Low Compression on Compression Test. - Yamaha Rhino Forum - Rhino Forums.net
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post #1 of 17 (permalink) Old 02-19-2012, 09:56 PM Thread Starter
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Low Compression on Compression Test.

06 660 with less than 100 hours on it. Compression tested it today and it is reading about 78 psi while cranking. It idles starts and runs good, has plenty of excel, spark plug is tan, and no smoke coming from tailpipe but the compression scares me. I am pretty positive the valves have never been adjusted we bought it from second owner who has had it for 3 years and never did it. Can tight valves be causing the compression to dip this low. When we run it it runs strong but at this point have chosen not to run it until we find out why compression is so low.
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post #2 of 17 (permalink) Old 02-19-2012, 11:52 PM
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Well, do not worry too much. These motors have an automatic decompression system that can cause a low reading on a compression test. When testing the compression you need to push the throttle all the way to the floor and hold it there while cranking, which in most cases raises the cranking rpms enough to deactivate the decompression mechanism. If the battery is a little weak, throw a charger on it first for a while to make sure there is enough voltage to crank it over fast.

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post #3 of 17 (permalink) Old 02-20-2012, 01:40 AM Thread Starter
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Well, do not worry too much. These motors have an automatic decompression system that can cause a low reading on a compression test. When testing the compression you need to push the throttle all the way to the floor and hold it there while cranking, which in most cases raises the cranking rpms enough to deactivate the decompression mechanism. If the battery is a little weak, throw a charger on it first for a while to make sure there is enough voltage to crank it over fast.
Thanks I will try that.
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post #4 of 17 (permalink) Old 02-20-2012, 11:25 AM Thread Starter
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So I did a compression test with the throttle to the floor and it went up but still does not sound like the auto compres. system is kicking off all the way. It went from about 75 psi with no throttle to about 90-92 with the throttle to floor. I think the battery might be a little low from the cranking or it could be my harbor freight tester. I am still pretty sure valves are a little tight. Does this sound like a good number or does it sound as if I might be in trouble.
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post #5 of 17 (permalink) Old 02-21-2012, 09:43 AM Thread Starter
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Anybody ? Wheres all the Pros at ?
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post #6 of 17 (permalink) Old 02-21-2012, 04:41 PM Thread Starter
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So I charged my battery and picked up a Matco tester and compression is at 120 psi. The book says normal is 188.3 but can be as low as 166. I think it will be ok and the other psi will be picked up in a valve adjustment.
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post #7 of 17 (permalink) Old 02-22-2012, 03:38 PM Thread Starter
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150 views and only 1 response WOW. I thought the forum was for getting info and advice and helping each other. Maybe it has just got overlooked but I would really appreciate some feedback if anyone can help.
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post #8 of 17 (permalink) Old 02-22-2012, 04:08 PM
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Maybe the 149 + you are also looking for advice. Did you check the valve adjustment? Let us know what you find. Do a leak down test and see if your valves are sealing.


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post #9 of 17 (permalink) Old 02-22-2012, 04:53 PM Thread Starter
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Maybe the 149 + you are also looking for advice. Did you check the valve adjustment? Let us know what you find. Do a leak down test and see if your valves are sealing.


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Valves are going to be done Monday but I was more curious if 120 sounded good or bad from a compression test. I did not put oil in hole before I tested it and I had read that you should warm up the motor before testing which I did not. With all that said is it possible tight valves would cause the compression to be that far off. As far as leak down test goes I have heard about it but do not know how to do it or what tools would be needed. I just know that the forums got some pretty smart people on when it comes to things like this and I am pretty sure alot of them were away on the weekend so that is why I waited till today to say something. If I + 149 others are looking for the same thing than someone needs to chime in that knows what they are talking about and help 150 brothers out.

Last edited by knoxxx29; 02-22-2012 at 04:55 PM. Reason: spelling
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post #10 of 17 (permalink) Old 02-22-2012, 05:20 PM
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Runs good + spark plug looks good + no smoke = no problem

We are closed...2013
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post #11 of 17 (permalink) Old 02-22-2012, 05:29 PM Thread Starter
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Runs good + spark plug looks good + no smoke = no problem
Thanks now that is what I am looking for. I value every opinion given and it is greatly appreciated now Rutt lets get back to buisness. What do you have that I want for my Rhino and no my wife does not need a grabbar nor a windshield. Any other parts would be considered.

Last edited by knoxxx29; 02-22-2012 at 05:30 PM. Reason: dumbass
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post #12 of 17 (permalink) Old 02-22-2012, 05:38 PM
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What's your wife's phone number I want to call her and find out if you are lying to me or not...

What we have on the website is it...we sell air intakes, fuel delivery kits, jet kits, MSD CDIs, aluminum dash bars, aluminum grab bars, and pop on/off windshields...

We also are a dealer for Hot Rod sheaves... mostly we sell our own unique products that we manufacture...

We are closed...2013
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post #13 of 17 (permalink) Old 02-22-2012, 05:49 PM Thread Starter
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What's your wife's phone number I want to call her and find out if you are lying to me or not...

What we have on the website is it...we sell air intakes, fuel delivery kits, jet kits, MSD CDIs, aluminum dash bars, aluminum grab bars, and pop on/off windshields...

We also are a dealer for Hot Rod sheaves... mostly we sell our own unique products that we manufacture...

See it is unfortunate that I do not know my Wifes number because I try not to call her because when I do I get in trouble. JK How about the fuel system is it something that would benefit my stocker with cdi ? PS my wife already got her windshield.
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post #14 of 17 (permalink) Old 02-22-2012, 06:25 PM
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You can mount the dash bar on reversed from what you see in the pics of ours with your windshield then she can have a grab bar to hold onto...

If you rode shotgun more often you would know she needs this...!

Yes if your 660 is all stock then a fuel delivery kit is nice to have...our premium fuel delivery kit for the stock pump comes with a primer and transparent fuel lines for easy trouble shooting...the primer makes sure there is gas in the carb prior to cranking...if your carb has never been tuned a bigger pilot jet helps a lot...if your intake is stock the intake is where you get the most bang for the buck power increase available on a 660 even with a stock exhaust...if you increase the air flow you have to jet the carb...

If you don't have a hot rod sheave already you surely want one and I can do that too...

We are closed...2013
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post #15 of 17 (permalink) Old 02-27-2012, 07:45 PM Thread Starter
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Just an update. Adjusted valves and the ones that are suppose to be between .004 - .006 one was .005 one was .004 and the other was .002. The other 2 that are suppose to be .006 -.008 one was .005 and one was .002. I cant remember which one is exhaust or intake but I am assuming this was my culprit to the low compression. Dealer came out on his day off to adjust for me and he said they were extremely tight on a couple. We put the home depot rais style snorkel and a 165 dyna jet and it ran 50 mph with no stumbles or cutting out. Dealer said it ran better than most of the Rhinos he deals with so that makes me feel a little relieved.
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