Yammy 60 4 stroke problem :(

[ShaneJ]

Guys I'm having some trouble with my Yammy at the moment I'm hoping you guys can give me some ideas or point me in the direction of someone that does good work on the Yamahas.

A few weeks ago I was down the tweed river and the first day in the water the motor when perfect as per normal. The second day I was 5 minutes into my travels and it started coughing and spluttering. At this point I didn't worry to much as I thought it was just some rubbish fuel.
I get back to Brisbane and fill up a plastic tank with some new fuel and separate fuel line. Put it in the water for a test which resulted in the exact same problem.
Got me stumped so I call Shane Macpherson to come and have a look. He pulls out the strainer filter from before the mech pump and the filter after the mech pump and they are both clean as. He also strips down the mech pump and checks it out and reported it also looked ok but it did have a tiny bit of gunk on one of the finger valves. He didn't go any further after this and asked if I could test it out again.
Put her in the river yesterday and hooned about for a bit and she was all good. Even headed across to Moreton island for a swim around the wrecks. All good mate..you ripper.
Packed up the picnic gear put it under the rear deck and I see the primer bulb so I gotta touch it. Give it a quick squeeze, yep shes firm(mid to late 20's type firm)..Just the way we like it. Start her up and head for home. Oh bugger.. 5 minutes into the trip and cough cough splutter splutter. Damm.. Something wrong with the primer or fuel line or something easy. I'll fix it.. Swap to the plastic tank with new fuel and different fuel line. Prime it, start her up and take off. All good again for five minutes until it started coughing and spluttering again. I go deeper this time and lift the bonnet. I pull the hose off the outlet of the little filter after the mech pump. Squeeze the primer and 2 pumps of aerated/bubbly fuel comes out. Hmm, to me that isn't right? Connect the hose, shut the bonnet start her up and take off. All good again for only 5 minutes or so. I repeat the process confirming that the mech pump and little filter are full of aerated/bubbly fuel. Prime the bubbly fuel out, put the cover on, start up and take off. Same thing again. After 5 minutes or so it coughs and splutters.
This time as its coughing and spluttering I give the primer bulb a good squeeze and she takes off again for a few minutes. I ended up doing this to make it home.

So is the aerated fuel normal? I cant imagine it is. So what could cause this? Diaphragm in the mech pump maybe?
Any ideas?

Anyone here used Shane Macpherson before. I'm probably going to call him again if I cant figure out the problem.

I'm going on holidays in a few weeks so I'm running out of time.

[BigE]

yatala yamaha.

bigE

[FNQCairns]

It's good you thought to squeeze the bulb in real time during the problem, it often clears up a number of things.

IMO based on what you say either the pump has a gasket/seal/o ring that is letting in air or the fuel line (includes under cowl) somewhere between the bulb has an air leak.

Injectors need a goodly amount of pressure to operate properly, it doesn't take much air for the pump that creates this pressure to start thrashing the fuel, and expanding it, not to mention the heat created. Seen the same thing on EFI cars..different but still the same.

At least it's not at risk of blowing up like a 2 stroke with more or less the sam symptoms.

Do these engines have a mechanical crank driven fuel pump?

cheers fnq

[ShaneJ]

I believe they have 2 pumps. The first mech pump and the second high pressure pump under the inlet manifold.
The problem is definitely under the cowl somewhere as I have tried 2 different tanks, fuel lines and primer bulbs.

It certainly seems like there is an air leak somewhere but buggered if I know where. With the cowl off and engine off I have looked for fuel leaks even while squeezing the primer but cant see anything.

[Lovey80]

Try taking the cowel off, leave the engine off and continue to pump the primer bulb even when it gets hard. It may force fuel out through where you have your leak and bingo you found where the air is getting in. Also have a real close inspection of where the fuel line connects to the engine. One of those o-rings may be gone or damaged etc.

Cheers

Chris

[cormorant]

It is a vacume leak letting air in not a pressure leak in the fuel line. Pessurising the sysyem may not find the leak. The fuel hose can act like a flap valve if the internal lining degrades but be sealed under pressure A stuffed valve in a bulb or blocked tank delivery pipe can cause enough vacume from teh fuel pump that it sucks air as a lesser resistance. You have changed tanks and hoses so an't see it being that.

Look for split or degraded fuel hose
Check o-rings on fuel connections both at tank and on the motor.
Check for cracked plastic / metal fuel connections. Check teh fuel hose connection is seating properly.
Check every joint in the line that is clamped or zip clipped and ensure they are firm and airtight.
As deduced it has to be before the high pressure pump andmore lilely befoe the low pressure pump.