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I posted this in a Yamaha forum but have received little help on the matter:
Slow oil leak into/thru the carbs, then down into the breather box, creating a puddle in the bottom of the box. Eventually this overflows & leaks into bottom cowling. The problem increases when the motor is tilted up.
The leak is NOT coming from the oil level sensor or body cap or the breather cap on the oil tank. It is definitely passing through the carbs. I understand capillary action could create this problem and there is a check valve in the oil pump itself (Kien pump) that could be at issue.
Has anyone else dealt with this and how did you remedy it? The motor runs fine, I'm just dealing with a little smoke at first crank and unwanted oil cleanup for the most part.
Thanks!

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Hello hc803,

I am not completely certain if your 1987 Yamaha 90 shares the same oil/fuel mix strategy as my 2007. On occasions I have observed oil residue in my air box and carb venturi's but not at the level you detail.

Are you using your oil injection pump or has it been bypassed (premixing). If you are still using the oil injection pump there may be an issue with its internal metering. Have you encountered above average issues with spark plug fouling from oil? If so, that would be an indicator of of a rich oil mix.

The smoking at start up is normal (unless it fogs the area as a mosquito killer) then you have a problem. Check out this website for information and possible assistance, www.simyamaha.com

Best regards,
Angel M.

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I'm not bypassing. This happens even when the motor has been sitting for some time, indicating that it is due to external temperature changes, hence the capillary action. It must be a check valve issue. I really just hate to spend $200 on a new oil pump to fix something so minor but annoying.

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Hello again hc803,

In your details you mentions external temperatures as a possible culprit, are you referencing cold or hot temps. Unfortunately So. Fla. doesn't see frigid temperatures but, we do get very hot and humid temps that get into the 100's as of late.

I have not ever observed your situation with my outboard. Do you use ethanol free gasoline? Ethanol fuels (@10%) has a tendency to cause interesting carburetor issues as it breaks down the 2 cycle oil that remains in your float bowls and fuel circuit.

I did a parts diagram review on simyamaha.com of our outboards, my oil injection pump/system differs from yours.

Ask for technical advice on the website I provided you, they have helped me in the past with my inquiries. Also research on this website www.screamandfly.com . There are boat owners that have modified Yamaha 90's that have "been there done that" with their outboards, perhaps one or more have experienced your oil weeping condition. Or as a last recourse do a google search of your condition, it may not be a rare occurrence for these earlier generation 90 2 strokes.

Best regards,
Angel M.

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I've never seen any outboard that didn't have remnant oil in the attenuater box. Nothing to do with oil injection or pre-mix. Typically what seems to happen in hot weather is the fuel expands in the fuel lines, carb bowl and perculates up the main fuel nozzle into the carb ventri, then goes one of two ways based on engine tilt angle, out the front and into the box or towards the rear into the intake. Some carb designs have a vent that allows the air to expand and vent out of the bowl, leaving the fuel to remain. Some designs have a drain in the bottom of the box that connects to a bleed hose that reconnects to the block for reuse. Check to see if there is a hose from the box to the block, it may be broken, disconnected or missing. Consider this normal if less than a tablespoon of oil is in the bottom of the box. Some boxes that are two piece design with a removable cover and the box remains attached to the carbs have a gasket that can often soften over time and start to leak into the engine pan. If you have abnormally large amounts of oil coming from the engine and starting to run down the midsection, that's a different animal and every bleed hose and oil injection hoses should be inspected and replaced.

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Hello fitz73222, I had mentioned the oil pump early in the thread only because hc803 had detailed an untypical puddle of oil in the air box which eventually reaches the lower cowling.

I have seen premix nightmares when instead of a 50:1 ratio the boat owner (or designated helper) mistakenly goes 100:1 in a 10 to 12 gallon tank. It won't kill the outboard but the spark plugs don't like it much (or mosquito's) especially in lengthy idle/trolling modes.

Very true that 2 strokes will typically have oily residue in and around carburetor/air box environment but, hc803 issue details seem to have begun recently as he is unfamiliar with it. Hopefully it will get figured out soon and with minimal expense.

Happy boating,
Angel M.

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Fitz I second that, a good explanation and as the OP said annoying but not avoidable. 30 year old mechanicals will show some oil.