Explain this

[TonyM]

I recently had the recoil off two different saws and noticed something strange. If I turned the flywheel in the opposite direction that the engines runs, there was significantly more compression than if I turned it the normal way. I know one of these saws needs rings badly. I suspect the other does as well. So why the difference in compression. Only thing I could come up with myself is the way the piston is tipping in the bore with the worn rings/piston etc, or the crank is not dead center with the cylinder bore making for different leverages depending on which direction you turn the crank. I don't think either of these is the answer, but I am at a loss to explain this. How do the rings know which way the crank is turning? It's a two stroke!

 

[spacemule]

Most engines will have more compression when turned in the direction opposite of operation. When you stop and think about it, the parts are loaded this way when the engine is running, hence the greater compression.

 

[glens]

The only significant difference between rotation directions that I can visualize is the timing of the magneto.  Try removing the coil and see if things don't even out.

Glen

 

[spacemule]

When rotated in one direction against compression, the connecting rod will push against the opposite side of the piston compared to rotation in the opposite direction against compression. Rotating the engine backwards loads the parts in the same direction as when the engine is running (you're pushing against the direction of force of the running engine). Since the parts are seated from pressure in the aforementioned direction, greater compression will result from this direction. Phew, what a mouthful. I know what I'm saying, but I'm not sure I'm conveying it correctly. There was an article in a magazine years ago on this very subject, but I can't remember the source. Oh well, later guys!

 

[Lawn Masters]

I dont really know why an engine would have higher compression when rotating the opposite direction. I suppose that theres something to do with a change in the side of the connecting rod that bears the load, and theres something to do with the rings but I'm not sure exactly what, I cant visualise this in my head so I cant figure it out.

 

[tony marks]

space mule.. why would a recoil turn the engine in the opposite direction its gonna go ,,in the cranking process....
by the way ,i sympathise in your troubled efforts to say what u mean. i got a little bit o what ever that is myself..
so i suspect im not understanding,, what u are sayin,, rite or something..

 

[Lumberjack]

Originally posted by tony marks
did somethin wrong sorry. it double posted so i edited this one out.

Just go to the edit screen, check the delete box at the top left, and click delete on the right.

 

[tony marks]

thanks carl.

 

[glens]

Hey rocketdonkey (LOL) he said <i>significant</i>.&nbsp; What you're talking about might amount to a few PSI out of very many, or perhaps as much as a couple percent, expressed another way.&nbsp; That's hardly significant in my book.

My money's on the magneto in this case.

Glen

 

[tony marks]

space mule,on rereading your post ,ithink i understand where u coming from better. that fact[me understanding] is a bit scary in itself.

 

[spacemule]

Hey Gland!

I think you might be surprised at the difference. Of course, the difference is negligible if spinning the flywheen fast, but I've had several saws that yielded a noticeable difference when turned slowly. The next time you have your recoil off, you should try turning the saw both directions just for kicks. What I usually notice is a a hissing noise as the compression is released when slowly turning the flywheel in the direction of rotation, but a tight seal when turned the opposite way. The more run time on the saw, the more the difference. I'd like to hear what one of the saw builder gurus on here has to say on this subject.

Tony, I didn't mean that the recoil would turn the saw in the opposite direction, rather I meant when your recoil is off and you spin the saw by the flywheel.

Rocket Donkey

 

[glens]

I just finished putting the coil and starter back on a high-mileage Homelite XL.&nbsp; I might be tempted to say I saw a little more difference with the coil off than I thought I would, but I'm also tempted to say any perceived difference is more attributable to my ambinondexterity.&nbsp; With the coil on, its interaction with the magnet creates a much more significant disparity.

But maybe, like Microsoft's paid-for third-party "analytical" studies, I found exactly what I wanted even if it barely has much to do with reality.

This is an interesting engineering study which has certain merits, no doubt, but I'd much rather see some hard data about <a href="http://www.arboristsite.com/showthread.php?threadid=10572">airbox pressures</a>.

Glen