How do I adjust my 3.0L for altitude?

Last season I took my boat out several times at 0-1000 ft altitude with no problems. When I took it up to a mountain lake at around 5000 ft altitude, I barely had enough power to plane off with 3 adults on board. I know little about engines, but is there something fairly simple I can do to improve performance at altitude and then reverse the procedure for running at lower levels? I'm assuming a mixture adjustment is needed. If possible, please provide pictures to I know exactly what to adjust. Thanks.

Re: How do I adjust my 3.0L for altitude?

A switch to a lower pitch prop will help the most. Just not enough air up there for you to do anything to the engine besides supercharge it.

Re: How do I adjust my 3.0L for altitude?

Use this site's search as I think there are a few good threads on altitude tuning.

I assume your going to be going running the boat at different altitudes 1000>5000', so it's basically, it's #1- less prop pitch, #2 leaning out the carb and #3 timing,,, or a combination of these until you find out what will work for you, maybe you can get away with just a prop swap?

I go to 9000' once a year and have dropped from 17 to 15 pitch prop and leaned the carb, it's much better, but I'm going to bump the timing a few degrees this year to see if I can get on plane easier.

Re: How do I adjust my 3.0L for altitude?

I bought the boat with a 13.75"/21 prop. It was slow to plane. After last seasons poor performance, I switched to a 13.75"/19 prop. I've only tested it at lower elevations so far, but it definitely planes faster than before. Hopefully that will be enough, but if not how do I lean it out? (ie. engine tuning for dummies type of explanation needed)

Re: How do I adjust my 3.0L for altitude?

You cold rejet your carb but I would only do that if the boat was going to be at altitude permanently. The problem is that at 5000 feet you have possibly 25% less oxygen and that is why you have 25% less power. Never going to make up for that with tuning.

Re: How do I adjust my 3.0L for altitude?

A friend said he wouldn't run his boat at that altitude because he didn't want to foul his plugs. Is that really a possibility? I'd still like to bring it on a week long trip this summer for some mountain fishing, but also plan to drive down to a lower elevation lake for some watersports. If you agree that messing with the mixture is not going to help anything, I'll leave well enough alone. Hopefully dropping down from a 21 prop to a 19 will make all the difference necessary. The high altitude lake has a 20 mph limit anyway and is fairly small. Do I want my passengers toward the aft while struggling to plane or spread out evenly?

Re: How do I adjust my 3.0L for altitude?

Hmmm, It struggled at 1000' with the 21P prop or at 5000'? If it was struggling at 1000' with the 21P and you put on a 19P for good performance at 1000' elevation, you will probably need to drop to a 17P prop for 5000'.

To help the boat plane, move your load towards the bow of the boat, people, coolers, anchors, etc. After you get on plane the people can move back to the stern for comfort.

On jetting, I've heard a few of the experts here say you don't need to worry about it up to 5000', but I do it on my Mercruiser 140HP because it's pretty quick and easy, 15 minutes tops on my carb. Plus, I do a lot of trolling and I don't want to foul plugs, the boat runs better too. If your running the boat at WOT or part throttle your probably going to get a hot enough burn to keep the plugs clean. Idling for hours, probably not, but you can accelerate the engine to clear the plugs. Heck, I've fouled a plug trolling at 1000' (idling) and just shifted out of gear, brought the RPM up to 2000 to clear the plug and then dropped it back into gear to troll.

Re: How do I adjust my 3.0L for altitude?

I have a house at Lake Tahoe(6200 ft) and run my boat up there a lot. Never have a fouling problem. I have a 15 minute no wake zone I have to go through to get to the lake and I have no problems even with a long term idle. My motor has a carb.

Re: How do I adjust my 3.0L for altitude?

With less oxygen available, if you change nothing you are effictively running richer at higher altitudes. So yes, after a while you could foul the plugs.