decompression lever


33 replies to this topic
  • matt4x4

Posted October 08, 2008 - 09:28 AM

#21

Hey Grey - It's getting close to halloween - let us have some crazy threads!!!

nice explanation!:lame: :bonk: :lol:

Trailscout - I probably should have stated that I was talking about the WR's Decomp lever and how it works and what it does and why it makes no sense to use it to slow yourself down, so I think confusion all around - my bad - I guess IMO - pulling the decomp. lever on a wr is about the same as pulling the clutch, you pretty much negate all resistance you could have had from the cylinder - the only difference would be drag of the extra components.

  • grayracer513

Posted October 08, 2008 - 11:10 AM

#22

in order to have the engines compression help slow you down, you pull the lever, (because) it doesn't do it automatically.

I just noticed this, and I spotted another misunderstanding, this time of the Auto Decompression system. The AD is a speed sensitive system, and is NEVER active at any speed over 700 RPM. That means if the engine is running, the AD is not working, on the gas, off the gas, whatever.

  • drtbk4ever

Posted October 08, 2008 - 11:58 AM

#23

Greyracer, I bow to your knowlegde oh wise one. :bonk: :lol: :eek:

Great explanation and I think I kinda understand it now.:lame:

  • WVrider20

Posted October 08, 2008 - 01:43 PM

#24

well im glad my question sparked such good conversation! lol. All I needed was the difference explained to me between the old engines and the new ones and how the decompression works. It now makes sense on why it would damage our newer bikes. As someone said, yes, it is correct that when you pull the lever the valve is held open, essentially stalling the bike, yielding no compression untill you let it go again, so my words were mixed up too. Nothing like a good ol fashion debate so everyone can get their facts straight!!

  • trailscout

Posted October 08, 2008 - 03:47 PM

#25

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.


for the record, the jacobs brake is used on fourstroke trucks. i drove a 350 dodge last summer that had one. loved it!

but they are not simple devices. when activated, a separate cam opens the exhaust valve on the compression stroke. when the piston passes over tdc, the exhaust valve closes and it is the vacuum that slows the truck down, not the compression. on gas engines, it is only the manifold vacuum that slows the vehicle. the compression and downstroke exactly cancel each other on deceleration. well, unless there is a jake brake on the gas engine. don't know if that is even possible. never heard of one.

  • trailscout

Posted October 08, 2008 - 04:01 PM

#26

[QUOTEprobably should have simply mentioned that the rocker arm type decompression mechanisms are not damaged by using them as an engine brake .[/QUOTE]


[quote name='creeky']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.[/QUOTE]

well golly, i guess i should have said the rocker arm type on the 1980 xt500. don't know why i got into this anyhow. the only time i ever used a compression release to slow down was on a 1978 it250. that was a while back. now-a-days i make most steep decents with the clutch pulled in anyhow.

thanks for correcting that for the xr600 riders. i wouldn't want to cause someone to blow theirs up. guess i'll go take a look at the parts blow up. can't figure how it'd work on a rocker arm in such a way that it'd cause a problem.

  • creeky

Posted October 08, 2008 - 04:34 PM

#27

A little more compression braking can be had by holding the kill button down.

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  • trailscout

Posted October 08, 2008 - 06:20 PM

#28

'course if we all wanted to be technically correct, we would shun the term compression braking and instead refer to vacuum braking which would be the more correct term.

but as always, when in doubt, just gas it!

  • grayracer513

Posted October 08, 2008 - 07:25 PM

#29

for the record, the jacobs brake is used on fourstroke trucks. i drove a 350 dodge last summer that had one. loved it!

but they are not simple devices. when activated, a separate cam opens the exhaust valve on the compression stroke. when the piston passes over tdc, the exhaust valve closes and it is the vacuum that slows the truck down, not the compression. on gas engines, it is only the manifold vacuum that slows the vehicle. the compression and downstroke exactly cancel each other on deceleration. well, unless there is a jake brake on the gas engine. don't know if that is even possible. never heard of one.

