Topic: 1974 cb750k combustion numbers?

[mr.brandon]

Hello, I have a 1974 cb750k that I've been playing with,  I just did a compression test and got 143, 125, 125 and 115. I know those numbers are not ideal, however I have someone telling me that it is absolutely necessary at this point to overhaul.  Maybe true?, I am wondering what the spec or ideal number should be without leak and all numbers even?

[PeWe]

150-160 PSI area. New pistons with fresh bores. Plus a head with good valves and seats.
If those numbers are at full throttle I should restore.  After a check for too tight valve lash.

I guess an old bike can have numbers like 140 PSI something.

[mr.brandon]

Yep, that's what I was figuring.  I will do a leak test and cross my fingers. Thanx

[Tim2005]

Has it been running recently? Motors can regain a bit of compression when they are in use, valves & rings seem to lose a bit of their sealing properties during storage.

[Stev-o]

The cylinder with 115 would concern me, the others are close to 10% of each other.  If you are using an automotive type tester, the numbers may not be accurate.  Is it close to be able to try to get it running?  [have spark and clean carbs?]

[HondaMan]

Has it been running recently? Motors can regain a bit of compression when they are in use, valves & rings seem to lose a bit of their sealing properties during storage.

One important thing to remember about these engines (all the SOHC4 type): when they sit for a long time the valve faces rust a little bit. When they are run frequently the valve faces keep polished and sealed. ALWAYS run them for 100 miles (IMHO) before looking at compression numbers, and you will be more than pleasantly surprised. I used to "fix" these bikes for local riders by putting some Marvel Mystery Oil in their fuel tank, then telling them to 'come back and see me in 100 miles', at which point the compression was most often back in-spec and close across the cylinders.

Englewood Honda, where I worked at the time, didn't appreciate that so much, though...