Hi Kevin
Ok first off yes advancing the timing will make a difference in the rpm of the engine! Now with that said on todays engines with the electronic distributors it is better to set the timing at the recommend total advance something like 30 degrees BTDC at 3200 rpm, go with you manufactures specification. Then let the idle rpm fall where it might! To do this you need a special timing light that has an ajustable delay! If you don't have one you can buy or rent one at most Auto Zones stores.
Next the tachs you use on the dash are terribly inaccurate to measure engine RPM you need a mechanics RPM test instrutment. Usually these instruments are accurate to 1 RPM or less. Most gauges on boats are inaccurate because of the enviroment they are in! A little moisture and you get corrosion and the increases the resistance of the electrical connections giving you false indications! The gauges are ok for most applications put not for truely measuring engine performance for setup! Whenever you get an indication that is not normal on a boat guage you must think wiring might be going bad! A good example of this is the twin engine boat one tach might say 3500 RPM while the other may say 3000 RPM but the synchronizer is dead center! Believe the synchronizer as what it does is beats the pulses from each engine against each other and any mismatch is display as percentage of RPM difference, no matter what the the tachs say!
As for your vacuum check again you need a mechanics RPM meter accurate to a couple of RPM. As a few rpm more or less will get off on the wrong foot! On the manifold you will see a small 1/8" plug. remove the plug screw in the spade addapter, plug the vacuum gauge into it, making sure you have a good seal at all connections. It is best to warm up the engine first as the choke circuit will change the vacuum reading!
Finally, seting the RPM out of the water is ok but not the best idea! You need to make sure that you can shift the boat in and out of gear with out stalling! Remember when you shift there is a momentary interupt where the timing circuit is cut to allow for smooth shifting! If you have issues shifting into gear either the RPM or interupt circuit needs attention. The interupt switch on most engines is on the starboard exhaust riser. Look for two control cables going to a quartant and two wires going out the micro-switch is there. Pushing and holding the microswitch will kill the engine!
Hope this help!
Mike -
