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gbernard

DECOMP IS A GOOD BRAKE

25 posts in this topic

HAS ANY ONE TRYED USING THER DECOMP LEVER AS A ENGINE BRAKE. TRYED IT LAST WEEKEND AND IT SLOWS DOWN THE BIKE JUST LIKE APPLYING THE REAR BRAKE ONLY ITS MORE CONTROLABLE. A BUDDY WAS HAVING TROUBLE GONING DOWN STEEP HILLS,KEPT LOCKING UP THE REAR(KTM400). SO WE TRYED PULLING IN THE DECOMP LEVER AND IT

WR400 89

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Used to do it all the time with my XR. Truckers have used this technique forever instead of breaking.

It sounds like your buddy needs to upshift a gear, let the bike roll, and use both the front and rear brake. The back brake doesn't do a whole lot on steep downhills. I sometimes overbrake too and it is a nasty habit to break.

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page 1-10 of my '00WR manual states: "Never use the decompression lever after the engine is started. The engine may be damaged if you use the decompression lever while it is running." is there something i'm missing here????

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Rumor is (from my dealer) that it causes the piston to bang against the valves . . .

dunno that I believe that, but that's what they told me. . .

------------------

'00 WR; YZ modified, DSP exhaust system & UNI air, Jetting by Clark, James & Bryan. . .

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GKessler,

Yeah, that is what they told me.

Recently I read an actual example of damage potential from, I believe, ColoradoThumpin' in this forum.

I think the story was he fell and a rock got stuck in his lever unbeknownst to him, so he unwittingly rode his bike with the compression release on.

Anyway the comp. release forces one ex. valve open by pushing down on lifter. At running speeds then the cam lobe is really whacking this part down onto the lifter. He didn't wreck his piston but his lifter was ruined by the extended abuse of the comp. release assembly being wedged between lifter and cam lobe spinning at 5k + rpm. It may have ruined his cam as well.

I should probably shut up now but I don't see how the comp. release will aid in engine braking. By releasing compression it erases all engine braking force.

Large trucks have "jake brakes" which do the OPPOSITE of a comp. release. A jake keeps the exhaust valve(s) CLOSED at all times, doubling the braking force of each cylinder (the motor "brakes" on compression stroke AND exhaust stroke).

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Large trucks have "jake brakes" which do the OPPOSITE of a comp. release. A jake keeps the exhaust valve(s) CLOSED at all times, doubling the braking force of each cylinder (the motor "brakes" on compression stroke AND exhaust stroke)

Not really. A jake brake (Jacobs Engine Brake) pops the exhaust valve open just as the injector is ready to fire. That way you still have the compression but then you have no power stroke because everything gets dumped out the exhaust right before it gets a chance to burn.

------------------

MotoGreg - The voice of absurdity

'99 WR400 - 'Cause thumpers rule and two-strokes drool!

'92 GSXR 7/11 (But I wanna get a dope 916)

I might let you visit my photo album for $3 - ENTER AT YOUR OWN RISK - DANGER LURKS WITHIN

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Hick,

I think you're referring to my post a while back on the YZ side. Jammed a rock in between the perch and the decomp. lever during a hard get off. Unfortunately, it was just enough that the valve would still close (enough that the bike ran fine) but, every time the valve closed, the bucket came up and contacted the decomp. cam. Anyway, it rolled a burr on the edge of the bucket. Like most lifters, these buckets rotate as the engine runs. The result of the burr was that about 180° of the bucket bore in the head was enlarged. It was not until the engine cooled and I tried to restart the next day that the exhaust valve bucket stuck in the downward position and hung the valve open.

Skipping the explanation of the pain it was to fix, I'll say that not only would I not use the decomp. anytime the engine is running, I check to make sure I have free play in the cable after every spill.

Ken

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I'v always mounted the compression release so I can activate it easily while riding specifically to take advantage of the excellent compression breaking provided when using the compression relese while negotiating nasty down hills. It is much more controlled than the rear brake. I'v Use this technique on Honda XL 500, XL 350; Suzuki DR 350 (broed and stroked to a 457); Husabergs-a 499 and a built 600; KTM-built 400; Husky-built 610; ATK's-three 560, 600 and a built 605; and my current WZ 420 with great results and zero problems. The key is not to slam on the comparession release to max stroke and hold it there but to just crack it and modulate it. After 27 years of doing this it will be impossible for any Yamaha represeative to convince me I'm going to toast my engine if used as I do and have always done and save my life. For extream conditions I will continue to use this technique.

This is not a continusous application like haveing a rock holding the activation cam against the shim bucket edge then riding a normal rpms. On the kinds of down hills where I use this technique I'm not screeming the engine or pulling power out of the engine with the throttle and combustion process.

Its a matter of developed riding technique, life and use of a consumable like a dirt bike vs busting up me. I will go for the save my ass over any piece of hardware every time.

Clark

[This message has been edited by Clark Mason (edited 08-29-2000).]

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Do you think Yamaha would design something like that???

If so then you'd damage the valves everytime you pulled in the decomp and kicked the bike to clear it or rash satrt the bike.

