Compression test PSI on FJ/FZ 600 motor?

Trying to figure out why this engine stopped.. First thought it siezed.. Deeper I dig in the less sure I am about that. Turns over nicely with a box end on the Aternator bolt, no odd noises from anywhere. Oil was clean. Cam lobes 'Fine' as are the Cam bearings.. So oil or pressure loss not evident. :?
Might have heat siezed. ..Don't have a Borerscope (too pricey that's why I need to know the Compression Pressure ranges.
Anybody have 'em handy ?

 

Range shouldn't be too far out from the XJ 650/750.
Specifics for the XJ's:
Standard 156 psi/11kgcm
Minimum 128 psi/9 kgcm
Maximum 171 psi/12 kgcm
Maximum difference
between cylinders 14 psi/1 kgcm
My advice to you would be that as long as you've got over 90 psi, you should have enough to support combustion. I've seen these motors run with 90 in a few holes but it certainly indicates heavily worn rings. If you pull anything out over 120, be happy.
Another question comes to mind if your finding a clean engine. Have you checked for spark? How about fuel flow? Gotta have those as well for the big bang to happen. Good luck!

 

Also, don't be discouraged if you pull lower numbers.

Remember that the engine is supposed to be hot when you do the test. Also, the longer that it sits, the lower the number will get. I think this may have something to do with oil draining away slowly?

The point is: If you get somewhere lower then you were hoping for, don't forget the two above factors.

 

Also, if it's been sitting a while, a squirt of 30W oil in each cylinder will help. The motor that's in my bike checked all over the place with no oil. Put a squirt of oil in each cylinder, and it balanced out to 130 across the board. It's now running beautifully...

 

As I recall from my old wrenching days (cars) squirting oil into the cylinder is a trick to see if low compression is caused by the rings. If compression comes up with oil then rings are the problem, otherwise look to the valves and /or head gasket.

Compression problems usually come on slow. Also, I'd think you would get some kind of poor firing even with low compression. My bet is you find the problem is with spark or fuel.

 

it seized on the highway, power dropped extremely suddenly, hit the kill switch, pulled the clutch and coasted to a stop
waited for it to cool an hour later the starter wouldn't turn it, could push it though with difficulty

now though it turns over fairly easily and nothing visibly wrong with the top end
it's definetly a seizure, not a fuel or spark issue

a dry(no oil) and wet (pouring oil through the spark plug hole to temporarily seal the rings) compression test are very different things and are used to see if the rings are what is causing the compression loss (so saying don't worry about low compression without adding oil to the cylinder is a bit silly)

just wanted the compression values to check which, if any, of the cylinders seized as that would have eaten the rings and ruined compression

 

sounds like your rings are actually gone and the oil temporarily sealed them

 

sounds like your rings are actually gone and the oil temporarily sealed them

your difference between the dry and wet values shows that the rings don't hold compression (rings should be able to hold compression bone dry) and the only reason you had decent values was that the oil was sealing the bad rings
getting those two results from the wet and dry tests is a test book example of having blown rings

 

According to my Genuine Yamaha Service Manual for the FJ600 L/LC and N/NC (works for the XJ600 as well)

Compression Pressure (at sea level): 1,079KPa (11kg/cm^2, 156 psi)
Minimum: 980 kPa (10kg/cm^2, 142 psi)
Maximum: 1,128 kPa (11.5 kg/cm^2, 164 psi)

Maximum difference between cylinders: Less than 98 Kpa (1kg/cm^2, 14 psi)

They recommend in the manual to repeat test with a few drops of oil squirted into the cylinder that reads low:

Reading: Higher than without oil
Diagnosis: Worn or damaged pistons

Reading: Same as without oil
Diagnosis: Defective ring(s), valves, cylinder head gasket, or piston is possible.

Reading: Above maximum level
Diagnosis: Inspect cylinder head, valve surfaces, or piston crown for carbon deposits

Here's the entire manual page: http://www.cs.toronto.edu/~relu/bike/DSC04659.JPG

 

Thank you.. V much!
My Haynes manual ( barely worth owning... unfortunately) has No such usefull info, not even Torque values.

