High speed overheating

[19cobaltcd]

I actually use very little trim, the boat rides nose high as it is, but it dosent seem to matter where the trim is. It has been hotter than hell here so I havent worked on it much, went out to the lake last weekend, took some friends for a ride, nice and easy, and it overheated more that it ever has. It was the most adults Ive had in the boat since the motor was installed. After a couple of hours it was okay and ran like it always did. Thank goodness a friend took the other people back to their camp. My son and I beat feet for the ramp. I am beginning to believe the carb just isnt flowing enough gas and it is running lean. Since the loading of the boat seems to make a big difference my next step is to start some serious tuning of the carb. It is a new quadrajunk replacment from holley, 650 cfm spreadbore. I've owned alot of holleys over the years and never had one run that lean out of the box. My cam is bigger than stock (not that much) and I had to use a 1" spacer under the carb to clear the thermostat housing, so maybe the combination of things is really leaning it out at the top. Will check and let everyone know. Thanks for all the help.

[19cobaltcd]

Just bringing it up to date. The motor is on the stand in my heated garage. Sure am tired of pulling it instead of just winterizing it. The last time it was on the water, I had 4 adults and my son in the boat. Couldnt hardly get it on plane. Once it finally rolled over and took of the temp guage was already climbing, by the time we got to the cove to park and drink a frosty it was so hot the paint burned off the heads and it deformed a pair of 35$ valve cover gaskets and leaked a quart of oil into the bilge. A friend took the other people back to their camp and my boy and I headed for the ramp (after considerable time to let it cool down). Ran like normal, took off fine, temp guage right where it is supposed to be, nailed the throttle a little ways from the ramp and the temp guage started climbing. Took the distributor to a local place and had them spin it, couldn't find anything wrong. Pulled the intake and heads, cant find anything obviously wrong yet. Gonna put a degree wheel on the cam and check it out soon. I'm still at a loss to what the problem is but it seems to point to the load applied to the engine. Dont have an oil cooler but my p.s. cooler and incoming hose were clear when I pulled the motor out. As a side note, whose boat is that under my screen name? Nice boat but not mine.

[Havasu47]

OK here's my .02. I agree you have a water restriction somewhere. Check the oil and power steering coolers for old pieces of water pump impeller from a past impeller failure. Check the riser gaskets for the proper gasket (there are many versions from Merc). Since it only overheats at high speed, a head gasket could be the problem. If the fire ring between the cylinders is damaged it will overheat under a load and NO water will be in the oil. Do a leak down. Are the head gaskets correct for your block / head combination? Check the area where the water passes through the transom very carefully. It can get restricted with corrosion even with a fresh water boat. The old 260 thermostat housings were not the best design. Make sure it is in good condition with no corrosion blockage.
Hope this helps

[19cobaltcd]

Thanks for all the help. I've been reading back thru the posts on here and in other areas of the forum. I have been using the head gaskets that come with the kit, (Fel-pro permatorque). Someone said us the marine head gaskets. What is the difference? I've done a couple of stock motors for other guys and they have had no problems with the head gaskets. (for 2 seasons) I've also been looking into putting tips and the exhaust thru the transom. Having a relatively stock 350, would the 3" tips be plenty big enough. I know in my years of hot rodding cars, bigger is not always better. The less back pressure you have the less low end you have. I dont want to lose my low end, (the boat is heavy). I'm thinking of putting the tips low so they are underwater at idle, (I like the quiet). Any pros or cons to this? Thanks everyone!!!!!!

[powerplay230]

I'm a rookie but one reason I have been told to use marine head gaskets- Here in Fl we have to worry about salt water. Use the wrong gasket and they disapear shortly after due to salt. Not sure of the other diferences....
Rookie Rich

[gnarley]

A couple of things I thought about??? You said you have a 160 stat, is it new? Have you ever put in water on a stove to watch it open & at what temp it opens & if it opens all the way?
The other thing I don't remember seeing was if you had an oil cooler? I had a similar situation in my boat but the factory cooler had been removed & I didn't know what should have been there until much homework was done. I put the cooler on & could run wide open after that! Before the cooler the alarm would go off around 3800 RPM & the oil got so hot you could hear the valve train ticking.... it was hot! If you don't have a cooler get one