* * * * * *
I've always had a chronic starting problem. I load tested the battery, it checked out. The starter was fine. Wiring was upgraded, but still no change. Mind you, the bike ran fine once it ever did get started, aside from the occasional stall. It just took forever to start. Something was preventing it from "catching" during startup. You need 3 things for combustion to occur: air, spark, fuel. I tested each one with the airbox open (by default this satisfies the air component). Spark checked out. Fuel ok, I could even visually verify the fuel pulses during starting. There would be an occasional backfire and/or knock from the vertical cylinder during the failed start attempts. An acquaintance opined that it seemed as if it may be lacking in some backpressure during startup. It was approaching time for a valve adjustment anyway so I put off any further troubleshooting.
* * * * * *
Fast forward a couple of weeks to when I did my valve adjustment.
With the cams exposed, I found a design flaw in my vertical cylinder head... Let's backtrack a bit. For those not familiar with desmodromic engines, desmo valves do not have a conventional closer spring. There is an opening rocker as usual, but the closing of the valve is done by a closing rocker. This closing rocker is held snug against the closing shim by a helper spring. This detail plays a key role in the design flaw I discovered.
Normally, one end of the spring hooks onto the closing rocker, the other end hooks onto a spring nook in the head.
This is my horizontal head. This is what it should look like. I'm pointing to the indented spring nook, and you can just barely see the end of the spring tang. With that indentation, there is no way that spring is going to slip off the nook:
On the other hand...
This is the vertical head. Note that there are NO indentations for the spring tangs. So basically in this case, the springs have nothing but their own tension to prevent them from slipping off their nooks. And that is exactly what happened with the right exhaust valve. The net result of this is that the closing rocker was lying limp against the cam, and at rest the opening rocker/shim was experiencing a gap equal to the sum of BOTH the opener and closer gaps (aka a loaded gap). At the bottom of the pic you can also see that the left intake and exhaust springs are also on the verge of slipping off.
People that responded to my thread in the forum said that the vertical heads they've seen DO have the spring indentations. Therefore I concluded that someone was sleeping at the factory when my head was rolling down the assembly line.
My solution was to machine out the indentations to mimic the horizontal cylinder head (sorry I forgot to take a pic before reinstalling the cams; I was very irritated by then). I verified that this is indeed how it is done at the factory--upon close inspection of the horizontal head, there are unmistakeable machining marks where the spring indentations are. So at the factory, the heads are cast so as the nooks lack the indentations, THEN the indentations are machined in after the fact.
I managed to get 3 out of the 4 vertical springs to reside in their newly machined nooks. The left intake spring seems to have been in that posture for so long that it has "remembered" this unnatural shape, so even with the indentation, it appears to want to leave its post. Time will tell.
It will be at least a couple of days before I get everything reassembled (I have a day job). Like I said in the previous post, the cooling system is half done; you have to remove the radiator to access the horizontal head. So it's a good opportunity for a coolant flush. So anyway, I won't know for at least that long if I have solved my starting problem.
No comments:
Post a Comment