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Auto-decompression (ACR)

PostPosted: Tue Oct 17, 2017 2:30 am
by gomes-nz
1999 DR650 with 115,000 KMs on the clock. Striped the top end to replace the leaking base gasket. Installed a JE high compression piston, and had the cam shaft reground to the 190 profile (as per the pro cycle specs).

Did about 300 kms. Torque and power much improved. And then bang and lots of noise.

Strip down of the rocker cover revealed that the automatic decompression mechanism, the weighted arm dislodged from the pin and then started hitting the rocker cover. Depositing aluminium shavings into the engine (joy). Inspection of the mechanism revealed that the weighted arm return spring was very weak and there was side play in the arm allowing the pin to dislodge and set it free to destroy.

Has anyone else had this problem? Is it result of my higher lift cam or just not very well designed component with a combination of lots of KM's.

Interested to know. tks Darrin.

Re: Auto-decompression (ACR)

PostPosted: Wed Oct 18, 2017 9:15 am
by BigBob
Not heard of this before.
Most DR650 riders use this site ... ad.135295/
I hope you find your answer there

Re: Auto-decompression (ACR)

PostPosted: Thu Oct 19, 2017 12:54 am
by gomes-nz
Turns out that the pin that holds the weighted arm on the automatic decompression mechanism had moved out for some reason, to allow another play for the arm to clear the pin. This allowed it to move out and strike the rocker cover.

Spoke to the Camshaft specialists this morning. They will repair the cam, and more importantly they are going to press the pin back in and then tig weld the pin on the lobe side. This will reduce the play in the arm, and stop is failing.

So moral of the story. If you are doing some CAM grinding work on your stock CAM, please check that there is no play in the auto decompression arm. Especially on high mileage engines.....of which there are a few about.

Should be back together quickly and back on the road. :D :D :D

Re: Auto-decompression (ACR)

PostPosted: Fri Oct 20, 2017 12:42 pm
by nug
Not heard of this happening before but certainly something to be mindful of.