Compression Test Goof Up

[sembrando]

Just performed a compression test on 1986 k75 with 101,000+ miles (odometer hasn’t worked in a while). Trying to get the old girl back in fighting shape before summer. 

This is my first compression test on this bike (I’ve owned it for 8 years and 25,000 miles) and anything ever. So I got lower numbers: 1-107 2-102, 3-107. Then, after I packed everything up I remembered that I didn’t open the throttle for the tests! Although, I did crank till it stopped climbing (6-8 cranks). I won’t be able to go back to it for a day or two. So my questions:

1) Has anyone experimented with throttle open vs. closed? Results?
2) From what I understand consistency is more important and cylinder 2 was just a touch lower than the rest. How great of a variance is normal?
3) Next up is checking valve clearance. Would a tight/loose valve affect compression?

Thanks for helping out.

[volador]

Checking Compression

Disconnect terminal 15 positive supply wire(green/yellow) on the ignition coils to shut down the high voltage side of the ignition

Pull plug out of the injection control unit to stop injection of the fuel into the cylinders (if not, the oil film will be washed out)

or
http://www.bmw-k100rt-page.eu/manual/Engine/compressionTest_2_Engine.htm

[Laitch]

1) Here's an explanation.
2) Acceptable difference is the standard. Approximately 10% difference between highest and lowest values among cylinders is generally acceptable.
3) Unless it were drastically damaged, probably not, but it could affect performance and eventually might damage the engine if not adjusted to spec—especially an exhaust valve.

[sembrando]

Thanks for the input. That article would suggest closed throttle is within a few percent of wide open throttle, so its probably still a little low on compression, but not terrible. More importantly they are well within 10% variance of each other.

If I redo the test and obtain drastically different results, I’ll post it here. Maybe I can find someone with leak down test equipment. Might not worry about it too much, runs decent enough as is. I think she’s ready for the next 100,000 miles.

[Laitch]

More importantly they are well within 10% variance of each other.

Not of each other, but rather within 10% of the piston with the highest compression value is the way to determine their condition. Your engine's initial test numbers were extremely low—a BMW poor rating is <120psi—but they are within acceptable range between highest and lowest; you should redo the test with wide open throttle.

[sembrando]

Ok folks,

Got her all warmed up again and redid the compression test with wide open throttle with some pretty interesting results. 1-170; 2-182; 3-180.


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These numbers are WAY higher than the 102-107 yesterday at idle throttle. I need a new battery because I wanted to experiment a bit with throttle position and compression, but the cranking was starting to slow.

Also seem to be within the 10% variance from the highest to lowest reading, which is nice.

Next on the spring tune up is a valve clearance check. I pulled the valve cover off, but since the head was still warm that'll wait until tomorrow. Being this is the first time I've ever checked valves on anything maybe someone could answer quick: do you count the reading as what feeler you can push in there or the last one to slide in easily?

[The Mighty Gryphon]

The clearance number is the feeler that slides into the gap with a little drag.

I like to see the widest gap that is in spec.  .008" for the intake and .012" for exhaust.   Those are the feelers I try first.  If they go, everything's cool.  .009" and .013" shouldn't go at all.

If they don't go, I go the the low number,  .006" for intake, and .010" for exhaust.  If they go, it's okay, but I'll make a note of the clearance and make sure I check again after the next 10,000 miles.  If they don't, for sure you have to be changing the shim.

I'm pretty fussy, and with the valve clearances, I try to keep them all about the same, with all the exhaust valves as close to .012" as I can get them.  My experience is that once the engine has some miles on it, the valves don't seem to close up very fast, so they hold the clearance pretty well.  It's worth spending a little time getting the gap the same for all cylinders to get the engine as smooth as possible.

[Laitch]

Being this is the first time I've ever checked valves on anything . . .

I'm pretty fussy . . .

I'm not fussy at all; I just set them within the published spec. In the end though, many of us seem to wind up at some point wondering why our moto's engines won't start regardless of what we do.
 

Here's a video.

[The Mighty Gryphon]

Ouch!

[johnny]

thats when members of the society of cappuccino craftsmen and dumpsters deloop them and list them on ebay for 10 large as ran before i delooper her...

[sembrando]

Thanks for the reply. Just to keep the saga going: checked the valves using Chris H.’s video as a reference for the cam position and for what is a “go” vs “no-go”.

Results (I put the number for the one that went with a small push the one that went with more than a little push; very scientific):

I: 1- .0025-.003; 2- .0025-.003; 3- .004-.005
E: 1- .008-.009; 2- .008-.009; 3- .007-.008

The good news: they all had some gap; the bad: they are all out of spec.

The consistent out of spec makes me think I did something wrong, but I started each time with the minimum and always a definite “no go.” Although, the thinner you get the softer you can push before it just flexes.

[Martin]

To minimize the amount of shims required shuffle them around from one to another to get some into spec. Once you get as many as you can into spec then work out what shims you need. Some workshops will let you do a swap for a small exchange of cash or booze.
Regards Martin.

[Laitch]

I: 1- .0025-.003; 2- .0025-.003; 3- .004-.005
E: 1- .008-.009; 2- .008-.009; 3- .007-.008

I can't understand your numbers. I think your millimeter to inch conversion—if that's what it is—is incorrect. Please explain how you arrived at it. According to your figures, all your valves are tight.

