Topic: Did a compression test,what do these #'s tell me??????????????????

[mario medeiros]

1975 cb750,runs great,lots of power (was told it has a big bore kit and a cam)leaks alot on the top end and smokes on start up and when you are on the gas.#1 is 125 dry 165 wet,#2  is 130 dry 180 wet #3  is 170 dry 225 wet #4 is 130 dry 210 wet? Thanks

[Rigid]

Tells you the rings are slightly worn, and the valve seals are shot. Ride on. Next question............  

[lone*X]

Smoking at start up and when on the throttle indicates you may have bad valve guide seals.  Looks to me like the dry #'s 1, 2, and 4 are a little low but close enough, but #3 is really wet due to the bad valve guide seals, adding oil even skewed it further.   The high #4 wet test may have been just a test anomaly.  Are you doing the compression test with the throttle held wide open? 

Add the issue of leaks and it sounds like it is ready for a good top end refresh.

[City Boy]

Hi.Your #1/2/ and 4 are weak.#3 is close to standard .How does it run for driveability?How bad is the smoking and how much oil does it consume?Assuming it runs ok and you and the environment can handle the smoking and you don't want to or can't repair,ride it.If you want to fix; rings for sure need replacement. You may or may not need pistons and or rebore.Check valves and guides and lap valves if all is well and replace valve seals.Obtain shop manual and have fun.Your requirements may vary.If not up to DIY find a good wrench!!     Rock On

[Grabcon]

The wet numbers don't mean crap if the exact amount of oil was not used in each cylinder. I use a syringe and measure the exact amount. Also the throttle must be full open. Using a compression tester from and auto parts store with a rubber hose extension I have never been able to get a recommended psi on any bike. What is important is that all cylinders is within 5%. My current 1974 CB750 has 120 psi using the above gauge and I have a rear wheel HP of 52. That is really good for Denver altitude.

Once you are confident that the dry numbers are good then do the wet numbers. the motor should be warm when doing either. The oil that you inject is meant to seal the rings. If the compression goes up either the rings are leaking or you put a lot of oil in the cylinder. I only use about a ml of oil when I test. If the compression does not change then the valves are leaking by.

[Hondawggie]

It would be telling if you could run the compression test 'hot' without adding oil.  The cold test is okay, I always thought you use the wet test to decide if a low-compression situation is rings (vs. not rings, such as valves) and agree with the volume of oil not consistent, reality is -- if you can get her warmed up then with throttle wide-open and re-run the compression test it would be interesting to hear what you got.   225! sounds real high although as you said it has had mods done.

[mrbreeze]

What you are really looking for is consistant readings between all cylinders. Engine should be at operating temp with the throttle wide open. If you find one that is signifantly lower than the others..........hone in on that cylinder. Do the wet test.......doesn't take alot of oil. All you are trying to do is temperarily seal the rings. If it comes way up....you probably have bad rings. If it don't change much.......bad valve/s...hole in piston..etc. If you really get anal and wanna pin it down..........leak down test...............

[mario medeiros]

Test was done cold (it is now just a bare motor in a frame as i am about to restore it) test was also done with the carbs off and each cyl was given a tsp of oil and those were the #'s i got.Also i thought 120 to 130 was a good compression #?Thanks

[Bull]

I just ran a compression test on my '76 which has been in storage and parked since '08.  First off, the numbers seem ultra low.  This was today in Fort Collins, CO at about 70 degrees F.  The motor was cold and the carbs are off.  Every cylinder was right at 60psi, except #3, which was just above 30psi!  I'm using the Universal tester from Harbor Freight, with the extension tube attachment.  The bike ran great back in '08, until it suddenly didn't.  I recently uncovered a melted electronic ignition and I haven't had spark since I started getting her back on the road.  Should I be seriously alarmed by the readings?? 

[MoMo]

I just ran a compression test on my '76 which has been in storage and parked since '08.  First off, the numbers seem ultra low.  This was today in Fort Collins, CO at about 70 degrees F.  The motor was cold and the carbs are off.  Every cylinder was right at 60psi, except #3, which was just above 30psi!  I'm using the Universal tester from Harbor Freight, with the extension tube attachment.  The bike ran great back in '08, until it suddenly didn't.  I recently uncovered a melted electronic ignition and I haven't had spark since I started getting her back on the road.  Should I be seriously alarmed by the readings?? 

I have had several motors with similar readings. I add a bit of Seafoam to every cyliner and the readings always come up far enough for the engine to run. I think the Seafoam frees up the rings that have become stuck from years of sitting. The latest was a 750f  motor with reading very similar to yours....Larry

[Duanob]

To add to MoMo's post once you get it started, ride the crap out of the bike and retest. You would be surprised how many ills can be cured just by riding it.

[70CB750]

Yep, engine is happiest when its running.

[Bull]

Thanks guys!