Re: How to - outboard compression check
When testing motors in the shop here, I usually bypass the controller and harness by hooking directly to the starter, that way I eliminate any chance of the ignition being 'live'. I normally just hook a jumper switch right to the starter terminal from a battery, that way even a magneto motor won't make spark and it eliminates worrying about stray sparks or possible damage to any electronics.
I either run the motor, of if it's been sitting a long time, I'll pull the plugs, give it a shot of light oil to make sure everything is well lubed. If the motor can't be started, I'll then do the compression check cold but only once I know all the cylinders have good lubrication. Don't over oil the cylinders, it will give a false compression reading and I try to make sure they are all lubed about the same amount. I'll crank the motor over several times to purge any extra oil as well before installing the gauge.
It's common for a motor that's been sitting a long time to read a bit low but what you are looking for is a consistent reading. You want to see all the readings within 10% of each other. For example, a 4 cylinder with compression of 120, 121, 123, 90, would indicate a problem being that the last cylinder is way lower than the others, but a reading of say 119, 123, 125, 120 would be well within range.
Keep in mind that compression gauge accuracy often varies, so stick to one gauge and look for consistent readings, not just pressure. I have 4 or 5 different gauges, I find that the cheaper ones tend to read lower for some reason. I like the Snap On set the best but it's a bit pricey for the do it yourself mechanic. One from Sears or similar will work fine for most outboards.
When cranking the engine to get a reading, crank the engine till the gauge stops advancing, and crank each cylinder the same amount of time.
The faster the motor is cranking, the faster the reading will climb, a motor that won't crank properly will often give a lower reading. This is one of the reasons I go right to the starter when checking compression, it eliminates all else and gives me an honest reading.
If you don't have a torque wrench, buy one. In general, spark plugs in an aluminum head are only tightened a fraction beyond hand tight to make is simple. I lube the threads with Never Seize and snug them up by feel. Most engine makers say to torque spark plugs in any aluminum head to 7 to 15 ftlbs with 15 ft lbs or about 180 in lbs being the absolute max most aluminum heads will take. This equates to about 1/16th of a turn beyond hand tight.
The majority of all outboards will give readings in the 115 to 150 psi range, any less and either the motor is well worn or the compression gauge may be suspect. A lot has to do with knowing your compression gauge and knowing how it reads as well.