response, repeat.. The needle does come in to play at about 1/3 throttle, but 30 years of tuning my own carbs tells me go for the airscrew first.

I do not pay attention to my plug color, but the performance throughout the powerband. 

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Thanks guys. I’ll try this out Friday when I go ride

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On 10/25/2018 at 2:50 AM, MXR said:

 

Hey ThumperTalk

I recently had my bike rebuilt and ported with the squish band height optimized. At break in, I checked after 30 minutes of riding slowly and the plug was a beautiful tan color, right in range. Later I started riding more aggressively and checked another new plug and it was more black with almost no tan. Would riding slower at barely any throttle be the pilot jet making it tab, and then riding harder at half throttle to 3/4 throttle be a needle adjustment. The atmospheric temperature did go up 10-15 degrees but I don’t think that’s it. Should I raise the clip or lower it? I still feel the lowend isn’t quite as crisp as it should be. Any advice is appreciated.

 

what needle is it ?

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what needle is it ?

I haven’t pulled it out yet, but I’m willing to bet it’s the stock one. I adjusted the air screw and it made quite a bit of difference. I have it at one turn out exactly, so it’s within spec. I’m still willing to try the needle to see if it helps even more though.

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Crisp lower throttle is a combo of pilot jet and air screw, try adjusting the air screw, warm it up , ride slowly in 1st and crack it wide open fast, tune out the response, repeat.. The needle does come in to play at about 1/3 throttle, but 30 years of tuning my own carbs tells me go for the airscrew first.

I do not pay attention to my plug color, but the performance throughout the powerband. 

You were right!!! I adjusted the airscrew according to Duncan Racing’s Keihin carb adjustment suggestions that I found on the web. Turned it all the way in until it almost died, and then backed it out 1/4 turn at a time until the revs hit their highest etc. it made a big difference. It was crisp, rolled on better, with more power, and lugged for the first time in a taller gear without dying. I’m exactly one turn out on the airscrew right now. Thanks for the advice.

 

Next question... the low and mid feel great, but when I really get on the bike, it struggles transitioning into the top end unless I fan the clutch. I messed with the powervalve, and it’s easier to transition when it’s at stock setting, but still seems to not want to go into the topend. I’m coming off a 450 four stroke, and I can’t remember if my 2 strokes 10 years ago were like that. Is that normal? Any suggestions?

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Excellent, glad that helped. Now to the upper end. The upper band is all needle and main jet size. Wide open throttle is purely main jet size, sputtering too rich, struggling too lean. From about 2/3 wound out it is a combination of needle and main jet. Taper plays some part but all but one bike I jetted was just adjustment or jet size. 

I am guessing you are lean on top. First thing I would do is adjust the needle position. The clip changes at what throttle position the needle is partially blocking the main jet. I always goof this up when I explain it, easier to do than explain. I think you need to adjust the needle to lift sooner or sit higher. That way it allows more fuel at the same throttle. Move the clip one notch down toward the tapered end. Usually you do not have to take the carb out, just unscrew the top and lift out, or loosen and pivot the carb. Keep in mind you may have to re-adjust you air screw slightly afterwards, it is all a balance. If that doesn't work go up one step in main jet size and repeat.

Your 2 smoke should have a smooth powerband with some top hit, no flat spot if jetted right, thrilling! The powervalve adjustment mostly effects torque off the bottom end, basically at low throttle it shrinks the exhaust ports to build torque not native to a 2 stroke, it gradually pulls completely out about half throttle, I wouldn't mess much with that adjustment until you have the jetting right.

Here is a chart that helps people wrap their head around jetting, not complicated but a balance and a science to do. Practice makes perfect. Good luck, hope this helps.

jet-chart.jpg

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