First, and most likely, is bottom growth or something fouled underneath. It doesn't take much bottom growth to have a significant impact on performance. Also check the condition of the running gear.

 

Did this develop just out of the blue or over a period of time. As LD said check the bottom and running gear.

 

Good advice above, but also make sure your tach's are correct as they are notorious for being wrong and this could certainly cause your problem. If you find that they are off, remove them, check and make sure the connections are clean and tight then cycle the small switches on the back several times through all positions (4-6-8) making sure you return them to the 8 position. This usually rectifies an inaccurate tach problem.

 

Are you sure your engines are in sync? Do it by ear without the tachs?

 

The boat is going to be pulled at the beginning of next season for bottom paint, so hopefully that will give me some insight and solution. I have had the tach issue in the past and fixed it with the 4-6-8 switch like mitch mentioned above. I usually sync the engines be ear anyways, but they do both show the same RPM when synced. I will update when I am able to pull the boat, get it painted, and check the gear. Could be a few months as it is currently snowing in KY

 

Again is this something that developed suddenly or gradually?

 

Also check your props. one of them may have some damage giving it less bite.

 

When the boat is in the slings, check the alignment of the rudders. If you hit something or grounded the boat it is possible that the rudders got out of alignment. The rudders on a 340EC are small so speed will have an effect on the boat pulling to either direction. The faster you go, the larger the effect which may explain why pulling to STBD is more noticable at speed than at idle.

I don't think this is a prop issue because enough damage to a prop to cause the boat to pull to STBD would almost assuredly be acompanied by a vibration.

For giggles, try a WOT test to see if both engines will turn up to rated rpm. Plug wires on a 30 year old boat are critical and the OEM Merc are known to break down. If you have a dead cylinder it could cause the pulling and would prevent the engine from turning up rpm's.