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Author	Topic:   Effect of Blown Poppet Valve
Newtauk1	posted 07-05-2009 05:59 PM ET (US)         Would a blown poppet valve result in a motor running rough at certain RPM? Can a blown poppet valve result in damage to the motor? I just noticed water dripping from the cover of the poppet valve. This is a 2001 Mercury 75hp two-cycle. I plan to replace the poppet valve and thermostat while I am at it.
outragesteve	posted 07-05-2009 06:20 PM ET (US)             From my experience and the Mercury manuals, the poppet valve is bascially a water pressure relief. As RPM increases, so does the water pressure. The poppet valve reduces the pressure so the cooling systems preforms properly. If you see a leak from the poppet valve cover, there is a leak between the inner and outer plates or the poppet diaphram is bad (not expensive) Do not, as others may suggest, run the engine without a poppet valve or thermostats. This is only done on race set-ups where there is a huge water flow and heat transfer because of sustained, high RPM. Good luck!
Newtauk1	posted 07-05-2009 06:25 PM ET (US)             Thanks for the response. Could I have caused damage by running the motor with a blown poppet valve? Motor never over heated, but I did experience bogging down at 1800 rpm. 1800prm is where I believe the poppet valve begins to open. Above 2500rpm the motor ran well.
Newtauk1	posted 07-06-2009 10:25 AM ET (US)             Should the entire valve be replaced or just the diaphram?
L H G	posted 07-06-2009 11:06 AM ET (US)          I wasn't aware a 2-stroke 3 cylinder Merc had a poppet valve, as they are usually used on V-6 engines, but since this engine is half of a 2.5 liter V-6, maybe so. they leak out the side of the block when they are bad. Anyway, Mercury's poppet valve system decreases engine cooling at idle and slow speeds, to increase engine heating. V-6 engines idle better at warmer temps. As speed increases, the poppet opens up, allowing full cooling at planing speeds. Doubtful you damaged the engine.
outragesteve	posted 07-06-2009 02:44 PM ET (US)             I quick check on the parts lists for the Merc 75 HP/2 stroke shows the 3 cylinder engines have a poppet valve and the 4 cylinder engines do not have poppet valve.
Newtauk1	posted 07-06-2009 05:41 PM ET (US)             Would a failed poppet valve result in bogging down at 1800rpm-2200rpm?
outragesteve	posted 07-07-2009 02:07 AM ET (US)             The poppet valve and cooling system in general should not cause the engine to run poorly in a given RPM range. If this engine is the infamous "shut off the fuel/carb function" @ lower RPM's (to save fuel) this may be the issue. The transition from the low to higher RPM set-up is around that RPM. I would get a Mercury repair manual (ebay) and familarize yourself with this fuel delivery system. Also, there is info on CW about this engine and the poor performance in the mid range. Good luck!
Newtauk1	posted 07-10-2009 03:21 PM ET (US)             Replaced diaphram. Had carbs adjusted. Running smooth again.

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