Topic: CB750 Cylinder Compression Range?

[Jim Harris]

One of my hobbies is buying low mileage Honda 69-76 CB750s, cleaning them up, replace a couple of parts, ride them for a while and then sell them (hopefully for a profit). Those engines are so reliable, especially the low mileage bikes, that I haven't had any problems yet, but my Honda buddy suggested that I should be testing the cylinder compression before I make an offer to help spot any engine work that may need to be done. I see in the manual where the original cylinder compression was about 170 psi, but what range compression should I be seeing in a 40 year old bike?

[70CB750]

Good engine cca 130 - 150, it depends on the testing equipment too.  What is more important that all the values are within 10%.

[Jim Harris]

Thanks!
I'm assuming that we want all 4 cylinders to have simlar compression? At what level will the compression be too low to fire and an indication that engine work is needed?
JH

[MCRider]

Thanks!
I'm assuming that we want all 4 cylinders to have simlar compression? At what level will the compression be too low to fire and an indication that engine work is needed?
JH

I think it will start and run decent, though sluggish at 85psi or more. Below that you'll have problems.

[bjbuchanan]

I second MCRider

You want the uniformity among cylinders, that is usually more indicative of a "healthy" engine. It means you don't have one really jacked up cylinder or something. A dry engine won't give good numbers tho. You want to have some oil in that cylinder it will get real numbers and usually higher too because the rings will seal

[Hondawggie]

I agree with 'bulletproof', I owned a '71 cb750 that had about 60,000 miles on it, and it had about 155psi across all 4 bores, and this bike had been neglected to an extreme.  It's a good impression these bikes make on you, no water cooling, no radiator, and it's easy to adjust the valves, the only problem is unlike a Z1 you can't do top-end work with the engine still in the bike, so if it does need top end work due to low compression, you have to pull the motor.  Good deal that you have compression in your favor.

[MRieck]

Compression test is good good BUT a leak is very important as well. Sometimes you'll have a bit of $hit betwixt the valve and that is where the compression test is at . Doing both shows you where you are at.

[70CB750]

The compression is better on regular ridden bikes too. It is good to have any reading, but mechanically sound engine gets better with use.

[Prospect]

I think anything below 70 psi would be too low to start. I always ask all kinds of questions regarding the engine and then I pull out the compression tester. It kinda freaks people out because it's a good BS tester. It gives you good basis to hagle the price down too.

[Rigid]

Compression testing on a motor that has been sitting is worthless.  Dry rings, carbon or light corrosion on valves or seats, stuck rings, and if you squirt oil into the bore to get the rings sealed up it is too thick to simulate a running engine's compression.  If it isen't stuck, get it running, ride it, change the oil and then test it.  Best not to test some cold, dry engine i am selling and try to haggle a lower price.  All you will get is a lecture on cylinder sealing properties and an invitation to take your low ass offer elsewhere.