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My CX 500 is only putting out 70psi / head right now. The bike feels pretty slow out of starts and has no real top end. Makes plenty of noise but not get up and go. After extensively tuning and cleaning carbs and ruling out other options I figured this was it. (Checked compression last...duhhh)
Question: Is 70psi / head low enough to drastically feel a loss in power? I know its not ideal...
And the manual calls for 171 psi....which seems ridiculous.
I have rebuilt engines before with brand new top end components and usually get 100-110 when the manual calls for 160, as an example. So im kind of skeptical...
A warm engine putting out 70 is lucky to run, much less run well.
Are your valves adjusted properly before the test?
171 is correct on a warm fresh engine after a few break in miles less and something is amiss. 120 is okay for a cold engine before break in.
70 is far far beyond time to rebuild. How many miles on the bike?
The CB360 Fresh engine compression is 170 PSI
A cb450 fresh engine is 185 PSI.
Not sure why you feel 170 PSI is ridiculous, it is normal for this type of engine.
Low compression can be leaking valves, bad head gasket, worn stuck or damaged rings.
A little engine work is in your future. As sonix said, it is amazing it runs at all....
Maybe you took the readings incorrectly. If you let us know the steps you took, type of compression gauge, we can see if there is a better way to get accurate results.
- Pulled both plugs
- Insert compression tester in head (other head open to atmosphere)
- Crank engine over w/ starter for about 15 seconds
- 70-75 PSI in L head
- 75-80 PSI in R head
Engine was COLD
Checked valve clearances, all good. But maybe the valves themselves are bad?
Maybe try another compression tester. I've heard of people on this site try to track down their low compression readings with a Harbor Freight gauge, only to realize the gauge was giving false readings.
Lot of variables.
Junk compression testers dont help.
actually a compression test tells very little useful information.
Get a leak down tester and test with that.
You will know if you are loosing compression and then be able to figure out where the issue is at the same time.
doing any of the tests cold is almost a waste of time really. The engine needs to be hot.
Why would 40K mean much? Dont rebuild your car engine that offten so why your bike?
Surfly, I agree a BAD compression test is useless, but done properly, does give some insight into whether the engine is working properly. Good compression is the indicator. Bad Compression means SOMETHING is wrong, but it is not good at telling what it is exactly.
I know you know that, just wanted to spell it out for others.
I see others post the same questions on here and Honda twins. Sometimes I answer both, just cut and paste. Lol. One wonders if they pick the best answer or the easiest answer.
Quick and dirty way to check cam timing is to rotate your engine in the running direction. The moment your intake valve closes with some clearance is about 105 degrees before top dead center. picture is upside down.
Is there a schrader valve on the tube that leads up to the compression tester? If not then you'll have to compensate for the added volume to the system.
However, even with that, 70psi is waaaaay too low. Chances are you have broken compression rings and/or bad valve seals. Time for a rebuild!
You can check to see if it's the rings by adding a some oil to the cylinder. Most people say to add "a teaspoon or so" but that's too much.
Normally, We accept that if adding a LITTLE oil to the cylinders signifigantly "ups" the compression readings, that this indicates poor ring seal.....HOWEVER, as the actual amount of oil added directly effects the volume of the compressed space left with the piston fully up, care must be taken NOT to add too much.....And, the amount/volume added must be known so the pressure reading can be corrected mathmatically.......
Let us ASSUME you added 1 tsp of oil.... (1 tsp = 4.92892 cc...for simplicity, we'll call it 5 cc)
You'll need to know the volume of the dome (on my CL200 it's 19.5cc) and the volume of the hose (about 1cc per1/4" ID hose)
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