larro Posted December 22, 2024 Report Share Posted December 22, 2024 Hello. As title i really can not get on with the compression on this bike. Is there anybody with experience with other year modern Ossa bikes that found the compression on 2013 bikes unreasonable to the point of cant be a***d to try to start the bike. I believe that Ossa changed something on the later bikes or earlier ones so the compression was not so high. So if they did how can i change it to the different compression rate because how the hell are you supposed to start the bike again in the middle of a trials section with out putting your foot/ feet down. Ta Quote Link to comment Share on other sites More sharing options...
mcman56 Posted December 23, 2024 Report Share Posted December 23, 2024 Curiosity made me look and I found this. It suggests that there is/ was a racing head available. However, it shows the racing head making more top end power and from my understanding more compression should make more low end while choking off the top. Is yours a 280 or 300? Could someone have put a 250 head on a 280/ 300 or a 280 on a 300? I would expect that to drive the compression way up. Does the engine knock (pink?) a lot when running. https://www.ossa-efi.com/home/engine/heads I had a Rev3 270 that was hard to kick and had very sharp power right off of idle. Adding 1 mm of base gasket in addition to the original made it easier to kick while softening the off idle power. It was a common practice, easy to do and easily reversible. I was told that someone used to make head spacers for Gas Gas to do the same thing so I once made one for a 250 Techno when trying to make it so my 100 lb daughter could start it. It was simply a 1/16" piece of sheet aluminium cut to match the head/ cylinder interface. One side sealed with the OEM o-ring. The other side was just flat surface to flat surface with a little silicone. IIRC, it was still too hard to kick but ran leak free long after it was owned by someone else. Quote Link to comment Share on other sites More sharing options...
larro Posted December 23, 2024 Author Report Share Posted December 23, 2024 Thanks for the reply. Yes i have seen this section on the heads it also describes how it puts the exhausts outlet out of position so more messing about. Quote Link to comment Share on other sites More sharing options...
kurtas Posted December 23, 2024 Report Share Posted December 23, 2024 If you can get your hands on a head for a 2011, this has a lower compression than the 2013 had. Not sure if the 2012's had the higher compression or not but I had both a 2011 and 2013 model. Depends how/what you ride but I found the 2013 to be a big improvement in power with the higher compression giving a much more torquey engine. Quote Link to comment Share on other sites More sharing options...
larro Posted December 24, 2024 Author Report Share Posted December 24, 2024 Thanks kurtas. How did you get around the compression on the 2013. Because on mine it is just solid sometimes and have to really jump on the kick lever and that is just going to cause something else to break surely. Is there a way to kicking these engines over as i have never had this before and it just does seem right. Quote Link to comment Share on other sites More sharing options...
kurtas Posted December 25, 2024 Report Share Posted December 25, 2024 Be 6ft tall and kick it like you mean it. I stood slightly forward of where you normally would so you could get a good kick at it once the starter gear had engaged. Quote Link to comment Share on other sites More sharing options...
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