carb gurus

[fxnteeth]

OK guys, I have an 85 ATC 250R that is bored to 260. It has a keihin 38mm airstriker, I think. New motor plugs not too bad. When I hit top end it sputters. I started with 180 main and have worked down to 160. It has gotten a little better, but I feel this should be getting too lean. Not sure what pilot or needle. To remove the needle I was told to unscrew the bolt that is in a spring on top of the slide???? I see the bolt and spring but have never seen a carb with this. Someone please tell me what to do? I have an fmf fatty with the fmf silencer, no air box just filter with cover. I need a starting point.
thank you

[Slingblade]

http://www.3wheelerworld.com/showthread.php?55086-Carb-Jetting-101-Terms-Tips-and-Jetting-Theory&highlight=jetting

^^Read this thread first. Lots of plug chops. I'm not sure what kind of keeper the pwk has, some carbs have screws and some have retainers that twist out of a slot, you'll have to remove the slide and pull the spring up in the cap, remove the cable and see. You're probably in the ballpark as far as a main size, but every machine is different, and lots of things to consider: reed type, mix ratio, temperature, etc. Here's a good trick I've found once your close to dialing it in. Ride your bike in the coldest part of the day, if it runs better, your on the rich side, if worse, it's lean of course. Hope this helps.

[fxnteeth]

I read the post. I am scared to run below 160, because with all the mods its seems I would be getting more air and need more fuel. Just scared Im running to lean. What else could cause the sputter? 1:32. 110 Octane.

[Slingblade]

The best thing to do is get a new plug and do a wot plug chop and read your plug. I think a stock main jet for a 250r is in the 142-145 range so your a bit richer than that already. Make sure your carb is clean, every hole, every jet. Make sure your petcock flows freely. Check your float height. Make sure it's getting a good supply of fuel or jetting won't help. I've learned that a rich or lean condition can cause a sputter, sometimes you can just "feel" if it's sputtering like it's running out of fuel (lean). Or if it's a bogging and sputtering (rich). Main Thing- Check your plug. I hope this helps, Maybe someone will chime in that has better posting skills than me.

[Tecate 50]

For starters you need to know for sure that you have an Air Striker carb. A 160 jet seems to be a bit small, but thats my opinion. Me and my buddies all run that carb on our Tecates. My current motor is a 256 cc cylinder with port work. I am currently running a 172 main jet and have been as high as 178 in the winter temps. My advice to you would be run a bigger main jet, just to be on the safe side, then work your way down. Secondly, check your needle position.....32:1 is fine. 110 octane might be a bit much depending on your cylinder pressure!

[fxnteeth]

Thanks for the response. How can I tell if it is an airstriker? There doesnt seem to be too much difference from the 180 main down to the 160 main. I want to remove the carb, clean it and maybe start over. This does have porting and is bored over. Can some one give me starting points on the needle, and pilot also. Something must be way off. I cant get a full wot run because it begins to sputter no matter what main I use.
thanks
Jarrod

[fxnteeth]

Update. When I pulled the unifilter off of the carb, I noticed that the rubber part of the filter had a tear around the clamp. Could this cause an unusually lean circumstance? The clamp did seem to be over the tear
Jarrod

[Slingblade]

You probably need a new filter but I don't think thats your problem, if it's leaking on the intake boot side, then it could cause a major lean condition. But you could have very easily sucked some trash in your carb if it is leaking. How does it idle? But most importantly, What does your plug look like?? Have you changed the plug? I would clean the carb, put in a new spark plug, start at a 170 main and go from there. I can't help you with the carb id, I'm running a pj. It is a Keihin and says pwk, right? As far as if its an air striker I don't know the difference, sorry. Do a search on what machine came stock with that carb (250cc mx bike), start with the stock pilot and clip in the middle. I know it gets frustrating, Hope this helps. I know someone on here runs an R with that set-up, maybe they'll help.

[nd4speed]

i run a 165 main on a quad vent 38mm Keihin (Air stryker?) on a Kx250 in any weather 50-90 F at about 1500ft above sea level. It's still a bit rich

So a 160 main or even 158 is not out of the question. I think it came with a 170 but it was too big. Every motor I bolt it to fires right up.

A bigger bore and a pipe often causes the bike to run richer.

[Just-Tri-It]

As already stated start with a new plug and do the chops. I have a Tri-z with the wrench report after market exhaust and a Keihin 38 running a 160. There are pics of plugs on some of the threads to compare your plug. It's also possible it could be electrical but if you start with the plug that would be best. Even though she sputters you could still run her a bit then get you a straight away toward home and open it up working the throttle a little and run it wot to home and clutch it and kill it at wot. Coast her in and pull the plug. If she's wet and on the dark side you are probably still in the safe zone on the richer side as oppose to lighter and dry and on the lean side. But every motor is different to a point. You are probably close. Make sure there are no leaks in the boots as you mentioned earlier.
Now a thought after you do this. You will have to be careful with this step but if you can get your hand on the choke or remove the air box cover but leave the air box and tie a string or small metal wire to the choke and get her going rather well and give it throttle and pull up on the choke which will give more gas and less air. If she is already running rich this will make it sputter even worse. If she sputters worse then you could try coming down to a 158. If there is no change when you pull the choke then it is probably air leak or electrical. But these are just suggestions that have to be thought on and used carefully and in small steps. You will need aftermarket exhaust also for all this to come together properly and I don't know if you mentioned that in the post.
Others have gone through this as well and its not a quick process unless you luck up and hit the right combination on the first or second try.

[bigbadktm]

i was running race gas on my stock ktm with an fmf pipe and it ran like crap, sputtered and all that. it could just be your gas, try some 91 or 93 octane whatever your area sells. mix a gallon of that at 32:1 and try it with your 170 main, needle clip in middle position, cant tell you what pilot I have never messed with mine

[nd4speed]

i had a wire that got cut on the radiator and was grounding out causing a loss of spark and a bog and bad gas mileage.

I changed the reeds because of that wire.

[fxnteeth]

Thank you guys very much. I will begin this weekend with all of your suggestions. This site rocks. Thank you again
Jarrod

[fxnteeth]

Another question. The bike idles great. I rarely choke it to start and it starts on 1 to 2 kicks. If I pull the choke, it begins revving very high. What does this mean from a carb setting standpoint
thanks