Compression test 90 psi every cyclinder

[RóisínDubh86]

Hi guys


First timer here, Iv been doing a lot of reading and found some great information on the K100, loving the website and the knowledge available on here which has brought me to post this forum. So I recently bought a 1986 K100 LT, my plan is to do a cafe racer build on it as I seen one at a bike show last year and really amazed me.


So like I said Iv been doing some reading on here and have heard how reliable and pretty damn indestructible they are, which is a major reason I went for the K100.


So before I went stripping her I done a compression test to see what the engine condition was, I know it's not the most reliable test but the handiest to do, so I got up to working temp, well I assume so as the cooling fan came on, had throttle fully open and charged the battery the previous day, the readings weren't what I was hoping, so I got between 85-90 psi on the 4 cyclinders, i believe they're meant to be up about the 140 mark??


Is there anything I'm missing with the test?? I didn't remover air filter but can't imagine that would make that much of a difference, so I'm thinking just general wear and tear, the bike only has 68k miles and Iv heard they can go for much more.


So before I strip the engine down is there anything I could be missing??


Any wisdom appreciated


Thanks 

[Laitch]

So before I strip the engine down is there anything I could be missing??
Any wisdom appreciated

If it runs well, idles well, and can overtake on the roadway without coughing like a cigar smoker, leave the engine alone and figure out what you are doing incorrectly to test it. Maybe one of these engines would wear out in such a balanced way that much within that little mileage, but I it seems mighty weird.

Did you find it abandoned in a bog with flora inside the fuel tank and mice in the air box?

Post a photo of it, then after you get your answer here, start a Projects thread with a Before photo prior to surgery.

[Chaos]

you could always squirt some oil in the cylinders and see if the reading jumps up, but it would be weird for all 4 cylinders to wear evenly. 

[The Mighty Gryphon]

I was always told that as long as the engine is running well consistent compression across all the cylinders is more important than high readings.

[The Dude]

Yeah.Consistency is the aim.Is your compression gauge accurate?It's repeatable at least,hopefully.

[Martin]

Compression gauge reading can vary between gauges, even readings mean more. As per Chaos you can trying adding oil to the cylinder or just ride it.You need to do the reading with the throttle fully open and the spark plug leads must be earthed or damage to the ignition module can occur. If your worried try another gauge that is known to be accurate.
Regards Martin.

[The Mighty Gryphon]

Couple things could be impeding the airflow necessary to get a good reading.

First is a dirty Air filter.  Have you checked yours?  You might want to remove it for the test.

Second is the air flow sensor.  The flapper will restrict air flow at cranking speeds.  You might want to hold it partially open for the test.  Be very careful while you are messing about in the throat of the sensor.  There is a fragile air temperature sensor in there that is easily damaged and impossible to repair.

[RóisínDubh86]

Thanks for all the feedback gents much appreciated


Hasn't been lying up fo long or so Iv been told, the guy I bought the bike off had been driving it up to a few months before i done the deal, it starts fine even without the choke, seems to run fine, havnt driven her as the previous owner had it half stripped already. But according to him it drove fine..


I was thinking the gauge although it's brand new and the first time Iv used it, will borrow another and do a retest to rule out the gauge, and I'll remove airbox this to eliminate the blockage to airflow. Gryphon is the sensor on the airbox??


Valve clearance is also something I'm thinking could be affecting the reading, if all where set wrong but again I wouldn't have thought it was make that much of a difference in readings??


Also I was thinking of the oil but I had no way of squirting the oil into the spark plug holes, will need to invest in a can, what oil is recommended for the brick and how much roughly, I drained the oil after test and it ran like water which probably doesn't help the situation.. So will throw a new drop of oil in on retest..


Again really appreciate all the replies guys

[Martin]

Engine oil about a teaspoon crank it a couple of times to get it to coat the bores. A wet test will determine whether it is rings or valves, but due to the consistency of your readings I don't believe there is much to worry about. Normally you don't get consistent reading with ring or valve wear. What brand compression gauge and where was it manufactured.


The air flow sensor is the small inverted U shaped device in the intake of the airflow meter, do not damage this. This is found inside the air box.
Regards Martin.

[RóisínDubh86]

Thanks Martin


The gauge is made by Sealey, pretty sure they're a uk company..
Yeah the readings are pretty consistent, so you think it could be the gauge giving me the wrong reading??
I'll change the oil and put a drop in the spark plug holes and retest with a different gauge.. 👍


Thanks again

[Laitch]

The uniformity of the low readings is what is interesting. Uniformity of compression among cylinders is good. 36% less than stock compression in each cylinder will affect power performance.
You just need to shake out the air filter. You don't need to remove the air box.
Just for review: the engine was warm, all spark plugs removed for the test, the throttle wide held open. My bike has a screw fitting on the handlebar near the throttle that I use to control the throttle for tests.
The valves could have carbon deposits on them keeping them open slightly resulting in these numbers, if your gauge and technique are ok.

A cylinder leakdown test will help you determine leakage and its location because you'll be able to hear it.

[RóisínDubh86]

Yeah that's what I'm thinking I'm not looking to race the thing but I'd like it running at its best specially if I'm putting the money into it cosmetically..


Yes was up to working Temp, throttle fully open, battery charged, so air filter removal is the only thing I didn't do..


Yeah a leak down test would be ideal, I don't have the equipment to hand though, will have to look into it..


