Weird overheating Problem

[Grady23]

I have an older (1997) carb SWII series 150 Yamaha that is giving me fits. The colling system was working fine, I did some carb modifications, fixed the float level, ran the engines for 20 minutes with NO PROBLEMS. Then when I re-started, it refused to pass water thru the pee hole. I've replaced both thermostats and the impeller. Here's the weird part. When I connect to the flush tube and run the engine at idle, it pees fine and doesn't over heat. When I run it on a single sided muff connection, it pees but has a weak stream. When I use the double sided muffs, It DOES NOT PEE and overheats. ANY IDEAS ?????
BTW --- when the flush hose is connected, the water flows down and out of all the lower unit water holes.

[DWinter]

Sounds like the muffs are not tight against the inlets to the lower unit

[DWinter]

Sounds like the muffs are not tight against the inlets to the lower unit. Make sure they are squashed flat.

[Grady23]

The only way I know to get them any tighter would be to put a "C" clamp on them. The twin muffs are new. I read somewhere in my Clymer book that there is a water pressure valve in the system --- Anyone know anything about it??

[Grady23]

I talked with 2 mechanics today and both said that Yamahas have a history of doing EXACTLY what I described. Both of them told me to take the boat to the dock and submerge the lower units and start them up. THey should start peeing right away.

[Sea Gristle]

What did he say was the reason? I have an '02 2 stroke and water always leaks around the muffs. I usually start up then jump over and hold the muffs down until the stream gets going good then let go. Seems like the suction holds it from then on. I have also tried running a hose to the flush fitting AND the muffs at the same time and it sets off the overheat alarm every time, but I can't figure out why it would...

[rj]

I also have a 97 2 stroke carbed yamaha 150,, it won't pee on the mufs, but it still cools well enough to run on the mufs, it won't overheat on the mufs,, in order to pee however, it must be submerged in the water

[Grady23]

Nobody seems to have or know the reason why these engines do this --- Just that they do!
I'm going to get a plastic 55 gal drum today and use that rather then drive 45 miles to the bay and find out it's really something else causing the problem. I should be able to run the engine for 5-6 minutes without any problem as long as I keep a hose running in the drum to provide replacement cool water. I'll keep everyone posted.