Compression Check
 

I am going to do a compression check on my starboard Merc 400 (454). What kind of reading should I expect? 110- 130 lbs?

Wannabe

That's low for an engine with not much cam. Leakdown is a much more definitive test.

In very good shape, 140lbs with less than a 10% variance from the highest to lowest. An engine with some hours will most likely be lower, like around 120lbs. The key thing to look for is a large variance and that will indicate a problem.

Remember- warm engine, throttle wide open.

This will be a cold check. Starboard engine backfiring 1300-1800 rpm and maxes out at 4500 rpm. Port engine no backfire and 5000 rpm. Suspect a faulty carb (holley) but will check # 4 cylinder- previous owner gave me heads up on this.

Wannabe

Take the compression tester and remove the tire valve from it. Then screw it into the hole you want to check and pressurize it. You'll have to hold the crank- a breaker bar on the balancer bolt will work. Listen in the intake , exhaust and valve cover breather. Poor man's leakdown test. If it's backfiring you may have a burnt exhaust valve.

It would be easier to swap the carbs than to do a compression test.

Chris- Thought about the switching carbs thing. Concern is If it is a valve I will mess that carb up to.

Wannabe

Yeah, you could pop a power valve.

Look at the bright side- it's February. This isn't a problem you want to find on July 1st.

I had an Thunderbolt IV ignition module go out which didn't allow the timing to advance. I don't know about your boat, but on mine it was quick to check the advance at ~3,000rpm.

Also, one summer I had an alternator dying on me. It wasn't completely dead but dead enough to let the voltage drop causing the engine to pop occasionally. After a long day on the water I noticed the popping got worse when I hit the trim tabs - that's when I put a volt meter on it and found the problem. The reason it didn't show up earlier was I'm very religious about charging the batteries before every trip.

Just a couple of ideas you can rule out with quick simple checks. Not likely the problem, but can't hurt to rule them out.

Is it a steady pop? If its a steady backfiring reapeatedly, it could be a cam lobe went flat, had that happen.

Mine backfired repeatedly thru the carb as soon as I cracked the throttle open.