 

just wanna get it clear...

the squirt in oil thingy , if the ring and or pistons are worn [except a big hole] there will be a increase in compression.

A squirt , would be the equiv of 2 to 4 table spoons...

 

2 to 4 tablespoons sounds like too much. You don't want to take the chance of hydraulic lock on the piston!

I would say 1 tablespoon at very most.


Also - having to place oil in a cylinder to raise compression does not ALWAYS indicate bad rings. If your rings have stuck to the piston, like when a bike sits for 5 years, then the oil helps to free the rings. When the bike fires back up, the rings pop back out and seal well.


Also: Just because a bike is hard to push doesn't mean the engine is siezed. In gear, you wouldn't be able to push it at all. My bike is hard to push in gear too!

 

Exactly my point by saying "if the bike has been sitting for a while".

Like I said, my motor runs beautifully now, I can go out and check compression now (since it's been running) WITHOUT oil and get perfect compression #'s.

 

Loss of power and hard to push can also indicate transmission problems such as seized/ collapsed bearings. Failure to start could be coincidental.

Gotta look at all the options but after reading your original post when the motor went belly up I would say you've done a main or conrod bearing. Letting it sit for a while may have let a bit of oil seep in and free up the bearings a little to ease the cranking now.

Youll soon know if it fires up.


Danilo's original post HERE

 

Interesting .. But WHY would either a rod or journal bearing just 'leggo" ?
They showed little to No wear 1500kms earlier when inspected/measured, oil was fresh (day before). Not argueing your thoughts, it's undeniably Fubar'd ...just trying to deduce what could concievably cause this

 

Hmmm, if you are saying the bottom end was stripped and checked 1500 km ago then that by itself leads me to worry about the bottom end.

Any number of contributing factors. contamination in the oil gallery?, bearings were greased in reassembly? Oil pump took the day off? Blocked oil pickup? Loose or leaky (sucking air) oil pickup?

My thoughts are based on if something breaks after it has been rebuilt then something wasn't put together right.

Only way to get to the bottom of it is strip it down. If it was me I'd pull it down ust to find where I might have gone wrong in rebuilding(If that is the case.)

 

Spot on HG , totally agree , also one could say "what if" the fuel tap leaked at some time and operator never knew it and cases filled right up and then motor got a pasting...... it would only have to happen once!

many unkown factors there....

 

I'm in agreement with the guys, pull the motor. To darn expensive to repair if you have a broken something floating around in the motor. Ounce of prevention at this point in my opinion.

 

Yes.. I thought that that might come up
Anything is possible... But after 1500 miles ??
C'mon,, on my planet, assembly errors show up in minutes not weeks.
Let me say that I built my first SCCA engine in 1969, tuned tz's for battle inna early 70's..Built Lotii and Jaguars, I have some small experiences with motors.
The entire engine went together well, Torqued,lubed and double checked..as in normal practice. No problems occured nor were expected...it's a really simple /dumb motor.. just way too many fasteners and Gawd is it Heavy!
The head recieved special attention: Ported, valve work , flowed, mm wde valve seats. It ran 'jes fine'.. thank you.
It did display a tendency to run 'hot' though, although mixtures were reasonable as deduced by plug readings. However never even been near one of these things before (son's Bike)
I remain unsure of just how hot the damned thing normally runs

Something 'let go' ..absolutely no arguement there... Only a full autopsy will reveal the actual reason.
Frankly I'm not overly interested in it, other than 'the Kid' wants to fix and rapidly 'lose' the thing.

 

So what planet is that ????

 

Incorrect bearing replacement is one example of something that may take 1500 k's to show up. Eg, each bearing housing must be measured and matched to crank sizes(numbers stamped on casing and crank) otherwise binding may occur if the wrong bearing shell is placed in the wrong spot.

Oil galleries can also get blocked due to the fact that parts have been disturbed and dislodged something. Many areas are hard to check for cleanliness.

Not having a go at your skills Danilo, just pointing out that it's usually the simplest, overlooked items that can contribute to the demise.