Ok 250r gurus here is the deal. I have a 88 250r bored .020 over, mild mx porting, ers cool head large dome, curtis sparks pipe and silencer, Boyesen reeds and Keihen 38mm air stryker carb 180 main jet, 50 slow jet, needle in center slot BR8ES plug. I just rebuilt the top end and when I got it back together and started it up it idles great but as I begin to ride it to heat cycle it I noticed that it begin to cough and sputter right off the bottom. I thought that maybe it was the oil from assembly that was causing the problem so I went ahead and heat cycled it a few more times. I then changed the plug and got into it. It still ran rough off the bottom, but the real problem begins at about 1/4 throttle and goes through 3/4 throttle. In that range it sputters, bogs and backfires, usually you can't get it past this range, but if you can get it to wide open it screams, it also continues to idle perfect. Plug always reads carbon fouled to me but a new plug never makes any difference. I have checked all of the jets several times. Also have a new cdi, spark is now as good as it gets on a trx. Any help would be appreciated.

 

Try moving your needle clip up 1 notch...

 

Omg. 180 main. I'm running a 185 in my cr500 punched. lol way too hi. for a stock ciclinder bike. I have a r set up almost the same. 35mm keinen 168 main 48 pilot jetted for sea level. so ther is a idea of what you should be close too. depending on you compression and elevation and temp, your jeting will be effected. My bike are jetted at sea level or around 400 ft above. ( mostly for glamis) my friend has a 89 mild motor stock cilinder. 220 psi and jetted at 162 48 pilot. might even drop the pilot one on his bike cause it was smoking on the bottem end a little. I dont know what your elevation is but 180 is way too hi. too be safe drop to a 170 and work yoiur way down, with plug checks, dont forget about the air/idle screw. hope this helps. keep posting this thred untill you get it. I will walk you through it. def drop your pilot as well to a 48. try 170 48 and re post with a plug check. late.

 

Thats a little high but not CRAZY high... im running a 170 in my bike, cool head, boysen reeds, pipe, drag port, 39mm carb.. I am running a TAD rich but I like it that way, it doesnt sputter unless cold. I should be running a 165 if I was running it on the fine line. If I were you, buy jets from 165-180 and jet down intill it cleans out.. But i'd adjust the clip as mentioned (though i am not sure if its right.. i dont know how to adjust those)

 

In that range, it would be needle position.
Up to 1/4 throttle would be the pilot.

Try leaning out the needle position if it is indeed backfiring.
Sometimes porting will make a motor more efficient, and leaning out the mixture is necessary.[img]i/expressions/face-icon-small-wink.gif[/img]

 

***** is right, the problem you are describing, you are not even into your main jet. Check out:

www.carbparts.com for a good explanation of jetting. Click on the Keihin link, then click on the Carburetor Tuning link, then jetting your Slide Valve Carburetor for a good illustration.

 

Thanks to everyone for all of the helpful suggestion. I seem to have the problem solved although I came at it through the backdoor. I put the 19cc dome in the cool head and used a thin head gasket to up the compression and bought some 108 octane fuel. Now the quad pulls like a tractor off the bottom and screams all the way through the top. Plug chops look great (paper bag brown) This is the setup that works for me.
1. Coolhead and head gasket like I described above
2. 182 main jet
3. 50 slow jet
4. Needle clip in the middle slot
5. Air screw 1 1/2 turns out