I had to replace the Intake Air Bypass (IAB) on my BF 225. So I disassembled the old one to see why it failed. The short answer is salt water corrosion in one difficult-to-access location. See below.
This should be a routine maintenance procedure that is not in the shop manual. If you are operating in saltwater, I believe this is an essential item to check regularly. If the IAB is not working properly you will experience less than optimal engine performance, especially at the high end – above 3950 rpm. You may also experience some surging in the 4000 to 4500 rpm range.
To check your IAB for proper operation, the next time you change out the high pressure fuel filter, check the IAB first.
• Open up both sides of the upper cowling by first removing the two bolts under the cover at the rear of the engine – those two below the exhaust outlet.
• Remove the six or so bolts on each half of the top of the cowling.
• Peel back each side of the cowling so it clears the exhaust outlets.
• (This is the same as you would do to get access to the HP fuel filter to change it out.)
There sitting in front of the HP fuel filter you will see the IAB control diaphragm. The diaphragm is held in place by two Phillips head screws that you typically remove to get to the HP fuel filter. Remove those two screws. Grab the lever coming out of the diaphragm and pull on it as if the diaphragm was being activated. With a bit of effort it should move until the spring arm engages (or nearly engages) the “stop” adjustment screw. Then let go and it should pop right back in place. If so, it is operating as intended.
If you can’t move it, or if it does not pop right back in place, then you need to remove the IAB and lubricate the shaft to the IAB bevels. Note that apparently Honda does not consider this a serviceable item, so from here on, you are on your own. The worst case scenario is that you cannot fix it and will need to fork out about $343 to get a new one. Part number 17130-ZY3-003ZA.
• First, remove the 10 mm bolts at each end of the frame piece on the back of the engine that the cowlings bolt to. This will allow you to pull down on the frame an inch or so in order to remove the IAB.
• Disconnect the vacuum hose to the IAB control diaphragm.
• Remove the ten 8mm bolts holding the IAB in place
• Grab the loose IAB in one hand and pull back on the cowling frame with the other hand to allow you to pull the IAB up and out.
• Carefully use a flat blade screwdriver to pop the diaphragm arm off its nylon pivot.
• Remove the two Phillips head screws holding the base of the spring mechanism in place.
• Use a flat blade scraper to gently separate the base of the spring mechanism from the IAB metal base. It will only move about 1/8th of an inch or so.
All of the corrosion will be behind that spring base plate. There is a rubber seal in there that prevents water and crude from getting further up the shaft. I would recommend using a right angle pick to get out as much corrosion as possible and then use PB blaster and/or WD-40 to clean and lubricate as much as possible. Work the shaft back and forth so to the point where it is relatively easy to move by hand. Then re-install, reversing the disassembly procedure.
This should be a routine maintenance procedure that is not in the shop manual. If you are operating in saltwater, I believe this is an essential item to check regularly. If the IAB is not working properly you will experience less than optimal engine performance, especially at the high end – above 3950 rpm. You may also experience some surging in the 4000 to 4500 rpm range.
To check your IAB for proper operation, the next time you change out the high pressure fuel filter, check the IAB first.
• Open up both sides of the upper cowling by first removing the two bolts under the cover at the rear of the engine – those two below the exhaust outlet.
• Remove the six or so bolts on each half of the top of the cowling.
• Peel back each side of the cowling so it clears the exhaust outlets.
• (This is the same as you would do to get access to the HP fuel filter to change it out.)
There sitting in front of the HP fuel filter you will see the IAB control diaphragm. The diaphragm is held in place by two Phillips head screws that you typically remove to get to the HP fuel filter. Remove those two screws. Grab the lever coming out of the diaphragm and pull on it as if the diaphragm was being activated. With a bit of effort it should move until the spring arm engages (or nearly engages) the “stop” adjustment screw. Then let go and it should pop right back in place. If so, it is operating as intended.
If you can’t move it, or if it does not pop right back in place, then you need to remove the IAB and lubricate the shaft to the IAB bevels. Note that apparently Honda does not consider this a serviceable item, so from here on, you are on your own. The worst case scenario is that you cannot fix it and will need to fork out about $343 to get a new one. Part number 17130-ZY3-003ZA.
• First, remove the 10 mm bolts at each end of the frame piece on the back of the engine that the cowlings bolt to. This will allow you to pull down on the frame an inch or so in order to remove the IAB.
• Disconnect the vacuum hose to the IAB control diaphragm.
• Remove the ten 8mm bolts holding the IAB in place
• Grab the loose IAB in one hand and pull back on the cowling frame with the other hand to allow you to pull the IAB up and out.
• Carefully use a flat blade screwdriver to pop the diaphragm arm off its nylon pivot.
• Remove the two Phillips head screws holding the base of the spring mechanism in place.
• Use a flat blade scraper to gently separate the base of the spring mechanism from the IAB metal base. It will only move about 1/8th of an inch or so.
All of the corrosion will be behind that spring base plate. There is a rubber seal in there that prevents water and crude from getting further up the shaft. I would recommend using a right angle pick to get out as much corrosion as possible and then use PB blaster and/or WD-40 to clean and lubricate as much as possible. Work the shaft back and forth so to the point where it is relatively easy to move by hand. Then re-install, reversing the disassembly procedure.