Topic: Compression test questions...

[catzor]

I'm a newbie when it comes to compression tests, so I have a few questions for you all about determining the health of my 550 engine. I've done a fair bit of searching, so I'll try not to be redundant.

As is, there are no carbs or exhaust attached to the bike. Instructions are always to do the test with the throttle open, but having the carbs off should accomplish the same thing, no? Will having the exhaust off affect things at all?

Okay so here's the wierd semi-empirical part of my question. I rented a compression tester from a local auto parts store the other day and (surprise, surprise!) it didn't come with the appropriate fitting. I only removed the #4 (Right hand cylinder from a on-the-bike POV) plug to see if the tester would fit. The conditions were like this:

Plug #4 out, #1,2, and 3 were in.
Carbs and exhaust off
Tach cable in
Paper towels stuffed in each exhaust port to block debris from getting in the engine

For no real reason other than just to do it, I kicked the bike over with the kick starter. I was surprised to see that the paper towels were coughed out of the #1, 2, and 4 cylinders, but not #3. I kicked a few more times and the towel stayed in the #3 cylinder. Could this have something to do with only having one plug out(#4) or, more likely, having the tach in (which mounts over the #3 cylinder), or is it a dreaded, albeit very crude, sign of little/no compression in the #3 cylinder? I'm going to run a real test this weekend, but I've just got this eating at my mind in the meantime...

[KingCustomCycles.com]

Dude, I have no idea what you just described........that being said, carbs off is the same as carbs open, it is all about airflow.  Exhaust stroke also pushes air (4 stroke engine) and if the engine hasn't run in awhile, you are wasting your time.  The rings need to be exposed to some oil from running and a bad test result can easily correct itself with running and loosing the rings back up.  I guess a tad more info might help here.  Thanks. :-)

[catzor]

I guess I made it a bit too complex. It was just that when I kicked the engine over, it did its thing and as the exhaust ports opened, it forced out the paper towels I had jammed in the ports to keep debris out. #1,2, and 4 ports all acted similarly, but the #3 exhaust port's paper towel stayed put, as if there was no pressurized air trying to make its way out when the exhaust valve opened. Is that more clear? 

The engine was ran about 1.5 years ago, so the rings shouldn't be totally FUBAR by now, but you're right about the oil. I guess I should just take a chill pill and wait to do the test as it should be done and get my conclusive results then.

[TwoTired]

Has the cylinder cover (cam cover) been off since last run?

If it was re-installed wrong, you could have a bent valve.
It's a common "mechanic" induced issue for the 550.


I don't think a "ring issue" will make much difference in exhaust port pressure pulses at kick start speeds.  A valve not closing...will.

[catzor]

Well, the cam cover hasn't been off to my knowledge. On the other hand, it was my brother's bike when it died and he rode it until it wouldn't really go any more. I recall it seeming very hot when he would pull in the driveway, and he complained of a loss of power, so I was thinking more along the lines of melted piston. I have no idea what sort of temps would be required to do such a thing, but the plugs don't look glazed so I'm thinking I might be overestimating the extent to which it was overheating. I know the carbs were in poor shape, so it may well have just been overheating to a non-piston-melting degree because of a lean mixture.