Topic: CB750 Engine Compression

[chundy]

I'm getting conflicting stories, some say 120 to 150psi.  Others say 170psi, what should the compression be on a cb750 to be acceptable?

[StevieMac]

Word at HondaChopper.com is anything under 100 is rebuild time. I'm no expert though, just passing on what I've read over and over again.

[Lumbee]

<100psi rebuild time 
100-120 psi  acceptable...I have a 78f that has 1 cylinder at 120, the other at 90 and she hauls a$$ (though I'm still gonna rebuild soon)  Its not like it won't run under 100psi, u'll just see a performance reduction.

120-150 psi excellent 
over 150    ...maybe 30 years ago when they rolled off the boat!!!

[Tim.]

Actually, new spec would be up in the 170 range (at least based on the CB500 manuals)

However, it all depends on the compression tester used, whether the engine is warm or cold, heck, altitude could impact it.

As long as the number is reasonable (i.e. 120+ I suppose) and CONSISTENT across cylinders, you're in good shape.  If you're concerned, drop some oil into the cylinder and try again.  If it jumps, then you've got worn rings.  If it doesn't, valves are to blame (or both valves and rings).

[kaysystems]

Checked mine last night. 500 miles since new pistons & rings..

168, 162, 157, 168

Think I have a bad valve.

David

[cb650]

Checked mine last night. 500 miles since new pistons & rings..

168, 162, 157, 168

Think I have a bad valve.

David

 That is only 11 #s and 8%. Sounds good. 
What does altitude do for readings?  Lower I would expect.  Got 150 both cyls out of my 360 in denver.



                 Terry

[Zeke]

Checked mine last night. 500 miles since new pistons & rings..

168, 162, 157, 168

Think I have a bad valve.

David

I doubt you have a bad valve.  Per manual, anything 170 +/- 27psi (or something like that) is normal.

Variance in material, machining tolerance, etc would probably account for having  slightly lower in one cyl -- I certainly wouldn't go looking for a problem If I was you.

Zeke