Follow-up to my initial posting.
After running the bike for about a month or so I decided to perform the initial 16,000 mile valve service. An inspection prior to disassembly revealed some light oil seepage occurring from both the front and rear valve covers. But nothing more than what one would expect from a seventeen year old engine that has never been disassembled. Upon removal of the front valve cover I noticed several spots of light brown (the color of road dust) appearing on the lower surface of the rubber sealing gasket where the gasket mates to the upper surface of the head. The upper surface of the gasket had been cemented to the lower surface of the valve cover from the factory so no indications of such discoloration were present. The valve cover gasket on the rear head showed no such discoloration. An inspection of the valve lash verified that all clearances fell within specifications ... as expected. No adjustments were made.
As outlined in the factory service manual, I reassembled the engine using new valve cover gaskets on both the front and rear heads. I also installed new O-rings on both the front and rear dowel pins. In addition, new O-rings were installed on both the timing hole and crankshaft hole caps. I also decided to replace the two O-rings which reside between the top of the throttle bodies and the base of the air box, although this is not suggested in the service manual. Once reassembled the engine started and idled without issue.
Given that no valve shim adjustments were performed I expected no change in the engine's performance. The engine always stared and ran without issue. My only concern with the engine had been the low frequency vibrations that I noted in my initial posting to this thread. However, immediately upon taking the bike for a test ride I noticed a considerable change in the engine's operating characteristics. Suddenly the engine accelerated in a much smoother fashion with a more linear and predictable throttle response. In addition, the popping produced by the PAIR system when up-shifting during moderate to hard acceleration was all but gone. The engine also maintained a constant speed without continual adjustments to the throttle. Previously, the bike had a tendency to drift up in speed. The engine was also far more comfortable at lower RPMs in sixth gear. Previously, maintaining, say, 55 mph at 2,750 rpm on a relatively flat surface was a struggle. Now the engine could comfortably maintain a constant speed of 45 mph at 2,250 under similar conditions. Also, the engine was far more predictable at parking lot speeds. The rather sensitive and somewhat bursty throttle response previously experienced was gone. All of these initial conditions of course point to a fuel/air mixture that is far to lean. Such a tendency for SP1s was pointed out by SubSailor in his response to my initial posting. This is not to say that the tendencies pointed out by SubSailor had been eliminated. Rather, they were suddenly far less severe and more in line with what I would expect from the machine.
In trying to understand why this has occurred, it appears that "False Air" may have been entering the Crankcase Breather system via both the front and rear valve cover gaskets, as indicated by both the light brown areas on the front gasket and the oil seepage from both the front and rear gaskets. The Crankcase Breather system is, by design, a "closed system." Any air entering the system would be introduced to the induction system as False Air thereby causing the fuel injection system to run lean. Too lean a mixture would of course produce the erratic throttle response, upwardly drifting engine speeds and PAIR system induced popping due to unburned fuel exiting the combustion chamber. The lean mixture would also burn hotter creating hot spots within the combustion chamber leading to pre-ignition thus increasing engine vibrations under load as I expressed a concern of in my initial posting. The installation of the new gaskets and O-rings appears to have restored the crankcase breather system to its intended closed system state thereby restoring the fuel/air mixture to a richer more appropriate level.
So what should be taken away from this discussion? My thought is that in addition to the recommended mileage interval of 16,000 miles for valve clearance checks it may be appropriate to introduce a maximum time interval as well. A maximum interval between checks of, say, 10 years would not seem unreasonable. On bikes such as mine that have had low usage during their lifetimes, or owners that have decided to delay the initial valve inspection due to the low probability of the valves actually needing to be re-shimmed, this would take into account the possible deterioration of the rubber used to seal the Crankcase Breather system. My guess is that the leaks were introduced by a contraction of the rubber components over time. Note, however, that the rubber seals were neither cracked nor hardened. In fact, they still appeared quite malleable. But obviously something had changed.
I would be interested in hearing if anyone else has encountered such difficulties on their bikes. Or if anyone can suggest additional maintenance items that should be accelerated simply due to aging.
Last edited by Enxss; 11-03-2018 at 12:13 PM.
Reason: Additional Comment