Yamaha 50hp 2 stoke high rpm misfire

Been putting together my new to me duck boat the past couple months and finally got it in the water this weekend. 1752 lowe with a tunnel hull. Motor is Yamaha 50 hp 2 stroke. 1998 model 50ejrw with jet drive.

The motor starts perfectly everytime, as soon as you hit the key it is on. At idle it purrs along without issue. Once you start increasing RPM it starts to miss around 3,000 RPM and continues up to WOT around 5500RPM. I visually check spark on all three cylinders and it seemed to have very strong spark. I checked compression and have 1) 112 psi 2) 105 psi 3) 112 psi. I pulled the plug wire off one at a time and ran the motor and when I had cylinder 2 unplugged it sounded the same as when it was plugged in so I assume that's my problem area.

I'm looking for direction on which route to go, should I be starting with a carb rebuild or does it sound like a ignition problem?

Thanks in advance

if you have good spark, id start with the carb.

Just a possibility that I've run into in the past: Sometimes the insulation on the wires coming from the stator or timing base under the flywheel can get worn through or cracked. When you open the throttle, the stator and timing base move. As it gets to a certain spot, the bared wires can make contact with the block, causing a short. I had the same issue with a smaller motor that would start and idle fine, but then had a serious miss around the mid-range. Checked the wires and fastened them, problem immediately went away.

you should be able to do a quick visual inspection of the wires. If you see any exposed or worn spots, wrap them with some electrical tape and run the motor. You should know pretty quickly.

Carbs are a possibility, but in my experience, they don't usually cause a miss, more often they cause a bog or sneeze.

Good luck!

Thanks for the help, I will check the wiring and order a carb rebuild kit awhile.

Where does one buy yamaha parts from online? I want oem quality.


One other piece of info that I thought of today: when I was running back to the dock I was at half throttle and the motor was missing, all of a sudden the motor surged and I had much more power and the mis Fire seemed to clear.... Then the motor complelty died. I check the gas tank and it was bone dry I had just run out. All of this happened in about 10 seconds.

This has me wondering if somehow the carb on cylinder 2 is letting too much fuel in and that's why it's missing?? Why else would it clear up just before running out of fuel. I'm not sure if that is even possible or not.

One way to rule out if your loosing fire on the #2 cylinder is to hook a timing light to it. Leave the boat hooked to the trailer and trim the motor up some and have someone give it gas while you watch to see if the timing light starts missing or not. That should let you know whether it's in the carb or do to electronics. Had a guy a few weeks ago thinking he had fuel pump problems checked it with the light and turned out to be bad power pack. Not sure how well this would work on a prop motor but it works for jets. I just barely back the boat far enough in the water to get the shoe in the water, that way most of the boat is still resting on the trailer.

Lil blue rude,

Thank you for the advice, I have the carbs pulled off right now and rebuilding them. It is a jet so when I get it put back together I will try what you recommended. Hopefully this weekend.

What's your thoughts on using a slack manometer on a yard stick to balance the carbs? Is this accurate enough?

Make sure to set your parking break on the truck to shouldn't roll it any but better safe then sorry.
If you have a way to connect a manometer I'm sure it couldn't hurt, I've never used one but I normally work on Johnson/evinrudes and they don't have any way to connect one, or not that I know of.

Well I have all the carbs pulled apart, these things look spotless inside so I don't have too much hope that this is going to resolve anything but it is nice to learn how to do it. I have the metal parts soaking in a can of berrymans right now. Can I soak the entire carb in carb cleaner with the butterly valve installed? I thought there might be some kind of plastic on the shaft so I didn't put it in with the rest of the parts.

I did find one thing on carb #2, this carb has the "emergency fuel enrichment" valve. I took this apart to find the diaphragm split badly. Could this have caused any trouble with that cylinder?

Just got back from the lake, it runs!! No misfire at all anymore

I put the carbs back together and also replaced the fuel pump and spark plugs while I was at it. I set the throttle and cdi linkage as per the manual. I realized my timing light is at my storage so I have to go get that to check it.

Thanks for everyone's input!