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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.

 

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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

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Hi Moses...My decompressor is erratic. That's probably why I had such a tough time to get the valves adjusted correctly. So when I start, I usually look for TDC and then press slightly through with the manual level actuated. Then I kick. If it doesn't start, I sometimes go back to the lever, but often not.

 

Whatever the case, the engine doesn't often kick back, and a slow prod through the travel of the kick starter will meet with only slight resistance on the compression stroke. But when it does get full compression, the feeling through the lever is pretty intense.

 

I suppose I'm not unhappy having it - I'm used to it.  I just need to take it into account for valve adjustment.  After my rebuild, the bike started right away and idled just fine, but the next day when I took it out for a shake-down run, it started to run poorly and refused to idle - almost as if it was running out of gas. I went through the carburetor about 4 times (even with a magnifying glass) trying to figure out what was wrong. It was extremely frustrating since every time I put it back together, it would run fine for a short while (once it was almost a day), and then stop.

 

Being that I didn't know what to do next, I checked the valves (although I really didn't suspect I could have done something wrong). The first time, I found one of the exhaust adjusters was very high in relation to the other one. (see photo) This was because of the autodecompressor.  I reset that valve, and the bike ran great again. But then it stopped. So I checked the clearances again. And once more, I found the valves were out of adjustment - this time it was the intakes. Somehow I must have been using a top dead center from an alternate universe. Once I corrected them, all was good and the bike has been running great ever since.

 

Now it's due for a clearance check and changing out the break-in oil. If I find anything out of the ordinary, I'll post.

 

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D

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Textbook photo, David!  The tall screw threads are a clear clue that the auto-decompressor was still "on" when the right side exhaust valve was set.  Thread height should be somewhat similar when the decompressor is fully released before the valve adjustment...Your comments on the auto-decompressor are very valuable to others.

 

Do you use the manual decompressor lever, too?  Or do you simply kick the engine through repeatedly without having to fiddle with TDC?  How often does it balk as you describe?  Is this ankle jarring?

 

Moses

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Hey Moses, thank you for being so thorough with your xr650r videos. I just purchased a nicely optioned 2000 xr650r. The previous owner claims to have checked out the top end 3k miles ago. He said new oem rings were installed as well as cam bearings and a chain tensioner, because they were there. The motor does look like it has new gaskets as well as some sort of sealant around the head. The bike fires up easily enough and runs well. Almost too easily. I feel like the auto decompression is giving me a false sense of what a 650cc single should feel like to kick over. I am going to do a leak down test just to give me the satisfaction that the bike is in good running order. I'll report back.

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Matt...To avoid inaccuracy or scaring yourself, I suggest backing off the valve adjusters enough to clearly seat the valves before running that leakdown test.  Still have the piston at TDC on the compression stroke, as the piston atop its bore here is the best point for measuring cylinder wear or "taper".  With the auto-decompressor, it's otherwise very easy to get a false reading from an unseated valve...

Here's some Honda XR650R fun, a ride I took two weekends ago at El Dorado Canyon.  For the record, the bike survived this outing without a scratch topside or bottom, I never touched a rock or obstacle, keeping both feet on the pegs unless stopped...

This supposedly "heavy" bike (add dual-sport equipment and be sure to top off the 6.3 gallon fuel tank!) never ceases to amaze me.  I'm a mechanical preservationist and almost left the bike home in favor of my single track, trail tested vintage (1984) Honda XR350R.  Thought I'd try it.   The day's riding ended up 179 miles of the Pine Nut Mountains, a lot of high desert and several secondary highways in the area...Enjoy!

I want to hear a lot more about your XR650R, Matt!

Moses 

 

Edited by Moses Ludel

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