Topic: Compression out of sync

[shotty750]

So, I checked the compression in each of the cylinders and got the following:

1-138psi
2-100psi
3-100psi
4-138psi

Trying to figure out what's up with the lower compression on the center two. Should I try adding a little bit of oil through the spark plug holes to see if the seal need a little lubrication?

This is a '77 CB750K

[carnivorous chicken]

Adding oil into the spark plug holes isn't to "lubricate the seal" -- it's to see if your problem is valves or rings. If it's rings, adding oil will give you a false higher reading on a compression test because it will make a better seal around the rings temporarily -- enough to get a reading. If your numbers don't come up, it's more likely the valves. A leak down test would be conclusive, but adding oil gives you a good idea of where the problem lies. Also -- check to make sure you did the test correctly.

[shotty750]

Is this a serious problem or pretty common/easily fixable? How would you go about this if you had the same problem?

[70CB750]

Not sure what tester you are using, but cylinders 2 and 3 are major PITA to screw the tester in.

First thing I would verify that you are really getting a good seal on those two cylinders.  Numbers look too "symmetrical" to me.

[shotty750]

I used a single compression gauge and tested each cylinder at a time. Used the kicker pedal to build compression. I believe there was a good seal as the pressure never dropped on either of the holes. I do agree that it is strange that they are so similar in pressure.

[carnivorous chicken]

Did you have a warm engine? Did you remove all of the spark plugs? Did you have the choke on? Did you have the throttle at full? The latter can give you a significantly lower reading.

If you do have low compression on your 2-3, you need to diagnose further. If adding oil brings the compression up, then it's almost certainly rings -- you'll need to inspect your cylinder and at a minimum replace your piston rings. If adding oil doesn't bring the numbers up then it's likely valves. They'll have to be removed and sent to a shop.

[shotty750]

Chicken/calj,

The bike is non-running....was getting help from forum member George and used his gauge. I'm not sure if it's a one way or not, I'll need to check later when I get home. All the spark plugs were removed when performing the test. The carbs are not attached.

I basically have a non-running bike that I'm trying to bring back to life. Compression was the first thing we wanted to check as we just installed the starter motor back into the bike (PO removed motor and it's electric start components).

If you do have low compression on your 2-3, you need to diagnose further. If adding oil brings the compression up, then it's almost certainly rings -- you'll need to inspect your cylinder and at a minimum replace your piston rings. If adding oil doesn't bring the numbers up then it's likely valves. They'll have to be removed and sent to a shop.

Does this mean I'll need to remove the motor from the frame and open it up?

Thanks guys, any help is more than welcome.....I am obviously a novice at this.

[MCRider]

Depends on your expectations. If you just want a bike to tool around on, those numbers are good enough. Watch your oil consumption. You'll never win any drag races.

If you want a crisp, sharp, like new, motor, then it needs an overhaul, which can be as simple as rings and a minor valve job, to an overbore, rings, pistons, and a more thorough valve job.

[carnivorous chicken]

I basically have a non-running bike that I'm trying to bring back to life. Compression was the first thing we wanted to check as we just installed the starter motor back into the bike (PO removed motor and it's electric start components).

This kind of changes things.  You should still recheck the compression, and add oil to see if the numbers come up. But if it's not warm, and hasn't been running, you can expect the numbers to come up some when it does fire up and oil gets circulating. I'd work on getting it fired up and go from there.

[70CB750]

Save the numbers, get her on the road, ride the crap out of her and check again.  Chances are she will be much better.

How about she had problems with 2-3 coil, it leads to 2-3 cylinders dirtier than 1-4, rings stuck in 2-3 and lower compression.

[shotty750]

Thank you for the advice guys. I guess my next task is to hook up the new wiring harness and starter assembly I just received. I'll try to get the bike running first and let you guys know.

[shotty750]

Save the numbers, get her on the road, ride the crap out of her and check again.  Chances are she will be much better.

How about she had problems with 2-3 coil, it leads to 2-3 cylinders dirtier than 1-4, rings stuck in 2-3 and lower compression.

I remember when I took out the PO's old spark plugs in 2-3, that they were very grimy and oily. It had supposedly been sitting for a year.