I got an '86 Honda 250R ATC and recently got an FMF fatty pipe for it. For the past coule days I've been trying to jet it, and it seems that with every plug I put in it I get a different reading. For example, I put in a Champion N5C and it ran a nice brown but could be a little fatter. I put in an NGK B8ES and it was running white? What's with it? I know it's supposed to take a Champion N3C but it doesn't want to run with that kind. is an N5C safe to use? Also it's got a little stutter between about 1/8 and 1/2 throttle. Do I just have to adjust my needle or is it my main?

 

If you are getting a miss in that part of the band, it could very well be the needle jet. Try raising the jet needle one notch. That could possibly take care of the plug reading variation also, since you didn't say what part of the band you were testing it on. The needle jet controls performance from 1/4 throttle to 3/4 throttle for future info.

 

For Jetting you have to run it at each different throttle position and check the plug after you run it at a particular position, then try the next throttle postion, etc. As far as plug readings, always do that with a USED plug, a brand new plug will not give you accurate readings (Thats a mistake allot of people make, they put a new plug in and then try jettting it and it doesn't read properly). Also, NGK's are the best plug to use (They last longer han the cheaper ones), your best bet is to use the stansdard NGK plug called out for your particular machine and get it jetted in properly and you should be on your way.

Good Luck,

Sickman

 

I might offer the suggestion of reading the following techical article written by one of the pre-eminent two stoke "Godfathers"---Gordon Jennings. He presents to date the most factual and well written technical article on spark plugs. He advocates using NEW plugs to check jetting indications. George Grabowski, and a host of others offer the same advice. Old plugs only give an indication of what WAS going on, not an indication of the immeadiate picture. An old plug's build-up of contaminates skews the results. While it may work for checking day to day operation, it's best to use new when checking a entirely new set-up.

Warm up your motor to operating temp. Install a NEW plug. Ride around for a few minutes(3-4) in all throttle positions and then perform the "plug chop" procedure. The 3 to 4 minutes gives the plug enough time to heat up to proper temp. and then give a true indication of what's going on. You'll be focused on looking at the "smoke" ring around the insulator, down in the "throat" of the plug. It's all detailed out in the article.

Go check out this article. Without a doubt the best there is.

www.strappe.com/plugs.html

Good day.

 

Thanks, Backcountry, for the excellent reference!

I think Gordon Jennings founded the magazine, "Cycle World" in the early '60s; regardless, the article you cite provides lots of solid info.

Tree Farmer

P.S. Met Jennings once, Lions dragstrip, Long Beach; I'll tell you about it some time over, um, refreshments!

T.F.