primer bulb goes flat at higher RPM

[JimBen14]

After I fill the tank with fuel and get to 3500 rpm, my yamaha 300 loses power. It may actually sputter to a stop and take a few minutes to start. I have checked the primer bulb and it is flat. Any advice as to what would cause this?

[panasonic]

Fuel tank vent not working. Try cracking the fill cap while running to see if this corrects the problem.

[fairdeal]

Certainly the tank vent is a prime suspect.

Something is preventing the fuel from leaving the fuel tank,
and getting to the primer bulb,
in sufficient quantity for the engine's needs;
so the problem lies between the bulb and the tank.

Inadequate tank venting,
clogged pickup tube,
restricted anti-siphon valve,
collapsed/pinched/occluded rubber tubing,
clogged filter element -
those are the possibilities.

[JimBen14]

Ran the boat for 20 minutes and it ran well. At 4800 rpms the engine sputtered and the bulb went flat. I took the cap off and got the engine to start, but could only manage 1300 rpms. Anything more than that it died. I put the cap back on after puttering for 25 minutes at 1300 and the bulb got full again. At the dock I tried to blow through the starboard side vent with the cap off and seem to be able to push air.

[99yam40]

well if it not tank vent problem, them time to look at the others listed above

[Donnie Coulsting]

Pull the pickup assembly and check for trash on the hose screen. Remove the line from the fuel water separator and use compressed air to blow through the line.

[JimBen14]

The Yamaha 300 had an issue today. It ran well for 1.5 hours, but as I was making my way home, the primer bulb collapsed and the engine would not exceed 2200 rpms. I was running at 3500 rpm for the 1.5 hours andI the engine lost power at 4500rpm's. I had a new bulb and all fuel filters replaced 25 hours as ago. Technician is going to look at the anti-siphon valve. Is this expensive and difficult to replace?

[fairdeal]

Is this expensive and difficult to replace?

Unless the boat was built without access to where the fuel line attaches to the tank,
removing the antisiphon valve is not at all difficult,
so should not be expensive.

The part is simply a 'modified' hose barb -
an inexpensive plumbing part with pipe thread on one end,
which threads into the tank,
and a 'hose barb" on the other end,
to which is clamped the fuel hose that foes to the engine.

[JimBen14]

The tech replaced the valve, $200, and said it was not as gummy as he would have thought. He also did a check for the venting and said it was fine. He has not been able to duplicate the bulb collapsing. Any thoughts?

[JimBen14]

I received a call and the tech is speculating that it may be an issue with the carbon canister filter. Unfortunately, they can't get this part and there is no ETA! Has anyone had issues with this part?

[DennisG01]

If he's not running it under a good load, he likely won't be able to duplicate the issue. Meaning, if he's just revving it up in neutral, that means nothing.

As far as other possibilities, I think others have hit on all the possibilities above. I don't think there's really anything else to tell - review what's been said above.

So this is a relatively newer boat, then. The canister is inline with the vent... I thought he said venting was good? Bypass the carbon canister.

FYI... over-filling the tank (topping off) can clog up the canister and render it junk. You may be able to dry it out... maybe.

[fairdeal]

I received a call and the tech is speculating that it may be an issue with the carbon canister filter.

If he means the carbon canister on the boat's fuel tank,
then it is certainly plausible,

as the engine sucks fuel from the tank,
air must be allowed to enter the tank to take its place -

in a modern "EPA compliant" fuel system,
that may only be possible through the carbon canister.

You could prove the issue very simply
by running with the fill cap off
or loose enough that it is not 'air tight' -
so air can be drawn into the tank through the fill,
bypassing the carbon canister.

[JimBen14]

Yes, he means the carbon canister. I will try running the boat with the cap loose as the anti-siphon valve was replaced. The old one did not look bad and the spring valve seemed to be working. Today the boat ran well for an hour up to 5000 rpm. But it did collapse once and I had to back off the throttle to 3000 to get to my marina. I then added 30 gal of fuel to get to half a tank and ran it another 30 minutes up to 4800rpm with no issues. I appreciate all your help.

[JimBen14]

Still having issues. Mechanic said it may be the carbon canister unfortunately it is not available. I put 30 gallons of fuel and within the first 30 minutes the primer bulb collapsed the fuel cap was loose, but not off. I ran the boat for 1 hour and it collapsed a total of 3 times. During the collapse, the RPM’s had to stay around 3000.
I understand the Perko carbon canister filter between the external vent and tank could be by passed, but this is against PA .The dealer mechanic said he could not do the by pass. Since this is NA and there is no ETA, I am considering having another mechanic do this. Is it difficult? The next day it ran fine for 1.5 hours. Perplexed!

[99yam40]

I am not sure that the canister being plugged up would cause the bulb to collapse if the tank cap was loosened giving it another place to vent.
something to think about and ask about