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I have rebuilt this carb but I don't understand the low speed adjustment screw and seat. Does it have a removable seat? There was no replacement needle valve or seat so I'm a little afraid to start messing around with it.

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There was a very good website out there for these engines specifically but it seems to be gone now. It answered every question you could ever have for old OMC 15 hp motors. The needle valve does have a seat way down in the depths of the carb and it's a pain to remove. I fought it for a long time with a dental type pick. I've heard using an appropriately sized screw (#8 comes faintly to mind) is a better way. Unless the seat is totally messed up, don't bother. The needle itself should be straight. It can be bent by being screwed in too far. I was able the straighten mine but they can be purchased (expensive though). It should be set about 1.5 turns out from all the way in. That's a good initial setting before fine tuning. I wish I never traded my 15- it was a great motor.

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Yes the 15hp is a keeper as were all the engines made on this block. This one will not run well at Idle and tends to die when backed off from running at speed. I can hear odd pitch changes as it runs and need to get this thing back to a reliable state. Oddly enough (though I believe you) the parts breakdown on the carb does not show a seat for the needle valve. It shows a retainer which I don't understand and may in fact be the seat that was mislabeled sometime back in the beginning of time. It also shows an odd cap that contains a lead ball that is right around the end of the needle valve on top of the carb but I have no way of removing and replacing that cap and so I haven't looked into that. It seems like a lead ball in an aluminum housing is just asking for corrosion problems.

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You might try looking at Mastertech marine site. There is a lot of outboard info. there.

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http://www.leeroysramblings.com/OMC_info/OMCinfo.htm

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That's the website I was looking for. I have no clue why it wasn't working for me the other day. Thanks for posting it!

I looked at the parts break down and there must have been a design change in 1979- 1978 seems to be the last year the seat (they call it a bearing but its just a tiny plastic cap) is shown. I assumed the carbs were the same... I guess I was wrong. Sorry for the bad input!

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I found a rebuilt carburetor on ebay for ninety bucks. I am extremely pleased with this engine and it runs like a top. It is now on the 12' Livingston and is reliable enough that my wife runs it two miles from Farnes Cove AK across the bay to Cordova, AK just for fun. It's a keeper!

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Mastertech marine is definitely the way to go. If you buy parts from him you can also give him a call and he'll help answer questions you may have. He has helped me quite a few times.

There is even a section of the website which shows a complete rebuild on a small engine. It might even be a Johnson. The site isn't working right now for some reason so he must be updating it.

www.maxrules.com

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I can't get that site to open either. I find that Ebay comes through about ninety percent of the time but you have to be patient. There were no carburetors for the 15hp for months and then a few trickled in but in the $200 plus range and then about three weeks ago the flood gates opened and the price dumped down to ninety bucks. Ebay is great for old parts.

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Try boats.net they have exploded views of the engines and the carbs, you can get parts from them as well...

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I guess I wasn't clear in my other post. I have a carb. from ebay and this thing is running like a top. Thanks for all the suggestions just the same.