Compression Testing Rules.

[Franki]

Hi People,<br /><br />I just bought a screw in Compression tester.. and I have some questions...<br /><br />I know that it is suggested that you earth out the high tension lead to stop sparking.. but is that necessary? (Also how do I do that on the outboard? on a car I'd usually just disconnect the coil..)<br /><br />I have a powerpack that cost me a packet and I don't want to fry it..<br /><br />The instructions that came with the tester said get it to operating temp, remove all plugs, earth out the high tension lead and run the starter and keep going till you get the highest reading.. remember the reading and the number of turns it took to get it? (how the hell are you supposed to count the revolutions on an engine spinning full tilt with no plugs? should I just time how long till I get the highest reading?)<br /><br />Is any of that different for an outboard?<br /><br />My engine is a Johnson 100HP 1972 model and I want to test it before and after I run OMC engine tuner through it.. (I am curious to see how effective it is.)<br /><br />Anyway, Any tips you guys could give would be much appreciated...<br /><br />rgds<br /><br />Frank

 

[G. Patton]

Re: Compression Testing Rules.

Frank.... Those instruction seem to refer to checking a engine that has a manual starter, not an electric starter such as your engine. It's not necessary to have the engine at an operating temperture, although you can do a cold test, then a warmed up test to compare the readings.<br /><br />Have all the spark plugs removed, the engine in a normal vertical position, then crank the engine over. For what it's worth, you can count the revolutions simply by listening to the sound if you like. At any rate, one can roughly judge the revolutions, or the length of time (approximately three (3) seconds. That would be fine. The compression on that model should be in the area of 100+ psi and even on all cylinders. Good Luck.

 

[G. Patton]

Re: Compression Testing Rules.

Frank.... I neglected to say that to avoid having power going to the powerpack, you can either disconnect the large main electrical plug at the engine, then crank the engine by using a small jumper from the battery side of the starter solenoid to one of the solenoid small 3/8" terminals (not to the ground one)... or you can simply remove the wire at the terminal block that supplies 12v to the pack when the key is in the on position. It's not necessary to do either as no harm will come to the pack, but I can understand your hesitation to do so.

 

[Franki]

Re: Compression Testing Rules.

Thanks Mr Patton, <br /><br />much appreciated..<br /><br />I was thinking it might be an idea to leave the key off, disconnect the small wires to the Soneniod and switch it manually.. in theory the motor should spin but no ignition...<br /><br />Thanks for the tips,, I am dying to find out what the compression is.. should be interesting..<br /><br />Incidently, 100psi doesn't sound like much all things considering.. (like the 130+ of other outboards..) would any benefit be had from shaving the head alittle?<br /><br />rgds<br /><br />Frank

 

[petryshyn]

Re: Compression Testing Rules.

Remove all plugs....<br />Engage the lanyard if you have one. That'll disable ignition. OMC recommends a warm engine. Give each cylinder atleast 5 compression hits. (count them as G.patton says) Try to give the same to each cylinder. Open throttle wide open while cranking....

 

[alcan]

Re: Compression Testing Rules.

Hi Franki<br />You will get varying results from the engine tuner (decarb). If your engine really needs it, the rssults may be great. Other times it dosen't seem like it dose anything. Except make a lot of smoke. The disabling of the ignit. is to protect your electronics. I use the remote start switch (hand held) in the maner you discribe. If the decarb doesn't help, at least it won't hurt.

 

[oldboat1]

Re: Compression Testing Rules.

I'm about to go out and buy a compression tester this morning, so this thread is good for me too. I assume the screw-in type is better than the plug-ins (if the latter is still made), but don't really know -- any thoughts on that? Also, what is the compression for a 10hp Chrysler ('76-'77)? Thanks.

 

[oldboat1]

Re: Compression Testing Rules.

OOPS -- wrote before I looked. My manual says the compression on the Chrysler is (should be?) 105-115. What is the lowest acceptable, and how much variation between cylinders is acceptable?

 

[Franki]

Re: Compression Testing Rules.

Cool, I'll do the remote start thing.. I have an button switch that'll do the job nicely..<br /><br />and I get the piece of mind of knowing that there is not power to any ignition components at all..<br /><br />Why were the early V4 OMC's using such a low compression? 100 is almost in the red zone on my tester...<br /><br />Plus or minus 5% is what I should be looking for isn't it? (plus or minus 5% variance between cylinders I mean)<br /><br />Thanks for all the help guys.. I am actually mildly excited at the prospect of testing this.. ;-)<br /><br />rgds<br /><br />Frank

 

[alcan]

Re: Compression Testing Rules.

Hi Franki<br /> Compression gauges are kind of like meat thermomaters. They all read differnt, and not always accurate. So long as you use the same gauge you will be able to determin the reading. Regaurdless of what color or number the needle points to. Yours may read a 100psi mine may read 125psi. for the same hole. I usually don't get too worked up untill I get about a 15psi spread. So long as your around 100psi +- your go to go (usually). The main thing is, dose the engine perform well? If yes don't worry. Good luck Mate.

 

[Franki]

Re: Compression Testing Rules.

Well, I did it..<br /><br />Here were my results..<br /><br />cyl 1 (top starboard) = 104<br />cyl 2 (top Port) = 106<br />cyl 3 (bottom star~d) = 101<br />cyl 4 (bottom port) = 104<br /><br />Do they sound good to you guys?<br /><br />I used the OMC engine Tune stuff, used the whole tin.. and blew out alot of stuff..<br /><br />Those measurements were taken around 10 minutes after the decarb.. <br /><br />looks like good results to me.. I think the guage is fairly accurate.. its not a cheapie and its brand new...<br /><br />Considering that some outboards have a compression of 160psi.. why do teh OMC V4's have such low compressions as standard?<br /><br />rgds<br /><br />Frank

 

[BKool68]

Re: Compression Testing Rules.

Looks like solid numbers to me.