WOT issue. low rpm

[Echo720]

i'm thinking a pitch issue but want some other input.

69 Shasta runabout. with 120HP Mercruiser I Stern Drive. the prop on it now was what came with the boat. I Believe 14x17. boat revs fine out of water, but when in the river under load WOT is about 3300-3500. boat runs smoothly down the river. just should rev to about 4500 right??? the carb is freshly rebuilt. running on electric fuel pump and has water seperating fuel filter. what would you suspect to be the problem???

Thank You.
J

 

[HT32BSX115]

Re: WOT issue. low rpm

Sounds like the engine is just plain worn out.

I had a 66 Mark Twain my dad bought new. It had a 150 Merc (6cyl GM)

The engine had never been apart. It turned 4500 new. When I sold it it would only turn 3900. It would only do about 120-130 psi compression.......... It was worn out.

I also had an 87 King Cobra OMC 460. New, it would turn 5100 or so. When I pulled the engine it would only turn about 4200 @ WOT. It had several (nearly) flat lobes on the cam. Compression was around 150-160 psi.

The engine wasn't worn out but the cam sure was.....You might look there.

Assuming you don't need to fix your carb, distributor or have a partially blocked exhaust, You probably have a somewhat worn out engine.

Regards,



Rick

 

[QC]

Re: WOT issue. low rpm

I would verify the tach first, and have you ever seen it run faster? RPM wise?

 

[Echo720]

Re: WOT issue. low rpm

rpm wise if you run it out of gear will rev to 4900-5000, motor is low hours. seems pretty healthy. i will definitely do a compression check. the tach seems finniky. sometimes it tweaks out when you first fire it. it will show a little more rpm than what it sounds like its actually putting out.but then idles at 900 and idles at 750 in gear.

also this is a little off topic but the boat has the 3 button trim switch. although trailer and down work drive wont trim. should this drive even trim?? being MC1.

Thanks again!

 

[QC]

Re: WOT issue. low rpm

rpm wise if you run it out of gear will rev to 4900-5000

Don't do that . . . it's meaningless. When we say WOT RPM, we mean in gear (in the water ) going as fast as possibel, trimmed perfectly, with a typoical load. This will tell us if you are propped correctly and getting the correct RPM. Tach needs to be right etc.

 

[Echo720]

Re: WOT issue. low rpm

okay. the prop is 16x17. I think its just got to much prop. the tack seems to be okay. motor is strong.

thanks for all your input.