2 stroke carb running rich?

[Seahawk4.5]

Hi All, I have 2002 50hp Yamaha (pre mix version). I had it serviced not long ago and ever since I think it is running too rich. The reason being whenever idling out on the water or motoring around at low speed the motor starts to run rough (like it is missing) and gets quite smoky. When the throttle opens up it misses and splutters for a moment before clearing and opening up nicely.

The problem is whenever I go to tune it at home out of the water on the muffs it will idle perfectly all day with little to no smoke and it will give a nice burst of smoke from the exhaust when you give it a short sharp rev then the smoke clears quickly which makes me think it is right... Hence the confusion.

I am running 50:1 mix of yamilube 2 stroke oil with 95 octane fuel, always fresh.

My question: should I dial the mixture screws back more (leaner) given all I have said above? ( I don't want to run it too lean for fear of doing damage to the motor) The problem only seems to be when under load so could there be another issue?

[robert graham]

Could be a small hole/tear in the fuel pump diaphragm causing fuel being sucked directly into the crank area and a very fuel rich situation....are your spark plugs wet with fuel?....Remove fuel pump with 2 10MM hex bolts, leave connected with fuel lines and pump primer bulb and check for fuel coming out of small hole/port on back of pump....should be none if pump is O.K.....New pump is about $50 and a very easy to replace, if that's the problem.....the idle mixture screws on top of carbs are generally set at 1-1/2 turns out from lightly seated and are not generally adjustable regarding operating fuel mixture.....good luck!

[99yam40]

If the pump is not the problem then ask to make sure the shop that did the service did a carb clean/rebuild and did the link and sync after reinstalling the carbs.
Hopefully you took it to a Yamaha shop to have the work done.
If you have had this problem since they worked on it, why have you not talked to them about it yet?

[Seahawk4.5]

Thanks for reply. Unfortunately I had the service completed by Mobile outboard mechanic who I won't be using again (long story). I have since asked him about the issue and he said the mixture was set right and "that is just 2 strokes for you, they don't like slow putting around!" I find that hard to believe as this isn't the first boat I have had... Hence me coming here for advice. In future I will only take it to a Yamaha dealer however am reluctant to spend another $300 on top of the last service if it is a relatively simple fix.

I will check the pump and see if the idle screws are set at the 1 1/2 turns from seated. Then go for there I guess!

Thanks for the advice! Will let you know how it goes.

[99yam40]

Find a Yamaha service manual for your motor
it will give you all the specs and procedures.

1 1/2 turns out is correct for my C40, but not what the 50 specs say in my manual, but it is not for a 2003.
It should be close though (1 5/8 +-1/4 or 1 3/8 +-1/4 depending on model)

If all jets/passages are clean then factory setting should run fine

If the carbs are not link and synced after reinstalling that could be a problem.

Make sure to check timing with timing light not just the visual indicator marks.

I found my C40 had a problem with that and had to replace CDI to correct idle timing

[robert graham]

If your thermostat/cooling system is not right and never allowing the motor to reach normal operating temperature it could cause motor to run poorly....maybe remove and check thermostat and pressure control valve....

[maintainerman69]

Could be a small hole/tear in the fuel pump diaphragm causing fuel being sucked directly into the crank area and a very fuel rich situation....are your spark plugs wet with fuel?....Remove fuel pump with 2 10MM hex bolts, leave connected with fuel lines and pump primer bulb and check for fuel coming out of small hole/port on back of pump....should be none if pump is O.K.....New pump is about $50 and a very easy to replace, if that's the problem.....the idle mixture screws on top of carbs are generally set at 1-1/2 turns out from lightly seated and are not generally adjustable regarding operating fuel mixture.....good luck!

are wet plugs a sign of being too rich? I have noticed that with my plugs a few times...

[robert graham]

Wet plugs is a sign of rich mixture caused by a variety of problems....could be the bad fuel pump....

[Seahawk4.5]

Here is the end result
I checked the plugs and cylinder 2 was very wet, the other 2 were ok. Upon further investigation, the idle mixture screw on the second carb was nearly a whole turn further out than the other 2 cylinders! So much for set right! No wonder one cylinder was missing.

Anyway the process I followed was to put the motor in a flusher bag to make sure there was back pressure on the exhaust (replicating operating conditions). I wound all 3 idle mixture screws in until seated, then backed them out to 1 & 1/4 turns out so they were even. Started the motor up and got it warm. Then starting with the top carb I turned the screw in slowly until the rpm started to drop, the backed it out to approximately half way between the starting position and the rpm drop point. Repeated the process for all 3 carbs. The final position is 1 turn out from seated.

It now runs like a dream and will idle away all day with no coughing and spluttering in sight!

Thanks for pointing me in the right direction all - much appreciated.