Looking for some trouble shooting advice. My boat runs like it is missing. With a good amount of throttle it will really run at full rev then go down and up. Kinda like in a car when you get bad gas. I changed the plugs and lower lube on it about 2 months ago. Ran fine until a few weeks ago. I added some ring free additive thinking I might have got some bad gas. It starts up just fine and does not die when I just let it idle. Any idea what might be wrong? Sorry forgot to add my motor is a 150hp Yamaha VMAX.

There are additives that preserve gas, but none that fix bad gas. If you suspect bad gas, borrow or buy a portable tank for testing.

J I am very stupid when it comes to boats. That is just a guess since up until now it has run perfectly. The motor is only a year old and I have taken good care of it. I also changed my plugs this year, but it ran fine after doing that. Could it be possible that one of the caps on the plug is loose? Saying that made me think of something else to check.

My guess is, bad gas...E-10 mix gone bad. Drain the tank, add fresh fuel and a double dose of SeaFoam. Run it and see what happens. If it set up over winter with the E-10 mix and no fuel preservative...you got problems. Drain it!

I added Sta-bil fuel additive to what fuel was left over the winter. It ran fine the first few times out. I did fill up the same day it started acting up. It is a place where I fill up my truck as well. Have not noticed it in the truck, but not sure what the tolerances are on my truck motor vs a outboard. Just find it weird that it starts right up and idles just fine.

Don't know exactly about your vmax, but your truck has sensors & computers that evaluate the burn & compensate for variances in gasoline. In general 2-strokes are more sensitive to contaminated or old gas.

Is that a carb'd motor? a sticky float valve will give you that kind of symptom - bowl not completely full but it can keep up with demand until you go full throttle for a few seconds then a cylinder drops out for a while. Don't ask me how I know, but try putting a float in upside down - you'll see what I mean

Weak fuel pump, similar kind of thing. Rule it out by squeezing the primer bulb when it starts acting up, see if it improves.

Fuel line? Primer bulb valve? Tank not venting?

Are you saying this is a 2007 motor? This outboard should be in warranty. If in doubt, put the gas in your truck and start over. Pull the cowling and check plug wires and fuel filter. Have seen some Yamys have a miss if plug wires not pushed on all the way. Good Luck

Stabil will not freshen up bad gas. You have to put stabil in it when you first put the gas in to stabilize it. Stabil keeps gas from going bad.

There are so many things that can cause an engine to miss. First thing, do a compression test. Then I would do a spark test. You could have a bad ignition wire, coil, ect. But there is literally 2 dozen things that can cause the problem. So if you don't know how to work on the engine yourself, spend a few bucks and have someone do it for you. That way you don't waste money on parts you shouldn't have to replace.

The bigger issue here is running an engine with one or more cylinders in an extremely lean condition on a two stroke is nearly the same as running straight gas in those cylinders. The lack of adequate lubrication in a dry cylinder will result in the same damage. Stop right now and get the problem fixed before you are looking at getting the cylinder honed and new piston and rings.

Bad gas is probably the last things I would consider. If it was bad gas, the engine as a whole would be having problems. It would be hard starting if it would start at all, stalling at idle, missing at idle etc. If there was as much bad gas out there as boaters claim, I think we would be seeing a 'few' more vehicles on the side of the road. Since it is only occurring at high speed operation, it boils down to a fuel delivery problem or an intermittent ignition problem that is only present at high speed operation (increased vibrations at higher rpm).

Someone above said there could be two dozen reasons for the engine to be doing this and that is true. There are probably more. The bigger issue with your motor is that the affected cylinders are chugging away just as fast as the others but with inadequate lubrication. That spells diaster.