compression

[Breeze]

I am getting ready to do a comp test on my 200xp. no problems just want to know. my question is do the test dry or wet or both ?boat hasnt been in the water for about a month.should i warm it up first or do it cold?emoScratch

 

[Bubbakat]

Take all the plugs out and do one cold and write it down relative to each cylinder and then warm it up and take another one and compare the two.
By no means never put oil in a cylinder to take a compression check. That will give you a false high reading. Its an old trick that car salesmen use to do.

 

[Breeze]

thanks bubba. wet cyls are also for checking the valves.I do believeemoCool

 

[hl&s]

Breeze - 11/14/2007 6:19 AM

thanks bubba. wet cyls are also for checking the valves.I do believeemoCool

Hope it checks out ok breeze. I would like to add that wet compression tests are still done and have long been a diagnostic aid in determining the cause of low compression and the extent , or costs, of repairs needed on 4 stroke engines. If adding a teaspoon of 30w oil to the cylinder raises the compression reading considerably, the rings are badly worn or the cylinder is scored. If the wet reading doesn't change, then a valve is burned or not seating properly. A lot of manufacturers also recommend that the throttle be held open during the compression test to increase the volume of air in the cylinder. The battery should also be fully charged to ensure the same rpm while checking all cylinders.

Not trying to contradict you at all bubbakat, you're the 2-stroke guru and a wet compression test isn't gonna tell you much on those.

 

[Bubbakat]

Good addition to the info in my book. I need to quit assuming that every one runs 2 strokes. emoSorry
What I need to do is upgrade my knowledge to 4 strokes. I have done the wet and dry checks on auto's but that was back when a shade tree was in use.emoTongue

 

[WLG]

It doesnt matter if you do the test with the throttle open or closed as long as you do it the same on all cylinders. And the compression readings should all be pretty close to each other. I think its like a 5% threshold on a 2 stroke engine??? Correct me if IM wrong. And thats a great idea to go ahead and do the compression test while the motor is running good...that way you have a reference to look back on in the future . I wrote my readings on my cylinders with a sharpie marker so I wont forget!!!

 

[hl&s]

If you're just looking to compare compression variation between cylinders the throttle opening wouldn't matter as long as it remained the same during the test.

The recommended procedure of choke off, throttle wide open does offer some advantages. At cranking speed, typically 200 to 300 rpm, it allows more air/fuel mixture to the cylinder increasing the pressure in the combustion chamber as the piston compresses the charge. The increased pressure can help identify smaller compression leaks, such as a head gasket, as the percent of compression loss will increase in proportion to the combustion chamber pressure. The extra fuel entering the engine, especially a 2-stroke non VRO, keeps the cylinder better lubricated while the test is performed. Also, if you want to accurately compare your readings to the manufacturers specs for your engine, then wide open throttle is a necessary step. I've typically seen 5 to 30 psi difference, depending on the displacement, between closed throttle and wide open throttle compression test. That much difference could lead to a mis-diagnosis. Even the length and material of the hose used to attach the gauge to the cylinder can affect the reading, more so on smaller displacement engines.

I was trained that any cylinder with a reading 15 psi lower, or higher, than the others was questionable. Does the 5% threshold apply to some of the later outboard designs ? It would only be a 7 1/2 psi variation on an engine with a 150 psi rating.

 

[Bubbakat]

My rule of thumb and personal observation is anything over 10lbs difference I tear down and inspect visually.

You are right on the mfg specs if you are comparing them to those specs then the throttle has to be wide open. On the two strokes that have at least 50 hrs running time will be out of mfg specs anyway so I do it with the throttle closed and is I get up to a 10 lb difference then I do a leak down test which is more complicated the most DYI people will do.