2-stroke jetting for dummies

JW381
Posts
10643
Joined
7/21/2009
Location
Harrisburg, OR US
7/23/2010 5:15pm Edited Date/Time 1/27/2012 9:24am
I know virtually nothing about jetting a bike, and would really like to learn a few tricks of the trade. I know it's alot more pivotal for us premix burners, so I need to at least have somewhat of an idea of what the hell is going on in that carburetor majiggy.

Any help, all know vital wrenches?
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mx5471
Posts
986
Joined
7/10/2008
Location
AL US
7/23/2010 8:11pm Edited Date/Time 7/23/2010 8:22pm
It's really pretty simple.Disregard humidity,and altitude if under 2000 feet above sea level. It all has to do with air temp,and what mods you made to the bike,such as reeds,exhaust,etc. You are in Oregon,which may have an altitude above 2000 feet,which may require a leaner mixture.A golf ball flies farther,a jet needs a longer runway to take off when it's hot,like 80's or 90's. It is because the air is less dense,as in everything expands when heated.When cooled everything compacts. The carbs job is to deliver the proper fuel-air mixture to the engine.Too rich,you foul plugs. Too lean,you blow the motor. The air -fuel mixture runs and cools the engine.You need fuel ,oxygen,and spark. Spark is a given,the tuning has to do with the amount of fuel relative to the amount of oxygen.When it's hot outside,the amount of oxygen in a cubic foot of air is less than when it's cold. Thus an engine will run richer when temps are up,and leaner when temps are down. Always use the same ratio of gas to oil when you mix your fuel. You can't tune the bike by changing the ratio.Start with a new spark plug,fresh gas and a clean air filter. Also,start with the stock settings for the carb,(jets, needle position).Warm up the bike and see how it idles,how it runs at mid throttle,and briefly at open throttle. Then ride it and see how it runs. If not too much power at wide open,it's too rich. If it pings(like a marble in a can) it's to lean. Ream it ,and then shut it down and look at the plug. It should be a dry brown color. Change the main jet accordingly,too rich go down 1 size,too lean go up 1 size. Then do it again. After that,you can fine tune the wide open with the needle clip on the throttle. If mid range isn't good then,you may have to get a different needle. In that case you need the table in the service manual to determine which one to try. The idle or pilot jet usually is ok unless it fouls while idling,but mx bikes aren't made to idle. Tuning is trial and error,and educated guesses.Sometimes you try something and are surprised at the results. These are the basics,take your time,and you will be fine. Good luck.
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JW381
Posts
10643
Joined
7/21/2009
Location
Harrisburg, OR US
7/23/2010 9:44pm
Damn, that was good stuff. Thank you for the lengthy explanation!

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