Twin 496 MAGs with Smartcraft. At low RPM both engines show the same GPH at same RPM.

Above approx. 3000 rpm, the port motor needs more throttle to stay at the same RPM and begins to use more GPH. At 3500 rpm, port uses approximately 4 more GPH. At full throttle there is a 300-400 RPM difference (4400 port vs 4800 starboard).

Both throttles are adjusted the same. What could the problem be? Any ideas appreciated.

Look for a plug wire that has fallen off a spark plug on the port motor. 496's are notorious for having plug wires fall off.

Thanks. The wires are all on, but I will pull all 8 plugs and check them.

Anything else I should check that could cause this?

What condition are your propellers in?

Julie

No dings.

Any 496 experts that might be able to help? I have two weeks of warranty left and something is not right.

I just paid for 3 hours of a Merc service guy looking at the computer and the plug wires ... nothing.

But the problem remains: 3500 rpm about 3 gph more on the port; 4000 rpm about 5 gph more on the port; full throttle is 4300-4400 on port and 4700-4800 on starboard. At full throttle gph is the same.

It seems like i am down a cylinder, but the computer has no fault codes or other problems (I have not had the computer on when the engines were loaded).

I don't want to keep paying my service guy to go on a witch hunt. Any help appreciated.

Have a leak down test and a compression test done.

Compression is good (all within 10 psi). No leakdown test done (yet).

If compression is the same, I doubt a leakdown will show you anything. As Julie brought up, you might try a differend set of props or swapping them and see if the problem follows the prop.

how many hours on them?