Author Topic: decompressor starting issue  (Read 180 times)

doh13

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decompressor starting issue
« on: January 09, 2021, 12:24:08 PM »
 From my understanding of how the decompressor mechanism works it should be fully "closed" when the bike is not running and the decompressor rod  will slightly press on the exhaust valve before the cam lobe  does to let off excess compression as the motor starts, then  centrifugal force from the running motor holds the decompressor "open".

 The problem may be  that the spring  has lost some tension or is not strong enough  and on certain shutdowns the part of the mechanism that keeps the valve open rests on the valve bucket on shutdown preventing the decompressor mechanism from fully closing/resetting.

My thought is that switching to a slightly stiffer spring or adding an additional turn to the existing spring may help the issue because the mechanism may now fully close on a shutdown before the motor fully stops spinning  which in theory should prevent the issue.  I checked the parts list for a 2020 sefr and mechanism looks to be the same p/n as the older models.  I do believe the intake cam timing has been changed on the newer models possibly to try and help with the issue?

The only problem i can see is that if the spring tension is too strong it may cause the decompressor mechanism to stay open at idle causing excessive wear. but if it were to work every time it would save a lot of wear on the starting system.

doh13

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Re: decompressor starting issue
« Reply #1 on: January 16, 2021, 06:13:35 PM »
Another reply to my own topic :)  I have installed a new spring and the decompressor mechanism  seems to return with more force. I have not tested it yet but it seems as if the old spring may have gotten weaker due to heat, if it works its a  cheap enough part to change on each rebuild.

erdat

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Re: decompressor starting issue
« Reply #2 on: January 17, 2021, 12:50:47 PM »
This might be a noob question, but how did you track the problem to a compression problem? What are/were the indication?
Cheers,
Erdat
'15 SE-R 250
Ankara, Turkey

doh13

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Re: decompressor starting issue
« Reply #3 on: January 17, 2021, 09:19:28 PM »
This might be a noob question, but how did you track the problem to a compression problem? What are/were the indication?
Cheers,
Erdat
The starter  motor doesn't have enough torque to overcome the engine compression fairly often for a lot of ppl but the bike will still start after a few presses of the starter ,the issue seems to be caused by the decomp mech not doing its job on each start to test you just roll the bike back and forth in gear before starting and it usually resets the decompression mech and will start normally. i believe it can be caused by a weak return spring.
« Last Edit: January 17, 2021, 09:23:59 PM by doh13 »