Why No Decompressor

[B Murr]

Just wondering, I have a 2 stroke 100cc villiers and I've had an XT600 43F , both are started manually and both have decompressors. Is there any reason why BMW wouldn't have fitted a decompressor to the R100 engine? I would have thought it would have made kicking an R100 engine into life a lot easier, or is there some fundamental reason as to why this wouldn't work with the engine/valve layout?.

 

[Rob Farmer]

Unless you're talking /2 and the like I don't think the airheads were ever meant to really be kick start bikes. The kickstart mechanism is pretty feeble and wouldn't last long as a serious means of starting the bike.

 

[Sean Kelly]

Interesting question. I believe that both bikes you mention are single cylinder many of which have a decompressor. My '60s Triumph twins manage without despite reasonably high compression ratios as do the GS bmws. But then try starting a big Vincent without one....

 

[Beemerboff]

Probably aesthetics and engine width had a part in the decision - it would be hard to get the smooth valve cover look with a cable operated system on top of the rocker box, and most centrifugal systems work on the overhead cams, so hard to fit them in on an airhead.
Easier to jut fit a car sized battery and starter----------.

 

[bowser]

if you want you can fit one, any number of spark plug decompressor valves available in various shapes and sizes

 

[B Murr]

if you want you can fit one, any number of spark plug decompressor valves available in various shapes and sizes

I never knew such an option was available. The one that utilises existing spark plug hole looks a bit cumbersome and the other option is one of boring another threaded hole in the cylinder head but I'd guess that would have to be underneath the engine and prone to sucking in dirt and water. With all of the people out there modifying and adapting engines there must be someone whose gone down one of those routes with an airhead, I guess if they had success the idea would have caught on in the airhead community. The only other option I can think of is a modified exhaust valve rocker which can be adjusted using a cable to open up the valve prematurely. The more I think of it the more I think its all just added complexity with little gain as the engine can be kicked into life even if its a bit of an acquired skill and depends on everything being in good shape including the riders legs.

 

[bowser]

they are often used on harleys with high compression, there are numerous ones available and some quite small but as you say not sure you really need them. However on the suck in bit they dont suffer from that, they let the compressed air out, there is feck all going in!!

 

[mavis cruet]

i know my gs kickstarter works through the gearbox, maybe others like my xt are connected closer to the crank, so create less oomph!? anyway, it takes a lot more force to move the gs lever than the xt's....

 

[Weekday Rider]

My Honda XR650R starts first/2nd kick with use of manual decompressor {it has a stage one hot cam so auto decompressor not an option). I don't think I could start it at all without.