Crazy RPMS

[Chaney101]

Heys guys, 90 Merc ELPTO. Yesterday I went out with the SS Prop. My RPMS in the morning where 6100-6300. I kept is easy thinking I was underpitched with just my wife and I. By the mid to the end of the day WOT was 5800. This is got me stumped as to why this would be? Prop is a SS Quicksilver 13 1/4X19. Gas is from last season if that means anything. Also when I stop sometime the engine quits. This thing only has about 5-8 hours on it.

 

[studlymandingo]

Re: Crazy RPMS

What was the temperature in the morning vs. what it was in the evening?

 

[Chaney101]

Re: Crazy RPMS

high 40's then high 60's Air

48 - 51 Water

 

[studlymandingo]

Re: Crazy RPMS

Gees that's chilly!!

My outboard runs quite a bit better in cooler weather and will turn a few more RPMs. When I say cooler weather though, I am referring to the high 60's though. I would imagine the 20 degree difference could account for some of the range you were experiencing.

The gas could have something to do with the stalling, what is your idle RPM? I would definitely get some good fresh gas in her and a can of SeaFoam. I would suggest a good parka and some long-johns as well

 

[Don S]

Re: Crazy RPMS

What year is this 90 ELPTO?
Are you sure the tach wasn't acting up on you? Did the rpms "Sound" different between morning and evening?

 

[Chaney101]

Re: Crazy RPMS

I am not sure Don I was more concerned with looking at the dang on thing as dumb as that sounds. It is a 2003. I will check this weekend and see what it sounds like. Is 5800-6000 still a bit high, or is that a good range for WOT?

 

[Don S]

Re: Crazy RPMS

WOT for that engine should be 5000 to 5500 rpm.

 

[Chaney101]

Re: Crazy RPMS

ok, this probably belongs in the prop forum.. but a 21 pitch from a 19 would probably be my ticket?

 

[Texasmark]

Re: Crazy RPMS

I have a '02 90 and I have a post on here where (Outboards other I think) where I easily run 6500 with a 21 HP SS on a 17' alum boat and it is obvious this is not enough prop for this setup. If I up to my 24 (Both Ballistics) It pulls it down to the 5800 area with my new plugs and all.

I did a decarb and it improved my idle (see below comment). Book says that 675 rpms in F gear and in the water moving is correct for idle rpm.

Temp can affect rpm's but you changed a lot for 20 degrees of temp change.

You didn't mention your boat length, type, weight which would help.

Get rid of last season's gas and put some fresh in there like 10 gallons of gas and a whole can (16 oz) of SeaFoam....auto parts stores have it...AutoZone, NAPA. Go to the lake and run the whole thing out at varied operating speeds.

By the time you have consumed that mix your engine should be running much better. Try it and let us know. Then keep your fuel fresh and keep it in there like 4 (or so) oz per 10 gal.

Mark

 

[Chaney101]

Re: Crazy RPMS

Thanks Mark. I have a 2003 Tracker Targa 17 Foot. I believe it weighs about 1100 - 1300 lbs. Does a 20 pitch merc prop exsist. That would be good middle point. I am afraid I will fall below 5500 with a 21. Is it true I will pick up speed with a higher pitch prop or is that a myth?

The current prop is a 19 pitch Mercury Quicksilver that I beleive is cupped.

 

[Texasmark]

Re: Crazy RPMS

Walleyehed is the prop expert. My experience with this current rig is that thin blades are more efficient. More efficiency means more speed and more rpm's. I have run 4 props on this rig and have proven that point .

You can look on here or go to Merc or MWC site and shop props. Yes a 20" exists.

I just looked up your boat on the www and it probably will do better with a lower pitched prop than I run. If you can duplicate your Merc prop and just change the pitch then you will have your best answer. If you change prop styles/types/mfgr's and all you will be guessing and have to start all over. What you select could be more or less efficient than what you are running.

HTH

Mark

 

[Don S]

Re: Crazy RPMS

Along with not knowing if your tach is correct or not, there is also a rev limiter on your engine.set for somewhere between 5700 and 6000 rpm.
So I would suggest you find out if your tach is right before you start looking at throwing different props at it.
Even if it is correct, the rev limiter may not like cold weather and you could actually be underproperd more than you think you are.
You REALLY need to know if your tach is correct or not.before you do anything.

 

[Chaney101]

Re: Crazy RPMS

What would be the first thing to check on the Gauge Don? Thanks for the input. I just want to get this figured out so I can enjoy myself instead of worrying the whole time.

 

[Don S]

Re: Crazy RPMS

There is nothing you can actually check on the tach itself other than the setting. And yours is probably right.
But it's accuracy needs checked.
Personally, since I work on boats for a living, I have several different tachs and constantly compare them for accuracy so when I do test an outboard, a diesel, or an IO, I know the tachs are telling me the truth.
Basically, you need to find a quality shop tach, or a shop that will test it for you so you know what your rpms actually are.

 

[Texasmark]

Re: Crazy RPMS

No rev limiter on my '02 90 hp (90 ELPTO). Not in the wiring diagram and surely none if I can run 6500.

Mark