Need advice on a runaway.

[truth609]

Hey guys, i was out testing my ski, with a jm 734 (i can provide specifics if needed) and all ran well. I started to notice a vibration on decel once it got near idle, i was thinking my pump might have come out of alignment, so i shut it down for the day. When i left, prob 45 minutes later, i revved it in the trailer to clear out the water and it ran away. Wide open for prob 15 seconds or so. Is the only thing that can cause a run away air in the fuel lines? Thanks!

 

[Brett Roman]

Sounds like a lean run off. How old is the engine? Maybe a bad crank bearing contributed to vibration and took out an oil seal leading to the lean run off....?

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[JetManiac]

Air leak in tank pickup assembly, air leak in primer or lines if on supply line, air leak under carb or intake, airleak in fuel selector switch, any fuel line on supply to carbs, or lean condition from carb settings or weak fuel pump, airleak in fuel pump or pulse line to cases, etc etc.

 

[Ducky]

Just let it run away, and if it comes back to you, it was meant to be.

 

[truth609]

Air leak in tank pickup assembly, air leak in primer or lines if on supply line, air leak under carb or intake, airleak in fuel selector switch, any fuel line on supply to carbs, or lean condition from carb settings or weak fuel pump, airleak in fuel pump or pulse line to cases, etc etc.

Chris, i don't have a primer, or fuel selector. I just installed new tank pick ups, the jetting is your specs for a b pipe/735. I've never experienced one like this before, thanks for the input. The motor is a 2015, wouldnt a leak in any internal seals cause a rich condition, being that the crankcase is pressurized more than the outside air?

 

[Brett Roman]

The stroke of the piston moving up by design pulls air/fuel through the carbs into the cylinder below the piston into the crank case. When the cylinder fires the downward stroke forces that air/fuel mixture up and out of the crankcase through the intake ports and into the cylinder above the piston. If you have a bad crank case seal air can be getting pulled in past the seal creating a lean condition because of the additional air.



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[JetManiac]

Can be from cyl base, case or seals leak but less likely. Look to the easy stuff first.

 

[Brett Roman]

Can be from cyl base, case or seals leak but less likely. Look to the easy stuff first.

Right on Chris, I was just going off the vibration he said he felt before he experienced the trailer lean run off. The vibration he meantion made me think crank bearing, when I think bearing I think seal. It's happened to me.

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[truth609]

I'll start by replacing all the fuel lines and the pulse line. Lets hope that vibration was the pump going out of alignment. Thanks guys!

 

[DAG]

Easy to find the leak if you do a leak down test. No need chasing gost and goblins with suspect leaks.

 

[Brett Roman]

Leave them to the pros

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