Yamaha 50hp 2 stroke problem

Toddboat

Petty Officer 2nd Class
Joined
Jun 13, 2013
Messages
134
I have a 2003 50hp 2 stroke Yamaha 50c outboard which usually runs great, but on the last trip out, it stalled and would not restart. It turns over and runs for a few seconds, but then stalls out again. Plenty of fuel in the tank, and there is a new fuel/water separator.
I noticed air in the oil line so I thought maybe oil wasn't getting to the cylinders. I figured maybe this would cause a loss of compression and might cause it to stall. So I checked compression and got 70, 70, 80. I shot TCW3 oil into each cylinder and turned it over a few times, then checked compression again. This time I got approx. 115 in each cylinder.
I figured I'd try to start it again and it ran perfectly (except for the obvious white smoke pouring out of the motor because of all the oil I had shot in there). But then after about 30 seconds of running well, the engine dropped to idle speed (which is normal), but then stalled out. I'm wondering if this could be because it had burned off all of the oil in the cylinders and was now back to having low compression again. I am thinking that it's the oil pump that may have failed, but I don't really know. I know that most people would say that the head gasket failed, or it's the rings or pistons that have issues, but I really don't think that's it. Like I said, the boat ran well that day, and it is well maintained. I'm thinking the problem is more along the lines of a gas or oil issue rather than major mechanical malfunction. I should note that the time it ran great was its first run after being stored for the winter. I started it right up and it worked fine until about an hour later. All last season it ran great with no issues. I'm worried that I may not have used enough fogging oil when I winterized it. I sprayed it in until it stalled, but I don't remember putting any into the spark plug holes directly. I wonder if I didn't coat it well enough and may have caused some rust inside the cylinders, or something along those lines. Not sure where to turn next. I know the motor runs great when I shoot oil into it, so that rules out bad gas, bad spark, bad compression, etc. But how do I keep that compression up? Could it be as simple as a faulty oil pump? Like I said, there is a part of the oil line that has air in it (about an inch long), so oil may not be reaching the cylinders. Could that cause low compression and stalling? Thanks in advance, I greatly appreciate any help!
 

Bamaman1

Lieutenant Commander
Joined
May 15, 2011
Messages
1,895
Re: Yamaha 50hp 2 stroke problem

If you have a new fuel/water separator, your motor might not have the suction to pull gasoline through that. You might want to splice the hoses back together (bypassing the separator) and see how the engine runs.

Yamaha oil injection is completely mechanical systems and they're very reliable . The electronics just power the pumps from the big external tank to the small tank on the engine. It's doubtful your problem is oil injection related.

Every time outboards don't run right, I immediately think of 90/10 gasoline and dirty or gummed up carbs. Is the fuel filter under the carbs clean? Have you checked the fuel pump diaphragm for deterioration (from 90/10 fuel)?

Pulling, cleaning, reassembling and reinstalling carbs is just over a 1 hour job. Cleaning carbs is one of those from time to time maintenance jobs. Often, you'll see sticky needle valves making 2 stroke engines stumble. Have you looked down the carb throats after cutting the ignition off. The drip, drip, drip tells the tale.

Now about your checking the compression. You've got to get the engine temperatures up to normal before shutting it down to check the compression.

Another question is what condition is your lower unit in? Lower units can affect engine performance if you have a gear or bearing problem. Many people overlook their lower units or don't change the lube often enough.

There are just so many angles to address.
 

Toddboat

Petty Officer 2nd Class
Joined
Jun 13, 2013
Messages
134
Re: Yamaha 50hp 2 stroke problem

Bamaman,
Thanks for the reply. I'll try removing the fuel/water separator from the equation and see what happens... good point, problems usually come from something recently changed. I will clean the carbs and check out the fuel pump and service the lower unit before worrying about the oil injection. Thanks for letting me know that the oil injection system is not the typical culprit. It's very helpful in directing my energies toward something likely more productive.
I'll post more info as I work on the motor and let everyone know about my hours of frustration and hopefully my eventual moment of pure elation:) Many thanks!
 

Toddboat

Petty Officer 2nd Class
Joined
Jun 13, 2013
Messages
134
Re: Yamaha 50hp 2 stroke problem

Bamaman,
Thank you for your tips... they worked! The fuel filter was so clean you could eat off of it, so I didn't think the carbs would be gummed up. I removed the three of them and took them apart and cleaned them really good with carb cleaner. At first, they appeared to be spotless, but then I noticed that each of them had some sticky residue where the rubber bumper on the float touches the bottom of the carb bowl. I suspect that the float may have been sticking in the open position, allowing me to flood the engine. After reassembling the carbs, I pumped the bulb and was able to get it harder than ever before. Once I did that, I knew I was likely going to have good results. The engine ran great, even at idle. I haven't tried it in the water yet, but this is a big step in the right direction. I must have left fuel sitting in the carb bowls over the winter.
On another note, the link that connects the arm that comes out of the lowest carb to the oil pump was broken. It is made of plastic and is very vulnerable. One strike against Yamaha for that one. I removed it and glued it together and put rubber bands around it until I can get a replacement. it put it back together and it works fine. I'm sure that you're right about Yamaha oil injection systems being very reliable, but this one plastic part in particular is weak between its outer edge and its threaded center. It should be thicker plastic, or it should be metal.
Anyhow, when I test the motor in the water, I'm going to bypass the fuel/water separator and run the motor on 50:1 gas/oil from a separate tank. That way I'll know when I switch back to the inboard tank whether or not the fuel/water separator is causing a problem of its own. Adding oil to the gas is just an insurance policy on my potentially iffy temporary oil pump repair. Funny thing is that the Yamaha manual cautions against running the motor on a fuel/oil mixture. It says it can start a fire. But in another section of the manual, it suggests running it on a 50:1 fuel/oil mixture when starting it after prolonged storage.
Just wanted to follow up because I find that the follow-ups are very helpful for future readers with similar issues. Again, thanks much for your input, it was very helpful in steering me towards approaching this fix as a fuel problem and not an oil problem.
 
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