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xs650 > > Motorcycle Systems > > Engine > > Compresion too high?


Compresion too high?
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AirHog
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Joined: Apr 28, 2009
Posts: 21
Location: IL

PostPosted: December 14, 2009, 2:08 pm    Post subject: Compresion too high?

I bought a XS in pieces, it was title a a 73. I figured the engine was probably old a and tired, but to my surprise a compression check revealed 180PSI on both cylinders. I did some looking around and found that the older engines had a longer stroke? and higher compression, but then i happened to glance at the VIN on the cases, and its actually a 1975 engine.

The engine runs great, and is very strong compared to my other later model XS's. Seems there is some porting done as well as witnessed by looking down the intake and exhaust. My question is what could have been done to increase the compression. Is there a long stroke conversion a you can do, or is it in the pistons, or heads?

So far there's no leaks, no detonation. Any other problems that can occur with such high compression on these?
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yamaman
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Joined: Jan 04, 2007
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Location: Perth Western Australia

PostPosted: December 14, 2009, 2:31 pm    Post subject: Re: Compresion too high?

Just guessing, but they'd be hi comp pistons, perhaps wiseco 750cc????

Keep premium fuel up to it for sure!

Probably a good candidate for xsjohns decompression mod, would help it live longer, if that's what you want????

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despecial
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Joined: Sep 26, 2009
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PostPosted: December 14, 2009, 3:11 pm    Post subject: Re: Compresion too high?

...the early 256 @ 135mm rod, 22mm piston pin, 26mm crank pin...8.7:1 to S650-023130 - then 8.1:1...TX650 were 8.4:1
...then the 447 @ 130mm rod, 20mm piston pin, 26mm crank pin...8.4:1
...the longest oem stroke are the european 533 models @ 140mm rod, 20mm piston pin, 29mm crank pin...8.4:1, 8.5:1
...the early models had higher crown-to-pin centre pistons giving them higher compression...447 pistons are also higher crowned than 533

...still, original compressions shouldnt be much more than 150

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xsjohn
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PostPosted: December 14, 2009, 3:37 pm    Post subject: Re: Compresion too high?

It will just tear itself up much faster and run hot as a pistol in the summer............bet someone lowered the deck or used popups.........popup pistons are great for racing but not much good for the long run..........mine went from 150 to about 140 or so with my method........will be hard to get 180 down .........dps shaved the tops off his to get it to about 9-1......at that point I would have raised the head 40 thou to bring it ot about 8.3-1 with the 700 pistons..........I keep posting this stuff but no one ever comments so I guess they think I am an idiot.......but I ride a lot of miles and need a workhorse that runs smooth and doesn't vibrate hardley at all and has absolutely no engine noises and no ratcheting...tck needle deat still.........and costs me nothing to do it...............

Last edited by xsjohn on December 14, 2009, 4:06 pm; edited 1 time in total
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KevC
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PostPosted: December 14, 2009, 4:05 pm    Post subject: Re: Compresion too high?

with you on that one xsjohn
ok for racers but with todays fuel, the lower compression gives a lot less grief
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AirHog
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PostPosted: December 14, 2009, 4:54 pm    Post subject: Re: Compresion too high?

I don't have any real intentions on getting it lower since i don't want to open up this particular engine. I guess I'll just put it in a not so daily driver, and keep it cool. So i guess the consensus is the deck was milled, or the pistons have been swapped.
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yamaman
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Location: Perth Western Australia

PostPosted: December 14, 2009, 4:57 pm    Post subject: Re: Compresion too high?

John, could you tighten up the inlet valve a tad so it closes a bit late?
aybe not enough to make a difference????

Here's Johns mod for de-compressing the engine:

650rider.com/index.php...pic&t=7891

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xsjohn
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PostPosted: December 14, 2009, 5:04 pm    Post subject: Re: Compresion too high?

wouldn't effect compression enough to measure if any...and would just take out the valve tips........closing advances cam timing..opening retards .........raising the head is stone easy after moving the cam...........if it were mine I would get rid of the popups if that's the case if the bike is to ridden a lot...if there is no popups then someone milled something to lower the deck....

xsjohn
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yamaman
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PostPosted: December 14, 2009, 5:20 pm    Post subject: Re: Compresion too high?

Fair enough. Toyota Prius (and I guess others) has variable valve timing. It keeps the inlet valve open till the piston is say 1/3rd of the way up, effectively de-compressing the motor! Basicly a modern twist on the Aitkinson engine:

www.animatedengines.co...nson.shtml

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dpmphoto
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PostPosted: December 15, 2009, 11:53 pm    Post subject: Re: Compresion too high?

Yeh but the xs is no prius
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