Originally posted by zenoahphobic
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Yamaha is almost paranoid about restrictions to fuel flow. It might be that their primer balls, and their balls only, are designed such that the check valves have very slight pressure to position them closed, so as to minimize any restriction to fuel flow within the ball. Not having as much pressure as other valves might cause the Yamaha valves to be fussy when it comes to their seating. That fussiness might cause the valve to not work as well as one would hope unless the ball is positioned vertically.
Once again, I am not saying to anyone to position their primer ball vertically. I am saying that if the dang thing is not working well then try positioning it vertically and see what happens. Might work for some. Might not work for all. There is nothing to lose and there is no cost. Once the system is primed then let the ball fall wherever it may. At that point it won't matter. The primer function is no longer in use.
The MOST common problem on two strokes
Extract from the link website above is provided below for ready reference.
Orientation
It is also advantageous if the primer bulb can be position in such a way that during priming it can be oriented vertically, with the direction arrow pointing skywards. This will allow gravity and the weight of the fuel inside the primer bulb to help with the operation of the one-way valves. If the primer bulb is oriented with the flow arrow pointing downward, gravity and the weight of the fuel in the line above the inlet check valve may spoil its operation, and the primer will not work properly.
This subtle but important point—orienting the primer bulb skyward—is often overlooked in many installation, but it really helps the primer bulb to do its job. This trick was shown to me by an experienced Mercury outboard mechanic, after I complained that the new primer bulbs he had installed did not work well. The difference in results is amazing, and just by changing the orientation of the primer to vertical, its operation is much improved. In just a few squeezes it should be possible to fill the bulb with fuel.
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