Topic: Need advice after compression test

[somnas]

Hello,

Just did a compression test on a CB500 -73 that's been standing for at least 5 years, which I bought earlier this year. Now my question is:

Should I with these numbers (after changing oil, spark plugs, checking cam chain and valves) try to get it running?

The numbers sure are better than I expected, even though they of course aren't optimal.

All numbers in PSI.

First dry test.

1: 117
2: 113
3: 80
4: 110

Pretty even except that number 3 cylinder...

I then poured a little bit of fresh engine oil into the chambers and got these numbers.

1: 185
2: 182
3: 150
4: 163

Very surprised about the high numbers on #1 & 2. Interesting to see #4 not being closer to #1 & 2 in the wet test when looking at how close they were in the dry test. The valves coming into play there I guess.

So, what do you think about its condition? You never know if it's right, but the tach is showing about 64.000km, or about 40.000 miles.

With no history on the engine, I'm afraid of things like the cam chain tensioner and perhaps piston rings gotten brittle over the years.

Thankful for any pointers on what action to take to be safe and making the most out of this engine.

Thanks

Erik

[pjlogue]

I would get the bike if you can and put a little time on the engine and then retest the compression.  Things like rings aren't going to get brittle with time.  The only thing that will harden and get brittle are "O" rings, or rubber parts. Your compression #'s are low for #3 but it might be because the engine hasn't been run in years. 

-P.

[somnas]

Thank you for your replies! I will then proceed to install what's needed to try and get it running.

To be more clear: The test was done with carbs removed and using the kick starter.

[pjlogue]

For an accurate compression test you need the engine turning over quickly (using electric start)  The kick start doesn't spin the engine fast enough to get an accurate reading.

-P.

[TwoTired]

Please read the FAQ
http://forums.sohc4.net/index.php/topic,369.msg476032.html#msg476032

The quantity of oil you added changed the compression ratio, as well as the sealing the rings better.

Further, the type of tester you use matters for absolute pressure reading.

Since you have found a cylinder reading different than others, a Leak down tester will let you hear where the pressure is escaping, aiding an accurate diagnosis.

[Jerry Rxman Griffin aka MuthaF'er]

Perhaps #3 was the open cylinder and the valve got a little rust/corrosion growing on the seat that needs to be cleaned off by a few miles?   

[BomberMann650]

Intake valve cleaner will break up the carbon and encourage it to leave the motor.
If it’s at a stage that it can be run for a while.

If you have a reverse cone muffler with a baffle, your bike will blow smoke rings until the cleaner is gone 😆