Auto Decomp Cam for Yamahas

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#1
Alright, BBR, Powell, Thumper racing,,, where are you at? Who do we have to wack over the head with a kickstarter to get to make a centrifical auto decomp cam for our Yamaha thumpers? You know like the ones the Kawies use on the KLRs.
Come on already, quit making very nice looking break peadels and tripple clamps and give me something I can USE. We all have gone round and round on the estart thing, thats not what I want. I just dont want to have to fool with the lever when I kick or when I flame out and want to bump start it QUICK when comming back down the hill.
Ok, so who's up to the task. I know some of you read this board,, come on, speak up :) .
 
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#2
... and then there are those who think Automatic de-comp devices just add uneeded complexity to an engine to appease a handful of riders who can't seem to master a simple concept. Yamaha did it the right way and provided us with a clean, simple and reliable valve train as a result. Good work Yamaha.
 
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#3
Plus the ability to clear out a flooded engine using the manual decomp lever. I'm happy with the manual decomp on the Yamaha, extremly simple to use and keeps complexity to a minumum. I see no need for an auto decomp system.
 
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#4
If Yame dose not want to put it on,,, fine, but I for one know I would use one If a decent aftermarket one was available . I never had any problem with the auto decomp on the Kawi. The valves on the oter hand,,, anyway, the decomp stayed open untill the RPM hit 1800. So clearing out was not a problem. I think It would save weight to get rid of the cable and lever, the only draw back is a small gain in the rotating mass of one of the cams. I rarly have any problem kicking it over, as a matter o fact, I kick it to hard and will probably need a new start lever soon, but the high wire act of trying to fumble with the lever stuck on a hill or comming donw the hill after a falme out and turn around bump start,,,,,,,,, come on,,, how nice would it be just to let the old clutch fly and have her fire up,,,,,,,??
 
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#5
The first time you have to take the top end of the Yamaha apart you'll understand while simplicity is a huge virtue. I wouldn't hold me breath waiting for the aftermarket to build something that requires monumental amounts of engineering with little or no chance for financial return.
 

XRpredator

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Damn Yankees
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#6
Auto decomp = pain in the posterior

My 83 XR500R had an auto decompressor, along with the manual, and I thought it sucked. My brother's bike actually quit running, and after a gob of unnecessary tearing down (looking for something seriously wrong, and causing problems of MoO_coW like proportions ;) ), we found the little nubbin that the kickstarter hits against to work the auto-decomp had broken off, causing the decompressor to ride against the valve, keeping it open. If we'd just unhooked the cable from the kickstart end, he'd have been able to keep riding!

Thankfully, my XR400 is manual only, as is Yamaha, the way they should be. It doesn't take a rocket scientist (or Charles Atlas) to get them to work right. Know the drill, and it'll start the first or second kick every time.

Or buy a KTM :p
 
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#7
If you position the compression release lever to a place that your thumb can actuate it, a bump start is very easy and you never have to loosen your grip on the bars. Mine is located on an imaginary line between the handle bars and the rear axle. This is also a better position than Yamaha had it for protecting the lever from damage when you dump the bike.
 

SFO

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#8
Husaberg had a auto cam for a while.
Thats exactly were I would put a hand gernade for maximum damage on my motor.
Retrofit of the non-ato cam was the only reliable option.
I also prefer a lever, BTW.
I have always disconnected the kickstart activated comp releases as well.
I like to have discreet functioning of my decompression.
 

KLX4smoke

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#9
The centrifugal decomp. system on my KLX is very simple and seems to be the perfect answer to me. I can't think of any good reason why this idea isn't used by the other manufacturers. Anybody got a good reason? When I had the head off, the thing is so small that I doubt that the "complexity", weight, or cost of manufacture could be the reason.
 

SFO

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#10
The problems that I have seen are these units come unglued and scatter in the head.
It also makes it hard for the aftermarket to make a cam.
 
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#11
Huh, mixed review, mostly against. I thought the Kawi rig was good, simple and reliable.
Can you explain how you mount your lever for a thum grab? I am not understianding the position and orientation. I moved mine to the right side so I can start the bike in gear with out juggeling the clutch and all.
 

dirt bike dave

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#12
Jason - for thumb operation, return the decomp lever to the left but have it facing the ground, on the bottom side of the bar.
 
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#13
Ahhhhhhhhh haaa
I see, I may give that a try and see if I like it better than being on the right. It is a real pain in the butt re-routing that cable for the right side.
 

dirt bike dave

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#14
With the thumb operated lever on the left, it makes it very easy to roll start the bike. Get it coasting in gear with both levers in, then release the clutch lever to get the engine turning - the tire will not skid on a slippery surface since the compression release is pulled in. When the tire and engine are turning, let go of the compression release lever to fire the eninge. It takes a little getting use to, but it sure makes bump starting on a slippery hill much easier.:)
 
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#15
I for one like the Kawi cam mounted auto decompressor. It is foolproof and really eases starting. I ran a Kawi 600KLR with a high comp piston. When the spring failed on the decompressor I could not start the bike, As you would expect from a 11.5 to 1 600cc engine. I did adjust the cam mounted unit to provide a little more opening to facilitate easy starting and boy did that help.

I believe the unit would work well on other big bore 4 strokes.

Chris