Update: ’99 Yamaha Big Bear Engine Reassembled

Sometimes the smallest thing can have the biggest impact.  A few weeks ago I started the reassembly of Ken’s engine, but I didn’t like how much “free play” the rear drive shaft had.  I did some research and learned that there were minor variances in the crankcase measurements after they are machined at the factory.  Since I replaced the left engine crankcase, the internal gear lash adjustments had to be done all over again.  Darn.  So, I followed the service manual’s instructions and retrieved 9 different measurements that were etched in various parts.  Those 9 measurements went into 2 formulas, and voila – I had the measurements needed to adjust the gears.  The adjustments are made by placing 2 sets of shims behind the gears.  Based on the measurements, I needed to replace one .15 mm small shim with a .20 mm size, and order a pair of .20 mm larger shims for the rear gear.  The latter 2 shims arrived in a few days, but the smallest shim took 10 days to arrive, delaying the reassembly for another weekend.

While I waited for the final shim to arrive, I had plenty to keep me busy.  The piston rings were seized in the grooves, so that required attention.  I soaked the piston in penetrating oil for a week, but it didn’t loosen the rings.  I applied heat with a propane torch, and that freed the top compression ring.  The second compression wasn’t impressed with the heat, and took 2 hours of nudging and gently pressing inward to finally loosen it.  I cleaned the light corrosion from the rings and cleaned carbon from the grooves.  The rings were reinstalled and now move freely.  I honed the cylinder to de-glaze it, so the rings should wear-in to the cylinder nicely.

Throughout the engine rebuild, I’ve replaced the oil seals as the parts were reinstalled, along with gaskets and O-rings.  The replacement stator housing has beefier idler shaft bosses, so I don’t expect a repeat of the cause of the engine failure.  The eBay-sourced stator housing came from a Yamaha Warrior, which used the same engine as the Big Bear, except it was painted a metallic gray at the factory instead of the Big Bear’s black color.  So, Ken’s engine will have a gray left side, and a black right side.

It would make no sense to rebuild the engine and ignore the carburetor, so I ordered a carburetor rebuild kit.  This carburetor was difficult to disassemble, since most of the screws were corroded.  7 of 8 screws simply twisted in two, so I had a mess on my hands.  Using the drill press, I drilled out each broken screw.  I was surprised that all of the screws were eventually removed without damaging the threads in the carburetor body.  I had expected that I’d need to re-tap the threads, but that wasn’t necessary.  I bought 8 new metric screws at the hardware store and reassembled the carburetor Saturday.

By the end of Sunday, November 8, the engine was reassembled.  I adjusted the valves, since it was more convenient to access the adjusters while the engine was on the work table.  I connected the starter to the battery using jumper cables, and the engine cranked over nicely.  I didn’t crank it for long, since I still have to add oil, but it sounded solid.

Stay tuned – next weekend I hope to reinstall the engine and start it up.

About Kevin Forth

Always learning, Kevin is an IT professional that likes to tinker with electronics, motorcycles, and whatever he can take apart.

View all posts by Kevin Forth →