Cylinder compression testing

[rbryson]

When I check the compression I get 150 psi in both cylinders when I have the throttle wide open, yet very different readings (120 left and 80 right cylinder) when the throttle is left closed. Is 150 psi normal for an R90/6? And, why the difference with the throttle open and closed? My Clymer manual does not say to open the throttle, neither does the BMW manual that came with the bike. However, the Chilton manual does say to twist the throttle to wide open when testing. I am using a new screw on compression tester and testing after the bike has been warmed up. Also, I pull both sparkplugs. Any thoughts?

[20774]

I would say 150 seems reasonable...probably higher would not necessarily be better...it's good that they're balanced left vs right. The reason you open the throttles (if you have slide carbs) or take the carbs off (if you have CVs) is to let the engine breathe easier and get a full charge of air. If there is a restriction to the air flow, you'll get lower readings. As for 120/80...that might be due to the way the air "leaks" around the carb throat when the throttle is closed, better in one carb than the other.

Kurt in S.A.

[38107]

You're doing it right. I usually disconnect and remove the carbs, a bit tricky on a hot engine. The pressure readings you are getting are in the right range, but more importantly, they should be within 10% of each other.

[PGlaves]

In order to develop meaningful compression the engine needs to suck in a lot of air to compress. Not too hard at running speed but not the same at cranking speed.

I do compression checks with the carbs removed from the heads. Wide open throttle is an acceptable approximation. Closed throttle is meaningless.

Differences between the two with a closed throttle are caused by carbs slightly out of synch (that is a very small slot for the air to go through) and/or deposits on the intake valves, a little burr in the intake tract, etc.

If they are close with carbs off and thus full air flow you are good. 150 psi is in the Excellent range for that motor. In fact - maybe a little carbon buildup in the combustion chambers.

[88bmwJeff]

You've gotten a lot of good advice so far. My only thought is how many cranks did you let the motor go to get the 150 psi. I've been told you should limit the number of compression strokes. Duane Ausherman recommends 6. Here's a link to his web page with his recommendations on doing a compression test.


http://w6rec.com/duane/bmw/compression/index.htm

[manicmechanic]

On whatever type of engine we test, we usually crank, kick, or pull until the indication stops changing.

[rbryson]

Thanks for the help. Isn't it amazing how one small detail (not having the throttle open) could have cost me a bunch. I still plan to take it in and have the heads checks for carbon. The bike has over 50,000 miles and the heads have never been touched. In fact it has only had four tune up ever.

[Andy VH]

At 50,000 miles, I wouldn't bother having the heads "checked for carbon" unless the bike is significantly "pinging" under load. Your compression numbers are a bit high but typical for the mileage. The only way to really check for carbon buildup is to actually pull the heads, and at 50,000 miles that's not necessary.

An old trick to help reduce carbon buildup is to mix a pint of Marvel Mystery oil in a full gas tank a few times per year. The Marvel Mystery oil is actually a refined kerosene solvent that can be quite effective at cleaning out carbon.

I do it to my 94 R1100RS about twice a year. And about 500 miles before my oil change is due I pour a pint of Marvel Mystery oil in the crankcase to remove sludge by the time my oil change is due. I also run Shell Rotella Full Synthetic oil in my bike. No problems and at 115,000 miles the bike is still running strong, never had the heads off, no reason to do so.

Try the Marvel Mystery oil and see if your compression test numbers drop slightly to indicate the carbon has been cleaned out.

[flash412]

I prefer leakdown testing. You can have leaky rings and crappy valves, coupled with coked up heads and get "good" compression readings. Leakdown testing is cheap and easy, accurate and intended to be done on a static engine.

[rbryson]

If you do a compression test without either removing the carbs or opening the throttle wide open then you will get misleading results. I almost paid to have a complete rebuild because I was doing the compression test incorrectly. Once I did it with the throttle open I got 150 psi in both cylinders, and saved a ton of money. Hope this helps.

[rbryson]

I agree that a leak down test will tell me more but what all is involved? What equipment do I need or it something only a shop would have? Please advise.

[Isamemon]

how much money do you want to spend
you can go full on shop with a ------snap on system
or
sears....with a ..........warranty...........kind of
or for a third of the price
go to harbor freight
http://search.harborfreight.com/cpisearch/web/search.do?keyword=leak+down
that easy
that cheap