HOW TO: RHC's Decompressor Mod - Easier Starting!
Posted: Thu Jun 12, 2008 3:11 pm
First off, we need to thank Ron Hamp (RHC) for really caring about the online community, and dedicating his time to making our sport faster and more enjoyable. That sense of helping one another out for free seems to get lost at the top level of the sport. But, folks like RHC and Kelstr understand the 'family' of motocross, and contribute here without monetary reward. I thank them deeply for what they offer our 'family'.
I read RHC's post about grinding down the backside of the decompression arm's lobe, to effectively reduce the amount of time that the exhaust valves are open during the kicking cycle. His theory, is that the stock decompression arm (weight assembly) gives too much duration of open exhaust valves, during kicking, making starting more difficult, especially with the 2008 model's cam, and especially if running the exhaust valves a bit tighter than spec (as most of those "in the know" do). However, this mod applies to ALL model year CRF250's. It seemed logical to me, and everyone who has been around this forum for a while, KNOWS that RHC is one of the most, if not THE most, knowledgeable engine builder/tuner in this country. I had a free night to spend in the garage, so I went for it, and took pics along the way.
What I didn't realize, is that the mod can be done WITHOUT REMOVING THE CAM! Great!
First, remove the seat and fuel tank:
Remove the plug wire, breather tube, valve cover bolts, and valve cover:
Its a good idea to check your valve clearances while performing this mod, so get to TDC by aligning the camshaft marks and crank marks (mine was a bit off in the pic), after removing the crankcase inspection plug on the right side of the bike (the cam lobes should be up and back from the valves:
My valves were: Both Intakes; .005", and both Exhausts; .009" (running exhausts on the tighter spec as builders recommend).
Roll the kick starter around to look at the decomp nub that you will be grinding off. When looking at the cam from the left side of the bike, the cam rotates the same direction as the rear wheel; counter clockwise. So, envision the area you will be grinding off...the backside. Or, wipe the decomp nub with some mineral spirits and make a mark on the backside of the nub, with a Sharpie pen, so you know where to grind:
Turn over the motor until the 8mm decomp retaining bolt on the right side of the cam, is at the top, so you can get a socket or T-handle wrench on it:
Remove the 8mm decomp retaining bolt (I did so with a T-handle wrench), and you may want to place a magnet parts catcher near the small setting plate, if you think it may fall into the head. If you are careful, the plate will stay on the bolt, and you can remove them at the same time:
Now, gently turn the motor over by the kick starter, one revolution of the crank (1/2 revolution of cam), making sure the decomp assembly stays in place with a finger holding it inward (you don't want it backing out, and catching on the edge of the head), until the hole that the decomp rod resides in is at the top:
Next, remove the decomp rod and spring, by pulling it straight out to the right, noting that the spring leg is wound under (clockwise) under the rod, and rests on the backside of the cam flat (this orientation will be important during reassembly):
End of PART 1, due to picture limits......
I read RHC's post about grinding down the backside of the decompression arm's lobe, to effectively reduce the amount of time that the exhaust valves are open during the kicking cycle. His theory, is that the stock decompression arm (weight assembly) gives too much duration of open exhaust valves, during kicking, making starting more difficult, especially with the 2008 model's cam, and especially if running the exhaust valves a bit tighter than spec (as most of those "in the know" do). However, this mod applies to ALL model year CRF250's. It seemed logical to me, and everyone who has been around this forum for a while, KNOWS that RHC is one of the most, if not THE most, knowledgeable engine builder/tuner in this country. I had a free night to spend in the garage, so I went for it, and took pics along the way.
What I didn't realize, is that the mod can be done WITHOUT REMOVING THE CAM! Great!
First, remove the seat and fuel tank:

Remove the plug wire, breather tube, valve cover bolts, and valve cover:


Its a good idea to check your valve clearances while performing this mod, so get to TDC by aligning the camshaft marks and crank marks (mine was a bit off in the pic), after removing the crankcase inspection plug on the right side of the bike (the cam lobes should be up and back from the valves:

My valves were: Both Intakes; .005", and both Exhausts; .009" (running exhausts on the tighter spec as builders recommend).
Roll the kick starter around to look at the decomp nub that you will be grinding off. When looking at the cam from the left side of the bike, the cam rotates the same direction as the rear wheel; counter clockwise. So, envision the area you will be grinding off...the backside. Or, wipe the decomp nub with some mineral spirits and make a mark on the backside of the nub, with a Sharpie pen, so you know where to grind:


Turn over the motor until the 8mm decomp retaining bolt on the right side of the cam, is at the top, so you can get a socket or T-handle wrench on it:

Remove the 8mm decomp retaining bolt (I did so with a T-handle wrench), and you may want to place a magnet parts catcher near the small setting plate, if you think it may fall into the head. If you are careful, the plate will stay on the bolt, and you can remove them at the same time:

Now, gently turn the motor over by the kick starter, one revolution of the crank (1/2 revolution of cam), making sure the decomp assembly stays in place with a finger holding it inward (you don't want it backing out, and catching on the edge of the head), until the hole that the decomp rod resides in is at the top:


Next, remove the decomp rod and spring, by pulling it straight out to the right, noting that the spring leg is wound under (clockwise) under the rod, and rests on the backside of the cam flat (this orientation will be important during reassembly):

End of PART 1, due to picture limits......