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BF 225 IAB Maintenance Item not in the Helm Shop manual

chawk_man

Silver Medal Contributor
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.
 
Follow-up. After I had been running my BF 225 for about two years (2010) I started noticing a surging of about 100 to 150 rpm in the 4000 to 4500 rpm range. I was told by several folks, including two maintenance shops, that this was the result of the VTEC system trying to decide whether to get into VTEC mode or not. However, once I replaced the IAB the surging problem has apparently disappeared. I'll confirm that later today when I take a long run down the Chesapeake on hopefully nearly flat water.

For those with the Helm Shop Manual, see page 18-15. The IAB has a set of baffles that mount on top of the engine air intake. At low rpm's, the baffles remain parallel to the base. However, when the engine reaches 3950 rpm, the ECU activates the IAB Control solenoid that opens intake manifold vacuum to the IAB diaphragm, which in turn, moves the bafflers to a 45 degree angle. When rpm's drop below 3750 rpm, the ECU turns off the vacuum, and the baffles (hopefully) return to their parallel state. Technically, what the IAB does is initiate an inertia charging effect which draws a higher volume of air into the combustion chambers under higher rpm's.

The bottom line, as implied above, if the IAB is not working properly, you will not get top efficiency from your engine at higher rpm's. One indicator that the IAB is not working properly is the surging effect in the 4000 to 4500 rpm range.
 
Thanks Jimmy. Like you, I enjoy solving a mystery.

All - after 5 hours of running yesterday, much of it in the 400 to 4500 rpm range, there was no evidence of surging.
 
Chawk.....I adjusted the valves on one motor today, so while I was back there I investigated the IAB the way you described above. I don't understand the directions, either that or my diaphragm is broken.

After removing the 2 screws the diaphragm is hanging, but attached to the arm that goes to the spring. The end of the arm that goes into the diaphragm is attached to a plate inside the diaphragm housing. This plate isn't secure.....it is loose inside the housing. I wasn't able to get any "pop right back in place" action.

I'm not sure what's going on, but it doesn't seem right.

Any ideas?
 
You stated" After removing the 2 screws the diaphragm is hanging, but attached to the arm that goes to the spring. The end of the arm that goes into the diaphragm is attached to a plate inside the diaphragm housing. This plate isn't secure.....it is loose inside the housing. I wasn't able to get any "pop right back in place" action. "

That's as it should be. I'll get back to that below.

It's the other end of the arm that you should be able to pull on against the coil spring and it should pull out until it reaches a "stop" bolt. Grab the lever coming out of the diaphragm and pull on it as if the diaphragm is 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 that doesn't happen, then you need to remove the IAB as described in my original post.

As for the arm as it goes into the diaphragm, push it in all the way, put your finger over the tube that the vacuum tube attaches to, and let go of the arm. It should come out just a little way then stop and stay there until you remove your finger.
 
I can't seem to find the part number or even the part drawing itself in the exploded diagram for my Honda BF250 XXA (2013) model.

Maybe I'm not seeing it, or I'm calling it a different name. It is the copper looking round/saucer shaped piece that you have to swing away after disconnecting a small hose so you can access the high pressure fuel filter.

I believe it may be called the IAB diaphragm.
 
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Just checked my IAB..... it was stiff and not moving back on its own but after a liberal dose of WD40 and some manipulation it's now much better and very snappy!

[although I now have an issue with the mounting screws which I'll put on another thread]
 
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