Topic: Compression Range for a CB750K2

[nomadwarmachine]

I was concerned with a cool cylinder #1, so after putting 30,000 miles on my CB750K2 I finally bought a compression gauge.  I did a series of tests on a hot engine, cranking it with the electric starter and all of the plugs out.  My readings were 130-130-140-140.  Apart from the slight leakage in #1/#2, I think that these numbers are good enough to keep me going for a while (bike has 42K miles).

Quick disclaimer.  I did not alternate a 'wet' test with oil in cylinders, so I do not have those numbers to compare to.  I did not have a syringe to get the oil in, and had already fried my right hand on the inner headers.  I also used the Actron tester, which has the rubber hose, and I expect leads to slightly lower numbers.

My Clymer states the range for compression on a CB750 as between 130-150, but the specs page on this site lists 170 as the stock compression figure (which seems high to me).  If the Clymer is right, then I feel like I am doing pretty well with my figures.  If the site is right, it looks like I have pretty low compression.  Which is it?

[SteveD CB500F]

If they're all within 10% they're fine.

170 ??

(Prepares to be shot down by resident techies) If air pressure is 14.7 psi at sea level and you have compression ratio of 10:1, then surely 140psi is fine.

[Terry in Australia]

I would have thought that 130-140 (+/- 10% between cylinders) is well into the healthy end of the spectrum, interestingly that old black junker K2 I bought just before Christmas last year had the following cold readings: 175,175,150,175. I've yet to pull the head off but I'm guessing it's running a big bore kit or at least high comp pistons because I've never seen those numbers from a stock engine. Cheers, Terry.

[nomadwarmachine]

Steve + Terry, I thought exactly the same until I did a quick search on the stock compression figures and found 170.7 psi listed on THIS site:

http://sohc4.us/node/65

I agree that, given atmospheric pressure at sea level and a 10:1 ratio, we should be looking at 140psi, but this suggests this calculation is wrong.  Any ideas?  Does anybody with an untouched K-bike have a real compression test to go by?

[TwoTired]

In the Honda shop manual, they have a picture of a technician doing a compression test.  The test apparatus is unlike any I've ever seen in person.  Can't see the whole tester.  But, it is interesting to note the very small diameter metalic tube extending from the cylinder to the gauge being read.  I would surmise that the tester adds very little volume to the cylinder and leads to higher readings, since with this device, there is now a much closer compresion ratio to that of the cylinder while in use, rather than one with the added volume of a large, long rubber tube between cylinder port and guage.

The stock compression ratio is only 9 to 1 with a volume of 187 cc displacement.  This leaves about a 20 cc chamber at TDC.  Adding 10cc of test aparatus seems and easy explanation for 20% lower test readings.

Anyway, your readings are all within ten percent.  Meaning you're good to go.  But, are you sure your valves were all adjusted dead equal across all cylinders?

And, just to give you something for worry. 
The cam wear can also effect comp readings.  Aren't they oiled from a galley at each end?  What would happen if the oil was restricted at one end?  Increased cam wear at that end, perhaps?

Have a nice day.
 

[nomadwarmachine]

Ah, even more to worry about!     Well, I am glad that there is an explanation for my readings, but I am still not convinced that these bikes had 170psi when they were put together at the factory.  Has anybody with a perfect unrestored example ever gotten a reading like this?

[Terry in Australia]

I am still not convinced that these bikes had 170psi when they were put together at the factory. 

I second the motion, or perhaps that should be, I second the notion! Anyway, I've never seen 170 psi in any of my CB750's, but I never did check my shiny new 1975 CB750F that I bought new (well and truly susperseded, no-one wanted the poor homely creatures here in Oz) in 1978, I don't think I would have known a compression gauge from a tire gauge back in those days, ha ha! Cheers, Terry.

[ofreen]

...my shiny new 1975 CB750F that I bought new (well and truly susperseded, no-one wanted the poor homely creatures here in Oz)

It's too bad you didn't hang onto that 'homely' '75 model, since it would probably still be going strong, unlike those F2s with their fragile heads.

