decompression lever

Does anyone ever use their decompression lever for braking? I read in the manual (01 WR426) that this is harmful in some way? It doesnt make sense to me, my dad has a 1980 XT500 and he has used his for braking since he bought it almost 30 years ago with no problems. Just wondering how it is with these bikes

your decompression lever holds an exhaust valve open, not really sure how that's supposed to help you brake - other than maybe that your bike won't make any compression if you hold it open, thus it won't produce any horsepower which in turn won't continue to accelerate you in a forward motion, getting off the gas has pretty much the same effect for me.

Maybe you should pull your hotstart out to slow down too... :lame: ..... or was that to speed up???

Valve held open; piston coming up. Sounds like bad news to me.

Well, to avoid a valve/piston collision - that IS calculated into the equation by a deeper clearance in the piston under that particular valve, but I think engine braking becomes null and void if you hold the Decomp open.

If I need a little bit of throttle I just use the idle screw.

Just kidding.

The manual says not the open the decompression valve with the engine running, bad juju.

bad juju.

shouldn't that be bad mojo?

Seriously though, all I can see the decomp lever doing is reducing engine breaking. If that is an issue, just pull in the clutch. Heck of a lot easier (and safer) than trying to get my thumb on that dam decomp lever, especially in emergency braking situations.

I, sometimes will engage the braking mechanisms

I, sometimes will engage the braking mechanisms

You aren't seriously trying to make us believe you use the brakes to brake? I suppose it beats dragging your feet to slow down.

You aren't seriously trying to make us believe you use the brakes to brake? I suppose it beats dragging your feet to slow down.

I kept wearing out my boots:bonk: :lame::bonk:

well what about when you hear 18-wheelers and dumptrucks use there jake brake? All that is is a decompression release. I dunno, maybe the old bikes were built better and can take it, I also have a built up 78' XL250 and the damn thing is bullit proof. Just wondering, thanks for all the responses!!

well what about when you hear 18-wheelers and dumptrucks use there jake brake? All that is is a decompression release. I dunno, maybe the old bikes were built better and can take it, I also have a built up 78' XL250 and the damn thing is bullit proof. Just wondering, thanks for all the responses!!

From Wikpedia

In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum, the balance coming from internal friction of the engine itself. Diesel engines, however, are unthrottled and hence do not provide engine braking from throttling losses.

a diesel uses a jake brake or exhaust brake to literally close the exhaust off to provide backpressure.

a gasoline engine will get the most breaking from just letting off the throttle.

test it youself...slow down my chopping the throttle, then slow down my using the decomp....it will slow much faster not letting the compression vent.

people used to use a compression release for braking on two strokes. since two strokes don't have valves, they of course functioned differently than those on current day four stroke motorcycles.

these devices worked by a one way valve that allowed the fuel air mixture to escape on the compression stroke, which of course prevented the engine from firing. then on the intake stroke the valve would remain closed even though the lever continued to be depressed. this would produce a drag on the motor in the fashion of a big rig engine brake, sort of.

i doubt that your dad used a valve depressing compression release as they were not used at that time and may not even have been invented yet.

they were easily installed on air cooled heads. just drill and thread a hole and screw it in.

harold

people used to use a compression release for braking on two strokes. since two strokes don't have valves, they of course functioned differently than those on current day four stroke motorcycles.

these devices worked by a one way valve that allowed the fuel air mixture to escape on the compression stroke, which of course prevented the engine from firing. then on the intake stroke the valve would remain closed even though the lever continued to be depressed. this would produce a drag on the motor in the fashion of a big rig engine brake, sort of.

i doubt that your dad used a valve depressing compression release as they were not used at that time and may not even have been invented yet.

they were easily installed on air cooled heads. just drill and thread a hole and screw it in.

harold

Well he still has the bike, its in our shed as we speak, and its a four stroke, that came STOCK with a decompression release lever, so it was deff. invented, I promise. And the "big rig engine brake" sound you are describing is exactly the sound it makes when he pulls it. Im not talking about pulling the lever everytime you need to slow down, obviously thats what the brakes and letting off the throttle are for. But, when on a steep decline, in gear, in order to have the engines compression help slow you down, you pull the lever, becasue it doesnt do it automatically.

i checked the parts on bikebandit for the 1980 xt500 and it sure enough has one. i wasn't into four strokes in those days so i guess i spoke a little too quickly.

on further thought the dilemma may be due to the fact that the xt500 and other four strokes back then all had rocker arms actuating the valve depression. from the diagram, it appears that the compression release simply pushes down on the rocker to hold the valve open. activating that compression release while running then certainly would not hurt a thing.

but today's bucket and shim engines use an entirely different setup. in this design the cam rides directly against the bucket which sits on top of the valve. the compression release is either a manually or automatically activated "variable cam" (for lack of a better term). this variable cam rides against the bucket and opens the exhaust valve. but this cam is not set on bearings that will sustain high rpms and the surface is not large, hard, or polished enough to ride on the bucket at high rpms. and that is where the damage occurs if activated at high rpms. damage to the depression cam and to the surface of the very hard bucket that the regular camshaft works against.

again, with the rocker arm type, this is not a problem. and the valve is not held open wide enough to contact the piston in either design.

harold

Ok, so the bike has a DECOMPRESSION lever, BUT - What you're saying makes absolutely NO sense:

in order to have the engines compression help slow you down, you pull the lever, becasue it doesnt do it automatically.

