How to test compression


10 replies to this topic
  • luvmythumper

Posted May 01, 2006 - 08:34 AM

#1

I had my bike apart and decided to run a compression test. It feels slightly down on power compared to my buddies bike, otherwise all is good. If I believe the prior owner, I have about 5k miles on the bike. It is a 2002 XR650.

When I ran the test it read zero with the decompression lever in and 30-40 psi without - well below the 160 they say it should have. There are a couple of things that I think could be contributing and want your guys advice. I did measure it with the throttle wide open and fully choked.

1. The compression tester stem is not long enough for me to get a wrench on it. I can only hand tighten it. Is this a big deal.

2. I measured it cold (had the stator out). Will this cause the large problem I am seeing?

3. When will the auto-decompression kick in.

How bad would it run if I only had 30-40 psi.

Normally I would tear in and check everything out, but we race baja in 4 weeks and the list of things to get done is long. I want to make sure I have a problem before diving in.

  • sorenlaf

Posted May 01, 2006 - 12:04 PM

#2

I had my bike apart and decided to run a compression test. It feels slightly down on power compared to my buddies bike, otherwise all is good. If I believe the prior owner, I have about 5k miles on the bike. It is a 2002 XR650.

When I ran the test it read zero with the decompression lever in and 30-40 psi without - well below the 160 they say it should have. There are a couple of things that I think could be contributing and want your guys advice. I did measure it with the throttle wide open and fully choked.

1. The compression tester stem is not long enough for me to get a wrench on it. I can only hand tighten it. Is this a big deal.

2. I measured it cold (had the stator out). Will this cause the large problem I am seeing?

3. When will the auto-decompression kick in.

How bad would it run if I only had 30-40 psi.


First, it SHOULD read zero psi with the decomp lever in - that's what it's for... :crazy:

Next, measure compression with throttle fully open and choke fully open - the closed choke ("fully choked") may be affecting your reading because the cylinder cannot fully fill with air.


Your other questions:

1. If you hear gas escaping, then it's a problem. If there's an o-ring where the tester hits the head, hand tight (in my experience) is okay, but I'm not there to listen, so I can't really tell.

2. cold is okay.

3. don't know, if you have a shop manual, it should detail the correct procedure.

I do not think it would run at all w/ 40psi compression, since it runs, I suspect you have a problem with the measurement.

I suggest you repeat with choke OPEN, throttle OPEN, and leave the decomp lever alone. Kick it over until the needle stops moving.


--Soren

  • luvmythumper

Posted May 01, 2006 - 12:25 PM

#3

Thanks for the info. Let me clarify:

1. I did run it with the choke open, my description above is wrong

2. there is an o-ring on the tester. I could not hear any air leak, but while kicking the bike over it may be hard to hear

3. The manual says it should be 86 psi with the decompression and 160 without. I am guessing they are referring to the auto-decompression.

I don't believe there is a way to disable the auto-decompress. I may try kicking it over slowly enough as to not engage the auto-decompress.

  • Xr650rhonda

Posted May 01, 2006 - 12:50 PM

#4

I think a leak down tester is a better way to go, it pin points the problem, easy to put one together if you can't buy one.

  • goofour

Posted May 01, 2006 - 02:29 PM

#5

I have a 2001 and tried a comp test after having trouble finding the compresion stroke when tring to start the bike sometime its their after three kicks somtimes it takes a lot more. With the carb removed I came up with only 60 lbs and a leak down test with a 100 lbs in held 98 lbs. I checked valve clearance the exhaust was .008 and the intake was .002 in, I adjusted the intake to .006 but it did not change the comp test. It did make kick starting much harder sometime I can drop all my weight on it and the motor won't even move. Are you having any of the same tbl?

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  • qadsan

Posted May 01, 2006 - 03:23 PM

#6

...after having trouble finding the compresion stroke when tring to start the bike sometime its their after three kicks somtimes it takes a lot more...

This was one of the symptoms I had a few years ago. Sometimes my compression stroke wasn't there when it should have been and it 'sometimes' took a few cycles to find it. I also had the engine 'rarely', but intermittently cutting out and feeling flat in terms of power at higher RPM like I was hitting the rev limiter when it wasn't, but my most common symptom was not being able to feel TDC when I was supposed to and that's what clued me into my auto decompressor problem.

My auto decompressor mechanism was failing, so I upgraded to a Stage 1 HotCam that doesn't use the auto decomp mechanism and never looked back. It's ran simply awesome ever since :crazy:

  • luvmythumper

Posted May 01, 2006 - 04:01 PM

#7

I don't have a problem finding TDC or a starting problem. I don't have a leak down tester so may check into that. Valves are in spec, not a problem there. Will play around with it tonight. Thanks for all the replies.

  • HawkGT

Posted May 01, 2006 - 05:52 PM

#8

[snip]...
3. The manual says it should be 86 psi with the decompression and 160 without. I am guessing they are referring to the auto-decompression.

I don't believe there is a way to disable the auto-decompress...[snip]


I'm almost certain they are NOT referring to the auto-decompress but instead the manual decompressor. The wording is "decompressor applied" vs. "decompressor not applied". Like you said, there is no way to disable function of the auto-decompress.

