2006 Mecury 4-Stroke rpm alarm help

[Sallie1029]

I have 2006 Mercury 4-Stroke on a 14ft Sylvan Super Snapper and the motor's rpm alarm is going off before wide open throttle. I throttle is probably at 90% when alarm goes off at 6300 rpm. Current prop is 8 pitch, top speed before alarm is 23 mph full loaded with two adults. What changes can be made to prevent this alarm and run wide open? Is prop pitch wrong? Thank you.

 

[WesNewell]

Follow recommendations here.
https://www.mercurymarine.com/en/us/propellers/selector/#!/step-one

 

[ahicks]

Prop pitch is too small. You need at least one size bigger.

 

[Texasmark]

Prop pitch is too small. You need at least one size bigger.

Yepper on that. Your Wide Open Throttle (WOT) upper limit is 6000 RPM as I recall and you are reaching it too soon on the throttle curve meaning you aren't "loading" your engine enough. Putting a higher pitched propeller on will cause the engine to bite the water harder, lower the RPMS and increase your speed in the process.

Rule of thumb is 150-200 rpm per inch of pitch change. Since most props are manufactured in 2" pitch increments, suggest you go to the iboats prop store and get yourself another identical prop with a 10" pitch which should put your throttle and your upper RPM limit in sync and increase your speed 3-4 mph or more....depending on your boat's reaction to increased thrust.

 

[TunaFish389]

I would venture to go even higher. If you are only at 90% throttle and hitting 6300 then I would be looking at 3-5p increase. Its hard to say till you get a prop that you can do full wot.

 

[Sallie1029]

I bought a Tiny Tach to validate my concerns. Does anyone know the spark degree setting I should use for my motor?

 

[Texasmark]

Remembering...........0-ish at idle and 22 BTDC-ish on my last 90 hp Merc 2 stroker. Going to bet a 4 stroker will be more like 12 BTDC at WOT.