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Yamaha 2 stroke issue

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cody0707

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I have owned my boat for about a year. My motor is a 1998 C150 Yamaha. The previous owner did not have a fuel water seperator on the boat. I plan on adding one. I also plan on changing the spark plugs and installing a new fuel line. I have already installed a new impeller. I have only had the issue I am about to explain happen to me on 2 trips so far. All other trips the motor has ran great. Here is what I am experiencing:

I can go out on my boat all day and have no issues. Then out of the blue when I try to put a decent load on the motor the motor begins to shake. If I back out of the throttle the issue goes away. This repeats for a few minutes then out of no where it seems like the motor corrects itself and I am able to continue on my way with normal operation of the motor.

Here is a video of what the motor is doing: https://youtu.be/pKwCiOKOztM

Does anyone have any clue on what may be causing this issue?
 
Docksidemarineservices said:
Make sure it is getting to running temp on both cylinder heads.
Click to expand...

I will check on this. I believe it is though. I have ran my boat at higher throttle for 3 miles. Then out of nowhere this issue pops up. Wait a few minutes at lower throttle and the issue resolves itself and I can run full throttle if I want.

The issue has only happened on 2 of my trips. Every other trip I have been on I was able to run full throttle and higher engine loads without this issue happening.

Since the issue is intermittent and isn't frequent I am hoping its just a fuel issue. The fuel I get is marine grade fuel not sure of the octane. I premix my oil to a 50:1 ratio. I am hoping it is just some gunk being knocked loose as the motor gets more use than the prior owner gave it. Prior owner stated the boat sat for some time and wasn't getting used often.
 
The reason you are checking running temps....
If the cylinders and heads are not reaching operating temps it will condensate water in the combustion chamber and short the spark for ignition. The condensate then evaporates and the engine runs fine.

Run the engine hard, wot, stop the boat and shut off the engine immediately and check the temp of the engine.

You will want to check this first before you do anything else.
 
Docksidemarineservices said:
The reason you are checking running temps....
If the cylinders and heads are not reaching operating temps it will condensate water in the combustion chamber and short the spark for ignition. The condensate then evaporates and the engine runs fine.

Run the engine hard, wot, stop the boat and shut off the engine immediately and check the temp of the engine.

You will want to check this first before you do anything else.
Click to expand...

I will check this the next time I am out. Should I just check the temperature around where the spark plug is going into the chamber?

Also, I will be replacing my spark plugs this weekend after decarbing the motor. I will have a close look at those too.
 
Make sure there is no varnish in the carburetors !----Plugged carburetor can now starve a cylinder of fuel AND OIL .----Also run with a timing light to see if there are ignition issues.
 
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