I was gonna change gaskets anyway so if I have to take the head off to check valves am I better off doing bearings and piston rings while I have it off?? Iv read the engines are best left alone if possible??

[Laitch]

. . . am I better off doing bearings and piston rings while I have it off?? I

I don't think you are unless you have time on your hands, have money to burn, have a dream machine as a goal, or riding the bike sooner than later just seems too scary.
 :giggles

Aside from regular maintenance, if machinery is working well, I leave it alone. A leakdown gauge and bore scope are only some of the tools serious mechanics should have. If you are considering complete disassembly of the engine, that's serious.

The only reason I know anything about mechanical repair is to keep my bike moving well so I can ride it—not work on it. If it looks a little ratty, that's ok by me. I let somebody else take the theft risk.

Of course in Vermont, everybody loves their brothers and sisters—spiritually, and physically when there's a chance. It's a wonder we have the energy to be as self-righteous as we are. Sometimes I like to go to Boston or Montreal though, where dream machines can disappear.

[The Mighty Gryphon]

Aside from regular maintenance, if machinery is working well, I leave it alone. A leakdown gauge and bore scope are only some of the tools serious mechanics should have. If you are considering complete disassembly of the engine, that's serious.

Back in the early 80's I attended an engineering conference on improving the quality of American automobiles.  One of the speakers was the VP of engineering for Ford Motor Company.

Two things he brought up have stuck in my mind to this day.  One was the incredible cost of manufacturing defects; even one bad part in 10,000 cost Ford millions of dollars in lost profits each year across all their product lines.   

The second was the drive to make the vehicle "tamper-proof".  Ford was spending millions of dollars on manufacturing process controls to build a "perfect" vehicle.  They were confident that they could build a vehicle that could reliably run up to 200,000 miles.  The one thing they couldn't control was the tampering with the critical parts by unqualified mechanics, especially during the warranty period. 

A major problem for them was the high number of warranty claims for problems that were attributable to owner "tampering" with drive train components. 

If it's running good, leave it alone.

[Laitch]

A major problem for them was the high number of warranty claims for problems that were attributable to owner "tampering" with drive train components. 

So that's the reason their trucks had all those black marks in Consumer Reports reliability surveys in the eighties. Sure it was.  :hehehe

To their credit, I managed to plow 20 hours worth of heavy snow with a failing u-joint in the front drive shaft of my '97 F250 and it squeaked its way through it with no problem.
  :clap:

[Chaos]

The one thing they couldn't control was the tampering with the critical parts by unqualified mechanics, especially during the warranty period. 

I'd say they're doing a damn good job fixing that problem.   :clap: Looking at the unfathomable computerized gee whiz offerings from BMW and others is what caused me to keep my K75.

[Martin]

Just ride it if it's not making horrible noises and blowing heaps of smoke. :nono Don't worry be happy, or one of the other seven dwarves.  :hehehe
Regards Martin.

[RóisínDubh86]

Thanks for all the support guys


Will be redoing the test hopefully this weekend with another gauge, will post the results then decide from there 👍

[Nine80seven]

Bought my KRS, '84, fairly cheap with factory trained BMW tech readout on compression at number two, 80 psi.  Bad valves?  Ruined cylinder? Oh well, lots-o-parts.  Rode it home.  Leaking rear master, both front brakes soaked in fork fluid and so on. 


Using screw in rubber hose type thingee with rubber o ring seat tried comp. test.  Didn't work.  Went, phweeeze.......


Dug out ancient Sears super nice antique hand held comp. test meter.  162, 156, 158, 158.  Repeated three times to verify.  Next did leak down with 120 psi at each cylinder.  Absoltootly no leak from muff or case.  83K, the rest is history as I wrench and ride this old bike.  Never know, sometimes, strike out, other times not.  And always need stash of spare parts.

[RóisínDubh86]

Nine80seven Good stuff man 👍




Hopefully I have some of the same luck, I'm a lil more optimistic now..


Cheers man

[Vespa no more]

Are you doing the test right?


Do some research - I'm not saying I know....


Did a test on my bike and got mixed results.


Then Martin came in with "The throttle needs to be at full throttle" when you do the test....


Can't see any explicit reference to this in the replies above (including Martin!)


As they say, if it runs good enough it probably is.


Guy

[RóisínDubh86]

Well I think so, yeah had throttle fully open, the only thing I didn't do that I've seen suggested when doing a comp rest is remove the air filter.. I can't see that making that much of a difference, plus the readings are pretty consistent so I'm hoping it's the gauge 👍


Thanks for the replies


All appreciated lads

[Martin]

Unless the air cleaner is blocked it should not effect the compression test. Sometimes low readings can be caused by the rubber on the end of the gauge not seating properly or sticky or dirty valve in the end of the gauge ( very few people replace or clean this valve) . Failure to seal can be due to hardening of the rubber on the gauge, incorrect angle when pushing it in, not keeping enough pressure on the gauge. The gauges that screw in eliminate these problems as long as you keep the "O" rings clean.
Regards Martin.

[RóisínDubh86]

Sorry just back on this now, ended up being a dodgy but brand new pressure gauge [emoji85] Didn't bother trying with another I'm just gonna assume she's ok [emoji1303][emoji1303] Thanks for the input

Sent from my SM-G950F using Tapatalk

[Martin]

 :2thumbup: Send the gauge back, they should replace it under warranty.
Regards Martin.