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Post by nighthawk on Jan 15, 2022 22:43:10 GMT -5
So I have been working on porting a 925 just for the fun of it and I believe I figured out the problem of spit back through the carb and I also think I figured out why they burned up so easily as it is caused by the same design issue. I will tell you what I discovered and maybe you can tell me if I am on the right track as I believe it could be fixed if you know what you are doing. I believe the blowdown is not enough from the factory. Meaning the exhaust port is not open long enough before the transfers are opened up. This would cause the exhaust pressure to exceed the crankcase pressure for a very small amount of time causing the excess pressure from the exhaust to push back down the transfers pushing the charge back out towards the carburator. This would also cause the saw to behave like it is running in a lean condition because the new charge up the transfers would be delayed until the exhaust pressure reaches a point in which it is lower than the crankcase pressure. In order to compensate for the delay the saw needs to be ran rich to ensure that it received proper lubrication because the transfer of the fresh charge is happening for a shorter amount of time. The fix is basically to raise the exhaust port roof some amount. I did not figure out the exact amount of time needed to fix the problem. I just went straight towards a hotter saw build. You could try a thicker base gasket but this would not change the blowdown timing so it probably won't work. The saw that I built I was able to remove the spit filter because the spit back problem disappeared in the process. Just in case you want to see what my 925 built on the hotter side is currently running like. I am not finished with it yet. It is trial and error at this point. www.youtube.com/watch?v=JjIC61hVZWE
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Post by onlyhomelites on Jan 18, 2022 23:52:37 GMT -5
Sounds great and it's vicious in the cut!
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