Thumpinhard25

Update:   No solution was found... not the greatest outcome but I wanted to update this thread.  I sold the Bear tracker as it was.   I will try to update the original post to state that to save people looking for the same problem in the future time. 

So here's the challenge. I have 7 days to make this ( never ran right) ATV and get it running, so that I can't take it to Colorado on vacation next week.  I also have to work 6 of those days.... So got some work ahead of me.  My next step is to pull the stock carb, and rip all the jets out of it, swap them into the ( new) zoom carb.  If it still doesn't run right, I should know for sure that it's not a diaphragm issue, or carb issue and more then likely electrical. We will see what happens. 

22 hours ago, davefrombc said:

Have you had a chance to try  another stock  carb on it ?. It  still sounds like a  lean burn problem  off idle.I'd look again  for a clogged  passage or a  small  missed drilled  hole  in one of the jet tubes .  Make  sure  float  level  is right  and  no  crud is coming from the tank to possibly restrict fuel  flow ..  Is the diaphragm  good /new and working as it should?  Is the  spark  plug  new  or known  good ? Some plugs  can be fine at  idle  but  under the higher pressures  when  throttle  is opened can  cut out  or  be  intermittent . 

I tried the zoom carb, and it wasn't right. Rebuilt stock carb was good " for a few mins". Im going to stick a new plug in it again. I tore the carb apart again yesterday, triple checked and all passages are clear. Float is set at 13mm. No fuel filter on tank right now, good flow from tank to carb. Throttle slide boot had no visible holes, or rips, same with the side rubber enrichment thing. what are the symptoms of a bad CDI?  

Well I finally found some time and I'm back at it again.  I went back to the stock carb, ditching the zoom zoom it had on it. ( stock carb had a stripped out pilot jet) but that's been removed and entire rebuild kit had been done back to factory settings. I put it on and it ran fine, for about 10 mins. My wife then rode it around the yard and said it did fine. I got home, cranked it up and it started fine, idles great, but as soon as you get on the throttle it wants to die. Seems to be mid throttle.  Pulled all the plastic and don't see any shorts, disconnected the brake safety switch, no change. It already has a new intake boot, but sprayed around all the carbs and boots with starter fluid and no idle change. has a clean plug in it as well.  

Man this is a great thread. Thanks!  I am having the same issue ( except it doesn't idle with choke off, idles up 1/2 choke, and revs high full). I haven't had time to mess with it but I'm about to go home and check it out again.  The orignal owner stripped out the pilot jet in the factory carb so it can't be removed, but it had a "zoom" carb and both carbs acted the same way.    However I haven't put a fresh plug in it. 

Sorry for the long update, since I posted this I blew the transmission out of my race bike, which ment tearing down the 450, while trying to rebuild my camper, and getting ready for a hard enduro competition on the one weekend that wasn't a race weekend. I have yet to mess with the bear tracker, however I did take it to the last event and it basically stopped running. Stumble is so bad it barely has power to pull itself, choke in, 1/2, out doesn't matter it wouldn't run.   So I'm hoping to get around to tearing it down in the next week or so, I will still update this page when I figure out what's up. 

Is it a mechanical jerk? Or a acceleration related jerk? Do you feel or hear something grinding or poping, or does the engine have quick losses of power? 

46 minutes ago, davefrombc said:

I'd  check  for loose or corroded  connectors  in the ignition circuits  first since it started  after bouncing  around .  Check the  voltage   from  rectifier  to  battery under  idle  and revved up. If  you  can  borrow  a compatible CDI  box,  you  could try  swapping it  out.  There  is the service manual  for the  1998 YMF250  in the  downloads section of the forum .  It will  have all  the  resistance readings  for  the  ignition  components should you  need to  check  the  stator,  pickup  coil and ignition coils. Most  likely  the  readings would all be the same  for  the  2001.  Very little  changes  from  year to  year for the  same  series  motors.

Check  components  cold  and  warmed up. Sometimes   they  break  down   when  hot   but are fine  cold.  One tool  that  isn't expensive    but is  often  overlooked  when  suspecting  spark  loss  under  load  is  an  inline  spark  checker to see  if   the  plug  is  firing  properly  under  load.

Here  is an example of the  spark  checker https://www.harborfreight.com/90-in-line-spark-checker-69023.html

Man this is great advice.  I'll pick up a spark tester on the way home. Now that I'm back at the shop ( and not in a field) I'll pull the plastic off and check all the connectors. I do believe I have a bare wire showing at one connector where the insulation has pulled back at the plastic.  I don't  think it's shorting but I'll double check.  The bouncing around and randomness of the sputtering is why I believe it's electrical as well. I was reading about some "parking break" governor or something. What exactly happens when the parking break is engaged? Is it possible to disengage it completely to ensure its not a faulty switch? Also I'll keep this thread up to date until I find the solution, for any future people to read.  

Well it's random. Sometime it starts cold and it's fine. Still has a little hiccup when I crack the throttle from idle, even with choke 1/2 out.  When it started running real bad ( as in wouldn't pull itself) it did it cold and hot.  I tore carb down and cleaned and it still did it. 

First thanks for any help guys, new to the forums and it's gotten me this far. I bought a 2001 bear tracker 250 for my family to get around the dirt bikes races. It wasn't running right and had a high idle. Previous owner thought it was a carb problem, so he ordered a new Zoom carb and new intake boot, but it still had the same high idle. ( I believe he had the throttle cable adjusted wrong, because it idled fine when I put it back together) So I tore down and cleaned the stock carb, and it started up and idled fine, but needed to have the choke 1/2 out to run.  I swapped to the new zoom carb and had the same issue. Choke half out bike ran ok. I played with the needle and the pilot screw with no change. ( bike did seem to be running rich though, but didn't get better when I tightened pilot screw or raised meddle) I Took it to the races and once I started bouncing around in the cattle fields the quad started cutting out real bad. It would barely run.  Sometimes it would run with choke off, sometimes choke 1/2, sometime choke full. I cleaned carb again, checked float level, and boots, no change still wouldn't run. Then suddenly it started running ok again.  This makes me believe it's an electrical issue rather then carb. I know there's no air leak, it's been checked, both carbs are clean( I've swapped them out a couple times, it starts up immediately and usually idles fine. But then bogs down/ or stutters under throttle. I saw the post on the guy who disconnected his rectifier and reconnected and suddenly it started working again, which lead me to believe it may be electrical and not carb.  So any advice on what I should check? Any way to test the CDI or rectifier?  Once again, I appreciate your help.