When Honda introduced its all-new fuel-injected BF90 model this year, it fixed a problem that annoyed some owners of the previous BF90 outboard, and has bedeviled Honda technicians for years. That old motor suffered from what I'd call a "lean bog," a flaw in the carburetion that could cause the motor to sputter and die when the throttle was advanced quickly from idle speed. This could happen when the owner was simply trying to punch the boat on plane, or when a boost of throttle was required to push the boat onto a trailer. The motor would kill, and the boat would begin sliding back off the trailer bunks while the frustrated owner muttered and fumbled around trying to restart the engine before he hit the dock or another boat. All in all, very un-Honda-like behavior.
The EFI on the new motor fixes this little problem, but that doesn't help the many owners of the previous-generation BF90, which was introduced back in 1998. And other than the bogging issue, I think the BF90 has been a great motor. It certainly is the model that put Honda on the map in the walleye belt. So since personally experiencing this lean-bog on a Honda BF90 last summer, I've been looking for ways to solve the problem.
Lean to be Clean
The carbureted Honda BF90 was tuned on the lean side at idle speed to keep it smooth and clean at trolling speeds, and also to help it meet emissions regulations. The duty cycle used by the EPA to measure outboard emissions is weighted 40 percent to operation at idle speed but just six percent at wide-open throttle. The duty cycle is supposed to reflect the way outboards are really used, and surveys show that most outboard boaters spend a lot of time trolling, in no-wake zones, and otherwise puttering around at idle or just above idle speed. So a motor that makes very low emissions at idle can be tuned to run richer at higher speeds, which can be good for performance and long-term durability.
Punch the throttle on the BF90, and the motor gets a big gulp of air. There's a split-second lag as fuel is drawn through the venturi of each carb, and it then has to travel all the way down the intake runner. By then the motor is starved for fuel and dies. Now, this does not happen in all applications, but it must happen frequently, because every Honda tech I've talked to is aware of the problem.
Living with Lean
One of those techs is Aaron Jacoby, the Honda man in the shop at Norton's Dry Dock near Princeton, Wis. A serious student of the Honda outboard, Jacoby knew exactly what I was talking about when I mentioned a bogging BF90, and had some advice for BF90 owners:
Get the revs up: "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," said Jacoby. 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. But Jacoby points out that the rev limiter on the carbureted BF90 is set at 6600 rpm, "so you got a lot of rpm left to work with." If your BF90 bogs, try dropping down one prop pitch size, and see if it helps.
Ease it up: Instead of simply dropping the hammer to get on plane, Jacoby suggests first gliding the throttle up to about 1600 rpm. This gets fuel flowing through the carbs and puts the motor past the lean spot. Then nail it and the boat should accelerate smoothly. How about pushing the boat on a trailer? Jacoby shrugged. "Guess you just have to learn to power it on to begin with," he said.
There are caps over the idle mixture adjustment screws on each BF90 carb, and to comply with emissions regulations, they are supposed to remain sealed as set at the factory. I have seen another Honda tech remove these caps and richen up the idle mixture in an attempt to reduce the lean bog, but Jacoby is not a fan of that method. "In the first place, its technically illegal," he said. "And while you may improve the bog on acceleration, you could end up with other problems." Richen up the idle, said Jacoby, and the motor may not run smoothly at low speed and it could start fouling plugs, especially during long periods of trolling. If you want to give it a try, however, here's the procedure. The caps are secured with epoxy, and you'll need a 100-watt soldering iron to melt that glue. Place the hot iron right next to the cap (keeping it away from fuel lines!) while you also grip the cap with a needle-nose pliers. (My finger is pointing to the cap in the photo) When the epoxy melts, the cap will come right off, and you'll see the slotted head of the jet. Don't try to pry the cap off - you'll just break off the head of the screw. Start by turning each screw out (counter-clockwise) two turns.
Be certain to adjust each of the four carbs identically, and keep notes of each adjustment so you know how to get back to the factory setting. And if anyone asks, I did not tell you how do this.
Editor's Note: Charles Plueddeman is the editor at large for Boating, the nation's largest boating magazine.