Low compression


15 replies to this topic
  • illusha

Posted October 03, 2007 - 10:50 AM

#1

I have this XR600 that I only had for a while - ever since I got it, it has been blowing a buch of black/blue smoke, worse when cold. Smoked quite a bit under acceleration, almost none at idle... still had power and everything, no wonder its called a big red pig :confused: I went to adjust the valves (.010 and .012), and then took a compression test, which showed a mere 30... it was always hard to start cold, now it wont start at all... So what's next?

  • cleonard

Posted October 03, 2007 - 11:07 AM

#2

There in an automatic decomp system that operates the right side exhaust valve. This makes for low readings. However, 30 is lower than it should be even with the decomp. If you back the adjuster way out on the right exhaust valve then the decomp system will not actuate the valve and you can get a true reading. Get a reading and then put some oil in the spark plug hole. If it goes way up then the rings are shot. If it does not change much then it's the valves.

Since it was smoking under accel the rings are suspect. However the valves usually go before the rings wear out.

When you adjusted the valves were they too tight when you first measured them? That would point to valves.

  • illusha

Posted October 03, 2007 - 12:23 PM

#3

well, all valves were really tight, BUT I am not absolutely sure I was at the right TDC at that time... I did readjust them a few times afterwards, so now they are done right... they do make quite a bit of noise for .010 gap that they have, and they are angled kinda weird inside of that gap... what does a good valvejob consist of and how expensive can it get?

I was taking the comp reading with the decompressor cable lose and off the bracket, so it shouldn't have mattered... what about the automatic decompressor from the kickstart though - didnt figure that out, it was getting late...

I will try taking a reading with some oil down the plug hole
Any other options or ideas?

  • Bennito

Posted October 03, 2007 - 12:52 PM

#4

I hope I'm not repeating what you already know/read. from the e-manual (can't remember where i found it):

warm up the engine
stop engine, remove spark plug, connect compression gauge
open throttle grip all the way
operate kick starter several times.
check reading.
spec is 71.1 +/- 14.2 psi.

since you're low, you'd then go to the decompresser not effected:
loosen the right exhaust valve locknut and turn the adjusting screw with care so that the valve does not interfere with decompressor cam.
operate the kick starter pedal several times again.
(the manual then specs that 0.82mm will release the half decompressor system).

compression to be 200 +/- 14 psi.

Low compression caused by:
improper valve adjustment
valve leakage
leaking cyclinder head gasket
worn piston rin or cylinder

oil down the pipe is, i believe, the conventional way to determine if its a worn ring or cylinder (Versus valves/head).

Good luck

  • martinfan30

Posted October 03, 2007 - 12:55 PM

#5

dont forget to disable the ignition and hold throttle WOT>

  • illusha

Posted October 03, 2007 - 03:29 PM

#6

ok, did some re-testing...

first, I can't warm it up, cuz it won't start... I did not know about having at WOT, however it did not make any difference - the reading was still about 30-35 regardles of the throttle position... I put 4 straw-fulls of honda oil down the spark plug hole - the first reading jumped up to 50, then the rest of them were at 35-40..........

so, does that mean that it could use new rings AND valvetrain?
I wonder if I could get away with just an adjustment... how does that decompressor work again? doesnt it push down on the valve?

...and I did not understand how Bennito gets 200 psi that other way... huh??? ...

  • cleonard

Posted October 03, 2007 - 06:24 PM

#7

You need to loosen up the right side exhaust valve, by where the manual decomp cable is. I think it needs to be like a few turns of the adjuster loose. Like Bennito says don't make it too loose or it could hit something. Then measure again. Do this before ripping into it.

As far as cost for a rebuild it's impossible to tell until you take it apart and survey the damage. Unless it has like 30,000 miles or seized, the piston and bore are usually OK. It might just need a valve job and/or new rings.

Even when everything is good with my XR600, I have never got that 200 psi. More like 160. It does depend on altitude.

  • illusha

Posted October 03, 2007 - 09:59 PM

#8

ah, I see... what will getting such a reading tell me?

what exactly does a good valve job usually include?
what else should I keep my eyes on when I tear it apart this winter?

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  • illusha

Posted October 04, 2007 - 12:48 PM

#9

well, figured out a part of my problem:
lesson learned: dont trust everything you find on the internet...

the first article re: "xr600 valve adjustment" that popped up in Google was:
http://www.4strokes....da/setvalve.asp


seemed like a decent source, and it clearly says in it to adjust the valves to "intake is .010mm and exhaust is .012mm"... so, I pulled out my trusty .010 and .012 gauges and went to work, not reallizing those are in inches... now that I look at the XR600 manual I found for free at www.repairmanualclub.com, I see that the valves should be actually adjusted to 0.10mm and 0.12mm (0.004" and 0.005")... so I will go try that and see if it will at least get me started first...

