Setting/Adjusting Timing on 150

My 90 Model GT150 is not turning up the RPM that it should..only getting 5100-5200 WOT with nose in the air and sitting on back pad...its on a 18ft Charger with 6" jack plate...could the timing be an issue with it not turning up like it should? has all new plugs, filters etc..and I belive the CDI box was recently replaced as it looks brand new...the motor was rebuilt not long before I got it....have heard some shops will "detune" and engine after a rebuild...So how would I go about checking the timing, and if needed, setting it? Sticker on air box says should be at 28 BTDC...do I check it with timing light like my truck? Flywheel has markings for #1 & 2 cylinders...

Yep use a timing light but you'll need a test prop or stand on the back deck while someone drives the boat (not to swift of a way to do it), you can't do it on the muffs as you need back pressure to do it properly. Make sure your throttle plates are opening fully at WOT before you mess with the timing. If you need to adjust it back off the stop one turn CCW.

Yea, Im not too crazy about standing on the back deck going across the lake if thats what your talking about lol...I did pull the breather cover and check the blades...they are level at WOT...I did notice the lil pad hits the stopper screw when im about 1/2 to 3/4 throttle....Im not sure how im going to be able to adjust timing if it needs it if I cant use the muffs..what RPM should the 28degree set at? is that at WOT only?

Thanks
Jimmy

Yep you have a different adjustment for idle timing also.