As soon as you pull the lever, YOU HAVE NO COMPRESSION ANYMORE, you are keeping it COMPRESSIONLESS (if there is such a term). The piston, under normal conditions, compresses gas every second upstroke which is work. Work requires energy which in turn would slow the engine if no fuel mixture is supplied (ie - no throttle applied = no gas to create more energy to keep piston moving). Once you hold open an exhaust valve, the piston just goes up and down without compressing any gas - it's now just sucking gas in and out through a permanent opening and it requires LESS work to keep moving it - Gravity pulling your bike which in turn turns the tire on the ground can provide the energy necessary to keep moving the piston indefinitely if you continue to go downhill and hold that lever down.

This is why the lever is called a DEcopression lever because you are NOT allowing the engine to build up compression anymore.

4 strokes are known to provide engine braking capabilities but 2 strokes don't, matter of fact, the compression on a typical 4 stroke is so high once you compress the gas usaing the kickstarter, you can actually stand on the kickstarter with all your weight (within reason) and it will support you until it bleeds off through the ring endgap - so imagine trying to start a bike without a decompression lever to help get you just PAST TDC so you can go through almost 2 crank/piston cycles to build up enough rotational speed to have the engine compress the gasses enough on the first compression stroke to actually get it to fire and catch and continue to run on it's own.

I hope this clears it up for you, if you don't comprehend this, I would suggest you find a steep hill, have your bike running and without throttle but in gear, roll down this hill, you will find your tire grabbing to slow you down with every second upstroke of the piston, the try it again at the same speed but hold the decompression lever open and see how you roll.

The sound is irrelevant, you need to feel what the bike is doing.

RE:Ok, so the bike has a DECOMPRESSION lever, BUT - What you're saying makes absolutely NO sense:

i'm sorry i worded my comments in such a way that they seemed to make no sense to you.

i probably should have simply mentioned that the rocker arm type decompression mechanisms are not damaged by using them as an engine brake but the type employed on the bucket and shim engines are damaged by using them as an engine brake. that would have been enough to clear up the original post wondering why his dad used the compression release as a brake but it's not done on current bikes.

regarding the decompression mechanism being automatic. the first of these that i know of came out on the 93 klx650. a year later it came out on the 94klx 250. all of the current fourstrokes that have electric start employ an automatically engaged decompression device that disengages at a certain rpm. that rpm is around idling rpm. the disengagement device works off centrifugal force.

harold

from wikipedia....

When the accelerator is released on a truck, its forward momentum continues to turn the crankshaft and compress air inside the engine's cylinders. When the crankshaft passes the top-dead-center position, the compressed air in the cylinder acts as a spring and pushes the piston back down the cylinder, returning the energy to the crankshaft and pushing the truck forward. Little of the energy absorbed by compressing the air is lost, so the engine does not effectively aid in slowing the truck. Of equal importance, even with zero accelerator input, there will be some trace introduction of diesel fuel (make and model dependent) which will still undergo combustion. Any power created from this will hinder efforts to decelerate.

In a gasoline engine, some engine braking is provided during closed-throttle operation due to the work required to maintain intake manifold vacuum, the balance coming from internal friction of the engine itself. Diesel engines, however, are unthrottled and hence do not provide engine braking from throttling losses.

from me.........

The jake brake on a truck works by releasing the compression on the up stroke which in turn creates a huge vacumm on the downstroke that would have otherwise been a firing or power down stroke.

i probably should have simply mentioned that the rocker arm type decompression mechanisms are not damaged by using them as an engine brake

I have worked on two XR600R engines in the past on which the riders used the manual compression release for braking purposes. In both cases, the tab on the rocker arm had broken off and caused some damage, in one case the enigne was pretty well done for.

This is getting pretty crazy.

The thing being missed entirely here is the real distinction between the Jacobs Brake on trucks, which is in fact a compression release, and the compression release used as a stating aid on thumpers: The truck is a two stroke.

Just as with Gasoline engines, there are two and four cycle diesels, and only the two stroke uses a compression release as a brake. The reason it works is pretty much exactly what trailscout posted, the energy absorbed by the compression stroke of a two cycle is mostly returned to the piston on the following dead power stroke by the air charge decompressing, spring-like. The same thing happens with two stroke gasoline engines, in which a compression release can also be used.

Four strokes, whether gas or diesel, are different. The exact same thing that happens to a two stroke happens to a four stroke on compression; the piston basically bounces back after the compression stroke. However, there is a good deal of air being pumped back and forth during the exhaust and intake strokes, and this absorbs power from the forward inertia of the vehicle and produces some braking. That's the difference.

What the compression release accomplishes on a two stroke is that it allows the dead engine to become an air pump by forcing air out through the release. Since two stroke diesels are supercharged and have no throttle plates, this can amount to a lot of air, and will slow the truck very noticeably.

The compression release on your WR, however is not designed for this. It raises one exhaust valve by pushing against the edge of the lifter, preventing it from closing completely. When the release is engaged while the engine is running, the lifter of the closing valve will strike the release with a considerable amount of force at any speed over an idle, and the potential for breaking the lifter is very real. If you tried to use it as a brake, it would create braking during the compression stroke (at least theoretically), but negate braking during the other two strokes, so it would be a wash at best.

The compression release on a thumper CAN be used to aid bump starting by allowing the engine to start turning after the clutch is released. Hold the release and clutch, roll the bike, release the clutch first, then the release, once the engine is turning, and roll the throttle gently open, pulling the clutch in once it starts. That's about the most you should do to it, speed wise, though.

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