[sembrando]

My feeler gauges are in freedom units (inches); no conversion was done; I simply wrote the number on the gauge. The two numbers, if that is what is puzzling is the one that went in with little effort and the one of greater effort; probably should have only put the smaller one.

I share your concerns on all of them being tight.

[Laitch]

My feeler gauges are in freedom units (inches);

How many freedom units equal 0.15mm?

[sembrando]

.0059 inches = .15mm

So clearances should be (in freedom units):

Intake: .0059-.0079 (.15-.20mm)
Exhaust: .0098-.012 (.25-.30mm)

[Laitch]

.0059 inches = .15mm

Good. On the intake valve, you should have been starting with a 0.006 feeler gauge.

Here's how it works. With an intake valve, if a 0.006 gauge will slide in but a 0.008 gauge starts to flex, the valve clearance is ok. That's it. That flex indicates a no-go. With an exhaust valve, if a 0.010 blade slides in but a 0.012 blade starts to flex, the clearance is ok. It takes practice.

All these valves are excessively tight as you have measured them.  I'm uncertain you understood the sequence, valve locations and cam lobe orientations as demonstrated by Harris. Review the procedure, try again and come back with go and no-go numbers—not how you did it previously.

[sembrando]

Thanks, Laitch.

I apologize that I'm not being as clear as I'd hope to be. I definitely did start with the smaller gauge when measuring (e.g. .006 for intake) with the cam lobe in the same kind of "up" position as on the video. In each case for each valve, none of the "minimum" gauges made it without flexing. So I just went down from there, until it felt like a "go."

I'm definitely up for giving it another go to see if the numbers are similar. Like you said, it takes practice. I'll let you know the numbers. Maybe I'll grab a photo or two for inspection.

[The Mighty Gryphon]

Is this "Sandra".  The bike in your signature with 100,000 miles?

If so, those measurements aren't unusual.  Over time the valves hammer the seat into the head.  You may be the first person to go in there since the bike left the factory.  As Martin mentioned, you might be able to shuffle the shims around to get things in spec on as many cylinders as possible before you go to the dealer to get the ones you don't have.

Here is a link to a source of less expensive shims.  Get the 29mm.

https://newmotorcycleparts.net/motor_parts/valve_shims.html

[sembrando]

OK, rechecked them twice over. I tried to be consistent in how much resistance the feeler moved in and out with. Again, starting with the minimum gap and working my way down until the gauge was able to move around smoothly without too much force (some articles I looked up on how to use feeler gauges mentioned the feeling of sliding a sheet of paper between glossy magazines).

I also experimented with the cam in different positions, looking for the widest point; that was helpful too. Again, in freedom units (inches):

Intake (minimum .006): .003, .0025, .005
Exhaust (minimum .010): .008, .009, .008

[Laitch]

starting with the minimum gap and working my way down until the gauge was able to move around smoothly without too much force
I also experimented with the cam in different positions, looking for the widest point

It's go or no-go. It's simple. There is no need to experiment to find the widest point. The point of greatest clearance was explained by somebody who knows the procedure. You can clarify it yourself by watching this video of how a cam lobe rotates and interacts with the shim and bucket arrangement of your moto's engine.



Some may not be surprised by your numbers but I am. If they're accurate, the moto would likely have idling, stalling, starting and fuel economy problems. Does it?

According to you, your compression numbers are good so intervention with valve adjustment is timely. If you've measured correctly and your numbers are accurate, the next procedure is to adjust the valves by removing the shims, calculating the size of a replacement that will give you correct clearance, then replacing the shim. The tools I use are a micrometer, an extendable magnet and Ken Lively's valve shim adjustment tools then deploy them in this way.



Wisconsin is a hotbed of Brick wrenchers so maybe somebody there will loan them to you if you want to use them. You can always experiment. 

Your next task is to adjust the valve clearances within spec then follow with a throttle body balance.

[sembrando]

Thanks again for the feedback.

The video is helpful and confirms what I was seeing as the cams went around.

Don’t worry, those numbers surprise me too, although it doesn’t run perfectly there are a list of things that I am hoping to sort out. Valves happened to be at the top of the list.

For the intake I’m pretty confident in the numbers, it was clearly a go vs. No go. With the exhaust (perhaps because of the thicker/less flexible gauges) I could stuff the thicker one in, but it wouldn’t move around easily.

The idle is ok once it is warmed up, mpg is around 40, there is a slight tick at low RPMs especially when it’s warming up.

Thanks for the valve adjust info,

[The Mighty Gryphon]

Because the valve clearance is intended to accommodate the thermal expansion of the valve stems, you might not see the negative effects of the clearance until the engine is fully warmed up.  Clearance also has an impact on how long the valve is open, and how much it is opened.  Tight valves are open longer and open wider, allowing the engine to breathe better at high rpm, but can cause rough idle and backfiring on closed throttle.

The danger is that the exhaust valve is not fully contacting the valve seat in the head and having the heat removed.  This, along with hot exhaust leakage is how most burned valves happen.  It's the reason why I try to get the clearance at the top of the spec in my engines. 

When the bike is running 85-90mph at 6-7,000 rpm in the desert in temperatures near 120F it's nice to have the exhaust valves making good contact with their seats.  This was a lesson learned the hard way with 60's vintage VW engines during my misspent younger years.  That was the reason you had to check the valves on those engines every 1500 miles.