I agree with the tank cap. If its sputtering and backfiring, ...its running lean. Next time youre out riding, lossen the cap evry 15 minutes or so, and see if the problem disappears. The other vent, is the tube on the bottom of the carb, and if this is plugged , itll have the same symptoms.
Next time it dies, loosen the screw at the bottom of the float bowl and see if any gas is coming out of that tube. If you dont have gas coming out, then thats the problem, ...tank vent or float bowl vent.
The stator output varies with engine rpm. At idle, it may be right around 12.6 volts. By 5000 rpm it should be 14.5 or less, but more than 12.8.

 

The last two posts make the most sense to me as it takes time to use enough gas to create a vaccuum in the tank from a plugged vent. I've had this problem with a rented Honda. Like they said, try cracking the gas cap if the engine hesitates. If that is the problem, you will know right away.

 

tank vent would give the performance problem....but why would that make it crank slow ? you guys lost me there.

 

Could be two unrelated problems. The dieing of the motor and the slow cranking.
Its an easy thing to rule out the gas tank vent, first , and then move on to other possiblities.

 

I still think it's going to be a low voltage deal. Low coil saturation do to low voltage from the battery, then starts to cut out, slow to crank, it's text book.
Kinda need more info to be sure. Are the lights dim when this happens? Need to put a volt meter on it when it does it to be sure.
Like Hondabuster said, easy to check and then go on.

 

Not sure about lights, I dont ride after dark anymore. I hope to have time to test it this week (weekend for me) and give some more insight.

 

I gave up, and decided to give the dealer one last warranty crack at it. He is thinking that maybe the volt regulator fried the CDI, and something inside it is failing after it gets hot. Im thinking the stator wire is coming loose under throttle due to vibrations. I unhooked the winch, battery tests good. I ran it with the headlights on and they got pretty dim as the engine started to sputter, that screams charging system to me! We will see what comes back and I will report. Thanks for all the help so far.

 

400PrarieXLT, are ya running without the battery? Even running with e dead one if ya hafta pull start the beast.

 

No, It wont run worth a poo at all without the battery, ive tried it when my stator wire broke last time. This time it will run good for sometimes 30 minutes at a time, sometimes only 10 minutes between sputters. Im thinking about the BF 650 or P700 though....... It might be trade in time depending on what they tell me.

 

I would be interested to hear what you find out the cause is !

I have the exact same thing happening to an '86 Honda TRX-125 Fourtrax, that would not run if cold, only if kept in the garage, and only for a short time (10-30 minutes).

I pulled the CDI, Voltage Regulator-rectifier, and coil, and tested them according to my Clymer's manual's specifications (resistance at various points in certain polarities), and according to my manual, all three devices were "out-of-spec". Although the coil fires and I am getting spark, the real tell-tale sign was my dry-sooty spark plug, which indicates a weak ignition. My issue is that I am trying to locate after-market equivelent parts, as this ATV is so old, many manufacturer parts are discontinued or used parts are scarce or of probable similar condition. So I am attempting to reach Shindengen directly for the regulator and CDI to find out if I can still buy them. I already have done Carb rebuilding, Valves adjustment, Compression Test, and new battery, all of which made it run better when it did run, but then it just dies.

I may have a lead on cheap after-market CDI ($69), and Coils ($25), and if the universal regulator works ($46).

Sincerely,
-CPUjohn