1997 40hp stalls at WOT

[Sgms1981]

Ok, I'll start at the beginning. I took my boat out a couple weeks ago & it was very hard to start. When it did start it idled ok & eased around the dock fine but had no power. Like 5mph at WOT no power. It was smooth, no stalling, skipping, smoking ect just wouldn't rev up. It would rev fine in neutral/ no load. There was gas all over the carbs, appeared to be leaking from several different places so I decided to rebuild them. Well that turned into replacing the whole fuel sys. New tank, hoses, ball, water separator filter, engine side filter, pump & carb kits. Now it fires right up & runs ok & revs up but then dies after about 30sec at WOT. After it dies it is very hard to start back up like the bowls are empty. Did I set my floats too low? I put the seam in the middle of the float level w/the carb body if holding the carb upside down. After it stalled the primer ball was still hard. Any ideas?

[99yam40]

myself I would check compression, spark to all plugs, and timing to make sure it is advancing.

there is no reason for a motor to die if the carbs have fuel in them.

sounds like carbs are running out of fuel or spark is going away

[Sgms1981]

When it dies it slowly loses rpm then stalls like a fuel problem not all at once like a fire problem but I will check spark & compression anyhow just to cover my bases.

[Sgms1981]

I'm no carb wiz kid or anything. The idle air screws may not be set perfectly but I wouldn't think that has anything to do with it. Please correct me if I'm wrong. No more leaks though so at least that's good.

[Sgms1981]

The boat has been sitting w/the motor on tilt for a little over 24hr now & now there is just a hint of gas in the mouth of the top 2 carbs. None on the bottom carb. That makes me think if I go any higher on the floats on those 2 that I'll have fuel leaking from the mouth of the crabs & that I may need to go a little higher on the bottom float. Does that sound logical?

[99yam40]

what does the sevice manual say about float height spec

[zenoahphobic]

I worry a bit when people fiddle with float height. In the past, yes adjust, because soldered brass floats aren't accurate in shape and solder weight.

Today the plastic floats tend to require nothing out of the box. I seen many and I have personally had no problems with them - they are a no touch item.

Having said that, the OP's symptoms ( slow to die/ fuel coming out) indicate floats are not shutting the fuel off at the appropriate level in the fuel bowl by overfilling.

Now that translates to what?, many people tend to be confused what high or low float level is. Somehow they often think they can change the float's buoyancy somehow. Although this is slightly true because the angle they sit at can be changed slightly, with the subsequent irregular shaped floats presenting a different buoyancy, however this should by and large be totally ignored.

Often one cannot or it is inconvenient to hold a carby upside down to level up to some point. And this way the float is heavier than in fuel and can compress the little spring in the seat pin. The other way would be to do a difficult practical test with fuel actually filling the bowl and determining the height. All fraught with difficulty, so the manufacturer provides some value to measure arbitrarily.

So what I am saying, make sure you are absolutely positive about the float height, because the chances are high that the fuel height will actually become higher when fiddling with such a delicate valve. Mr Townsend will no doubt know exactly how to do this procedure.

[Sgms1981]

what does the sevice manual say about float height spec

My book says .55-.63 from the body of the carb to the bottom side of the float. It's looking like I'm going to have to take em back apart & verify the position. Before I do that I'm going to try & run her one more time. I fixed a hose that was somewhat pinched, the hose from the fuel/water separator to the primer ball. I doubt that's going to fix it but I'm at least going to check it out before I dive back in to the bowls.

[TownsendsFJR1300]

Agreed with Zeno, the floats rarely have to be adjusted.

Generally, checking them upside down and their level is even with carb body should be good. Obviously, checking them per the manual is the way to set them and rule them out.


It sounds to me that perhaps the fuel tank (portable I'm assuming) is not venting and after 30 seconds of WOT, the tank has a vacuum, not letting fuel get to the engine. Try loosing the filler cap and running again.

I'd also make sure all your fuel lines are clear, innerds NOT coming out (from ethanol fuel) and clogging things up. A pinched line as you noted, will also cause havoc more so at high RPMs when you need the full flow of fuel to the carbs..

