The magazine recently purchased a 2000 Honda XR650R motorcycle with less than 1000 original miles on the bike. An honest and forthright previous owner could not start the engine, and given the mileage, we agreed that the problem was stale winter (ethanol) fuel—which had eaten up the plastic tank screen! I trailered the cycle home and looked forward to restoring the fuel system and completing the dual-sport conversion process. The motorcycle will be an HD video shooting platform for remote backcountry documentary filming and off-pavement event coverage, so the XR650R's low mileage seemed a huge asset.
After 20 hours of prepping the cycle for a Nevada dual-sport inspection, including some minor tuning and checking out the bike's general condition, I discovered that the 650 thumper engine would not start. Consulting two friends with XR650Rs, I assumed that my starting technique was the culprit. A heavy cardio workout later, it was apparent that the compression was lacking. I did a quick compression gauge check and discovered a 95 PSI cranking compression issue. (To assure an accurate reading, make sure the auto-decompression mechanism is not holding the valves open when performing a compression test!)
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My next step was a cylinder leak down test, which I have covered in detail with an HD video how-to feature. (Click here to view the HD video how-to procedure and results!) The leak down test pinpointed leaking intake and exhaust valves; a look inside the cylinder (through the spark plug hole) also indicated scoring at the upper wall of the Nikasil cylinder plating. Time for an upper engine tear down, which I cover as a step-by-step HD video: Click here to see the how-to series at the 4WD Mechanix HD Video Network's freshly launched "The Off-Road Motorcycle Channel".
So, how did a "bulletproof" Honda XR650R motorcycle, with less than 1000 miles since new, end up with a worn out set of valves and leaky piston and rings? The answer is two-fold: 1) the motorcycle had an incomplete "uncorking" job with a partial "Honda Power-Up Kit" installed and 2) a leaking aftermarket air filter had seeped abrasive dirt into the air stream and through the engine's intake system. The air filter issue is notorious for pitting valves, scoring an upper cylinder and damaging piston rings on motorcycle and automotive engines. An incomplete uncorking calls attention to an ever important problem for any motorcycle engine modification: the need to re-jet the carburetor to compensate for improved intake flow or a less restrictive exhaust system!
The "Power-Up Kit" opened up the exhaust cap on the muffler and reduced exhaust backpressure (basically an HRC end cap). The cycle is a non-California model, so there was no restrictive "D" molding in the intake manifold grommet. (The non-California intake manifold is considered the unrestricted air intake, often sold as the upgrade for better breathing.) The pilot jet had the Power Up Kit's #68 sizing, though not the specific "68s" style. However, for some unfathomable reason, the main jet in place was still the ultra-lean, original 125 main jet! The most important ingredient in the uncorking process for a Honda XR650R motorcycle engine is the unrestricted rubber intake manifold and a jetting change to a 175 main jet and 68s pilot jet (base line at sea level). Exhaust modifications help further, the stock OE muffler's exit flow and end cap are ridiculously small for a 650 thumper!
How important is the 175 main jet and 68s pilot? Important enough to be the OEM jet sizing on all Honda XR650R engines sold outside of North America! The U.S. engines were leaned to the limit by E.P.A. requirements, and California models (XR650R AC designation) were even more restricted by a draconian reshape and air flow restriction in the rubber intake manifold and the intake air box. So, this uncorked, non-California model had its exhaust opened and uncapped—plus the removal of air intake box restrictors. The EPA regulated (non-California) cycle already had the open, round intake manifold. It was in severe need of the 175 main jet, however, which the dealer neglected to install with the power tuning!
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The result is clear. Despite the low mileage, this engine was busy overheating its upper cylinder and valves. Upon tear down, which you can follow in the HD video linked above, the engine's upper cylinder looked more like a Baja 1000 Race finisher than 1000 miles of reasonable recreational riding. Fortunately, the previous owner had used quality lubricant and changed oil filters regularly. The XR650R's unique liquid cooling (the only XR to ever offer it!) also helped minimize damage and isolate the wear to the cylinder plating, piston skirts, rings and valves. The main engine assembly is still in near-new condition.
I'm now in the process of rebuilding the upper engine. The head and cylinder have been sublet to L.A. Sleeve Company. My approach will likely be an alloy iron/chrome/moly, patented L.A. Sleeve liner in place of the factory Nikasil. Though many are fans of Nikasil, I'm not thrilled that's its only a "plating" thick. This Honda XR650R has a lot of work ahead, and it must be ultra reliable.
I may consider a Stage 1 Hot Cams alternative, mainly to eliminate the auto-decompression mechanism on the OE camshaft. Compression ratio will remain the stock 10:1 with the new upgrade piston, enough squeeze for higher altitudes and the limit for kick starting and long piston ring life! Quality machine work, a fresh cylinder and upgrade piston choice, renewed valves and proper assembly technique will have this motorcycle living up to its legendary reputation—with a 175 main jet and 68* pilot jet in its Keihin carburetor! Follow the rebuild how-to coverage at the magazine's The Off-Road Motorcycle Channel—in 1080P HD video!
*Note: The carburetor had a straight 68 jet installed, and I will see if the 68s is necessary. There is a difference in the flow between these two varieties. Honda's Power-Up Kit does call for the 68s and also a needle change if necessary. I will fine tune as needed, though that would be anticipated at northern Nevada. Our home base is 4,500 feet elevation and high desert; the average/mean elevation at Nevada is 5,500 feet!