Yamaha 25 – fuel issue?

I have a 2007/2008 25hp Yamaha two-stroke that I’m trying to diagnose an issue on. The motor is new to me this summer, bought it from someone who only used it 1-2 times a year in the Boundary Waters.

On the first few trips out this summer the motor ran like a top at wide open throttle and would troll for hours on end at the lowest setting without issue. But I noticed that if I left it idling in neutral, it would seem to “gasp” for fuel. Everything was smooth as butter and then it would come very close to stalling out before saving itself and coming back to a normal idle. Only happened in neutral at idle. It seemed odd, but it never actually died. The more I’ve used it, it’s gotten worse. Now, I can barely keep it idling in neutral for 30 seconds without it dying, and I’ve also noticed that while trolling at the lowest speed it will kill from time to time. Running WOT is still perfect.

I have run probably 6-8 tanks of fuel through the outboard this summer so I don’t think it’s an issue with old gas. What should I be looking at in terms of fuel delivery?

Thanks!

If you’ve run 6-8 tanks this summer, I’m not betting on bad gas.

I’m assuming this is a carbed outboard.

What you’re describing sounds to me like a “lean sneeze”. That is where the engine is running just a little lean and can’t quite maintain itself at idle so it occasionally “sneezes” by briefly stalling.

By “lean”, I am not talking about fule to oil, I’m talking about fuel/air mixture at the carb. Have you tried adjusting the carb to richen the mixture a little?

At the dock with engine warm, I would richen by about 1/8 turn increments (turning mixture screw out is rich) and allow it to run and see what happens.

There are plenty of videos online showing how to adjust an outboard carb. Don’t worry about the brand, the principles are the same.

If doing this adjustment in the driveway, I’m not a fan of using garden hose muffs to make carb adjustments because the backpressure of having the lower unit underwater is not there when running on the garden hose. Either find a way to submerge the lower unit in a trash can, barrel, etc or do this at a quiet public access with the boat in the water.

Grouse

Could also be a fuel line problem. If your primer bulb feels like it has air in it when it acts up, put a whole new line and bulb in. Your fuel pump could be getting weak as well. They only last so long.

Next time it gasps, pump the primer bulb. If it temporarily resolves the issue, I’d replace the items listed above.

Both are relatively cheap fixes and are considered maintenance items, especially if you run oxygenated fuel.

Thanks both for the ideas.

TheFamousGrouse wrote:

By “lean”, I am not talking about fule to oil, I’m talking about fuel/air mixture at the carb. Have you tried adjusting the carb to richen the mixture a little?

At the dock with engine warm, I would richen by about 1/8 turn increments (turning mixture screw out is rich) and allow it to run and see what happens.

Would that be screw #16 on this diagram?

dirtywater wrote:

Thanks both for the ideas.

<div class=”d4p-bbt-quote-title”>TheFamousGrouse wrote:</div>
By “lean”, I am not talking about fule to oil, I’m talking about fuel/air mixture at the carb. Have you tried adjusting the carb to richen the mixture a little?

At the dock with engine warm, I would richen by about 1/8 turn increments (turning mixture screw out is rich) and allow it to run and see what happens.

Would that be screw #16 on this diagram?

Yes.

Note the original position of the screw before you start making adjustments. Carefully record every adjustment you make. That way you can return it to its original position if this turns out not to be the fix.

Try to warm up the engine before making any adjustments if possible. If you cannot keep it running long enough to make an adjustment then open it one quarter turn and see if that improves things. Adjust from there.

It goes without saying or at least it should that you should check all other obvious things before making any adjustments. Are the spark plugs clean and correctly gapped? No there is no such thing as a standard gap.

As per the other comment above there are other possibilities but before we start ordering and changing parts I thought we should try the simple stuff first.

Grouse

Took the boat out for a couple hours today. Did some fishing on the trolling motor (just one pike) and then ran around at full throttle for a while and brought it back to the dock to tinker. Unfortunately neither the adjustment to the fuel/air mix or pumping the bulb made a difference. I will look at the plugs next although it seems odd to me that there are no issues at higher RPMs. I took a video of the issue but it’s too big to attach here. I’ll see if I can compress it.

I’m not opposed to having a tune-up done since the motor is new to me, but I’d like to try to see what I can do on my own first. Appreciate the feedback.

It is not odd at all. The idle jets in a carb are much smaller than the high speed one. There is less vacuum to say, run a weak fuel pump.

iowa_josh wrote:

It is not odd at all. The idle jets in a carb are much smaller than the high speed one. There is less vacuum to say, run a weak fuel pump.

Right that makes sense. I was saying it would seem odd if it was a spark issue given these symptoms.

If this helps at all…

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^^^^ Dirty jet or bad fuel pump (some motors have just one fuel pump, others have two)

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Pull the carbs and have them ultrasonically cleaned, new fuel filter, and plugs, things that should be done anyway and start there.