Accepted Answer

Hi XXXX XXXXXX,
The port engine will always turn less RPM's than the right engine. However, there should not be as much deviation as you are detecting.
The reason one engine turns more RPM's has to do with the results of the design. The hull has a bit of a V to it at the jet pump inlets, and both engines spin the same direction. That produces the net effect of one pump being loaded uphill and the other being loaded downhill, so to speak. An analogy would be cutting wood with a power saw. If you cut against the direction of saw blade rotating, you encounter more resistance, where as if you tried to cut with the saw blade turning in the opposite direction, it would tend to run away.
There are several reasons you could be encountering the deviation in RPM's. In no particular order, it could be manufacturing tolerances of the read out on the tach's, one set of injectors not opening up completely, damage to a prop, or a problem with the one engine.
If you have your boat on a lift or trailer, get underneath it and check the props for damage. Any signs of damage are cause for concern. You need to check them from the front, which involves looking from the intake grate back at the props. You cannot see them well enough looking from the pump outlet forward.
Then you will need to determine if it is a tach issue or not. There are two easy ways to do this. One involves connecting a digital tach to each engine and running them at full throttle. Yamaha gives about a 400 RPM tolerance for differences in RPM's at full throttle. If you don't have a digital tach, the next easiest thing to do is switch the tachs from the right tach to the left engine and left tach to the right engine and run the boat at wide open throttle and see if you get the same readings as you did when the tach's were connected L to L and R to R.
When checking the tach's in this manner, remember that you should expect to see up to 400 RPM less on the one engine compared to the other. Suffice to say if all of a sudden the former good engine is now not turning near 10,000 RPM, the problem you are experiencing is limited to the tach read out, not an engine issue. If the 8800 RPM engine is now turning 9600 RPM, you have a tach issue.
If you do get the same results no matter what tach is connected to what engine, then I suggest pulling out the spark plugs of the offending engine, see if they are all similar in color, then doing a compression test on the engine. Also make note of the air filter while digging down to the spark plugs.
Beyond that, you are going to have to take it to a dealer to have them plug it into their computer for system analysis.
If you need any more help, let me know.
Thanks
RSRBOB

Expert:	RSRBOB
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Answered:	4/10/2009

Technician

Former Yamaha Factory Service Rep, Dlr Line Tech, Serv Mgr, G/M