Decreased WOT speed and RPM's

[BigTerp]

1994 Johnson 50/35 on a riveted 1996 Tracker 1648. With 2 guys and fishing gear I use to get upper 20's spinning at 53-5400 rpm's. Nowadays I'm down in the lower 20's spinning 51-5200 at WOT. Compression is good and even on the motor which has 75 hours after a complete rebuild a few years ago. The hull has seen it's share of bottom dings, dents and a hole or two. I know my top end is effected by the abuse the hull bottom has taken, but what about the WOT rpm's? Will increased drag from a beat up bottom decrease WOT rpm's?

 

[DaleH]

Normally aspirated motors can lose 10% or more in hot, hot weather as the air is thin. If you also switched to ethanol fuel you’d see a 10% loss too, as its combustion factor is lower than MTBE fuels.

What is the MAX rated RPMs for that motor? In Spring (or cooler weather) with a clean hull, 1/3rd fuel and you aboard and I’d like to see it no more than 200 RPMs off the maximum.

 

[BigTerp]

Normally aspirated motors can lose 10% or more in hot, hot weather as the air is thin. If you also switched to ethanol fuel you’d see a 10% loss too, as its combustion factor is lower than MTBE fuels.

What is the MAX rated RPMs for that motor? In Spring (or cooler weather) with a clean hull, 1/3rd fuel and you aboard and I’d like to see it no more than 200 RPMs off the maximum.

It definitely runs better in the fall/winter, and I do think hot, humid weather is a factor. Didn't get it out at all last fall/winter to waterfowl hunt (new baby kept me home) so it's been awhile since I've been able to run it in cooler/dryer weather for comparison. I have only run it on 93 octane ethanol fuel (as non-ethanol isn't available locally) at 36:1 gas/oil ratio. I run it 36:1 because it seems to run best at that ratio after it was rebuilt, so I never changed. Maybe I should consider 50:1? Not sure if that would help at all or not?

Manual states fuel throttle operating range as 4500-5500. When the hull bottom was pristine (which it is far from now) I was getting in the 5300's for WOT rpm's.

 

[BigTerp]

And FWIW, when doing a hard turn at WOT and the boat cuts loose to slide through the turn, my RPM's jump back up to the 5300 range. Less hull resistance sliding through turns, right? So make sense the rpm's climb back up in that situation. Which tells me the decrease in RPM's at normal WOT is due to the increased drag from my hull bottom condition.

 

[dhoganjr]

Have you sharpened and shimmed the impeller recently. Can easily lose several hundred rpms there. In turns it could be getting enough air to allow it to spin up more.

Is the gas all new and fresh or topped off with new.

Just a couple ideas!

 

[BigTerp]

Have you sharpened and shimmed the impeller recently. Can easily lose several hundred rpms there. In turns it could be getting enough air to allow it to spin up more.

Is the gas all new and fresh or topped off with new.

Just a couple ideas!

I have not shimmed and sharpened the impeller recently. I'll get on that before I take it back out again. At this point I'm living with the decrease in top end speed (which I think is coming from my beat up hull bottom), but the decrease in RPM's is concerning. Hopefully a cleaned up and shimmed impeller will fix that.

I keep my gas pretty fresh. Less than a month old and topped off with new.

 

[JoshKeller]

wish id have seen this post before answering on another site lol. unless you have higher than normal compression, running 93 octane is not letting you get full power from your motor. they are designed to run on 87, and the 93 is harder to ignite. combine this to the 36:1 ratio, you are seriously richening the motor. try a tank of 87 with 50:1 and a new set of plugs

the rpm rising when you slide is just the motor sucking a little bit of air. mine does it also.

 

[BigTerp]

wish id have seen this post before answering on another site lol. unless you have higher than normal compression, running 93 octane is not letting you get full power from your motor. they are designed to run on 87, and the 93 is harder to ignite. combine this to the 36:1 ratio, you are seriously richening the motor. try a tank of 87 with 50:1 and a new set of plugs

the rpm rising when you slide is just the motor sucking a little bit of air. mine does it also.

Makes sense. I'll give this a try.