Jump to content
Sign in to follow this  

XR400 decompression lever when going downhill

Recommended Posts

Is it ok for my XR if I use the decompression lever like a jake break while going down hill? It really helps to crawl down the steep technical stuff.

Thanks

Share this post


Link to post
Share on other sites

If you want to risk bending a valve. I would not. Just downshift if you need to go slow.

Share this post


Link to post
Share on other sites

i believe the way a jake brake works is to open the exhaust valve before tdc on the compression stroke. this allows the intake charge to be compressed but not ignited. the energy to compress the charge is delivered from the drive train, hence slowing the vehicle.

one problem with use in the decompression lever as a jake brake is that the exhaust valve is open during the compression stroke. unless you can modulate the use and only open prior to tdc on the compression stroke. sorry but i doubt it. therefore, no energy is being absorbed by compression the intake charge. also, you cannot turn off the spark. therefore, there is still ignition of the fuel.

then as rockjockey mentioned, since you are opening a valve and you don't know where the piston is, there is the possiblity the the piston will crash into the valve.

bottom line, downshift and carefully apply the brakes. i've never descended a hill that i couldn't find the right gear and modulate braking to get down successfully.

Share this post


Link to post
Share on other sites

I use compression release lever to start bike on down hills and also to clear out top end when flooded so I have some problem with statement that valve and piston could collide. I just think trying to use compression release as jake brake is poor substitute for proper use of both brakes and riding in correct gear.

Don't be in too low a gear. This can result in rear end hop and loosing traction. Just up shift and use your brakes.

Share this post


Link to post
Share on other sites
since you are opening a valve and you don't know where the piston is, there is the possiblity the the piston will crash into the valve

If that could occur, then it would happen during kickstarting as well as during running. Opening the valve with the decompression lever will not cause a piston/valve collision. In theory, it could, but then it could also happen during kickstarting.

I agree that it is not an effective braking mechanism. I can't see how using it incorrectly could cause a castastrophic failure.

Share this post


Link to post
Share on other sites
If that could occur, then it would happen during kickstarting as well as during running. Opening the valve with the decompression lever will not cause a piston/valve collision. In theory, it could, but then it could also happen during kickstarting.

I agree that it is not an effective braking mechanism. I can't see how using it incorrectly could cause a castastrophic failure.

yeah, that is true. i'm an idiot. i use my decomp lever to start also. so the motor goes through all 4 strokes with a valve open.

correction, no collision of piston and valve. nonetheless, only minimal help if anything during braking. it would only allow, no compression to occur and hence a weak power stroke.

go ahead use it all you want.

Share this post


Link to post
Share on other sites

Keep in mind that there are no real R.P.M.'s being generated during start-up.High RPM's could lead to valve float and catastrophy.(Quoting my mechanic). I just dont take any unnecessary risk with my engine or wallet.

Share this post


Link to post
Share on other sites
I just dont take any unnecessary risk with my ... wallet.

What are you doing in motorcycling? I find that it is a monster risk to my wallet. Of course, I would argue that it is a necessary risk. But I'm pretty sure my wife doesn't see it that way.

Share this post


Link to post
Share on other sites
I find that it is a monster risk to my wallet.

I dont. Ride smart and it wont be.(Of course I'm not running 6 bikes either). I agree on the necessity of it.

Share this post


Link to post
Share on other sites

I was going for ironic humor, but that is fairly hard to convey with typing.

I need more friends willing to spend their own money, rather than mine. Oh well... you have to go with the friends you've got.

Share this post


Link to post
Share on other sites

You can take your XR400 up to its 9500rpm rev limiter with the throttle WFO and then grab a hand full of compression release lever and shut the engine down with it. The piston will NEVER touch that right-hand exhaust valve.

I used to use the comprression release on my XR400 all the time when racing. It worked excellent for the steep sandy downhills. I raced it for two years in the desert and motocross. I still use the compression release on my ATK's to slow down.

Share this post


Link to post
Share on other sites

OK Now I am not 100% sure, but how many people have looked at your head and seen how the decomp system works it is a tiny spring loaded pin that realeses the pressure behind the valve.. I dont think the decomp lever actually opens a valve.. It just releases the pressure via the lil spring loaded pin... Flame on guys roast me!!!

Share this post


Link to post
Share on other sites

is it ok to have the deomp lever asembly (cable, lever) disconected?

Share this post


Link to post
Share on other sites
OK Now I am not 100% sure, but how many people have looked at your head and seen how the decomp system works it is a tiny spring loaded pin that realeses the pressure behind the valve.. I dont think the decomp lever actually opens a valve.. It just releases the pressure via the lil spring loaded pin... Flame on guys roast me!!!

I'm not sure if you're talking about the automatic decompression release, but there is a lever arm attached to a cable that activates the rocker arm which pushes the valve down on the right rear exaust valve. The travel of the arm will not push the valve far enough out to be struck by the piston, but i do agree that it is a poor substitute for proper brake/throttle control. While it won't hurt anything to do, i'd save it for emergency situations (and for startup/clearning the top end ect).

It is ok to have the decompression lever removed, but its pretty useful. When you dump your bike in a sandy/rocky uphill in 80degree weather, its nice to be able to use the decomp to get moving again quickly. Its one cable, and one lever that you can really tuck away anywhere. You can also modify the lever/cable so that it can be used as a "backup" for your clutch, incase something happens in a fall. The only time you'd really HAVE to use your decompression lever is if you removed the automatic decompression function from your cam (or replaced your cam with something not stock). The 400 isn't hard to kick over, but it sure is helpful to have.

Share this post


Link to post
Share on other sites

When you are at the start of a dead engine drop banner race, kill your warmed XR400/600 engine by revving it up to around 4000rpm. Next pull the compression release, hit the Kill switch and hold the throttle wide open all at the same time.

The cylinder is now very clear, just like using the Scott Summers post crash startup procedure, but without having to use the kick starter. Place the piston at just after TDC.

Your XR400/600 will light up on the very first kick when the banner drops!

.

:D

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×