MarkinAR

Most of the Chinese clones from TSC, Lowes, etc, are OK for what they are.  Reliability seems to be ok for a few years, value is pretty high.  They're not going to run 20+ years trouble free like a Japanese one and repair parts and support are hard to come by.  Like anything imported; decent value for the money in the short term, depreciating quickly over time.

No sweat, most people spam random posts to get to ten.

I would visually check the shifter shaft in the middle of the shifter lever moves when you step on it, sometimes they strip.  If the shaft is moving, then yes, you may have a shifter fork issue.  That's pretty rare on these machines, but I guess possible as anything metal wears with time.  It's a tough one, because you really don't know until you pull the motor and split the cases to get to the gears.

You posted the question late in the day on Friday and then reply 3 times over the weekend that no one has replied?  Calm down and give us a bit, some of us only look at this forum when we're bored at work.

I can't say if it's a shift fork without seeing it, but if it is, those parts really can be hard to come by.  The better option is to pick up a similar year Bayou (I think all of the 300's are the same regardless of year but can't confirm) cheap that's trashed and use it for experience tearing down and as a parts bike.  Those were extremely reliable machines (as were the 250 2WD) so finding spare parts is pretty tough.  I bought one a couple years ago for $500 complete but trashed and it took me a couple of weeks to refresh it and sell it on again reliable and ready to rock.

Assuming the replacement regulator is working (not Chinese junk that may or may not work), then it's pretty odd to be at 0V.  What are you using as a ground to check voltage?  at the battery?  Check the ground wire from the battery, if it's not grounded properly you won't get any voltage.  Or are you checking directly off of the regulator?

Is it a circular squeak like a wheel bearing or a suspension squeak like rubber on metal?

You have a sticking throttle cable, butterfly, something like that.  Or a massive vacuum leak between the carb and head.  It's either too much throttle from mechanically hanging (cable or butterfly) or vacuum leak.  I'm leaning towards mechanical because if it behaves closer to normal taking half the fuel out, that almost rules out too much air via vacuum leak.  If it were mine, I'd take the carb off and strip it down.  Clean and check every passage and movement of the needle, choke, butterfly.  I'd also check the throttle cable to make sure it moves completely freely not attached to the carb.

Those fuel pumps are bad about crapping out, but it's not their fault.  Modern gas of the last 10 years or so has ethanol and that turns to yellow crystals in the fuel pump quicker than you'd think.  I pulled one apart this summer that had sat 6 months and the inside of the fuel pump was so bad I couldn't clean it.  Just had to replace it.

Not without pulling it out and splitting the case.  I've had one once that actually twisted the head off the drain bolt.  Sometimes they have never been apart and the iron plug is welded to the cast aluminum housing due to lack of maintenance.

To answer your questions:

1 and 2: middle drive gear is fed by the engine oil.

3: I usually just go with whatever 90W ish hypoid is cheapest.  These don't make a ton of power or go fast so there's not much heat generated.

Didn't the big bear then have a shift adjustment rod on the left side of the motor?  Hooks directly to the back of the shifter?

technically you can put most on their side and save the oil, but I've never done it.  Too easy to just drain it and replace it for me.

You can try oven cleaner, sometimes that will bubble the paint but shouldn't hurt the plastic.  If you can get it to bubble a little, a pressure washer will do the hard work.

Lots of times on paint that is that badly done, you'll get the paint off only to find that they sanded it with 36 grit and ruined the plastic.

So here's one I've never seen, the Nikasil is flaking at the top of the stroke on the top ring on both cylinders.  Both cylinders are clean, nearly zero vertical scratching, just flaking at the top.  The flakes are just enough that compression is bypassing the top ring and going into the crank case.  It actually shot the dipstick out.  Weird.  BTW, if this were a 90's Honda, this wouldn't happen.  A quick hone and new rings and a Honda will live forever.  Stupid pull-harris.

I usually just strip all that black tape off, directly solder the wires, and heat shrink it good.  You can get a box of heat shrink at Harbor Freight dirt cheap.

If there a switch on the brake lever, then that switch is likely bad.  Should be a two wire and you can check it for continuity with a multimeter.  I usually just bypass the neutral start though so it will start in any gear by grounding out the neutral safety switch at the case.  But that does come with the warning that it'll start in gear....

Second issue....if you turn the key forward another position and it dies, most likely the ignition switch is bad.  You can check the phases of that switch also with a multimeter for continuity.

If it fired on carb cleaner then it's a fuel delivery problem.  It really should run for a second or two on carb cleaner...if it doesn't do that, then I'd suspect fuel being an issue as well as intake in some fashion whether it's a bad intake boot, sticky valve, broken timing chain.  Lot of possibilities.

Nothing rips like a 2 stroke wound up tight.  I've looked for a decent 250r forever around here and the 3 i've found in 2 years folks wanted a mint for.  One day....

Usually if there is a mismatch between CDI and harness, it has to be all of one or the other.  Also, stator may differ since it's a different wiring harness.

The sound doesn't go away with RPM, it just gets faster than the ear can easily hear.  Man, that sounds bad.  I'd take a look at the timing chain if it has one...not real familiar with that motor.

My guess would be you almost have to take what you can get on that machine.  Since it's not super popular, there probably isn't the aftermarket support of say a HL1000 or Outlander.

Choke cable into the carb body maybe?  It's been a WHILE since I had one of those carbs apart so I can't remember.

@Finnforest It's fine to run the same size tires front and back.  If you were to run it on pavement for miles the ratio difference would become an issue over time, but for a 300 that will never be fast or used for long distance highway travel, it's fine.

Stator output is not constant as it is RPM dependent. Likely minimum would be at or above 12V with high RPM pushing it to quite a bit higher with the regulator keeping the system around 13ish so it doesn't burn the electronics out. 

Love the intelligent banter above on CDI's.  For a simpleton like me, I'll distill it down:  If everything else tests good, it's a bad CDI.  There's no at home test for an average DIY guy like me to conclusively test a CDI....so all things ruled out, it only leaves one option.

Occam's Razor, if you will.

There's not much of anything you can't do yourself with a little time and research. Good work. 

@SashaGraves21 If the noise seems to be in the general tire area I would suspect a bad wheel bearing. Bearings heat and expand under use so that probably explains why you hear it after riding a bit. But that's just a guess. 

Ended up trading this one as it sat for a 9mm.  Not a bad trade having only $50 in it.  As far as an ignition switch, I'm not sure.  Lots of Suzuki's had them, but that's about all I can remember in older stuff.  Newer Can Am's have a 3 way but that's a chipped key that requires an ECU.  I would bet something like the link below would work, but can't confirm since I don't have either available to check.

https://www.hqpowersports.com/p/ignition-key-switch-1987-1995-suzuki-lt-80s-lt-80-s-atv-new/