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Author Topic: Decompression lever  (Read 1816 times)

LarsBloodbeard

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Re: Decompression lever
« Reply #15 on: October 18, 2012, 03:01:48 AM »
Here's a slideshow of how to detach the cable without risking dropping anything into the cyl (note: you might need to loosen your cable).  I took it off the bike first just to make it easier to see what's going on.  My valve is uncharacteristically clean, since I just rebuilt my engine.  I also lapped the decompression valve along with the intake and exhaust valves, just to make sure everything seats nicely.

Hopefully this forum supports animated gifs...

Phlakaton

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Re: Decompression lever
« Reply #16 on: October 18, 2012, 11:27:32 AM »
GIF anim worked great!  That is really helpful man.  Thank you!
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LarsBloodbeard

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Re: Decompression lever
« Reply #17 on: October 18, 2012, 04:16:49 PM »
GIF anim worked great!  That is really helpful man.  Thank you!

No problem.  After unscrewing it from the head, just take the cotter pin off, pull that valve out and clean it up.  If it's pretty gunked up, you might be able to squeeze a .22 bore brush into the body, or perhaps .17 (I've found gun cleaning rods to be pretty useful for my motorcycles)... I didn't bother with that though, just soaked it in chem-dip for a few days and all the gunk washed off.

barenekd

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Re: Decompression lever
« Reply #18 on: October 19, 2012, 06:28:24 PM »
Those things look like what we put in 2-strokes in the '60s and '70s. They made for great engine braking. I think what put them out of style is that they sucked in as much dirt as they blew out air!
They worked like jake brakes on diesel trucks and they really slow you down without locking up the wheel.
Bare
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LarsBloodbeard

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Re: Decompression lever
« Reply #19 on: October 19, 2012, 07:16:28 PM »
I was thinking of plugging mine for that reason.  I'm concerned about air leakage when I don't want it, both in and out.  Gonna wait till I try out the engine rebuild though... I might need that lever for kickstarting now that I have decent compression.  The electric start is crap on my bike.

tooseevee

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Re: Decompression lever
« Reply #20 on: October 19, 2012, 07:19:08 PM »
Those things look like what we put in 2-strokes in the '60s and '70s. They made for great engine braking. I think what put them out of style is that they sucked in as much dirt as they blew out air!
They worked like jake brakes on diesel trucks and they really slow you down without locking up the wheel.
Bare

              That's inneresting - please 'splain in a little more detail. I unnerstand how decompressors work, I unnerstand engine braking, I unnerstand engines. I just don't get how this works. How do you get compression braking with a decompressor?

                I'm sure it will all be clear as mud if you just clue me in to what I'm missing.
2008 ACE Head AVL Classic
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LarsBloodbeard

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Re: Decompression lever
« Reply #21 on: October 19, 2012, 07:29:45 PM »
              That's inneresting - please 'splain in a little more detail. I unnerstand how decompressors work, I unnerstand engine braking, I unnerstand engines. I just don't get how this works. How do you get compression braking with a decompressor?

                I'm sure it will all be clear as mud if you just clue me in to what I'm missing.

I don't really know HOW they work for engine braking, but they do.  I suppose it probably floods the cyl with too much air, plus not as much air coming through the carb = even less fuel.  So basically no combustion, or very weak combustion.  Perhaps someone with more knowledge can chime in...

I used to have one on my 3-wheeler and I'd pull it if I needed to slow down in a hurry.  It was kind of a cool setup in that it was a lever that stayed open till the engine started to turn then it closed itself.  So it gave you a bit of extra oomph when pull-starting.

barenekd

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Re: Decompression lever
« Reply #22 on: October 19, 2012, 11:34:50 PM »
It works because it make a pump out to the engine. If you don't have a decompressor, when the piston goes up on a two stroke, it compresses air in the cylinder. When it comes down the compressed air helps push the piston down, so nothing is really gained. The decompressor forces the piston to push the air out of the cylinder as the piston goes up forming a vacuum in the cylinder, but then has to suck it back in to fill the void as the piston goes down. All that heavy breathing is hard work!
But while it's sucking in that new air, it also sucks dirt!
Some guys did use air filters on them, but I think it lost efficiency.
Bare
2013 Moto Guzzi V7 Racer
2011 Black Classic G5 (RIP)
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Arizoni

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Re: Decompression lever
« Reply #23 on: October 20, 2012, 12:02:32 AM »
It's kinda like the "choofer" I have for my 4 wheel drive truck.

It is intended to be used as a tire pump that screws into one of the engines spark plug holes.  A very handy thing to have after you've deflated your tires to drive over long distances of very sandy areas and want to re-inflate them for highway use.

Anyway, it has one way rubber flapper valves on it so when the engines piston is going down on the intake stroke, outside air rushes in thru the flappers.
No fuel/air mixture is sucked into the cylinder because there is a high vacuum in the intake manifold that is also trying to suck air into the cylinder when the intake valve is open.

  Another valve between the choofer and the tire closes to keep the air from being sucked out of the tire.

When the piston is on the compression stroke, the air pressure closes the flapper valves and is pushed past the one way valve into the tire.

On a V-8, there are enough cylinders still operating that the engine idles quite nicely but that choofer makes one hell of a racket while its operating. :)
Jim
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LarsBloodbeard

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Re: Decompression lever
« Reply #24 on: October 20, 2012, 12:59:06 AM »
But while it's sucking in that new air, it also sucks dirt!

I don't think this is an issue with mine.  If you look closely, you'll notice there are two seals.  One is smaller, just above the threads.  The other is after the vent holes.  It vents into the head.  I assume into the crankcase... Or maybe that's what that mystery hole was inside the exhaust port.

Ice

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Re: Decompression lever
« Reply #25 on: October 20, 2012, 01:53:40 AM »
maybe that's what that mystery hole was inside the exhaust port.

It is !

I have the head off mine right now. Lets see if I can take a decent pic or two.
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LarsBloodbeard

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Re: Decompression lever
« Reply #26 on: October 20, 2012, 06:22:44 AM »
Cool!  I was wondering what that odd hole was...  I didn't give it much thought since it seemed to be there for one reason or another.