20' sunrunner prop question

[nailem]

hi, im new to boating and put my new 1986 I/O 20'sunrunner in the water for the first time yesterday. it has a 5.0L mercruiser with a alpha 1 outdrive. the motor seems to be running fine (other than a ruff ideal). but the boat will not get out of its own way. from dead stop to WOT it just took forever to get up to plan. there is no way you could pull even a 50lb skier out of the water.

the prop that is on the boat is a 15.5x21. my tach seems to be inacurate so i can not give you a true WOT RPM. it did come with a spare prop that is 14.5x17.

from my little bit of reading last night i thimk i should have some were around a 14 or 14.5x19 or 15.5x18 i know i can just try it but does the 14.5x17 seem to small?

the boat sat for 2 years so someone else told me its not the prop at all but something in the motor. what do you think?

 

[rndn]

Re: 20' sunrunner prop question

If it's rough at idle, something must be wrong. When is the last full tuneup the motor was given? Did you give it a compression test?

 

[steelespike]

Re: 20' sunrunner prop question

What he said; most likely a motor problem might be as simple as a plug wire off.be sure you have wide open throttle,good compression and firing on all cylinders.

 

[nailem]

Re: 20' sunrunner prop question

i already changed the distributor and rotor. the wires all look good and i plan to change the plugs this weekend. the boat did sit for about 2 years before i bought it. i changed the oil and out-drive lub. drained the old fuel and replaced with new. is the fuel filters other than the water separator?

the motor sounds good and is firing on all cylinders. should i have replace the ignition module?

what do you think about the prop size?

thanks you!

 

[steelespike]

Re: 20' sunrunner prop question

According to my info all the 20 footers are 260 hp.however weights vary from
about 2,700 to 3,500 some details of your boat might help with the weight.
As you may know I/Os can be tail heavy and slow to plane.Without your max rpm and speed its a crap shoot.No doubt the 17 will help your hole shot but at what rpm result.

 

[nailem]

Re: 20' sunrunner prop question

steelespike, i know what your saying, i have to figure out that tach. i havent had a chance to start yet. im not real familiar with how they work or were they get there signal from.

as far as the weight goes its registration says 2,900lbs so figure 3,400-3,600 loaded. im bringing home a digital tach from work used to calibrate rpm of fans and motors. i'll stick a piece of reflective tape on the harmonic balancer or crank pulley and it will give me an accurate rpm. i plan to use this to calibrate the factory tach. if that does not work i'll have a passenger hold it every time were out and yell to me the results. lol