Gas in Exhaust Rough Idle - 1996' Yamaha 50 Four Stroke

Hi everyone,

I have a 1996 Yamaha 50HP 4 stroke that is giving me fits. She starts up fine and runs fine at different RPM's. Might idle rough like it is missing on one cylinder but smooth out with a small tweak of the throttle. Seems to smooth out under higher RPM's and at WOT runs like a top. Idle or go slow and the motor runs rough, it might quit and then start hard after being run. After an extended period at idle you can see quite a bit of droplets of gas hitting the water, spitting out the exhaust. Can smell gas from the time the motor cranks up, its pretty strong right out of the gate. I didn't notice and gas under the cowling, just spitting from the exhaust.

Looks like the top cylinder is the culprit, which is also the one that has the oddball carb. The carbs have been rebuilt with no difference. Compression is good on all 4, the only thing not done at this point is to have a new carb installed on the top cylinder.

Any ideas where I should look? I hate to drop $400 on a new carb if its just a shot in the dark.

Thanks for your suggestions.

Re: Gas in Exhaust Rough Idle - 96' Yamaha 50 Four Stroke

Have you ever had the valves inspected/adjusted - it kinda sounds like you have a sticky exhaust valve.

These aren't hydraulic like in almost all cars and many outboards (now adays) - they do need periodic maintenance..

Re: Gas in Exhaust Rough Idle - 96' Yamaha 50 Four Stroke

Working on a four stroke outboard is different from others. If you don't have a vacu-mate to adjust the carbs with, you are wasting your time. Then you need to do a leakdown test to check the valves.

Re: Gas in Exhaust Rough Idle - 96' Yamaha 50 Four Stroke

Thanks for the replies. Many things I can do but I think I will bring it to my local shop to have the carbs and valves adjusted.

Re: Gas in Exhaust Rough Idle - 96' Yamaha 50 Four Stroke

Update.... Valves look good. Tore down the #1 carb, cleaned and rebuilt. Cranked her up and I have the same deal going on.

Pulled all of the plugs out, cranked the motor over by hand and noticed lots of gas coming from the #1 cylinder spark plug hole at each revolution. The amount of gas is more that could burn or probably ignite for that matter.

Any other suggestions?

Re: Gas in Exhaust Rough Idle - 96' Yamaha 50 Four Stroke

Well, After countless hours of troubleshooting this motor everything kept pointing back to the carb. I cleaned and rebuilt the carb again, to no avail, so I broke down and shelled out $370 on a new carb in a gamble that something was just not right internally. Bolted on the new carb, cranked it up, dropped it in the water and she runs like a top. Still would like to know what the deal is with the old carb but it's just not that important to me anymore.