Even with the manual decompressor applied, there is still some compression created in the combustion chamber--should be 87psi on a fresh engine when the test is performed correctly (engine warm, choke off, ground plug wire, throttle wide-open, keep kicking until the gauge stops climbing--it will take several, maybe 5 kicks). The 160psi number should be seen when the decomp lever is NOT pulled. Note that the auto decompressor is still active at that point. The auto decomp cannot be disabled manually--it disables its self once the engine is spinning at or above ~800rpm. Normal kicking gets you about 400rpm. 400rpm is what you should kick at during the compression test.

I would think that kicking over a 650cc piston at 10:1 compression would be nearly impossible if not for (at least) the auto decompressor doing it's magic behind the scenes [shrug]. CR500s are only 500cc at 6.8:1 (low speed) compression and they can be a bear to kick.

I've done a compression test and saw ~87psi with the compression lever pulled. I didn't look for the 160psi reading with the lever not pulled because I have a very hard time kicking through TDC with only the auto-decompress helping me--basically I can't do it well enough to get a proper guage reading.

  • qadsan

Posted May 01, 2006 - 07:37 PM

#9

...I would think that kicking over a 650cc piston at 10:1 compression would be nearly impossible if not for (at least) the auto decompressor doing it's magic behind the scenes [shrug]...


It's a piece of cake kicking over our 650R's without the auto decomp mechanism. Even my kids do it! No joke!

The auto decomp mechanism doesn't do a whole heck of a lot in terms of making the XR650R that much easier to kick over when compared to the auto decomp mechanism on some of the other bikes such as the YZF/WRF bikes. On the older WRF's I've done, installing a newer updated factory Yamaha cam with the auto decomp mechanism definitely makes a noticible difference in the ease of kicking those bikes over.

It's actually quite easy to remove the auto decomp mechanism from the 650R's cam and I've done it plenty of times for other people. It's just a matter of removing the cam, pulling off the cam gear (NOT by the ears or they'll break!) and removing everything between the cam gear and the lobe. There's a drilled hole in the stock cam directly underneath the auto decomp mechanism that provides oil to the mechanism and you simply weld it shut. Two seconds with a mig welder and its done, or you can epoxy / JB weld it shut, etc, but I prefer to weld it shut. If you leave it open, then the oil from that hole takes the path of the least resistance, which is back down inside the cam chain cavity and I'd rather make better use of the oil / pressure, which is why I weld that hole shut.

As long as the bike is properly started (kick from slightly past TDC), there will be no kickback issues. But, if you try to kick it like a two stroke, then you run the risk of possibly being seriously injured if the engine momentarily spins backwards while starting the bike. Start it from just past TDC and there will be no issues for a bike without the auto decomp mechanism.

  • HawkGT

Posted May 01, 2006 - 08:40 PM

#10

It's a piece of cake kicking over our 650R's without the auto decomp mechanism. Even my kids do it! No joke!

The auto decomp mechanism doesn't do a whole heck of a lot in terms of making the XR650R that much easier to kick over when compared to the auto decomp mechanism on some of the other bikes such as the YZF/WRF bikes. On the older WRF's I've done, installing a newer updated factory Yamaha cam with the auto decomp mechanism definitely makes a noticible difference in the ease of kicking those bikes over.

It's actually quite easy to remove the auto decomp mechanism from the 650R's cam and I've done it plenty of times for other people. It's just a matter of removing the cam, pulling off the cam gear (NOT by the ears or they'll break!) and removing everything between the cam gear and the lobe. There's a drilled hole in the stock cam directly underneath the auto decomp mechanism that provides oil to the mechanism and you simply weld it shut. Two seconds with a mig welder and its done, or you can epoxy / JB weld it shut, etc, but I prefer to weld it shut. If you leave it open, then the oil from that hole takes the path of the least resistance, which is back down inside the cam chain cavity and I'd rather make better use of the oil / pressure, which is why I weld that hole shut.

As long as the bike is properly started (kick from slightly past TDC), there will be no kickback issues. But, if you try to kick it like a two stroke, then you run the risk of possibly being seriously injured if the engine momentarily spins backwards while starting the bike. Start it from just past TDC and there will be no issues for a bike without the auto decomp mechanism.


Yeah, I guess the Hotcam cams don't use the auto decompress and no one complains too much. "...nearly impossible..." is surely an overstatement. I suppose what I was trying to get at is that without the auto decompress OR manual decompress it wouldn't be fun or easy for me (I kickstart in sneakers and such when street riding). As long as the manual decompress is there perhaps the auto decompress is redundent.

  • HawkGT

Posted May 05, 2006 - 03:22 PM

#11

Unfortunately, I have to report back and admit I was totally wrong [embarrassed]. I must have completely mis-remembered what I did and what results I got when I did a compression test once before. I just did another compression test and with the manual decompress pulled the reading was 0psi. When I didn't touch the manual decompress and just kicked it over (my ankle!) I got various readings up to 175psi. Holding the throttle open makes a huge difference in the readings.

Basically, I DON'T really know for sure how to do what the service manual says. I guess I don't really get how you tell if the auto decompress is active or not.

Sorry for the bad info.





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