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Discussion Starter #1
I'm in the market for a boat and my budget does not allow for a EFI motor, so ill be most likely buying a boat with a carb motor.

now with my ATVS thyere is a big difference in performance with my one carb bike VS the EFI bikes which have not effect what so ever.

while looking for a boat i have noticed that some folks post they have a prop for altitude, but how does a prop change over ride not getting enough air through a carb with out jetting? likie have done with my carb ATV

So i guess my question is does the prop make that much difference?, and is it necessary to run a boat along the front range most of the time and a few trips a year to the high country
 

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I'm in the market for a boat and my budget does not allow for a EFI motor, so ill be most likely buying a boat with a carb motor.

now with my ATVS thyere is a big difference in performance with my one carb bike VS the EFI bikes which have not effect what so ever.

while looking for a boat i have noticed that some folks post they have a prop for altitude, but how does a prop change over ride not getting enough air through a carb with out jetting? likie have done with my carb ATV

So i guess my question is does the prop make that much difference?, and is it necessary to run a boat along the front range most of the time and a few trips a year to the high country
Yes, the prop matters. At altitude a motor develops less torque. One needs torque to spin the prop. Air (and fuel) into the cylinder, through the carb, can only be changed by increasing the air pressure to the cylinders. How can one do that? Turbo or super charger... Or reduce pitch allowing the motor to spin the prop faster...
 

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Just get it propped and jetted right for where you will use it the most, you might need a smaller prop for optimum performance at high altitudes or a bigger one for lower.
 

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Prop definitely helps, for the reason that TALO outlined.

I

I also found it important to adjust the motor so it runs leaner. If I didn't lean it out, the rich air/fuel mix(due to less air up dere) would foul the plugs. I you foul your plugs, you're gonna have a bad time.

A quarter turn of the mix screw did the trick and the boat ran like a top in the mountains.
 

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The rule of thumb is 2-3% decrease in horse power per thousand feet elevation. EFI or carb doesn't matter, air pressure matters. Boat performance relies on torque. Reduce pitch when torque drops. As Chuck points out, this assume the engine has the right air fuel mix.
 

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Think of the prop as being able to change the gearing of the transmission. Smaller pitch allows it to spin faster.

I am able to use the same prop on the front range as places like steamboat, Granby and spinney. It takes longer to get on plane, but works. If I go up to Turquise or down to Powell, I need to change props.
 

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Be careful when leaning the carb out for altitude and then running at lower altitudes. Running a engine lean can cause serious damage. Its better to run rich at altitude. Also leaving that altitude prop on at lower altitudes can cause the engine to over rev, again causing damage.
 

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EFI meters the amount of fuel so that it is appropriate for the altitude. Nice to have. 8). Put a supercharger on it and you've got more air going in so even more power. I was just told by a guy at Crowley that you don't need to change props when you go to altitude with a supercharged Verado but it makes sense to me that you could/should.

Carbs are older technology and you can adjust the air fuel mixture with the mixture screw so you are dialed in for the altitude you are running at. If you adjust for front range you'll get good gas milage, smooth running/power and won't foul your plugs. Then you go up to a higher elevation with those same adjustments and you'll be running with less air so that means too much gas so you'll be running a bit "rich." Motor will start and run ok at first but as it warms up you'll start to foul out plugs, with coughing, stuttering, losing power. You can adjust the mixture screw by turning it in until it runs the best, and you're good. Or you can just clean your plugs as needed which is probably what most do. If you've adjusted for higher altitude And then you come back to lower altutude and don't re-adjust you'll be running "lean." Leave it that way and you can overheat and can destroy your motor. You can also change out the main jets on your carbs for the altitude you use the boat at the most.
 

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For the OP, you are going to need an extra prop anyway. If you hit a rock you gotta have that backup! Might as well buy one with the next lower pitch and that way you've got it with you when you go to altitude. You should of course be checking for fishing line wrapped around your prop shart every couple of trips or so -- so you'll be adept at changing out props if/when you ever need to. :)

So check the prop, pitch will be labeled in it. If you have a 19 for flat land then your new prop will be a 17. If it's a 17 then you can pick up a 15 as your backup/high altitude prop. You'll feel at altitude it in your "hole shot" (easier to get up and moving with lower pitch, think water skiing), and easier to get to top revs on the motor.
 

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Re: carburated engines. The mixture screw is for the idle circuit. It will not compensate for high altitude main jets when running at lower altitudes. Must change the mains.
 

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Re: carburated engines. The mixture screw is for the idle circuit. It will not compensate for high altitude main jets when running at lower altitudes. Must change the mains.
True.

I'm getting ready to change out main jets on four carbs on my 2000 125hp Merc motor. Snagged up has the same motor and has been talking me through it. 8)
Unless I end up buying that Verado. . . :smile:
 

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My last boat had an Evinrude 115 two stroke. Ran like a top at Big Mac, but really complained up at Dillon or Green Mountain.
 

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So i guess my question is does the prop make that much difference?, and is it necessary to run a boat along the front range most of the time and a few trips a year to the high country
Changing the pitch on a prop is like changing the rear gears in a car or truck.

