85 HP RPM Questions

[plottrunner]

I have been watching this site for a month now and the amount of info on hear is remarkable.... I bought an 1986 Bayliner Capri 1600 with an 85hp Force on it a little over a month ago.... I have used this site to go through all the troubleshooting that I have had to do....I changed the impeller put new plugs in.(previous owner had plugs with the electrode on them ) Changed the fuel pump diaphragm and decarbed with sea foam.. Compression is 1-136 2-133 3-135 so I think that is good..My question is, I finally got a 20 pole tach installed and went and ran some tests yesterday.... I was at 8400 feet above sea level and I was hitting 3700 RPM's at WOT. I am not sure what formula to use to figure RPM loss at elevation... I'm not sure how fast I was going as I forgot my GPS and the speedo is broken. As I spend most of my time at lakes above 5000 feet I am trying to figure out which prop to buy... I currently have a 13x19 on it and I know if I go down to a 17 pitch that I will gain about 400 RPM.... I need to make sure that if I put a 17 on it that I wont have a problem with to many RPM's when I am fishing lower elevation lakes ie Lake Powell and Lake Mead..... So I need to know how many rpm's I will gain with each 1000 feet of Drop... Thanks in advance for your replies....

 

[Frank Acampora]

Re: 85 HP RPM Questions

As you go up, air get less dense so there is less available for your engine to burn. Since the proper ratio of air to fuel is about 14 to1, at 8400 feet elevation, standard jets in the carbs are too rich. You need to drop down two jet sizes. At 5000 feet elevation you need to drop one jet size. I don't know what size is in your carbs, but as an example, if you have .074 jets, at 5000 feet you would install about 072. There is a chart in an Intertech manual that does give jet sizes for various heights above sea level. A good Merc dealer should also be able to tell you what jets to use.

Since you are burning less air and fuel, you are making less horsepower and yes, you also need to drop down in prop pitch. Going to a 17 pitch prop will definitely help RPM and may also increase speed.

 

[plottrunner]

Re: 85 HP RPM Questions

Thanks for the Reply Frank...Now that leads to another question... I fish from 2000 ft to 9000 ft so I really didnt want to mess with the Jets and just change the prop.....If I do change the jets and I go to say Lake Mead at 1090 above sea level.... will it make my motor run to lean... I really dont need the speed or hole shot at the higher elevation lakes so running to rich is fine with me as long as it doesnt hurt the motor.......

 

[Frank Acampora]

Re: 85 HP RPM Questions

Running rich won't hurt it, it just reduces power and fouls plugs easier. If you are willing to live with that, then sure, leave in the original sea level jets. I can see where it would be a pain changing them from week to week or more frequently.

You do, however, want to prop the engine so it is running somewhere in the 4500-5500 RPM range. That is where it will run best and deliver the most horsepower available at your elevation. Just remember that a large change in prop pitch will necessitate an adjustment to idle speed.

 

[plottrunner]

Re: 85 HP RPM Questions

Ok Thanks Frank....Im going to test it at a local lake thats at 5000 feet today and ill take my GPS....Ill post the results later....