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Need help identifying this threaded hole on BF90?

Embarq82

Member
Can somebody tell me what this hole that is circled is? Just got the BF90 and I believe it is a 1998. Took it out for its maiden voyage and died under load so I'm thinking carbs need to be cleaned for sure, but it starts first try and idles fine. What does this port/hole go to?

Thanks
Andrew
 

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The threads are for a flushing adapter, and before you consider running out and buying one of those, most find flushing with the engine running on muffs works just fine.


If it's starting and idling fine, it's probably starving for fuel at higher speeds. That could be anything from a plugged vent on the gas tank, to a gas line/gas line fitting sucking air, to a plugged fuel filter, etc. I would eliminate those potential issues before pulling the carbs down. If the carbs were the issue, it's almost a guarantee they wouldn't be idling well at all. Idle jets usually the very first thing to plug up..... -Al
 
I have a 1997 BF90. I had to plug the Flush port due to the check valve leaking. I bought a stainless and a brass plug..I used the stainless plug and put 2 wraps of thread tape on it. I also bought the round cover online. my engine sat for 7 years before I got it..I had it running in a couple of minutes but I had gas coming out of top carb. compression was good and even so I started throwing money at it...After new filters, hoses, cleaning,flushing etc. still had gas coming out of top carb.. I ended up having a Honda dealer rebuild and sync the carbs.. that was 2 years ago, no more carb problems.
 
Alan thanks, and yes i either use muffs to flush and I just bought a giant 100 gallon tub to use as well. And to Roberts point about plugging the flush port - I may also have a vacuum leak somewhere because when I took it on the water it didn't seem like it wanted to run properly, and that may have been the cause. I ruled out vapor lock. Changed the fuel filter and ran it idling for a good 15 minutes, then shut it down and tried to restart and it would not start. I then waited about 30 minutes changed the spark plugs and put tape over the flush port and it fired up and ran another 15 minutes just fine. I'm really hoping it could be the flush port plug that fixes this issue. My plugs were pretty black from carbon build up.

Robert - thats encouraging to hear the motor is running good! I don't know entire history on mine but I hope to get it up and running great like yours. I'm going to order the carb gaskets and seals and clean up the carbs real good, and attempt to sync them also.

Anybody know the exact size and thread type of that plug?
 
I can pretty much guarantee that any flush port issue, and whatever might cause a vacuum leak, will be problems that are NOT associated with each other.

If the check valve, located within the flush port, is working properly, there is no need to plug the port. If it's NOT working properly, it can cause all sorts of cooling issues.....

Older Honda 90's that have been completely serviced top to bottom, make GREAT running engines! I have 2 of them. One on a pontoon boat, the other on a 22' deck boat. Both awesome motors....

I've done 10-12 of them over the last few years, and carefully done, they're tough to beat!
 
I think its 1/2" pipe thread. The Flush port is part of the cooling system, if there is a problem with the check valve water will come out. Alan suggested checking out the fuel lines and filters. Old fuel lines can break down and put little pieces of rubber into the carbs. Our engines were built before Ethanol was added to our gasoline so fuel lines were not ethanol rated i think??.... Bulk fuel line is pretty cheap. I have a water separator/fuel filter between tank and engine. I have a spare filter in the boat just in case.. EDIT, I see Alan Has responded again with excellent advise. I am a slooooow typer I guess
 
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