new prop? Seems it should go faster merc90

So IVe had my boat out 10 or 12 times and Love it! ITs 16 foot with glass hull...Unfortunately It only revs to 4400 rpms and usually a max of 33mph...once i got to 35 with wind at my back. My guess is that the factory prop (this looks as old as the outboard it self) is probably pitched to sealevel...

1/ Im at 5-6000 feet opperating range...is a new prop in order?

2/ Is 4400 rpms all shes got? seems strange and i would think 5000rpms or more should be easy as cake for a 90 horse...

just digging for some advice or helpful something...Goal is 40mph without repowering of course.

if the 90 is running correctly then the engine is over pitched. Unless the boat is water logged or improperly trimmed. Running over pitched is hard on the engine and can cause failure.

You can forget running forty with that pair.

When you drop the pitch to get the motor into it's correct rpm range, you will still be lucky to hit the low/mid 30's.

So if 40 is your goal, dump the 90 and hang at least a 150, maybe a 175 on the back - otherwise, enjoy 30 mph

LOL...thirty aint that bad i guess lol...well can anyone answer the proper rpms for my outboard to run at at WOT? right now its 4000-4400 depending on how many people are on board.

Contact a Merc dealer and find out the recommended jetting for your altitude for that engine, THEN look at your prop.
If it's fuel injected, then go 1" less pitch on the prop and see how that works.

Regards, Andrew.

ITS AN 81 so it definately is not fuel injected LOL. I can rejet my 4 wheelers easy are the carbs on an outboard similarly easy? I want to do this all myself but I may just take it to the dealer and tell them to find a prop that lets it rev a little easier....At this point its not as much about speed I guess but dont want the motor to have to overwork itself when I have 4 people aboard

THE LONE SHARK said:

ITS AN 81 so it definately is not fuel injected LOL. I can rejet my 4 wheelers easy are the carbs on an outboard similarly easy? I want to do this all myself but I may just take it to the dealer and tell them to find a prop that lets it rev a little easier....At this point its not as much about speed I guess but dont want the motor to have to overwork itself when I have 4 people aboard

Click to expand...

The dealers will have a chart they can look up showing jetting for certain altitudes, and some workshop manuals also contain the data as well. It's as simple as reading teh chart, buying the jets you need and installing them.
With props, you can see if you can find somewhere that will let you "try before you buy" and see what works for you, if the boat is always heavily loaded, you will need even less pitch again.
Or you can just buy a cheap used prop in the pitch you want to try, give it a go and then spend on a good prop.

Regards, Andrew.

MY DEALER has a lake behind them so I may just tell them what i want to accomplish and take their recommendation and let them run it out back..

...thanks for the advice

Yes you need to get your rpms up. +1 on the correct jets. Then get an rpm number....should be well above the 4400 and cut your pitch till you get at the top of your engines rpm rating with normal load at WOT. About 150-200 rpm per inch is what's commonly used. Course if you change prop brands or types, or styles that can skew the results.

Then get a couple of cans or so of Sea Foam (auto parts stores) and per directions on the can, add it to the gas/gas mix to rid your engine of the carbon it surely developed with your lugging it.

Mark