Boat Tach

Help! I have owned a 1998 Sea Ray 210 Signature Open Bow with a 220 horsepower 5.0L Alpha I MCM stern drive for about 10 years. Purchased it with 40 hours on it and it now has approximately 600 hours.

A couple of years ago it started backfiring periodically under load. The problem has become increasingly worse and is now a problem every time we take the boat out.

There is definately a connection between the tachometer and the backfiring. Under load, the tach begins jumping/bouncing around sporadically, jumping up to 5000 or 6000 rpms; however, the engine does not surge. Then the backfiring, sputtering and popping starts right after the tack begins jumping. It's extremely frustrating when trying to pull a skier up and the boat is jerking and losing power as it backfires. Once up to speed, the ride is usually normal and uneventful, until making a turn with a skier or something else that puts it under load.

We've had a lot of repairs made that the mechanic thought would fix the problem, but it's still as bad as ever.

The repairs made include:
· Tach replaced
· New wires from tach to engine
· New distributor and distributor cap
· New spark plugs
· New fuel pump
· New coil
· New fuel filter
· Compression test (normal)
· Valve test (normal)

Any help/suggestions are very much appreciated!