Jet Performance at high altitude

[Johnny Utah]

Does anyone have any suggestions on how to increase Mercuruy Jet Pump performance at High altitude. I have a 1992 200 hp EFI Mercury w/factory pump. It was tuned up at low altitiude but I am running it at high altitude (4500 ft to 6000 ft) for the next month or so. It is powering a 21 foot Super V and sometimes I can't even get on plane. At Sea level the pump can get me up to 30mph with 5 passengers. I am hoping that it is the altitude that is causing the lack of performance and there isn't anything wrong with my motor. Is there anything I can do to help a little? How about different octane gas or a different fuel to oil mix? Any suggestions would be great.

[Starfish]

If it's an EFI there shouldn't really be much of anything you need to do... carbureted motors would need to be re-jetted to run leaner, but an EFI should be OK. You just can't get as much horsepower at altitude because there's not as much air, so you can't burn as much fuel.

Are you already running a stainless impeller and is it shimmed properly? Sounds like things aren't running quite right.

[Johnny Utah]

I don't have a stainless impeller and I am not even sure what shimming is. Where is a good place to get a stainless impeller? Are there different pitches?

[Starfish]

Your local boat shop should be able to set you up with a SS impeller or contact the manufacturer of your pump or you can order them on-line... just try ebay or google. Good question on the impeller pitch, you can check whether they make one with a lower pitch. Shimming is just adjusting the tolerances within your pump to make sure the impeller is running the right distance from the housing so you don't waste energy by having water bypass the impeller. The tolerances loosen up with time because of wear, especially with an aluminum impeller.