NOTE: I know of 1 guy (make that idiot) who decided he'd pull the decomp in while on the limiter in 2nd going up a hill to see what it did.

Will it made the valve with the decomp attached to it hit the piston... and ues it made a mess!!

Hes the only one I've ever heard of doing it.. and I've seen the results.. trust me.. not good!

I've done it out of curiousity (I know.. curiousity killed the cat! or was that stupidity :) ) and I felt a little uneasy with the lever jumping up and down on me! :D

------------------

--

**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Vist the Rubber Chicken Racing Online Shop

Discounts for ThumperTalk members.

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Originally posted by Hick:

I should probably shut up now but I don't see how the comp. release will aid in engine braking.

I really should listen to that little voice more often...

Moto-Greg thanks for the Jake Brake lesson.

Ken, sorry I confused you with CoThump.

Clark, now I'm curious, do you feather the release while engine braking to keep the wheel spinning and not lock up on slippery hills? Or are you saying that the comp. release actually helps slow your bike down?

I guess I'm just going to have to try this because I don't see how it would help any.

It seems like pulling release would erase all engine braking and allow engine to freewheel, which is what it does when you use it for starting. Shows you what I know...

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although i'm not a registered member of this forum, i've been here numerous times per day since i bought my 2000WR in june. i've looked up to everyone who has shared incredible info here but, no offense to anyone, i part ways on this subject. if my manual states not to use the decomp lever w/the engine running because engine damage may occur, i will abide (see posting #3 above)! this subject is not like bike weight or performance claims & i treat yamaha 's published "Caution" in this regard with respect. i can't imagine that the '98 or '99 WR manuals state much different. again, no offense to anyone's personal techniques.....

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I talked to a couple of MX boys today and they said that pulling the decomp in while goin into a corner with the throttle closed lets the bike freewheel more like a 2st into the berm/corner.

------------------

--

**Ride it like you Stole it!**

Matt Porritt

99 YZ400F

Vist the Rubber Chicken Racing Online Shop

Discounts for ThumperTalk members.

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Manuals are also full of content for liability and warranty reasons. Things like injury of death can occur if you ride without your helmet or jump off a 50ft cliff. :)

Clarks experience shows us that the lever CAN be used without damage under certain circumstances. I'll continue to use mine as such.

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Why would you use the decompression lever instead of the clutch lever? Why not feather it? I'll bet the FAST pro's don't even think of doing a dangerous stunt like that!

Erik

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THANKS TO EVERY ONE FOR THERE WORDS. NEXT TIME YOU ARE ON A DOWN HILL AT LOW REVS GIVE IT A TRY,IT REALY DOSE SLOW YOU DOWN :)

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why not try pulling the throttle on tuff down hills it gets them over with a lot quicker!

sam

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bobwombat,

I agree with your "better safe than sorry" philosopy but I also have to believe Clark when he says this method results in no ill affects. I also agree with Bryan Bosch on manuals sometimes being overly zealous to the side of caution.

Example: My KX 500 manual stated that the bike required decarboning and new rings. After the first ten hours!!! I'll be damned if I'm gonna put a new top end in a brand new bike! Even the dealer laughed at that suggestion.

I tried the comp. brake trick a few times yesterday afternoon. I have a GYT-R thumb lever so I'm especially equipped to take advantage of this. You could feel the cam lobe hitting the release, it transmitted this nicely through the thumb lever. This method does seem to slightly increase and somehow smooth out the engine braking of my 426. I think the smoothing is more noticeable than the increased stopping, especially on steeper hills with good traction.

So I'd have to say that it does increase the braking, but I don't have any idea why. It makes sense that it would smooth things out a bit though.

By letting the lever "flutter" against my thumb I guess I am holding valve open on compression and letting cam push release out of the way (and avoiding too much friction/force between the two) on exhaust.

You learn something new every day...

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hick, understand your thoughts & pls know i would be the last to question those here @ the forum who i look to daily for expert advice. as an ex-2 stroker, the WR has engine breaking alone that exceeded the rear brake capability of my ktm300exc! while i'm still getting used to the wr rear, i find the front brake excellent. i suppose if i needed just that little xtra braking capability that the decomp lever may provide, i'm in a world of @#$% anyway! besides, if there's only 1 wr in the world that would experience problems as a result of such a technique, i'm sure i'd get it. all said, that's why we gather here, right? to exchange ideas, etc, the info is incredible & those who post it all even better! ride safely, bobwombat....

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Maybe I'm a minority, but I have encontered problems resulting from improperly using the decompression lever. I was using it to start the bike while going down a steep hill. The decompression lever hit the lifter bucket and caused it to mushroom slightly. I finished the race but after the bike cooled down the lifter was stuck open. This resulted in absolutely no compression and the bike would not even attempt to start.

It took three weeks for thebucket to come in. My dealer and racing buddy told me to never use the decompression lever in that manner. He races a YZ400 in the "A" class. At the time, I didn't know alot about working on my three month old ride so the dealer fixed it. It wasn't cheap and I was fortunate that I did not have to buy a new head. The mushroomed bucket casused damage to the head, but they were able to repair the damage.

Be careful.

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