Looking for fresh opinions on a bit of a headscratcher I'm working on

[RIDE-RED 250r]

I'm working on my '85 ATC110.

I just replaced the valves and lapped them. Carb is clean and valve lash set at .005" on both valves.

It's running MUCH better now and has more power than it ever had since I got it. The valves were blowing by so bad the intake valve had carbon buildup on the back side of it! LOL!

So now my head scratcher: It starts much better, and pulls much harder now... But, it has somewhat of an unstable idle and when putting along, easy on the throttle in low range in just about any gear it will begin to stumble and skip and lose power. I have flipped the choke on when it does this to see if it was stumbling due to fuel starvation but choking it only makes it worse. I'm about 95% sure its right in the pilot jet circuit because all you have to do is hammer the throttle and it recovers and takes right off. I have tried dialing in the idle mixture screw and idle speed screw and have not been able to adjust this problem away.

It just seems like its too rich on the pilot jet to me for whatever reason. I don't recall what size pilot jet is in it and had to call it quits last night and did not remove the carb yet. I did find out its supposed to have a #35 pilot jet in it for stock spec. I dont have any jets for a Keihin PB carb kicking around so I went ahead and ordered a new pilot jet already. Funny thing is, #35 is the smallest pilot jet available... I'm hoping to find a richer pilot jet in this thing when I get the carb off... If not, guess I'll be hoping somebody took a drill bit to it! LOL!

I'll be checking ignition timing too, I have to get my timing light back from my Dad.. Tore the garage apart last night looking for it! LOL!

[LastFoolerInVA]

From what your describing it does sound like it should be the air screw..... however; it may also be the incorrect or damaged needle on the throttle slide... the air screw, pilot jet & needle are the only adjustments that affect fuel delivery at low rpms... if the jet is correct & playing with the screw doesn't do anything, I would get another needle, or at least check the one that there...

[yaegerb]

Hey Joe,
so all you did is clean the head/valves/carb? Did you install and oversize piston, cam, etc? If not and everything is stock then my first step would be to set the carb up stock, the way it was from the factory, along with the idle mixture screw setting. 0-1/4 throttle position is pilot so to me it sounds like the pilot in the carb is too fat.

That being said....if you set everything back to stock/set up per factory specs then my next look would be at the o-ring that sits on the idle mixture screw. Typically those lose their ability to seal after long periods of time and will wreak havoc on your carb adjustments. I would look to that next if it doesn't run correctly for you.

[RIDE-RED 250r]

The engine has a .030" over piston in it and brand new valves. The old valves were shot. Seats didn't seem too bad, and I lapped the new valves in. I should have had the valve seats ground to make the valve job "complete", but I didn't do it this go round.

I'll check on that o-ring. Matter of fact, at this point I don't think it even has one.

But yes, the problem is isolated to idle and just off-idle. Once you begin to tap into the mid-range (slide needle and main_ it cleans right up and runs great.

And like I said, I tried flipping the choke on half choke while it's sputtering and it only makes it worse. That's why I think it may be rich on the pilot jet for some reason.

Also, I believe the idle mixture screw is a fuel regulating screw as it is between the slide and engine. I have tried every position wit the mixture screw from just cracked open to over 3 turns out and have not been able to tune this problem away.

Regardless of what pilot jet is in it now, I will put the new one in once it gets here and see what happens....

[LastFoolerInVA]

air screw=idle mixture screw.... my bad....2 stroke on the brain....

[TecateDan]

Had a similar problem with my Tecate for two years and it wound up being a worn slide. It was leaking air and you couln't jet the bottom circut no matter what pilot jet. New Kiehin carb and it ran perfect with in 20 minutes.
Old carbs and knock off carbs suck



I'm working on my '85 ATC110.

I just replaced the valves and lapped them. Carb is clean and valve lash set at .005" on both valves.

It's running MUCH better now and has more power than it ever had since I got it. The valves were blowing by so bad the intake valve had carbon buildup on the back side of it! LOL!

