Topic: Compression investigation. Valve train problems?

[Tretnine]

So, I've got low compression on my 'good' motor. Roughly - 90, 90, 80, 90 - There are only two ways to get low compression, right? (Aside from holes in your pistons) and they are bad valves and bad rings. On an 11k motor could the entire valve train be bad? Is it worth buying a whole new valve train... could it be full of crud and preventing good mating of surfaces? My low compression cylinder is not firing correctly, or at all. I'm also getting smoke coming out of the vent tube. I've done maintenance on this bike, assume there is proper carburetion. Is it possible to solvent clean your valves while your engine is still in the frame?

[eurban]

Depending on your compression tester and its attachment apparatus your "low" readings may be source of false worries. The volume of the tester assembly essentially adds to the size of your combustion chamber and can give you significantly lower than expected numbers. Read this thread by Two Tired before you get too worried.  http://forums.sohc4.net/index.php?topic=39971.0  Your test procedure can also affect the numbers.  Are the carbs removed or the throttle held wide open?  Is your battery / starter up to snuff?  Variation is typically what you should be looking for rather than absolute numbers.  Your variation is greater than 10% but not too terrible.  If the motor has sat up a while it may improve with running.  How does the bike perform?  Are there performance problems?

[Tretnine]

this particular compression gauge has read in the 110-125 range on other cb550 motors. It seems to pull strong but the one cylinder is not firing correctly, if at all. So, I'm assuming the compression is actually as low as it's read.

[scottly]

Squirt motor oil in through the sparkplug holes; if the compression goes up, suspect the rings, if not, then suspect the valves. If this is a motor that hasn't been run for a while, the rings may be gummed up, and it may improve as you run it.

[eurban]

this particular compression gauge has read in the 110-125 range on other cb550 motors. It seems to pull strong but the one cylinder is not firing correctly, if at all. So, I'm assuming the compression is actually as low as it's read.

Typically, engine wear, corrosion from sitting up etc, operate in a chaotic fashion and it would be unlikely that three out of 4 of your cylinders have deteriorated so significantly yet evenly.  I still suspect your gauge or your technique in using it (throttle wide open/carbs removed, all 4 plugs removed, good seal to the head, strong battery/starter motor or strong legs??).  If 3 of your 4 cylinders are only making 90 psi then it is also doubtful that your engine would run well enough to "pull strong"  Again, has the engine been sitting up a while?  Have you adjusted the valve clearances?  Your low cylinder may be the only one with real issues and they might improve with running.  Have you considered fuel or ignition related issues with the cylinder that is not firing correctly?

[Tretnine]

Please assume the compression check is mostly accurate. Again, I've used this same compression tester on more than one engine and received 110-125 evenly across those motors. This one is even and low, what could that mean?

I know it's not typical, but say the other parts of the engine were sealed and another let moisture in, or for some reason there is atypical carbon building up on the valves. The motor sat for a while so I'm looking at speculation. My dad tells me to put seafoam in everything. Other ideas?

This weekend I will swap carburetors and make sure the spark is even. (Maybe dig out different coils) Eurban, your best point is that the motor does pull nicely even with the low numbers. I will use the gauge on my other bike and see what those readings are just to cross check the integrity of the tool. It might be getting worn.

[Nikkisixx]

So, I've got low compression on my 'good' motor.

This is like military intelligence or jumbo shrimp.    But you say the motor runs well? 

Is it possible to solvent clean your valves while your engine is still in the frame?

No.  Either the guides wear, or the seats and valves, or both.   "Mechanic is a can" will not fix that.  It would be unusual in a 11k motor to have this happen, unless the bike was treated like a $2 hooker.  A little blow-by smoke out of the breather is normal as well.  If the bike runs well otherwise, it is more likely something less catastrophic.  I'd keep fiddling with the tuning if it were me.  For instance, tight valve lash can lower compression.

[Tretnine]

So, I've got low compression on my 'good' motor.

This is like military intelligence or jumbo shrimp.    But you say the motor runs well? 

Is it possible to solvent clean your valves while your engine is still in the frame?

No.  Either the guides wear, or the seats and valves, or both.   "Mechanic is a can" will not fix that.  It would be unusual in a 11k motor to have this happen, unless the bike was treated like a $2 hooker.  A little blow-by smoke out of the breather is normal as well.  If the bike runs well otherwise, it is more likely something less catastrophic.  I'd keep fiddling with the tuning if it were me.  For instance, tight valve lash can lower compression.

Compared to the dancing primary chain on my other bike, this 11k motor was supposed to be the 'good' one of the two. I'll get back in there this weekend. Different carb setup and 'liberal' settings on the valve clearances. What do you guys run on tappet clearances?