Engine

Catalina Direct Discussion Topics / Catalina 22 Discussion Area / Engine / Altitude effects
 
 
Author Altitude effects
Paul McLaughlin
Walnut Creek, CA

Boat Name: Tiare

Model/Year: C22 Swing keel, 1982

Hull No. 10890

Hailing Port: SF Bay
02/06/2006 7:05 AM Pacific Time

I have an older 4 stroke Honda BF75. It's been a very reliable and rugged engine and always gotten me where I need to go. I am very meticulous about maintaining it.
I use the motor in the SF Bay (sea level) almost exclusively, sometimes a local reservoir (elevation 100' if that). I am planning a trip to Lake Tahoe this summer. Can anyone give me any advise about how the motor will run at 6225'? Will I need to tune it differently?
Michael Smalter
Webster, NY

Boat Name: Marrakesh

Model/Year: 1986

Hull No. 13645

Hailing Port: Rochester, NY
02/06/2006 3:11 PM Pacific Time

There is less air at 600ft than at sea level, so a regular carburated engine will likely run a bit too rich. However when I visited Colorado last September, I saw that the octane of the "regular" gas was 2 points less than what we have in NY. So if you get a fresh tank of lower octane gas, that may counteract the difference in air-fuel ratio.
Greg Baker
Charlotte, NC

Boat Name: Sea Sharp Minor

Model/Year: Catalina 22 - 1984 - Swing Keel

Hull No. 11823

Hailing Port: Lake Norman Sailing Club
02/07/2006 2:14 PM Pacific Time

Sorry but the difference in octane is not going to help with the mixture.

If it is an older outboard it should have a mixture control. Once the engine has warmed up and you are running at cruise, screw the mixture control clockwise until the engine just begins to slow down, then back out to where it just runs best again (probably 1/4 turn or so. Now it ought to run fine. What you are doing is to set the carb to use less fuel with the thinner air to maintain the correct air/fuel ratio. Don't forget to redo the adjustment again when you return to lower altitudes as too lean of a mixture is a very bad thing.
Michael Smalter
Webster, NY

Boat Name: Marrakesh

Model/Year: 1986

Hull No. 13645

Hailing Port: Rochester, NY
02/07/2006 6:02 PM Pacific Time

Greg is right. I was wrong. Lower octane gas won't offset the fuel/air mixture adjustment required at high altitudes.I did an internet search to find out why the octane is lower in the Mountain states. Here's what I found.

"In the mountains lower octane fuel can be used. At 6000 feet the air pressure drops to 80% of sealevel thus reducing the effective ML compression ratio from 10:1 to 8:1 which prevents acceleration pinging.'
 
 
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Catalina Direct Discussion Topics / Catalina 22 Discussion Area / Engine / Altitude effects