Yesterday I mounted the rebuilt engine into the frame and reconnected the back axle and drive shaft.  The original starter cable was broken, so I took a quick trip to Kalkaska and bought a generic automotive-style ground cable to use as a replacement.  It was the same length and worked perfectly.  After cleaning the electrical connections I tested the ignition for spark.  I could see tiny sparks as I cranked the engine.  I added some gasoline to the newly-cleaned fuel tank.  I switched on the fuel petcock and the carburetor filled with gas.  I pulled the choke out, turned the key and pressed the starter button.  I was pleasantly surprised when the engine roared to life after just 1-2 seconds of cranking.  I also tested the pull starter, which worked very well too, starting the engine again with a single pull.

I pushed the ATV onto the lawn and started it again.  I put it into first gear, thumbed the throttle, and the engine revved but the machine didn’t budge.  I was able to shift up and down, but there was no forward motion.  Darn.  I drained the oil and pulled the right side cover to examine the pair of clutches.  I removed the bolts holding the clutch pressure plate in place, and could sense that the clutch was disengaged.  It appeared that there was nothing wrong other than the clutch adjustment.  I thought I had adjusted the clutch properly a few times, but something must have hung-up inside.  I manually actuated the clutch rod inside the engine and it appeared to be working well.  I reassembled the clutch and could tell that it was now engaged, holding the clutch plates tight.  I closed up the engine and refilled the engine with oil.

That was all it needed.  I restarted the engine, stepped into first gear, thumbed the throttle, and the Big Bear lurched forward.  I drove across the street, around some trees, and back home again, with Enzo chasing me the whole way.  I spent the remainder of the afternoon adjusting the brakes and installing the body panels.  Tomorrow I’ll change the gear lube in the front and rear axles, and it’ll be ready to hand back to Ken.  It’s not perfect.  I never did find a replacement muffler, so it’s louder than it should be, but it’s running again.

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.

I found a few more large pieces of aluminum in the crankcase, so I was concerned that there would be damaged transmission gear teeth.  I inspected every gear and they appear to be in good condition.  I’ll remove each gear and shaft to thoroughly clean them prior to reassembly, to ensure that any bits of ground-up metal are removed.  I also need to flush the tiny oil galleys in the engine casting for the same reason.

I compared the replacement alternator stator housing with the original.  The replacement casting is noticeably more reinforced than the original, so I don’t expect a repeat failure.  Ken’s original stator and wiring, however, appear to be in better condition, so I’ll move his stator to the replacement casting prior to reassembly.

There will be the usual housekeeping prior to reassembly.  I need to remove the old gasket material and thoroughly clean everything, which takes some time.  I also need to replenish my supply of Loctite, since my tube is almost empty and many screws on this engine use a thread adhesive.  Stay tuned to this channel for updates – hopefully the engine reassembly will begin next weekend.

Earlier I observed that the steel gears had ground away sections of the crankcase.  As expected, I found the aluminum bits in the oil filter.  I’ll have to ensure that no aluminum grit remains in the engine or the external oil cooler lines.

When the work resumes, I’ll be removing the front and rear U-joints from the engine.  I’ll also be removing the transmission shift barrel once I get some stuck Torx-head screws removed.  That should allow me to split the crankcase and remove the crankshaft and transmission pieces.

The Yamaha Big Bear was a utility ATV that was sold in both 2 wheel drive and 4 wheel drive configurations.  Ken’s machine has a 348cc single-cylinder engine – small by today’s standards, as Yamaha’s smallest ATV, the Grizzly, comes with a 708cc engine.  After some research, I learned that the Yamaha dealer was right – many parts for Ken’s engine are hard to find.  The 2WD version was more common, and Ken’s engine configuration was made for only a few model years.  Wonderful.

I pulled the right side engine cover to inspect the clutches and look for signs of a problem.  This machine has a pair of clutches – a wet clutch like a traditional motorcycle, and a centrifugal clutch mounted on the crankshaft.  That centrifugal clutch allows the bike to start and stop from any gear, and to allow the engine to idle without pulling in the clutch lever.  The clutches looked clean, with no debris or signs of failure.  I moved to the left side of the engine.

On the left side of the engine I noticed that only 2 screws held the engine cover on.  I should have noticed that earlier, as the rest of the screws were in a bag that the dealer gave to Ken.  The dealer put just enough screws back in place to hold the cover on.  I pulled these screws and removed the left engine cover.  The issue was plainly evident.  The starter idler gear mount had broken away from the crankcase casting.  In addition, the crankshaft was stuck as if the engine had seized.  I found a rogue bearing jammed between the alternator rotor and the crankcase.  Once the bearing was removed, the crankshaft rotated nicely.  Regardless, the broken mount meant that the engine crankcase would have to be replaced – probably the largest component of the engine.  Given the relative rarity of Ken’s engine configuration, I wondered if I could even find a used crankcase.

The best source for old engine parts is eBay.  eBay provides a global source for parts at great prices.  I was quickly able to find available crankcases for the older Yamaha Big Bear models, and for the 1999 model year, plenty of 2WD crankcases.  After more online hunting, I found 2 available crankcases that matched Ken’s engine, one from a seller in Michigan.  After comparing pictures and part numbers, I purchased the part.  That’s the good news.  The bad news is that I’ll need to do a complete engine rebuild to install it.

Interestingly, I’ve noticed a difference between Ken’s crankcase and the replacement crankcase.  When comparing the section that broke in Ken’s engine, that area is noticeably stronger and reinforced in the replacement crankcase.  I suspect that Yamaha made a change to the castings to correct the weakness that caused Ken’s part failure.

I’m looking forward to the rebuild.  Until the new parts arrive, I’ll be removing the rear axle and drive shaft to prepare for the removal of the engine.  I’ll post updates here as parts arrive and I make progress.  Wish me luck!