Compression Release

Compression Release

Here is an idea that I never implemented.

Back in 1976, Yamaha introduced an air-cooled 2T street bike called the RD400. It had an automatic compression release that was simply an approximately 5mm hole from the cylinder bore into the exhaust port. This made kickstarting easier but had a negligible effect at operating speed (ask any racer who plugged the hole). Unfortunately, I figured it would be much more difficult to implement that strategy on a water-cooled cylinder. And who knows, maybe a trials engine would be negatively affected by it? Seems like a no-brainer that back in the day some manufacturer must have tried it.

Then I found a compression release valve intended for a chainsaw. I think the big-inch Harley guys use it also. You manually depress the button to open the valve, then combustion pressure automatically closes it. I considered making something that would use a 10mm sparkplug and allow the compression release and sparkplug to occupy the original 14mm sparkplug hole. But this seemed mostly like a good way to achieve very poor combustion.

If you conduct a search, you will find a 2010 thread on Thumper Talk about a guy who does this mod to Honda CR500 heads. He does very nice work, but I was not sure if I could communicate exactly where it needed to go on the OSSA. I looked at several proposed locations. I could do the machining myself and found a local welder who was willing to tackle the job. I even made a simple adapter that replaced the spark plug so I could see how much effort it took to kick. It seemed workable, but I chose not to do it for several reasons:

1. OSSA heads are not exactly common. If something went wrong, I could be in trouble.

2. Although this makes sense for an MX bike that is only started a few times per day, it is less inviting for a trials bike that needs to be started 60 times during an event.

3. I figured if I could make the bike start reliably on a single kick, I would not really need it. So I focused my attention there instead.

Valve closed

Valve open

Kicking effort with compression release open was tested by replacing spark plug with this homemade adapter

Possible location 1

Possible location 2

Possible location 3