Administrators Moses Ludel Posted December 4, 2014 Administrators Share Posted December 4, 2014 Many of today's dirt motorcycles have electric starting, and this is a very good idea for the contemporary ultra-high compression four-stroke singles! For these operators, unless the starter fails to work, manual kicking has become a thing of the past... Despite this trend, there are still many kick-start Honda four-stroke motorcycle engines that have an "auto-decompressor" start mechanism. This device, typically mounted at one end of the overhead camshaft, unseats an exhaust valves during crankshaft rotation. The aim is to relieve compression as the piston approaches TDC (top dead center) on the compression stroke, only during kick start phase. Once the engine starts, the mechanism disengages the ramp/cam that opens the exhaust valve during kick starting. On my Honda XR650R engine, the OEM camshaft was still in place when I purchased the bike. The engine did not run, and repeated attempts to kick it over proved futile. Eventually, I ran a leak down test and discovered a considerable loss of compression caused by leaking intake valves. Despite this low compression, the kick starter repeatedly balked as if the auto-decompressor was stuck off. Even with leaking intake and exhaust valves, the engine would not kick over easily. The auto-decompressor was stuck in the off-position and acting as if not even there! Large piston displacement gave a false sense of "compression". The bike had set for some time, and perhaps this accounted for the condition of the auto-decompressor. It felt like the mechanism was sticking in the released position, providing no decompression. Even with the low compression from leaking valves, I found myself using the hand lever decompressor to "free" up the compression resistance and what felt like a jammed kick starter. Note: Wondering why an engine with leaking intake valves would still be hard to kick over? This is not hard to explain with the volume of air that this huge piston can displace, an overwhelmingly large amount compared to the amount leaking off from valve face-to-seat seepage. Given the kick start resistance, I never suspected worn, leaking valves. In fact, all four valves had a fair degree of seepage. By the time I committed to the leak down test that led to the upper engine rebuild, I had researched the XR650R engine. I'd also had enough experience (i.e., cardio workout) with the auto-decompressor to seek an aftermarket replacement camshaft without the auto-decompressor. I was not opposed to using a manual decompressor lever and knew from my first experience with a BSA 441 Victor how to find that special point, just past TDC on the compression stroke, where the engine kicks through without kicking back. Part of my rebuild was the choice to install a Hot Cams Stage 1 camshaft. This is a milder performance approach that provides more bottom end and midrange power—and a camshaft that eliminates auto-decompression. I'm good with my decision to eliminate auto-decompression and am curious how others have made peace with the factory start mechanism. Am I the only one whose auto-decompressor has stuck or jammed? I could have rebuilt and polished the OEM auto-decompressor, as the mechanism can be rebuilt. I opted out. Is everyone else happy with the stock camshaft and auto-decompressor? Please share your experiences here...An auto-decompressor is not unique to the Honda XR650R, they come in many forms and configurations, like the kick-start mechanism on my '84 XR350R, still a valve unseating approach, that has worked flawlessly and easily for three decades. From the XR600R forward, the auto-decompressor attaches to the camshaft and has a one-way clutch mechanism. How does that work for others? Moses Quote Link to comment Share on other sites More sharing options...
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