Hello,
I have been trolling around a wile and lots of great info around here. Thank you !
The problem i having is i picked up a 1986 lt250r. The airbox has been removed. It ran great before the airbox was removed but now wont start? Jetting? Where do i start? it is the factory mikuni carb.
Thank you
From Bartlett Racing...
"OK get a lot of questions on properly setting up carbs. Here goes.
In order to properly jet a carb you need to first get it to start. Start by dividing bore size of carb by 1/16 and 1/8 these #s are your starting points. Set your slide height as close to the 1/16 as you can. To do this the cable should have slack in it and you use THE IDLE SCREW to get the slide height correct. Then adjust your air screw to 2 turns . Light it up.Once its starts determine if its rich( wont stay running) or lean (engine races).
At this point adjust the pilot size accordingly to what you find, if its lean go bigger, rich smaller. Re try until the bike will idle somewhat once it runs dial the air screw in and out it should raise and lower the idle. Get it to run properly with no surging.
Once the pilot is set correctly move on to the needle and brass ( mikuni )
Keihins are only needle adjustable they have no brass emulsion tubes for fine tuning and in my opinion are a inferior design to a carb with them. So you have a good idling machine now take it for a ride holding it at 1/4 throttle. Listen to what it does, a slight up grade is best for this never, do it down hill.
Does it surge, race or load up. If it loads up lean it out, LOWER THE NEEDLE BY RAISING THE CLIP. If the engine races and revs by itself its lean so RICHEN the needle.
Repeat this for 1/2 and just under 3/4 throttle. Write down what it does at each throttle setting. Be sure to hold the throttle constant., if it runs away pull the choke and kill the engine and immediately richen the needle. Before re trying. If you can't richen a needle enough it may be necessary to change the emulsion tube. If the needle is good in one or two positions but not the third it may need to be changed to differnt profile.
Once you get the needle dialed in re check the pilot circut. If it still good check your main.
To check the main, you need to do a Wide open run shifting through the gears at Max rpm. START RICH AND WORK DOWN. put a big enough main in the machine to choke it, so it won't rev. Now work down from there one or two sizes at a time untilled it starts to pull clean and rev out on top. This will be the richest main for that given condition.
Now replace your plug with a brand new properly gapped. 020-.024 " plug and make a couple hard passes. Then do a Wot plug chop. Note the color or lack of where the insulator meets the threaded base. No color means its lean black sooty rich. Differnt riding conditions will dictate what you want for color and is for a completely differnt discussion. General riding likes a dark brown to just black. Have fun riding."
I've used this method and it worked for me on my 87.