Always a trade off, 4:10 gears in a car will get you off the line quicker, but you loose top end speed compared to 3.23 type typical car gears.

Same with props.

I had a ski boat that I had 2 sets of props for and the one prop could get me on plane instantly, but I'd loose 10 mph almost on top end.

I currently run an 18.5 ft glass fishing boat with a 115HP 4 carb'd Johnson. There is a big difference between sea level top end and Green Mountain top end speeds based purely on the altitude difference, but I don't use 2 different props, just not worth it IMO, just takes longer to get on plane at a Green Mountain type altitude location.

Hope that helps.

PD
 

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Discussion Starter #14
Thanks for all the info guys it hasd helped I was getting mixed messeages from my basic understanding carb of motors way back in the day. it just didnt sound right that a prop could over come the air/fuel, but it makes senes lined out like gears.
 

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Everyone has explained things well. Air/fuel ratios make some difference between carb and EFI. But prop pitch is the biggest performance variable when boating at altitude.

I will add that if you prop your boat for the front range and take it up to 8000+ feet, the prop will bog down the motor. Bogging down the motor is never a good thing and can lead to future issues. So having multiple props will give you better performance and is better on the motor.

And I will repeat what Albow said. Having two props gives you a backup in case you run the prop into something. I have had two multi-day trips become a few hours because my buddy did not carry a spare prop. It is a long drive to Blue Mesa and back for an hour of boating.

And if you get a boat that is underpowered, multiple props becomes even more important.

So do not fear that a carb motor will be underpowered at altitude. You can change the jets and fix that. Do fear than an underpowered boat on the front range will be very underpowered at altitude, unless supercharged. And plan to get two props with different pitches. One for the front range and one for the mountains. It only takes 10 minutes to change a prop. If you go to lower elevations, like Powell or Mac, you may want a third prop.
 

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Some say running 91 octane can damage and engine but up here on dillon it makes a huge difference and i've put alot of hours in without any damage. Using a good additive can also help. I switched to a four blade prop for up here. I don't get as good a hole shot and it takes slightly longer to plane but it will run about 4 mph faster at 4500 to 5000 rpm.

Whatever you do, let a professional mess with the carb jets if you decide yours need adjusted.
 

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Take a look at the Doel fin hydrofoil which is bolted to the lower unit. Really helps the hole shot, especially when loaded with passengers and full live wells. I've been running them for years.
 

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My II Cents

IMO

At higher altitudes less air less power (torque) no matter what the motor configuration, boat, car truck diesel etc. Super charging helps by cramming more air into the cylinders.
It’s all about air density or the amount of air (oxygen) available for combustion. Temperature is also a factor, cold air is denser than warm air.

The comment made about lugging an engine is the best advice given. Bad for the motor.
A tachometer is important so you can monitor Wide Open Throttle RPM”S. Every outboard motor has a Wide Open Throttle rpm set by the manufacturer, same as the redline rpms for all other motors.
When I had smaller outboards without a tachometer I propped by ear.

The correct prop avoids lugging the engine by allowing the motor to run near the Wide Open Throttle rpm and that avoids lugging the motor.

Jetting a carbed motor is the only way to get the best performance and fuel economy. I jetted my 125 hp Merc. for 5 thousand feet so I could run it at lower elevations if needed. The boat is used in South Park the most which is about 8700 ft.
I re jetted all 4 carbs per Merc’s specs for the correct jet size for 5 thousand ft.
It runs better starts better at 8700 ft even though it runs a little rich being it’s jetted for 5k ft.
It uses less gas and oil (oil injected) after re jetting it.

With a carbed motor the question is “How concerned are you about performance and fuel economy”.
Then consider the cost if you are not able to do the work your self.

When I bought my boat the dealer tried to get me to upgrade to an EFI motor pushing better fuel economy, and performance. There’s no doubt they are sweet but I personally could not justify the cost for these reasons.

Most of the lakes do not require going long distances to fish so fuel economy was not that big of a deal to me.

The loss of some performance was not an issue either. I’m not racing I’m fishing.

The biggest factor of not upgrading to an EFI was the maintenance and repair factor. It uses computers to run it and takes a dealers computer to fix it.

I can work on my old school carbed motor. GET a manufactures service manual.

If I was back to fishing the Great Lakes I might have went with EFI for the fuel economy alone.
 

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True.

I'm getting ready to change out main jets on four carbs on my 2000 125hp Merc motor. Snagged up has the same motor and has been talking me through it. 8)
Unless I end up buying that Verado. . . :smile:
ALAN..... Buy the VERADO !!! :eek::eek:;D
I DO notice a "very slight" loss of power at altitude....above 8500'..... Not like My older (carburated) Honda, though....
 

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As some have already mentioned, just prop the boat correctly. A 4 blade prop tends to be a more efficient design when compared to 3 blade models. Just remember that you should go down at least an an inch in pitch for a 4 blade compared to 3 blade to achieve best performance in the power band RPM's.
 
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