Yamaha 40 stalls out, with video

The idle jets on that motor are notoriously small. I owned one for 5 years and dealt with quite a few carb issues. The common issue when the carbs were messed up was stalling. It usually happend during initial acceleration or when coming off planr and backing down on the throttle.

No matter what the owners manual states that motor will not run correct on e10.

 

I had to have all new carbs installed but that was partly due to another issue. Many people that I spoke with that owned that motor had similar issues.
You will most likely need ultrasonic cleaning. Past that do yourself a favor and don't run anything else other that regular unleaded.
Also put a good fuel filter in line to try and help keep things out of the system.

 

Ok just watched video that is the 2 stroke. I was referencing the 4 stroke. My bad.

 

Carb or fuel system problem. Sounds like clogged idle jets or low speed jets.

 

Ask and ye shall recieve

There's no direct injection on that motor, just carburetors ("controlled fuel leaks" I call em)

My first two things to look at would be the fuel pump and carbs.
On the fuel pump, there's a gasket/diaphram in the middle with a tab that sticks out on top. Is it white or black?
Also, run the boat again, and right as it sounds lime its about to stall, press the choke for a second or two and pay attention to see if it keeps the motor from stalling or if it still stalls like it has been.

Going from e10 gas to non ethanol usually causes a few problems because the non E fuel breaks all the little stuff loose in the fuel system that was put in there by the e10.

To me it looks as iff that stuff has broken loose and is sitting in the bowls of the carbs, the motor starts for a second then the idle jet starts pulling fuel up from the bowl like a straw and starts sucking up the debris and getting clogged, resulting in stalling.
Or the fuel pump diaphram is ruptured and puring fuel in the crankcase and stalling it.
Did you notice an abnormal extra amount of fuel sheen coming out the exhaust when you were running it on the hose?

 

You have a fuel delv. Issue that motor is not running on all 3 cylinders in that video. I would say its a carb issue, you can check the fuel pump very easily, pull the 2 bolts off that hold it it the block and prime the fuel bulb, if gas squirts out the back of the fuel pump its bad. Next trying running it on a external tank and hose if there is no change then its inside the carbs.

Cut the black and white fuel pump diaphragm does not include the 70tlr motors.

Also you can disconnect the quick disconnect fitting on the engine/from the fuel hose and pump some gas into a clear glass bottle and look for contaminates or water in the fuel system. 

I have herd that sound to many time, I'm 99% positive in my diag above.

 

Creek
Just goes to show my detailed knowledge on little motors ;D

 

Buddy I didn't realize it either until that thread about 6 months or so ago that me and you kept telling the guy he needed to switch it out, and when he went to buy one they told him he was crazy. I went back and pulled the bulletin up, back then. I just happened to remember it this time. Lol

 

Where are you located? I have some down time on Friday, also throw a set of plugs in it before you take it in, I have been having a lot of NGK's going bad lately. Long shot as if 1 plug was bad it should still run on 2 cyc. But worth a shot.

 

Yep you are, good luck let me know If I can be of any help.

 

I have a '01 250hp yamaha that was doing something very similar to what yours is doing and it was a split fuel pump diaphragm. Primer bulb would go soft once it pumped up and had raw fuel coming out the prop. It would crank up and sound like it was dropping a cylinder or two then die. Replaced the fuel pumps and she runs like new! This E10 really screws up gaskets and rubber parts, i only run rec 90 now in all of my outboards… hope this helps!