Breaking carb drain screws

[dcwyard]

I have a Yamaha 2002 f90 That keep breaking carb drain screws. I put in 4 new screws and rings this spring and when I drained carbs yesterday, 3 of the heads were broken off of the screws. What is happening?

[99yam40]

over torqued maybe

[dcwyard]

Breaking Carb Drain Screws

Every year since I bought the boat, I have had to replace at least one each year. I just snug them down to seat the rubber o ring. When fall comes, the head seams to have just blown off. (I guess this would also explain rough running towards the end of the season. I never put 2 and 2 together on that one.) This spring I replaced all four, just give me a new starting point. The last trip out it ran a little rough, and when I went to drain the gas this past week end, three were broken off. Not sure what is going on but I would have to say with three plugs broken and fuel pouring out of the the carbs, it could not have run at all.
I have no problem screwing the broken threaded parts out so I would not think over tourqeing would be my problem. I use fresh fuel each spring and add a can of "Sea Foam". I am using Yamaha parts.
Thanks for the reply. I will be very gentle next spring and see what happens. If nothing else, I may have explained my rough running towards the end of the season.

[robert graham]

Are you talking about a 10MM brass hex head drain screw with an O-ring on it?, or something else?. I can't imagine my drain screws with the heads just coming off of them. Maybe send us a photo of what you've got?

[dcwyard]

Photo included?

Yes, that is what I am talking about. The brass plugs. I find the heads sitting in the bottom of the engine case and the thread part is still in the carb bowl. I have worked on lawn mowers, tractors and such for years, and they all use a similar brass plug, and have never seen anything like this happen.
I will try to attach a picture. This is the first time I have used a forum of any sort. Hope it works. These parts are not mates. Just a couple of pieces that were on the platform next to the motor.

[mitch]

me either but i wold say that it has something to do with different expansion rates of the materials used brass and aluminum .

I know that a o ring is the correct seal but could you also incorporate a fiber washer so as to allow for the expansion of the aluminum to stop it pushing the heads off ?

[robert graham]

Yep, that's the same carb drain plugs as mine, but it's a mystery to me why they would break off like that. My 90HP Yamaha is a 1999 and the plugs are all just the original ones, with no breakage, and I always tighten mine down good and tight, use a small screwdriver instead of a wrench so not to put too much torque on them. I believe Rodbolt mentioned that he gets several broken plugs every year from folks overtightening. Maybe your Yamaha dealer or service department can shed some light on this. Please let us know what you find out on this.

[OffShore1]

Good God how do you back the screw out with the broken head? It just happen to me on a 70

[boscoe99]

Drill and pick it out.

Got a NAPA nearby? They can probably do that for you for a few buck.

[Bluefish]

Are the bowls aluminium could it be electrolysis at work here?? mybe try aluminium plugs to see what happens

[boscoe99]

Over tightened is my guess.

[zenoahphobic]

Over tightened is my guess.

Most likely cause. The carb drain plugs have holes through them leaving small area for the size of the head. You see in the first pictures a bit of gouging on one of the heads.
Electrolysis would take much longer but some sort of crystalline could have occurred because of poor quality brass. Too tight and change in temperature is all that would be needed for them to suddenly snap, my guess.

[panasonic]

Over torqued at one time or another....use a good fitting screwdriver and just snug them up!

[boscoe99]

Lightly snug.