Sometimes it takes some added incentive to give the old steed a good service and check-over. My nudge came from a volleyball-size rock. It was just one little broken bone that allowed walking, but kickstarting a 450 was out for a couple of weeks. The bike had been running fine and strong, but at 76 hours was taking a few more kicks to start, and the off-idle response was off. So I expected the worst as Yamaha’s Jonathan Belding checked the valves and cams. Like before, the valve clearance was at the tight side of the specs, but it hadn’t moved a bit in the last 35 hours of run-time. The cams and auto-decompression system proved to be in good shape. I’m crediting Maxima Lubricants for partial responsibility for keeping the interior of the engine looking new.The Keihin FCR carb has a coated plate on the slide that is designed to be replaced as it wears. The slide in my bike showed the coating was completely worn away at the bottom corners. A very thin rubber O-ring seal goes on the back side of the plate, and it is extremely sensitive to chemicals. For that reason I try to keep strong chemicals away from the carburetor, but the O-ring was all wrinkled like it had been hit with contact cleaner. I replaced both parts, and response and a cleaner sound on deceleration were the results. Even now at 100 hours on the meter, the bike starts easily and runs strong. While the carb was off I added a Scotts Performance brass fuel screw to aid tuning.I did have one odd failure. The YZ450F uses a reinforced rubber hose to route oil from the clutch case to the oil reservoir. The hose attaches to a steel fitting that is an interference fit in the aluminum sidecase. I noticed a little leakage, and when I tugged lightly on the fitting with two fingers, it popped right out. Some Superglue got me riding for the day, and I’m sure that some epoxy would have fixed it permanently, but Yamaha preferred to replace the cover. We found a small nick in the fitting. It must have lightly tagged a rock, and that worked it loose in the case.
The clutch was still working fine when we checked, and the hub and basket looked good. Just as at 40 hours, the plates were a little blue, so we replaced them and lubed the cable for the first time. The bike even had the stock grips, but at 80 hours I tried a new grip from AME that has been working great. The replacement but stock sprockets lasted 60 hours since I’m running an RK X-ring chain. It has gotten accidental adjustments when I’ve put anti-seize on the adjuster bolts, but the chain hasn’t really stretched that I can tell. I put on new Dirt Tricks Ironman sprockets at 100 hours but kept the RK chain.Amazingly enough, the bike used only one set of tires for 30 hours! The Pirelli MX Extras wore like iron despite four days at the track and a little over 400 miles off-road. The traction was good for any tire unless the terrain was hard and dry, but excellent considering the life. The front tire wore out before the rear. I used a 120 rear, and it is a tall tire. I think I would have preferred the 110. Since it has been all track lately, I’m back to Bridgestone 403/404s.For motocross I have been trying a Jardine exhaust system. It is rated at 99 decibels, so it’s too loud for off-road and is louder than I would like for the track, but it fits, makes excellent power and is holding up great. For off-road I switch back to the FMF Q4. I repacked it, and it is still quiet and has excellent performance for a 96-decibel pipe.I serviced the suspension with Maxima suspension fluids at 100 hours on the bike. I’d say the shock had 50 hours on it, but it still looked pretty clean inside. The fork had more like 80 hours on it, and it was dirty and long past when it should have been changed. But the Shock Soxx are doing their job, and I haven’t had a single fork seal problem. Even the levers and brake pads look good.I was already convinced that the YZ is a great bike, and my favorite of all motocross models to use as a combination trail/track bike. At 100 hours the top end has never been off, and the valves have never been adjusted. What a great bike!