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XR4 Decomp. Lvr. mishap!

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Posted by: MadLawnGnome---------------------

I had my beast on the maint stand today, while it was warming up. I was going to change the oil, filter, and usual maint. A friend of mine came over, we had a few cool barley waters while I let it cool back down before draining the oil out. After I put in the fresh oil and filter in I fired the bike back up. My non-motorcyle riding friend asks me if he can sit on the bike, I said "sure go ahead". He climbs up and sits for awhile. I turn my back (big ooops) and the bike dies! I turn and ask him what happened, he says " All I did was squeez this lil black thingy right here" (decomp lvr).

What can hitting the decomp while the bike is running do? I assume this is very bad! Bike still runs fine but is there anything I should check out?

MLG


Posted by: Gary B.---------------------

I've done this accidently on both of my current bikes. (TIP- don't have the lever above the clutch on one bike and below on the other!) No apparent damage. Of course this was, like yours, at an idle. I don't think I'd want to try it at at 8 grand. In that case you would possibly bend valves.


Posted by: biglou---------------------

The bike died because of the loss of compression (decomp=decompression). That is all. Valve clash is not an issue.


Posted by: xrsforever---------------------

No damage,in the ole days(early seventies)bikers would screw a crompression release in the extra spark plug hole for ease in slowing down. It sounded like a giant fart. At speed this would let dirt into the cylinder and promote rapid wear. At an idle with no dirt blowing it should do no damage.


Posted by: DualSportr---------------------

Didn't anyone teach your friend not to touch levers unless he knows what they're for??

Actually, some guys use the decomp to kill a bike. As long as the things not being revved to the moon at the same time, no harm can be done.


Posted by: VintageDirt---------------------

You don't really think that Honda would put that lever there if they thought that it could destroy the engine if placed in the hands of the consumer?


Posted by: stormer94---------------------

Vintage dirt,

Good point. There's no way they could put it on there if the thing was guarnateed to shell the engine if you happened to pull it while riding.


Posted by: TexKDX---------------------

Quote:
Originally posted by xrsforever
No damage,in the ole days(early seventies)bikers would screw a crompression release in the extra spark plug hole for ease in slowing down. It sounded like a giant fart. At speed this would let dirt into the cylinder and promote rapid wear. At an idle with no dirt blowing it should do no damage.


Modern compression releases don't work this way - they hold an exhaust valve slightly open mechanically. This applies to the XR and the Yammy F motors for sure, and probably the rest of the modern bikes. I've had the Yammie and Honda apart and have seen these.

I've heard of some people who use the release to kill the motor. This supposedly makes it easier to start based on the stroke it dies on and the mixture in the cylinder or something.

At any rate, no harm done. You could tell him otherwise though and at least get a case of malt beverage out of him .


Posted by: MadLawnGnome---------------------

Ahhhh guilt is such a beautiful weapon!!! I think I'll tell him that he trashed the top end and it'll cost me like $5000.00 to get it fixed! Just kidding he's a good guy.

Oddly enough though i just found out my valves are in out of adjustment..easy fix though


Posted by: VintageDirt---------------------

Quote:
Originally posted by TexKDX
You could tell him otherwise though and at least get a case of malt beverage out of him .
Mmmmm...frosty malt beverages.


Posted by: TexKDX---------------------

Quote:
Originally posted by MadLawnGnome
Oddly enough though i just found out my valves are in out of adjustment..easy fix though


Easy on the XR that is! I'm growing fond of no radiators and screw valve lash adjusters in my old age. Too bad the KLR uses shims. The motor doesn't even work over 5500 RPM, so the weight savings in the valvetrain is of no real benefit. All they do is add to the maintenance cost.

BTW, Motion Pro makes a right handy feeler gauge for the XR. I believe it is .002" on one end and .004" on the other, but don't quote me on that. It is in the service manual in the garage.


Posted by: DeBarker---------------------

Quote:
Originally posted by xrsforever
No damage,in the ole days(early seventies)bikers would screw a crompression release in the extra spark plug hole for ease in slowing down. It sounded like a giant fart. At speed this would let dirt into the cylinder and promote rapid wear. At an idle with no dirt blowing it should do no damage.


Dood... the compression releasee's you are referring to were on 2 strokes!
This is totally different.

But still... there is no danger on this guys XR400.

But i wouldn't use it to kill the motor all the time either... because the little mechanism isn't meant to be taking those little hits at 1000 rpm.

It's meant to be used at 0 rpm.


Posted by: gremlin---------------------

There are two reasons why we have a manual de-compression lever.

The first and obvious reason is to aid starting.
The second and less known, to enable better control of your bike.
Does anyone know anyone with a truck?
Ask them if they have an exhaust brake, its the same thing..
Next time you find yourself going down a steep wet or slippery slope and think you are going too fast, USE THE DE-COMPRESSOR LEVER instead of the brake, all it will do is slow you down without locking the back wheel.
What it wont do is blow your engine..try it..


Posted by: SFO---------------------

Let me get this straight, my 426 has a Jake Brake?
Multistage?
If I am going down a hill too fast I can hit my Jake?
(Truckers let 'em roll...)
The Nitro Express,yea man, gotta listen to more trucker music.
Maybe the local truck stop can give me some jetting tips for the FCR?


Posted by: TexKDX---------------------

Hello Rich, are you listening?


Posted by: Hondaxrguy---------------------

Ya, my dad told me the same thing. He used to ride with a guy that had some engine brake thing on his bike. Anyways, I tried it one time while riding down a hill, and guess what happened. The engine died. That's it. I think I actually started going faster. Now, I have a little "tick" in my engine just before TDC........

Jeremy


Posted by: DualSportr---------------------

s'not the same.

Fourstrokes have engine braking - that is, you let off on the throttle on a steep downhill and the engine will slow the bike down by reducing the rpm's (a two stroke will "freewheel" and typically gain momentum).

Opening an exhaust valve (with the decompression lever) will only allow the compression (which is what slows the bike down) to release prematurely - this will make the bike roll faster down the hill.

The jake brake on a diesel engine works differently - I have a vague idea on how this works, but I know someone who can explain it precisely... RICH???? Help!

BTW, it IS possible to bend a valve if the compression release is actuated while the bike is running at higher rpm, or if the decompressor is incorrectly adjusted.

I know it seems funny that Honda designed something that could be damaged so easily by rider error - but in the same vein, ANY bike is easily damaged by someone who doesn't understand how it works. I've seen minibikes throw parts after junior has ridden around all day in 2nd gear - no matter what speed he's going.


Posted by: Gary B.---------------------

I agree with DualSportr and Hondaxrguy. the last thing I'd want to do is pull the decompressor anytime while riding.



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