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  #1  
Old 10-17-2013, 06:04 PM
cakey cakey is offline
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Location: Fairbanks, Alaska
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STIHL BR600 and BR550 Compression

I've been trying to apply some recently won knowledge and tools to a badly beaten fleet of 20 or so STIHL backpack blowers, all but one is a BR600 the other is a 550. I know that the 550 and 600 are very similar so i think this particular question would be interchangeable between the two models.

I was reassembling the BR550, i adjusted the valves and did a compression test, it was 88 psi or 6 bar. I readjusted the valves and to my surprise the compression went up to 142 psi or 9.8 bar. I wasn't expecting a 38% change from what i felt to be a minor adjustment, however my question lays in that the guidelines I believe are 7.5 to 8.5 bar or 108 to 123 psi. The blower seems to start and run fine but is their a down side to running at a compression 13% greater than the guidelines? Or is this a boon and I should be glad with it?
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  #2  
Old 10-17-2013, 06:18 PM
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Sharpcut 1 Sharpcut 1 is offline
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If you have a good cylinder, but some carbon build-up on top of your pistons, you are going to see an increase in compression, because the top of the piston is now "domed" with carbon, compressing into a smaller chamber. I would'nt worry about it. Should be running 89 octane in today's 2 strokes/ 4Mix. Does it spark knock?
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  #3  
Old 10-17-2013, 06:47 PM
cakey cakey is offline
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It runs smooth, no knock and we only use 89 octane fuel. I can't imagine that the fuel oil mix has always been ideal though. I also did a leak down test on it and got almost no leak. 4% or less i think, although my process for testing leak is poor and needs refinement so the validity of that measurement is suspect.
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  #4  
Old 10-17-2013, 07:32 PM
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Jeff in AL Jeff in AL is offline
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I wouldn't waste the time on a compression test... Compression reading will vary depending on amount of carbon build up in cylinder anyway. But if you insist on doing so, Min compression would be 7.5 Bar or 109 Psi. Specs are 7.5-8.5 Bar or 109-123 Psi. IF higher then 10 Bar, you have a problem with the decompression on the cam.

The better way to test any of the 4-Mix units is to do a leakdown test. As long as there is no more than a 10% loss, the upper end is fine! You can also determine at the same time where the loss is from if more then the 10%!

By the way, there is a tool that is used on the 4-Mix units that will lock out the flywheel once set at TDC on compression stroke... Thus you will get a correct reading each time. The same tool is used on ALL 4-Mix units!
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Old 10-17-2013, 08:20 PM
cakey cakey is offline
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I've started doing both tests on our blowers, I'm the curious sort and much of the time the time it takes to perform the extra test in inconsequential. About 20 min ago i ordered that part to stop the flywheel after reading Jeff in AL's post. Currently i use a pair of vice grips, a socket and a 3" extender. It leaves much to be desired. Thanks for the help guys. I'm left to assume its just carbon build up. Next time that particular blower comes across my bench i will treat it with EDTA acid to see if that has any effect.
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  #6  
Old 10-18-2013, 05:27 AM
tigerepairdotcom tigerepairdotcom is offline
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Try to do the compression test on that same 140 lb blower warmed up. I have 2 br 600's both test at 150 or so psi and drop to about 125-130 or so after running for a sec. Super rare and hard to find Technical data sheet All kinds of goodies. Theres also a decompression cam inside the engine to facilitate easier starting.

http://2.imimg.com/data2/UM/WS/MY-18...en-blowers.pdf
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  #7  
Old 10-18-2013, 05:30 AM
tigerepairdotcom tigerepairdotcom is offline
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another thing i only run stihl HP ultra oil in my br 600's heres why http://www.stihldealer.net/stihlhpul...r-_e4y4ea.aspx
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  #8  
Old 10-18-2013, 11:19 AM
cakey cakey is offline
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Thank you tigerepairdotcom i hadn't though to warm it up first although i should have. I have recently been switching crews over to HP Ultra oil, focusing on the crews with new blowers. this particular br 550 has been around for years, as have many of the blowers and in the past the crews used whatever oil was around. I'm trying to change old habits but it is what it is, at least i can give them the tools.
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  #9  
Old 10-18-2013, 09:17 PM
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Jeff in AL Jeff in AL is offline
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Quote:
Originally Posted by cakey View Post
I've started doing both tests on our blowers, I'm the curious sort and much of the time the time it takes to perform the extra test in inconsequential. About 20 min ago i ordered that part to stop the flywheel after reading Jeff in AL's post. Currently i use a pair of vice grips, a socket and a 3" extender. It leaves much to be desired. Thanks for the help guys. I'm left to assume its just carbon build up. Next time that particular blower comes across my bench i will treat it with EDTA acid to see if that has any effect.
cakey,

Which lockout tool did you buy, the 4282 890 8600 OR the 5910 890 8600?
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  #10  
Old 10-18-2013, 09:21 PM
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Jeff in AL Jeff in AL is offline
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Quote:
Originally Posted by tigerepairdotcom View Post
Try to do the compression test on that same 140 lb blower warmed up. I have 2 br 600's both test at 150 or so psi and drop to about 125-130 or so after running for a sec. Super rare and hard to find Technical data sheet All kinds of goodies. Theres also a decompression cam inside the engine to facilitate easier starting.

http://2.imimg.com/data2/UM/WS/MY-18...en-blowers.pdf
Have you ever asked a Stihl Dealer for them?
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