08 bf90 hp random drops in rpm when running above 5k rpm

[g8tors09]

Have an 08 big foot Honda outboard with a real bad problem. It will run ALL day long at speed when I don't exceed 5000 rpm (the tach goes to 7000) I mean I have driven for 1 1/2 hours straight at 5000 rpm and no problems what so ever. When I push it above 5000 (doesn't matter what number it is 5500 6300 etc etc) the engine will randomly drop in rpm and then immediately go back to normal. It will do this continually until I back it down to 5000 rpm then no issue what so ever. Bump it back up then the mess begins again. Thought it might be water in the fuel but then it should be doing that ALL the time no matter the rpm. It is really bad because I am missing about 5-8 mph (It's a 26ft pontoon) and it is really dangerous because the boat basically stalls for a micro second and you are thrown forward. It feels like a stall or hesitation but it ONLY does it above 5000 rpm. Any thoughts or ideas?




ADD ON: This problem is very random in nature. I have gone flat out for 5-6 minutes and then it will start happening. Back off the throttle to under 5000 rpm and bam it WILL NOT HAPPEN. Only over 5000. (or 5000 on my tach) That being said you are not going to go flat out or even near the top due to safety issues for the passengers being thrown around.

[Jonvee]

The inside liner of your fuel lines maybe getting soft and will suck close when you ask for more flow.

[slingshot]

The first thing you should do is give is a full tune up. Plugs, oils, fuel filter. Your symptoms sound like a fuel delivery issue. To rule out your fuel tank, lines and such get yourself a separate fuel tank (3 gal for example) and lines and hook it up directly to your OB and see if things clear up. If it runs good on your aux tank then you know where the problem lies. If the problem persists then you know the problem is with your motor. I’m inclined to believe your problem is in your fuel delivery system because your OB would throw you a code if it was having problems. Good Luck.

[Bamby]

It could be a fuel delivery issue as others suggested, but there are still some other possibilities that could come to play. First check the oil, some four strokes have a tendency to make oil and effectively overfill the crankcase. An overfilled crankcase can create problems especially at higher rpm's.

Another potential is the rev limiter. Your motor is designed to top out at 6000 rpm's and the limiter will kick in a few hundred rpm higher. If your tack isn't set right or faulty you may be actually running up against the limiter and creating all the motor problems because it will miss fire at wide open throttle, the limiter interrupts the ignition.

Another potential problem actually could be your prop.

The BF90 needs to be propped to run 6000 rpm at wide-open throttle, and that’s with a full load of fuel, gear and passengers. Too much prop pitch puts extra load on the motor when the throttle is dropped – it’s like trying to start a stick-shift car in second gear – and this exacerbates the effect of the lean carb tuning. This is especially true if the prop is an aggressive stainless steel model. Aluminum props will usually slip more on hole shot and let the motor rev quickly through the lean spot. If you prop to 6000 rpm with a heavy load, you’ll need to watch the tach with a lighter load and avoid over-revving the motor. If your BF90 bogs, try dropping down one prop pitch size, and see if it helps.Source

[dunny961]

I was thinking the rev limiter also. Should be running at 6000 rpm max. Your tach could be a little off and your running against the limiter causing the engine to cut spark to the plugs.

[g8tors09]

I just had a tune-up done a little while ago. I did run ethanol fuel for the first couple of years.........Next time I am out I will try the seperate tank and go from there!

[dunny961]

When I push it above 5000 (doesn't matter what number it is 5500 6300 etc etc)

How high of an rpm does your motor run up to? You say 5500, 6300, etc, etc? It sounds like it will run way above 6000 rpm's! 6000 rpm should be the max that you run at.

[g8tors09]

Wow I don't know I have just been going by the factory tach on the boat. It goes up to 8000. That being said when this issue starts occuring there is a considerable amount of rpm left in the engine so even if the tach is off it is happening nowhere near the top end of the range. There is a little over 2000 rpm's left once the problem starts.

[slingshot]

Your Honda should run from 5300 to 6300 rpm with the proper prop. With just you in the boat and minimal fuel you should be on the higher end of that. As you put more weight in the boat (load) your max rpm will go down accordingly. Also, yours is not a "Bigfoot" OB. That monicker belongs to Mercury.