Hey fellas,


It's been a while since I've been on the forum (been laid up recovering from another back surgery). Anyhow, I've got another project I'm trying to sort out for an older gentleman neighbor of mine. It's a 2002 Yamaha Bear Tracker 250. It cranks nice and easy, idles good, etc. But when riding if you get above say 1/2 throttle (doesn't matter which gear), the thing will split/sputter/hurk & jerk. You let off the throttle a little and runs good. Air filter looks brand new.

1) Easy things first, disassemble & clean out carb (he swears he only used Pure gas ethanol free) = No change.
2) Ordered kit and did a complete carb rebuild (including float, float pin, air valve, and all internals) = No change.
3) Looked into spark side next. Although all looked good, I replaced the ignition coil and plug and ensured all were OEM items = No change
4) Checked valve lash, timing chain, timing, compression, adjusted to all within specs = No change.
5) He even had a new carb in the box (not sure if it's OEM or not, but I even threw that on there at some point) = No change.

What am I missing? I've done hundreds of carb rebuilds/adjustments, vale jobs, top end rebuilds etc over the years.........Stator is my next option, but I'm still looking for the specs to try and ohm it out/test it out.

Any other thoughts are anyone know the specs I'd be looking for when ohming out the Stator/testing it's output?

I'd really like to get this ATV going for the old fella. These old guys talk keep my busy with these type of projects. Up until now, I've always been successful getting their rigs fixed no matter the make/model/problem.

Thoughts?

Roger

 

Wife had the same make and model, great machine. She had a similar problem creep up at one time and it turned out to be a leak in the rubber intake. There was a crack in the rubber and it idled great but the crack would open up under acceleration and cause it to spit and sputter. Had a friend with the same machine and he tried to up the performance by opening the airbox for more air to come in. Same problem as you describe, idled great but would not accelerate, sealed the airbox back to original status and all was well. Don't know if any of this will pertain or not, just some things to look at.

 

Thanks for the info. I did forget to mention I also replaced the carb-to-engine intake assy with boot because someone had wrapped it all up in tape and work clamps. I didn't really look at the air box too much though. I'll certainly have a look at it when I can get a free minute to tinker on it again.

Now that I have the correct NGK plug with resistor I'm going to run it some and pull/inspect the plug end....that should give me an idea where to go as well. Old plug wasn't the right one someone put it at some point.

Thanks for the input and y'all keep 'em coming.

Roger

 

Have you checked the voltage at the battery with engine running? If it goes over 15v due to a faulty regulator, it can upset the CDI, this tends to happen as revs increase because the alternator is producing more power.

 

Finally had a chance to look at this one again (on a question with 3 other golf cart/at jobs. Regulator seems good, holding steady @ 14.55-14.60vdc. trying to find Stator ohm specs now,.but internet has been down. I hate trying to search online for stuff like this on my phone.

Found specs, looks like stator. Ohms changed drastically after getting the atv warmed up and then retesting. Got it on order and will update yall with results once I do the R2.

 

Well, after some lost USPS shipments and other delays, I finally got the new Stator Assy in this week. Got around to changing it all out yesterday. Cranked right up, rev'd up fine, jacked the rear off the ground and went thru gears, no problems, no skipping. Drop it back on the ground and take it for a short run........exact same thing still happening. Above say half-throttle it's like it's losing spark or something. Yet I can only duplicate it under load and not with rear wheel in the air.

So, what is next. Let's recap:
- cleaned and rebuilt carb
- new spark plug, new coil, new stator assy
- new fuel lines and filter
- compression, valve lash all checked good
- Voltage regulator and battery check good
- new air intake boot, cleaned air filter, ensured no air gaps/cracks/etc in filter box

Am I missing something? (CDI or Voltage Reg) I hate to give up on it for the old man I'm trying to fix it for. (He's already had 2-3 different people mess with it before me). But I also don't want to be throwing hi money away on shotgunning parts. Usually when I get my hands on something like this it ends up being something simple stupid in the end.

 

If you are getting a steady 14.6v and nothing over 15v when you rev up, there is nothing wrong with the regulator, (nor with the first stator for that matter). That leaves the CDI and carb. If someone has messed with the carb, fitting wrong jets etc, they could have upset it so badly that cleaning won't help. With the CDI, it is a lot of money to throw at it with no guarantee that this is the cause of the problem, but so was the stator. Double check that all wires to CDI are reading what they should before going for a new one. Some 250 Yam CDIs are on eBay at the moment, dirt cheap Chinese copies, quality is questionable, but if the fault goes away when you fit it, you have found the fault.

Just had another thought, some Yams have a rev limiter when in reverse, could the CDI be thinking you are in reverse and limiting the revs going forwards, due to a faulty sender unit in the gear selector mechanism?

 

Agree 100% and thanks for confirming my thoughts on CDI. I replaced the stator due to bad ohm readings previously. It would read good resistance when cool, but after running, the white/red stripe and white/blue stripe wire off the stator would read open at times and if I did get an ohm reading it was 150 ohm difference from when it was cold or hot. Who knows...maybe one is leading to the other. And with several people messing with thsi thing before I got my hands on it, who only knows what they did....lol

I did rebuild the carb after doing a cleaning initially (and ensured correct jetting). I've found that more times than I can count (from when someone brings me something to fix and first words are: "my buddy looked at it and did....etc, etc, etc") You all know how that goes...hahaha

Thanks again for the advice. While I did ohm out and visually inspect CDI to stator wiring, I may give the cheapo CDI a shot first. I'll eat that cost in the end. Seems the next logical step though.

Rev limiter? I may have to look into that as well. Had a golf cart that ended up being a Rev limiter issue sorta similiar to this a while back as well.

More to come

 

Well, after receiving not 1, but 2 faulty CDI's over the past couple weeks, I finally got a good one (OEM, not China knock-off). I gave that a whirl once with no luck.

Problem still exists. I've gone over everything again. I have some Yamaha ATV diagrams, but not for this one in particular. I'm now firmly believing what merryman suggested, seems something is going on with the Rev Limiter circuit. I only see one switch at the bottom left of the engine (just behind the foot shifter). That is the Neutral switch. I isolated that after starting (won't start without that grd signal) and no change. I thought perhaps that grd may have been dual purpose. So, going with this theory of the Rev Limiter circuit. Any idea of where to start (which wire to the CDI)? I"m not seeing any wires at or near the Fwd/Rev handle or foot shifter other than that Neutral wire.

Thanks Fellas. This one's kicking my tail for sure. Got my other working backing up daily because of it.

 

Found a wiring diagram, and it shows the reverse light as just a switch to earth and a pos feed to the bulb, no connection to the CDI. What the diagram does show is a mysterious "Controler" teed into the orange CDI to coil wire, for an electric choke, however the title is ambiguous as to if the diagram is for a Bear Tracker or Bear Tracker Clone. To be honest, with all the new bits, you have eliminated almost everything except wiring, I would get a correct wiring diagram and trace every wire that goes to the CDI. I tend to use a test light where possible, as I have been fooled before, getting voltages with a meter which have subsequently proved to have no power to back up the volts.