I need some help, I have been pulling my hair out with this thing since September. Resurrecting an old 1989 Yamaha Big Bear 350 that has sat for the last 8+ years since it was last started at my parent’s house. Unfortunately it had fuel in the tank all this time and the petcock on so, tank and carb were completely gummed up. The float must've stuck open too because when I drained the oil before even attempting to start it, about 5 qts of oil/gas mix came out of the crankcase. The ATV is completely STOCK, exhaust, carb, etc. Also, it has lived a very gentle life, originally my grandfather's he babied it and never ran it hard. While it has lacked periodic maintenance this ATV has never been abused in regards to it operation. Here is what I have done:

1) New oil and filter.

2) New battery and solenoid.

3) New petcock and in-line fuel filter (post petcock, pre carb)

4) New spark plug (gap set within spec of 0.024 to 0.028)

5) Adjusted valve clearance (intake within spec of 0.002 to 0.004, exhaust slightly out of spec of 0.006 to 0.008, adjusted accordingly). Also inspected to make sure timing is correct.

6) Temporarily patch 3 each 1/4 inch diameter holes in stock muffler (Used JB Weld Extreme Heat and fiberglass roving cloth for reinforcement, temporary repair to rule out any exhaust leaks causing issues, will eventually buy a new muffler)

7) Extensively cleaned the fuel tank until spotless before installing new petcock and inline fuel filter.

9) Cleaned the carb and installed all new O-rings, seals and jets that are same as stock jets from carb rebuild kit. I even replaced the square-cut O-rings on the needle jet plastic slide body that you can get to when you remove the torx tamper-proof screws. Thoroughly cleaned carb with solvent, scraped out all junk that had built up in passages, float bowl, etc. Blew air through all passages, etc. It is SPOTLESS and I have since re-cleaned it just to be sure an additional 5 times. Finally just bought a cheap aftermarket carb from amazon to see if that would help and no difference.

10) Been running 91 octane gas and have changed the fuel a couple times in the tank and I’ve also run sea foam thru it.

11) Inspected and cleaned air filter.

12) Inspected intake boot, no cracks not even dry-rotted at all.

13) Installed a new starter, the old one had a broken spring for the brush.

14) Compression is 120psi with engine cold

15) I’ve even put a new ignition coil on it with no change in behavior.

So here is what my issue is:

It started and idled just fine for the first month or two but now starts to miss even at idle. Press the thumb throttle and it will build revs if you baby the throttle but eventually starts to miss and then back/after fires, mostly after fires through exhaust. When I hear the missing I let off the throttle and it goes back to idling just fine however the last month or so even the idle has gotten rougher. Only if I quickly goose and hold the throttle will it miss, after fire and then die. Of note, of the 40+ times I have fired it up it has ran FLAWLESSLY (good power, cruise around the property, pulled a trailer, drug some logs, bury the throttle) 5 TIMES. Never back-to-back. Weird.

Anyways, I'm at a loss. Based on what I have done air and fuel should be fine which points to the issue lying in the ignition system. I have pulled the spark plug multiple time to get the fuel/air mixture right. Currently I have the fuel screw 1-3/4 turns out and a spark plug that is a light brown on the tip (not overly rich or lean).

So moving on to electrical I have inspected the entire harness, all wires and plugs are in really good clean shape and no chafing, good contacts, etc. I'm getting a low 14's voltage charge back to the battery so its charging fine but could the ignition pickup coils be screwing up? Will a failing stator allow it to idle but then not produce a good enough spark higher in the rev range?

I feel like I have ruled out fuel and air and that it seems to be an ignition issue. So weird that it has run perfectly a handful of times but then will misfire and have no power 90% of the time. I’ve put new gas in it multiple times and been through the carb and the amazon replacement carb COUNTLESS times. I’m about ready to roll this thing off a cliff. It seems like the idle has gotten worse over time to... when I first started working on it it would idle flawless but now even the idle it starts to miss (sound effect like: pum, pum, pum, pum, pum-pum, pum, pum)

Thanks for any help!

 

Intermittent electrical faults are often the worst to find. I had a 600 Grizzly in before Christmas which ran fine (though wasn't the best at starting) until it got hot, then either misfired, or cut out and always resolutely refused to re-start until it cooled down. Previous owner had fitted a new carb, present owner a new coil, so I did the resistance tests on the power and exciter coils etc, everything OK. Got a Yam dealer to check, he said the stator tested OK but would suggest getting the stator rewound anyway, as that CDI never gives intermittent trouble. So sent the stator to the re-winders and they reckoned the power coil was suspect, fitted a new one and sent the stator back. Result the bike runs fine now.

Alternatively, I had a Honda 300 years ago that had a flat spot at just above idle. Open the throttle very slowly and it revved out fine, open the throttle as normal and it would stall. I did the same change carb, coil etc, as you have done with no result. So tried a new CDI as I had it in stock, instant cure!

 

Thanks for the reply. I’ve got a new RM stator assembly on order with the pickup and pulsar coils. Hopefully that solves it. Between the 3 white wires I’m getting 1 ohm which is in spec and explains why I’m getting good battery charge back. but on the other plug (yellow, blue and grey) I get 170.4 ohm between blue and yellow and then nothing between the others. Something suspect there. Which if I’m not mistaken those are for the pickup & pulsar coils. What are the single red and white/green wires for? No ohm reading between those...

anyways I’m hoping I’ve just got a intermittent fault within the stator components somewhere because if that doesn’t fix it then it only leaves the cdi. If I have to go down that road does anyone have a source on 1989 cdi... I can only find ‘90 and later?

 

Only got the book for the Bruin but it says between red and white/blue 0.085 to 0.105 ohms for "charging" coil. Between 459 and 561 ohms for pickup coil between white/red and white/green. Someone in Australia is selling a new CDI on eBay, but wants big money for it. It is NLA from Yamaha. Dealers may know of a work around so a later CDI could be used, but my guess is a faulty stator coil.

 

Thanks merryman. I should be getting the new stator by Tuesday or Wednesday. I’ll report back if it solves the issue. Fingers crossed it is not the cdi.

 

SOLVED. Received and installed the new RM stator on Monday. The quad runs flawless now and has every day since install. The old stator was just below allowable spec for both the pick-up (170.4ohm, spec is 220ohm +/- 20%) and source coil (223ohm, spec is 315ohm +/-20%). So that may explain why 99% of the time in ran like crap but would still idle, not a full failure but just out of spec enough that it couldn't keep up as the revs climbed. Thanks for the help.