Topic: CB 750 compression

[Markcb750]

After five days of finding adapters, ( My thanks to the guys at Advance auto parts on Rt74!) I got these compression readings on my '76. 

      
#1   139   7   7.75   5%   
#2   122   -10   -9.25   -8%   
#3   135   3   3.75   2%   
#4   129   -3   -2.25   -2%   
mean   132            
Average   131.25      

This is with the carbs off the bike and in my parts cleaning tank,
cold engine, dry combustion chamber. 

I have not adjusted valves. P.O. clains to have adjusted about 2K miles ago.

Given the increase in volume imposed on the system by the gauge, adapter & hose I think the 132 psi mean pressure is good.


I think this is great results for a 32 year old motor, might be a slight indication that #2 needs valve adjustment, but still under a 10 % variance from the mean compression.

Anyone with experience to confirm/dispute my "belief" ?


      

[TwoTired]

I'd have no qualms running it.

But, I'd adjust the valves myself rather than rely on hearsay from a PO, and with unknown prowess of the work performed.

Cheers,

[kghost]

I'd have no qualms running it.

But, I'd adjust the valves myself rather than rely on hearsay from a PO, and with unknown prowess of the work performed.

Cheers,

+1

[Markcb750]

Thanks.  I agree. I will go over the valves next.  I have been trying to get a feel for the motor.  I had bought the bike with the intention of a tear down, but it seems to run so well I think I will just do cosmetics.

I was in Milpitas, CA the day after the October 17, 1989 earthquake.  Stayed in a hotel there just off a freeway.

I was there to analyze a failure and repair a prototype Vertical Machining Center I had helped develop with a NY machine tool company.

Memorable trip!

Learned valuable lesson about how much (little?) Loctite salesmen and application specialists knew...

[KingCustomCycles.com]

You are good to go.  Check them again after a few hundred miles and see if they are headed North or South.  I would guess the numbers will improve.  These are good old engines. Yearly compression tests are common on aircarft.