So now my head scratcher: It starts much better, and pulls much harder now... But, it has somewhat of an unstable idle and when putting along, easy on the throttle in low range in just about any gear it will begin to stumble and skip and lose power. I have flipped the choke on when it does this to see if it was stumbling due to fuel starvation but choking it only makes it worse. I'm about 95% sure its right in the pilot jet circuit because all you have to do is hammer the throttle and it recovers and takes right off. I have tried dialing in the idle mixture screw and idle speed screw and have not been able to adjust this problem away.

It just seems like its too rich on the pilot jet to me for whatever reason. I don't recall what size pilot jet is in it and had to call it quits last night and did not remove the carb yet. I did find out its supposed to have a #35 pilot jet in it for stock spec. I dont have any jets for a Keihin PB carb kicking around so I went ahead and ordered a new pilot jet already. Funny thing is, #35 is the smallest pilot jet available... I'm hoping to find a richer pilot jet in this thing when I get the carb off... If not, guess I'll be hoping somebody took a drill bit to it! LOL!

I'll be checking ignition timing too, I have to get my timing light back from my Dad.. Tore the garage apart last night looking for it! LOL!

[RIDE-RED 250r]

Had a similar problem with my Tecate for two years and it wound up being a worn slide. It was leaking air and you couln't jet the bottom circut no matter what pilot jet. New Kiehin carb and it ran perfect with in 20 minutes.
Old carbs and knock off carbs suck

Thats a very good point Dan. I am using the original carb. The slide and slide housing seem to be in good shape visually, but that doesn't necessarily mean it isn't sucking air.

When I get the new pilot jet, I'm going to borrow my brother's fancy meter that can measure RPM's with the amp-clamp on the plug wire and along with a timing light and set this thing up by the book. If after all of that it isn't right I guess I'm going to have to look into a new carb.

I did putter around with it more since my last post. I backed the ignition timing off a bit and that seemed to help quite a bit. But at the moment im only out about 1/2 turn on the idle mixture screw which seems indicative of a rich condition down low. I ran it a few laps around the yard at that just off-idle RPM and the plug came out black and sooty. I'm still leaning toward being too rich on the pilot jet...

Thanks for the replies thus far guys. It really helps to have fresh ideas from other people when something is giving you fits...

[82 250r]

idle or off idle is the area where a pesky vacuum leak will be the most obvious.
That o ring might be the cause.

[RIDE-RED 250r]

I just looked up the fiche for the carb on Bikebandit and it shows no o-ring.... And I don't think Keihin makes the PB carb any more.....

[yaegerb]

Interesting....I could have sworn there was an o-ring on that carb.

[RIDE-RED 250r]

Interesting....I could have sworn there was an o-ring on that carb.

IDK, the way the fiche on Bikebandit shows the breakdown it looks as though you would have bought the mixture screw as a set.. (pretty sure it's NLA now)... But in the diagram I can only see the screw and spring. Did you check it out?

Just for the heck of it, I pulled the screw and put a little bit of Threebond on it just in case for the time being..

[82 250r]

Don't know if this is your problem...but while were on the subject...

When i clean my carbs and then blow them out with compressed air, bubbles and carb cleaner always come out the air screw threads. I figure that could be a source for a vacuum leak.
I now put a small amount of Loctite thread sealant on the air screw threads to be safe.

This is what i use because it doesn't harden and i can still adjust the screw.

http://i282.photobucket.com/albums/kk261/bmwic155/P1010159.jpg (http://s282.photobucket.com/user/bmwic155/media/P1010159.jpg.html)

[86T3]

I was messing with a 125 m carb this winter, couldn't get the thing to run right cosistently. I finally looked on dratv and found that carb kits are ridiculously cheap. http://www.dratv.com/cakiat84.html.

They come with a new pilot and main also, maybe you could return the pilot you bought if you havent used it yet