Until researching it further, I'm willing to grant that such a device exists, but I do want you to realize the diesels don't have manifold vacuum (because they don't have throttle plates.) The a device operating as you describe would negate the compression rebound effect by blowing off pressure at TDC. BTW, what year Dodge with which engine?

... can't figure how it'd work on a rocker arm in such a way that it'd cause a problem.

When the cam returns the valve to the seat, it is designed to do so gently by slowing the valve as it seats. When the compression release is operated, a very simple cam rolls up under the rocker, or down on the lifter, preventing the valve from being reseated (thus, the common British nomenclature:"valve lifter"). The cam cannot have two profiles, and since it's designed to ease the return of the exhaust valve against its seat, it fails to gently set the lifter or rocker arm against the compression release cam. At speed, the battering these parts can take is somewhat excessive.

  • trailscout

Posted October 08, 2008 - 08:57 PM

#30

Until researching it further, I'm willing to grant that such a device exists, but I do want you to realize the diesels don't have manifold vacuum (because they don't have throttle plates.) The a device operating as you describe would negate the compression rebound effect by blowing off pressure at TDC. BTW, what year Dodge with which engine?

When the cam returns the valve to the seat, it is designed to do so gently by slowing the valve as it seats. When the compression release is operated, a very simple cam rolls up under the rocker, or down on the lifter, preventing the valve from being reseated (thus, the common British nomenclature:"valve lifter"). The cam cannot have two profiles, and since it's designed to ease the return of the exhaust valve against its seat, it fails to gently set the lifter or rocker arm against the compression release cam. At speed, the battering these parts can take is somewhat excessive.


correct. diesels don't have manifold vacuum. but when the compression release allows the gases to escape through the exhaust valve on the compression stroke and then closes on what would have been the firing downstroke, a vacuum in developed in the cylinder chamber. the piston is going down and both the intake and exhaust valves are closed. BIG vacuum. that is what generates the braking on the jacobs.

the dodge was an 07 cummins owned by idaho parks. i used it to pull a trail cat around northern idaho. but all the fourstroke big rigs use the same setup. been around a long time. i recently looked into adding one to an old international 1800 i own. about $2k. not a simple upgrade.

the area on the cam that gently seats the valve happens to be named the "seating ramp". like i said, i haven't actually been into the rocker operated releases. just looked at the parts blow up. i guess if the release just partially depressed or partially held open the valve, then the cam would sure enough be hammering on the release mechanism everytime it came around.

not sure how wv's dad got away with it. i initially figured it was one of the one way valve type that was once popular on two strokes. but the parts blow up "looked" like it worked on the rocker. maybe wv will pull the cover and take some good pics for us.

and i'm not sure where you got the idea i thought a cam could have two profiles. i guess i'm not too clear sometimes.

  • grayracer513

Posted October 09, 2008 - 06:22 AM

#31

and i'm not sure where you got the idea i thought a cam could have two profiles.

Nowhere in my post have I said that you did.

  • matt4x4

Posted October 09, 2008 - 07:55 AM

#32

trailscout - paranoia!
He wasn't correcting you - Grey was simply explaining how it works - it had nothing to do with your posts.
I think if someone were to correct someone else it would go something like this:

your thoughts on a cam having two profiles are incorrect......

  • trailscout

Posted October 09, 2008 - 11:23 AM

#33

Nowhere in my post have I said that you did.


ok. i guess i assumed you were addressing me and since you were explaining that a cam could not have two profiles you must have thought that i thought they could have two profiles. and i further assumed you would not think i thought they could have two profiles unless i had said something to indicate i did think that. i had mentioned the jake brake used another cam so i thought i had mis-stated it somehow.

wow. there i go thinking again.

  • trailscout

Posted October 09, 2008 - 11:46 AM

#34

trailscout - paranoia!
He wasn't correcting you - Grey was simply explaining how it works - it had nothing to do with your posts.
I think if someone were to correct someone else it would go something like this:

your thoughts on a cam having two profiles are incorrect......


matt, i've heard if one is not paranoid one is just not paying close enough attention, er, something close to that.




 
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