If you have a brake vacuum gauge, (and pull the carbs), you can put the gauge on the inlet side of each carb, turn the carb upside down and try to pull a vacuum. A good needle and seat(upside down) will hold a vacuum so you can rule a leaking needle out..


*I just had a small engine in for repair (not an outboard) with a cap that didn't vent.. Ran great for a little bit, then started acting up. More so with a full tank as there's less air inside and the stoppage of fuel was faster.. Kinda hard to find as that doesn't occur often at all. Used a vacuum brake bleeder tool to pull a vacuum which it did(and shouldn't have-it was clogged).


.

[Sgms1981]

Agreed with Zeno, the floats rarely have to be adjusted.

Generally, checking them upside down and their level is even with carb body should be good. Obviously, checking them per the manual is the way to set them and rule them out.


It sounds to me that perhaps the fuel tank (portable I'm assuming) is not venting and after 30 seconds of WOT, the tank has a vacuum, not letting fuel get to the engine. Try loosing the filler cap and running again.

I'd also make sure all your fuel lines are clear, innerds NOT coming out (from ethanol fuel) and clogging things up. A pinched line as you noted, will also cause havoc more so at high RPMs when you need the full flow of fuel to the carbs..

If you have a brake vacuum gauge, (and pull the carbs), you can put the gauge on the inlet side of each carb, turn the carb upside down and try to pull a vacuum. A good needle and seat(upside down) will hold a vacuum so you can rule a leaking needle out..


*I just had a small engine in for repair (not an outboard) with a cap that didn't vent.. Ran great for a little bit, then started acting up. More so with a full tank as there's less air inside and the stoppage of fuel was faster.. Kinda hard to find as that doesn't occur often at all. Used a vacuum brake bleeder tool to pull a vacuum which it did(and shouldn't have-it was clogged).


.

I will definitely check the vent soon as I get a chance to put it back in the water(hopefully in the next day or two). The lines, filters, and primer ball are all brand new & I NEVER put ethanol gas in my boat. I really hope fixing the pinched line did the trick but I have my doubts only because it seems I never get the easy fix.

[Sgms1981]

Compression is 110psi on all 3 cylinders by the way.

[Sgms1981]

I'm pretty sure I've found my problem. It seems the fuel line connector where the line from the tank clips on to the motor is causing a major restriction. I pulled the line off where it goes into the small filter mounted to the side of the motor & squeezed the ball & all I got was a little trickle then I squeezed the 2 sides of the connector together in my hand & squeezed the ball & plenty of fuel came out. Of course I won't know for sure till I go run the boat again but I feel pretty confident that this was my problem all along. I'll post back after I get a chance to put her in the water.

[TownsendsFJR1300]

I'm pretty sure I've found my problem. It seems the fuel line connector where the line from the tank clips on to the motor is causing a major restriction. I pulled the line off where it goes into the small filter mounted to the side of the motor & squeezed the ball & all I got was a little trickle then I squeezed the 2 sides of the connector together in my hand & squeezed the ball & plenty of fuel came out. Of course I won't know for sure till I go run the boat again but I feel pretty confident that this was my problem all along. I'll post back after I get a chance to put her in the water.

That would cause a restriction no doubt...

[Sgms1981]

Well I ran the boat today & was sure it was fixed. It ran great for about 4hr then right before I put it back on the trailer it started acting up again. I was doing 31-32mph all day then I whooped it around 180 real hard one time & when I got back on it it felt like I only had about 3/4 throttle. It was all I could do to get to 20mph. No stalling, ran smooth just didn't have the power like it should. I shut it off for a few minutes, it fired right back up but still weak. I'm at a loss. I'm so tired of beating my head against the wall w/this damn Yamaha. All you ever hear is how great a motor they are & I've always believed people when they said that but I'm starting to have my doubts. I've had this motor for a little over a year & I've already had 10x the trouble I had from my last motor (72 mercury) & I ran it for 10+ years.

[TownsendsFJR1300]

What did you do at the fuel tank, post #12?

Mod, direct connection to the tank, ?


It ran great till you spun it around 180 degrees. That changed something, period.

As you probably addressed something in the tank/fuel system, it seems your issue is there...


.