[Terry in Australia]

...my shiny new 1975 CB750F that I bought new (well and truly susperseded, no-one wanted the poor homely creatures here in Oz)

It's too bad you didn't hang onto that 'homely' '75 model, since it would probably still be going strong, unlike those F2s with their fragile heads.

You're probably dead right offy, but that's like saying that you should stay married to an ugly fat chick because she'll be more reliable than that beautiful babe down the road with the heart condition? Sure, she'll probably die at the most inconvenient moment, but hell, a few years of bliss has got to crap all over years of having to look away from that fugly ol' pig? Ha ha, no disrespect intended, I'm just drinking a lot lately, Cheers, Terry.

[ofreen]

You're probably dead right offy, but that's like saying that you should stay married to an ugly fat chick because she'll be more reliable than that beautiful babe down the road with the heart condition? Sure, she'll probably die at the most inconvenient moment, but hell, a few years of bliss has got to crap all over years of having to look away from that fugly ol' pig? Ha ha, no disrespect intended, I'm just drinking a lot lately, Cheers, Terry.

Of course, the only difference between my ugly fat chick and yours is mine is a blonde and yours is a brunette.  To extend your analogy further, that would explain why I have more than one bike.  I keep the old reliable one to do all the mundane chores like hauling my butt back and forth to work or the hardware store, while I have other flashier models that are more specialized in their purpose.  The big question becomes how to make that work with women.  I await your insight.

[uny2]

I just did a compression check on my 70' K0 with 44k miles and got #1-139.5 #2-151.5 #3-150.5 #4-149.5.  I used a new Mityvac digital compression tester and decided that with all pressures within 10% of each other that I was ok not to pull and rebuild the motor.  I still have to adjust the valves, but when I synchronized the carbs it made a huge difference in how it ran.  Would I be crazy to take it apart to put new rings in it with these readings?

ED

[Jerry Rxman Griffin aka MuthaF'er]

Ed,

Those numbers look great especially if the rings have 44K on them. I think 150 is more of the norm. The 139.5 is off slightly but unless it's giving you problems I wouldn't open it up. If you haven't pulled an engine you don't realize how big a job it can turn into.  Wait until you need to take it apart.

[ofreen]

I just did a compression check on my 70' K0 with 44k miles and got #1-139.5 #2-151.5 #3-150.5 #4-149.5.  I used a new Mityvac digital compression tester and decided that with all pressures within 10% of each other that I was ok not to pull and rebuild the motor.  I still have to adjust the valves, but when I synchronized the carbs it made a huge difference in how it ran.  Would I be crazy to take it apart to put new rings in it with these readings?

ED

You should check your tappet clearances and adjust if necessary before doing a compression check.  That being said, odds are good that if you run the engine a little and recheck, the number 1 reading will be there with the others.  Rings gaps can line up.  I've seen the same thing happen with aircraft powerplants.  A cylinder will read low on a leakdown test.  After a couple of times around the patch, it will check out fine.

[uny2]

Thanks for the reply guys.  I am contemplating pulling the engine so that I can paint the frame and thouroughly clean/detail the engine.   I just pulled the engine from the 71 parts bike it was a pain.  If I can find a way to paint the frame and clean the engine without pulling it , it would save me a ton of work.  Can you pull the valve cover with the engine in? I wanted to polish it.

ED

[CB750F2]

G/Day. The reason why the required compression pressure is greater than atmospheric pressure multiplied by the compression ratio is because of the fact that when a gas is compressed there is a rise in temperature which in turn causes the pressure to rise further. Valve overlap reduces the temperature effect - otherwise the compression pressure would be much higher. Other factors to consider would be the  increase in volume at TDC due to the compression gauge and the restriction of the air filter. It would be interesting to do the test with the air filter removed.
The following link explains everything.  http://www.pumaracing.co.uk/comp.htm
Hope this helps. Pat