mercury 2 stroke 90hp 2001 rpm

[clinerfhmerc90]

Compared to my brothers 2 stroke Johnson, my Mercury 90 - 2 stoke runs higher RPMS. It will be over 6500RPM on full throttle with me and half a tank at 44MPH on my 17 foot crestliner fishhawk. Obviously I do run the boat like that very often but it does seem like it is too high.

Will a different prop improve this? Does this motor have higher RPMs than other motors? I just had the voltage regulator? replaced and did not pay much attention to the RPMS prior to that, but I am pretty sure that has nothing to do with the RPMS.

I am not a mechanic by any stretch of the imagination but would I like to know if anyone has any thoughts on this before a spend a bunch of money having it looked into.

 

[Barnacle_Bill]

Re: mercury 2 stroke 90hp 2001 rpm

That motor should be running 5000-5500 RPM at WOT. Are you sure the tach is working properly?

 

[Texasmark]

Re: mercury 2 stroke 90hp 2001 rpm

I have an '02 version of that engine and even thought the recommended rpm range is as Mr. "bhile" said, I run 5600-5800 regularly and have been since Dec. '04 with that engine. I quit worrying about the upper recommended limit on an '89 6 cylinder "tower of power" that I ran at 5800-6000 on a 17' Ranger Fisherman for about 8 years and my bro-in-law is still running it with the same prop. And then there are the Mercs that the back yard mechanics race and run much higher than that with nothing more than maybe balancing the pistons...some may not even do that. I realize that they do porting and carbs and racing lower units and lots of other things, but the point here is what is there to hurt....about the only thing I can see would be that you might break off a piece of a reed valve, but that Merc has a special functioning valve for improved performance so I wouldn't worry about it either.

If you are really worried about it, you can add 4" to your pitch and get back in range but that might result it a higher top speed and I know you don't want that (Big Laugh....just kidding). I currently run a Ballistic 24XP that I bought from iboats and I am over 50 gps at 5800 on my 17' boat on a cool day. Adding a 200# passenger only drops top end about 1.5 mph and a couple hundred rpm........(lick you lips, grin) I used to run OMC's for years and my gut feel is that they used to be designed around lower rpm's and larger cubic inch blocks....but nowadays, some of the Mercs sport quite large CU's for their hp.

After reading threads on here for the last several years, I'd much rather be there than on the low end when you look at the engine damage that results from loading up a 2 cycle engine. To each his own.

My 2c,

Mark

 

[clinerfhmerc90]

Re: mercury 2 stroke 90hp 2001 rpm

Thanks for the info. I do question the accuracy of my tach, but the description from Mark seems very much like my boat. How do you check a tach?

 

[Silvertip]

Re: mercury 2 stroke 90hp 2001 rpm

Thanks for the info. I do question the accuracy of my tach, but the description from Mark seems very much like my boat. How do you check a tach?

You use a known accurate shop tach to compare the two.

 

[henrycoleman]

Re: mercury 2 stroke 90hp 2001 rpm

Its from the start, but the motor should run with the accurate timing.

 

[steelespike]

Re: mercury 2 stroke 90hp 2001 rpm

Tell us about your boat and your prop size.

 

[clinerfhmerc90]

Re: mercury 2 stroke 90hp 2001 rpm

I will check prop size this weekend when I am back in Brainerd. I know my prop is pretty clean, there are very few nicks in it and what is there is very small. We have a sandy bottom and can you shear a prop down ? When the size of a prop is discussed is the smaller number a diameter?

Thanks.

 

[Texasmark]

Re: mercury 2 stroke 90hp 2001 rpm

I will check prop size this weekend when I am back in Brainerd. I know my prop is pretty clean, there are very few nicks in it and what is there is very small. We have a sandy bottom and can you shear a prop down ? When the size of a prop is discussed is the smaller number a diameter?

Thanks.

Usually measurements are "Diameter and Pitch", also the pitch is sometimes indicated by XXP. On that engine, the diameter will be 13ish inches and the pitch will be a much larger number for your boat. It runs a 2.3 gearbox so the pitch runs 15% higher than a 2:1 gearbox which is common in the next hp range up. Like my current prop is 13 3/8 x 24P. If it were a 2:1 it would still be higher pitch than dia, like a 13 1/2 x 19P.

Sandy bottoms are just sandpaper, unless you really get bogged down. Then you might spin a hub, but you really have to be churning up the bottom. Little knicks and dings aren't that big of a deal usually....nothing worth raising an eyebrow over from a performance standpoiint.

Mark