I have a 2004 Mercury 90 hp, 4 cyl, 4-stroke on a 1974 21' Boston Whaler (the old banana boat style). It has many, many hours on it (over 6000) but was running OK; a little rough idling but otherwise fine. It would run up to just over 5000 RPM and push the boat at a little over 20 kts. Last weekend I changed the engine oil & lower unit lube, did a compression check (all cylinders within between 180 and 195 lbs), and changed the spark-plugs. When I put it back in the water it wouldn't run up to 4000--about 3800 is all it would do, and would only push the boat to about 12 or 13 kts. I replaced the fuel filter and drained 3 of the carburetors, the fourth I couldn't get to with the lower side cowling on and I did this in the water. This made no difference.
I guess my main question is whether this is most likely a fuel (i.e. carburetor) issue or an ignition system issue. I guess the first thing I'll do today is replace the plugs again in case one or more of the new plugs I put in are bad. This seems unlikely but it's the only thing I did that could cause this problem, and it certainly seems like something I did last weekend since it was OK when I pulled it out of the water last weekend.
Any thoughts would be appreciated.
I guess my main question is whether this is most likely a fuel (i.e. carburetor) issue or an ignition system issue. I guess the first thing I'll do today is replace the plugs again in case one or more of the new plugs I put in are bad. This seems unlikely but it's the only thing I did that could cause this problem, and it certainly seems like something I did last weekend since it was OK when I pulled it out of the water last weekend.
Any thoughts would be appreciated.