Author Topic: decompressor starting issue  (Read 784 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 »

N3YMY

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Re: decompressor starting issue
« Reply #4 on: January 29, 2021, 09:09:14 AM »
I just had the decomp spring replaced on my 2015 SEF, it made no difference...

doh13

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Re: decompressor starting issue
« Reply #5 on: February 09, 2021, 08:03:06 PM »
I just had the decomp spring replaced on my 2015 SEF, it made no difference...
dang that's no good. i suspect ill have the same issue as before when i try the bike in the spring. Next time i am into the motor i may try an extra turn on the spring.

Dangermouse

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Re: decompressor starting issue
« Reply #6 on: March 20, 2021, 06:17:04 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.

The problem with the 250/300 SEF is that the engine stops with the cam at a point where the de-comp can't reset.

I have not tried a 2021 model yet, but I'm told that they have a new camshaft and should have fixed the issue.

The way around the problem in the earlier years is when you stop the engine, leave it in gear and release the clutch.
Pull the bike backwards slightly until you hear the engine roll over backwards a little (does not need much)

The bike will fire first time every time 100%
Hope that helps.
2019 300 SEF Factory
2011 TE449
Admin - Sherco Riders & Owners FB group
Just an old quick trail rider having a go.
From Qld Australia 🌏