rpms

[terry]

anyone know if advanceing ignition timing will cause you to lose rpms on the top end?my boat should be running 4600 and best i get is 4200 at wot with myself and full trim.i quess 400 rpm would be good for 5mph more?check carb at wot and there is no room for adjustment,carb is a holley.how do they govern these engines in these boats.only other area i can thin to look would be the pump impeller.

[Rich_V174SS]

What is the boat and engine, and what size prop is on there now? If this is a Mercruiser I/O 140 there is no "governer". As long as the throttle plates are opening fully, and fuel flow is good, compression is good, and it's propped right it should run 4600 rpm. If the ignition timing is advanced too much the engine will knock and ping. Initial idle speed (800 rpm) timing should be set at around 6* BTDC.

[terry]

its on my baja-403 olds. only other thing i can think of is when they rebuilt the pump they used the wrong impeller for the jet.

[OleRed]

I never kept up with all the jet applications, but familiar with some, but I'm wondering ... did the 403 cid come out that boat, what year is it ?
Most applications came out with the 460 Ford and the 455 Olds, some with big block chevy, and only a few with the small block chevy.  Reason I'm asking, it's not uncommon to see someone build up a new motor for a jet boat, not being familiar with the cam, pistons and heads required to turn the pump up to speed, and having the pump re-built to spec's, would certainly pull down the rpm's if the impeller damaged to begin with.
I do know that timing is critical on the motors, and yes, too far advanced will drop rpm, and like Rich said, that causes detonation "ping" and you can usually hear that, poping back through the intake valve, and you can sure hear that, both can cause melting of the piston ring glands, score cylinders, knock out rod beaings, burn valves, any or all of which will destroy the motor.
I would suggest getting on line, search for the timing spec's for that motor, mainly the total advance spec's, spark plug gap, set them to spec's to know your motor is safe.

[terry]

78-403 origanal to this boat.plug gap .60. timeing 18 degrees is running at advance 20 degrees.wonder if i should go back to stock timeing.im going to split the bowl in the fall when i winterize and check impeller.

[bellj]

Just a quick note about whether some came with 403's. The Switzer jetboat that I spent so much time on all those years ago now came straight from the factory with an Old's 403 in it - so while maybe not the most popular engine, some of the manufacturers did offer it along side the larger 455 options.

With that said, while ours ran good initially, after a dealer winterizing incident (long story), it didn't last much longer, and as evidenced later with fuel consumption, a 20' jetboat application was a bit much for it.

Being young and naive, we tried to replace it with a '72 Old's Toranado 455, and soon learned the lesson that "built for marine applications" is more than just bolting on the old special marine parts (especially when the RPM's increase two fold over what the engine has been used to running most of it's life).

But as I mentioned about the fuel consumption - once it was rebuilt properly, you could literally hear the rear quads shutting down on the carburetor...and the fuel consumption dropped in half from the usual 40 gallons/hour down to 20 gallons/hour. Of course everyone runs their boats differently, and we maybe rodded around too much, although we tried to be conservative a little because of that high consumption.

I think that the Old's 403 would have been a great engine in the 15 - 16 foot range - but the 18 - 20 foot range was a little much for it in our case. Now just because the factories were including them like in our case, maybe it wasn't quite as "marinized" as we did later on the 455 - so maybe some manufacturers included better modifications than what our manufacturer did - your's might be different.