Unless you buy the good muffs that are around $25 you probably won't get enough water. We use the flush port on our Mercury and never had a problem. Those cheap muff are rated for 30 HP and under I think.

 

My yamaha HPDI 150s do not piss on the muffs. I ran them in a trash can with the intakes fully covered and they piss fine

 

I have always had a problem with the big Yamahas. Never had an issue with any other outboard or IO. I have seen IOs collapse the hose with their sea water pumps but the Yamahas always seem to struggle. I have never tried one but I was told the Sierra pump kit has a much stronger flow. Try using one hose on your flush port and one hose on the muffs unless you have a drum big enough to get your water pump below the surface of the water. If you have installed a new pump make sure your lower unit is deep enough in the drum or you will melt your pump as it will not prime itself.

 

Best to run the big Yammies on the flush port. They don't get enough water on muffs for the tell tale to be steady. Just don't run the motor much above idle because even with hose connected to the flush port they just don't get the water they need and it is possible to overheat them.

 

How do you like that 300 Yami? I just got a 25 mako with one on it. Seems like a strong motor. Hours on yours? about 30 to 1 oil burn? max rpm about 5600?

 

i had a honda 50 4 stroke and i had to put duct tape over the vent on the lower unit. a real pain but it worked. this will be my first year with my 300 hpdi so i'm interested to hear the solution.isn't their a flush port up near the top of the motor?

 

I have Yamaha F250's, Put Duct Tape over the bottom pickups of the lower unit. Then run the muffs on the top pickups. Mine piss, it takes a minute then it will come out steady. Not as good as in the water .
Hope this helps. I have the muffs that feed the water to both sides of the muffs.

 

The big motors with the dual pickups need to have the low pickups duct taped off in order to get enough water to cool at idle. Even then, it depends on the water pressure from the hose. They like a lot of water.

 

I never trusted the muffs for my Yammy 250 4strk. never could get a good piss hole flow. I fill a green trash can I use in my landscape bus. and have piece of mind. I have the better muffs that are kind of rectangular.

 

That must be a big trash can you have. I borrowed one of those plastic roll around trash dumpsters that they use at hospitals and schools and that worked great.

 

CeltricCross & AdamBomb have it right. Do not exceed 2000 RPM on the muffs. Double muffs with direct water flow mandatory.

Do not run motor on the flush port. Specifically stated in the Owner's Manual. Motor should be off on the flush port.

 

Thanks for the relpies. I don't want to use the flush port on the engine that makes me nervous. I'll try the trash can. Classof73 I have the outboard on a Robalo 247 and I love the boat and motor. I don't think mine is proped correctly because I'm only getting about 5200 RPMs and the top end is too low. I'm taking it back to the dealer next week. The motor started right up after the winter and everything appears good. Once I get the prop figured out it will be perfect!

 

Sometimes it helps to take off the intake screen as well (beyond taping off the lower pickups).