What is wrong with my engine? Air getting in fuel? Yamaha F40, video attached

[Troutman561]

Ok, This problem had been developing recently and only occurs at WOT. As you can see in the video, I run the engine WOT and it does fine for about 25 seconds and then starts cutting in and out. If I pump the ball every 10 or so seconds, it will not do it but that is annoying to do. It also stops when I back down to 3/4 throttle. I ran the engine on a remote tank eliminating the possibility of a air leak forward of the racor. My next move is replacing the primer ball, hose, and attachment to the engine. I am hoping the problem is not the engine or worse case, the carbs. I hope someone can get me pointed in the right direction and maybe help me troubleshoot this issue..

[DFreedom]

Sounds like a leak somewhere, the trick will be narrowing it down. I have a 70 Yamaha and a couple of weeks ago started the motor and it ran fine for a minute or so and then cutoff. I finally used the emergency fuel enrichment switch and it started but would only stay running a minute or so. I took it home and later discovered that plastic bowl on the fuel filter (inside the cowling) was not tight so it was leaking. I figured out the motor would start because I was priming it but, once the bowl emptied out, the motor would stop. After tightening it, I have had no problems. Just a thought, good luck.

[njterrier]

What size "microns" is the racor? 10 mu should be fine.

Check the fuel pump as well. It is a mechanical, pump essentially a flapper. The internal rubber membranes of the pump eventually dry out/crack from ethanol. Also the gasket behind the pump could be letting air in too. The fuel pumps are not very expensive, I would look at that.

Scott

[welder]

Bad bubble/fuel line/sucking air, clogged filter, bad pick up tube in the fuel tank, loose fuel line connection.

If it doesn't cough and idle or mid range throttle and only at High Speed , I would say filters.

[Troutman561]

I dont think it is filters as I have changed both twice since it began...

Anyhow, I have an update from today. My friend Josh and I changed out the fuel primer bulb and the clip that goes to the engine from the fuel line and that fixed a long standing low speed stalling problem. We also pulled the fuel pump apart and everything looked to be in good shape including the diaphragm. Unfortunately the WOT problem still exists and pumping the bulb still temporarily solves the problem. Any new ideas? Could the fuel pump still be a problem eventhough it visually checks out?

[njterrier]

When you ran the boat on an aux tank, did you run WOT ?.

If the boat has a built in fuel tank, could there be an obstruction partially blocking the fuel pick up?

Were the carb(s) ever cleaned/rebuilt?

I would also look into and clean the small fuel filter on the engine itself.

I had a fuel issue last year, ended up having to have the carbs cleaned. Problem solved. I Have a yamaha 115 2s.

Hope this helps

Scott

[njterrier]

I know that it could be a wild goose chase trying to diagnose the problem.

After I had the fuel/carb problem resolved, I ended up taking a "basic marine mechanic" course through a local adult education program offered in my township. Best $300 I ever spent. I am sure that they offer a course down by you.

On a side note, I was able to work on a 1985 Mariner 40, which is a yamaha motor. I helped with rebuilding the carbs, not difficult at all, If I remember, the owner bought the kits for $40 each. Also we found so much debris in the small fuel filter, and we also found pieces of the same debris in the high speed circuits in the carbs. A good soaking in carb cleaner, blew air through all the circuits, new gaskets, and the motor ran like new.

When I get a chance, I will scan some info and diagrams I have, and post it.

Hope this helps.

Scott

[VeroWing]

Check fuel tank vent for blockage, and check all fuel lines for collapsing under pressure.