  • illusha

Posted October 04, 2007 - 03:12 PM

#10

well, now its adjusted again, I'm getting better and better at this... now it starts ok again, still smokes white and blue, the compression is still at 40, regardless of if I pour oil down the plughole or not - so am I correct on assuming that this points towards a valve job?

again, what all should a good valve job include? thanx guys!

  • cleonard

Posted October 04, 2007 - 03:44 PM

#11

Smoke can come from worn out valve guides, but this is rare. Usually bad valve seals smoke a little at startup only. Smoke usually comes from shot rings. This happened to me and when I took it apart the ring end gap was about 6x spec. The piston and bore were fine so new rings and a hone fixed it. The cost was low. About $45 for the rings and I did the hone. Unfortunately, I looked in the crankcase and saw trans gears that needed replacing. The gears alone were over $200.

It really depends on what the condition of the head is. If the cam and rocker arms are good then it's just the valves and valve seats. When I had to take mine apart I had to replace 2 valve seats and the 2 valves. I think it ran me about $250. If more valves are bad or your cam/rockers are shot then it will be more.

I did these repairs on two different occasions about 2 years apart. Each repair included a gasket kit. The hardest part of either repair was removing the old gaskets. It was a major pain!

These costs are all just the parts cost or the machine work that I can't do. I do all my own wrenching when I can.

Until you take it apart you will not know. With the head the problem may be obvious or it may not. Spray some carb cleaner in the ports above the valves. It will leak through the ones that need attention.

  • illusha

Posted October 04, 2007 - 04:07 PM

#12

so, when I take the head off, how do I know what will need replacing inside? - is it just looking for uneven wear? I've done head gaskets on my 6.2L diesel, so I think I should be able to handle this...

also, in the bore and piston, do I just look for scuff marks? I'm sure there are some expected, so what is the threshold?

also curios, how often is work like this needed on our engines and does anything specific cause it?

  • martinfan30

Posted October 04, 2007 - 09:55 PM

#13

so, when I take the head off, how do I know what will need replacing inside? - is it just looking for uneven wear? I've done head gaskets on my 6.2L diesel, so I think I should be able to handle this...

also, in the bore and piston, do I just look for scuff marks? I'm sure there are some expected, so what is the threshold?

also curios, how often is work like this needed on our engines and does anything specific cause it?


maybe try to turn the cyl. head upside down and fill the combustion chamber with a solvent. you will know pretty quick if you need a valve job by the solvent leaking into either exh or int ports. valve seals are a mandatory and if you need a valve job by the above test have the machine shop check the guides for you.

  • cleonard

Posted October 05, 2007 - 08:45 AM

#14

maybe try to turn the cyl. head upside down and fill the combustion chamber with a solvent. you will know pretty quick if you need a valve job by the solvent leaking into either exh or int ports. valve seals are a mandatory and if you need a valve job by the above test have the machine shop check the guides for you.


I do it the opposite way. Hold the head with the combustion chamber down. Spray carb cleaner into the port above the valve. Enough to get some liquid standing on top of the valve. Hold the head above your head and look for carb cleaner to leak through. A small amount of seepage is normal as far as I can tell. It's not a perfect seal. If it flows through you found your burnt valve.

You need the proper measurement tools to check the piston and the bore. The max out of round and taper is only .002 for the bore. I have a set of micrometers and bore gauges. If you don't have these take the parts to a shop that does. It can be worn out with no scuff marks. I have never seen a worn out piston in any of my bikes, but I have seen worn out rings and bores with too much wear.

If you don't have one get yourself a manual before tearing into it. Aftermarket ones have the critical info, but the actual Honda manual is best.

  • illusha

Posted October 05, 2007 - 10:41 AM

#15

sounds like its easiest to take the cylinder and head to a shop and have them deal with all that... thanx again for all your ideas guys!

  • martinfan30

Posted October 06, 2007 - 09:15 AM

#16

I do it the opposite way. Hold the head with the combustion chamber down. Spray carb cleaner into the port above the valve. Enough to get some liquid standing on top of the valve. Hold the head above your head and look for carb cleaner to leak through. A small amount of seepage is normal as far as I can tell. It's not a perfect seal. If it flows through you found your burnt valve.

You need the proper measurement tools to check the piston and the bore. The max out of round and taper is only .002 for the bore. I have a set of micrometers and bore gauges. If you don't have these take the parts to a shop that does. It can be worn out with no scuff marks. I have never seen a worn out piston in any of my bikes, but I have seen worn out rings and bores with too much wear.

If you don't have one get yourself a manual before tearing into it. Aftermarket ones have the critical info, but the actual Honda manual is best.


good idea. i use my method cuz im used to doing it with auto cyl. heads. kinda hard to hold them up with your method LOL! a little seepage is normal.





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