BF50A high altitude jetting

Hello:

I am in the midst of re-jetting my 2001 50 hp Honda. I live and sometimes fish at 5500 ft but mostly fish at 7500/8000 ft. The engine currently will not get over 4200 rpm wot at 7500 ft lake and is a bit hard to start in the am. I am putting in 120 main jets, down from 130 sea level jets, and reducing the mixture adjustment 1 forth of a turn, sea level adjustment is 2.5 turns out which represents 10% reduction on the mixture screw and 8% reduction on the main. My question is, am I going to lean on the main? The mixture screw is easy-previous owner drilled out the caps but to go back on the main jets is a pain in the rear. This set up works great on my Honda ATV's but I am not sure with the outboard.

Thanks;
Mike

The 50, as you probably know, has three size of main jets. 130, 128 and 125. The standard is the 130.

Honda's chart for high elevation indicates that between 5000 and 8000 ft, you should use the 1st smaller jet. That would be 128.

It also says that if you run the engine modified for a higher altitude at a lower altitude, the engine will be running too lean.

Even though you are at the potential crossover to the 125 jet, I would not recommend going all the way to a 120...especially if you use your boat occasionally at the lower end of the altitude range.

Your power is reduced to 82% of full horsepower at 5000 ft and down to about 73% at 8000 ft.

As you have noticed already, the rpms are greatly reduced due to lack of power. That means your 50 hp motor becomes anywhere from a 35hp to 40 hp. That means, you will need to change the pitch of your prop to get your rpms back up to the 5500 range and increase the life of your motor.

The idle mixture screw adjustment will mostly have affect on only idle. The chart does not mention any change there.

The above is based only on what I am reading on the altitude chart. I have no actual experience with modifying motors for high altitude.

You may want to contact a Honda dealer in your area or in an area that deals with these motor used at altitude to get their opinion.

I am sure, like anything else in life, a chart is a guide and there may be exceptions to the rule...so just reporting what I am seeing.

Hope this at least gets some other opinions generated.

Mike

OK, after wasting 25 bucks on jets that I will not use, I settled on the 125 main. I am already propped down BTW. That is life at altitude. My motivation here is to get the engine running as it should. If I could get 5/5500 wot, I would be happy, even 5k along with easier starting in the morning. In powersports--atv and snowmobiles, we go 8% off the sea level jetting and about 10% leaner on the fuel air mix. That seems to work well. The difference is that it is not likely that I will bring those machines down here, where as the boat, I may. On these carbs there is a very small screw on the upper left side of the carb that is 2.5 turns out. I have set each in .25 turns in hopes of easier starts. and smother running. I thought the idle screws are the ones with springs on them. Anyway, catch me up. All in all, If I experience no change the engine does what I need, I am just puttering in hopes of making it better. Oh, I also went from a 7 heat range to a 5 on the plugs. So in the event that I do run the boat down here and get on the main jet, if it starts to over heat, I will just shut her down and run the aux motor. I think the 125 will work well for me and not freak me out, I hope so.
Thanks for your input, I love these forums and the quality input from fellow boaters.
Mike

The screws with springs that you can see are used to adjust the carburetor balance.

The screws on the upper left that have the caps cut off are the screws used to adjust idle air/fuel mixture. Backing those screws out usually makes the mixture richer.

I have never increased the heat range on a Honda 50 HP spark plug. I am not sure what benefit it will have at altitude.

Mike

Hi Mike;

I have gone to the 125 mains, finished the project and started her up yesterday. The engine seems to run well in Denver, 5600 ft at my house. After warming up(I think it was warm enough), the idle is up to 2000rpm and does not seem to be dropping down. I did lean out the idle air mixture screws 1/4 turn but did not touch anything else. Is there a way to get that idle down to 1000? This engine is used mostly for higher speed transportation from one spot to the other and the smaller engine for trolling, I will be on the main jets most of the time. I see the three screws with springs but am not inclined to monkey with them as I have no time to balance right this minute. Suggestions:
Thanks;
Mike

The top two screws that you are referring to are used to vacuum balance the carburetors.

The screw on the bottom carburetor is the throttle stop and it is used to adjust the idle speed. If turn it counterclockwise, it should reduce the idle, unless the carburetors are already fully closed.

You should get the idle down in any case to prevent any damage to your lower unit when you shift.

Mike