09-21-2005, 02:07 PM
I recently took my boat up to donner for the fisrt time, and my boat performed terribly. I have a 115 mercury four stroke, and I couold not even get the boat up on plane! It seemed as if my rpm's were cut almost in half. Is this normal? I drove it at Folsom the day before and it performed great. Any suggestions????


09-21-2005, 02:23 PM
I'll ask a stupid question. Are all 4 strokes fuel injected? Tracker, it wouldn't be normal if your 4 stroke was fuel injected but would be normal if it was carburated.

If you are back home now, hook it up to the hose and see if you are still rpm limited.

09-21-2005, 02:52 PM
Even if you have fuel injection you still lose about 2-3% of your max hp per 1,000 ft of elevation. There is just less oxygen taken in with each intake stroke. The O2 sensor can adjust the mixture to minimize the loss, but it can't get around the limitation unless the engine is supercharged.

I use a lower pitch prop for high elevation. You should re-prop so that your engine can turn at least 5,000 rpm at WOT at the highest elevation that you fish.

Check your plugs after a run at high elevation. They could be fouled.

09-21-2005, 08:26 PM
I just had the motor serviced last week. I also took it to Folsom again today and it ran like a champ. I will be at donner tomorrow, so I guess I will see if it happens again. I will post my results.


09-21-2005, 08:31 PM
I had a 19ft Bayliner with a Mercury 140 I/O. I had three props, 21 pitch for the valley, 19 for the lower elev foothill lakes and 17 for the sierra lakes. Like Doc said without the smaller pitch at the higher elev I couldn't get the boat on a plane with the thin air.

09-21-2005, 11:20 PM
What's the elevation at Donner anyway? I've never run my Suzuki built 140hp on Donner, but it has run very well on lakes at 7000ft. I know that my motor has individual fuel injection to each cylinder and that amount of fuel injected depends on the oxygen content in the air as measured by the motor's oxygen sensor. I'm not sure how the 115 Yamaha is injected or maybe Donner is such a high altitude lake that all motors have a problem on it.. Larry

09-21-2005, 11:29 PM
Donner is about the same elevation as Tahoe (~6,200 ft). He is just over propped and couldn't get his boat on plane. Once a boat gets on plane, it takes much less hp to keep it there. As little as a 2" decrease in pitch can make all the difference in the world if it can get a boat up on top of the water.

09-22-2005, 08:38 AM
I haven't had to downsize the prop on my 140 Suzuki-built motor at the high elevations. I think this is because of the way the motor is fuel injected and because my boat planes fairly easily. It does lose power at the high altitudes, but still has no problem getting my Lund up on plane. My previous boat was a 20 ft bass boat with a carbeurated 200 hp 1993 Evinrude. Up at Eagle Lake the only way I could get it to plane was to have Connie and our dog both move up near the bow when we took off. Eventually solved the problem on that boat by changing the jets and putting on a smaller pitch prop when I took it to Eagle Lake. Like Doc says a couple of inches lower in pitch can make a big difference. Lets the motor quickly rev to higher rpm where it has more torque and horsepower.. enough to get the boat on plane.. Larry

09-22-2005, 02:08 PM
Doc is right. You are losing about 18-20% of your hp due to elevation. A smaller pitched prop will make a world of difference.

I have a new 30 honda with a 12 pitch blade. At 1000' can go 29mph unloaded and 25mph with another person. At 6000', i could hit 22mph by myself but only 14mph with others with the 12 pitch prop (would not plane).

Now with the 10 pitch prop, I hit 23mph loaded at 5000'.

From what i found, each decrease in 1" of pitch would be a gain of 150-200rpms, 2"=300-400rpms, 3"=450-600rpms, etc.
Look at your owners manuel to see the max rpms of the engine and look at the prop to see what size/pitch it is. 2" might work but ask questions on this board (those who have that same engine) or ask your local dealer for recomendations.

If your local dealer has a good price on the prop, buy from them but if not,www.boat-props.com has a good selection and great prices.

09-22-2005, 06:14 PM
I took the boat to donner again today and had no problem getting the boat up on plain. I discovered however that last week when the dealership installed my kicker and control box, they kinked my main motor throttle cable. I took it apart and straightened the cable and the problem was fixed. thank's for all of the great suggestions.


09-22-2005, 07:21 PM
Now how were we supposed to figure that one! ;)

09-23-2005, 10:04 AM
Doc, what about an inboard jet at high elevations? Mine sucked. Is there any easy fix? Its a carbed 351 Redline :'(
My little yamaha kicker starts better at high elevations than it does in the valley. I loved your tip from before on not to use high octane. I used to buy it everytime i went to the mountains. A spare impeller would cost me $400 and changing it is not just a cotter pin and a nut as you probably know. I would only really do it if I was going to say pull a skier/tube. Any thoughts or just SOL?
2Shakes ;D ;D 8)

09-23-2005, 10:12 AM
You're probably SOL. :P

Jets are so inefficient in the first place that altitude really messes up their performance. The impellers are generally matched with the performance of the engine at sea level.

09-23-2005, 10:49 AM
2Shakes, that is why I bought the ThunderJet. I wanted a boat that was tuned on average for 4000 to 5000 foot elevation. A jet MFG'd in Idaho was a way to get there from here, stock. My question for Doc is how is it going to perform at sea level? Be nice. MM ;D 8) ;D

09-23-2005, 10:56 AM
Doc thanks for the response :'(

Metalmouth I have a solution. We take your boat at high elevations and mine at sea level ;)

Doc buys the beer :o

2Shakes ;D ;D 8)

09-23-2005, 11:03 AM
It's On!! ;) 8) ;)

09-23-2005, 11:12 AM
An engine set up for 5,000 ft will run too lean at sea level. You probably wouldn't notice the difference in performance from what you had at 5,000 ft, but there is a chance of the engine overheating if operated at low altitude.

But you guys seem to have worked out the solution among yourselves. ;D

09-23-2005, 11:17 AM
so run a bite richer above 5000ft than around 1000ft or lower i have a 77 envirude 9.9

09-24-2005, 07:01 AM
So Tracker, the way I understood you to say was that the 115 Merc. ran fine with out changeing the prop right? I have a 115 Merc. on my DC but haven't taken it up to Tahoe yet but would like to know if I need to get a prop change or what.Thanks, Brettski