Hot spot on intake manifold

I have a 1990 7.4 merc with a bravo 1 outdrive in a 2855 bayliner, freshwater cooled and fw cooled risers. My question is I noticed a rust spot on the port side of the intake manifold when I bought the boat last summer, not bieng my first bayliner I assumed that it was water dripping or puddling on the top of the manifold causing rust. I ran the thing in the driveway last weekend and while it was idling and warming up I happended to brush my hand across the vale cover on that side and I could feel a bit of heat, tracked it down and the rusty spot on the intake was hot, not bad at idle but to hot to leave a bare hand on for more than 2 or 3 seconds. I put the boat in the water and it dawned on me as we idled away from the dock that it has seemed to run rough for the first few minutes until I get it up on plane when I start out cold, and the light kinda went on in my head...I think I have a bad lifter on that cylinders exhaust side! Am I right on this? <br /> One other question for don or the other techs, I know the merc riser and elbow gaskets are not the best design, do I need to replace the FW part of the riser/exhaust manifolds too or do the hold up better than when they are exposed to salt water... I am going to do the risers here very soon and suppose i should just do the whole thing and replace all the hoses while I have the cooling system drained....just wondering if its necessary

Re: Hot spot on intake manifold

There is an exhaust crossover port on both heads that go through the intake manifold to heat the choke coil for your carbs automatic choke, and that exhaust air is HOT. It will burn the paint off the mainfold on both the left and right side of the carb. It's normal. You might be able to use some Hi-Temp black paint to cure the problem of the rust. And all boats suffer from the problem, not just Bayliners. Just scrapped a 7.4 out of a Grady White about a year ago because of rust.<br />Regarding your manifolds and riseres, the risers sit on top of the manifolds, and are always raw water cooled. The manifolds is what is Antifreeze cooled.<br />The Big Block engines also get a lot of rust on the bottom of the manifolds (also heat caused) and with a little water added can flake off large sections of the manifold and the bolt heads. If they are ok, and the ports around the water and exhaust sections where the riser fits looks ok and is nice shiny metal and not black carbon deposits, then you should be ok. But they are worth checking after a certain age.<br /><br />

Re: Hot spot on intake manifold

Thanx Don I guess I hadn't thought of that, and with the cold miss...well shouldn't be to hard to track down,<br /> as for the manifolds I have the 4" spacers between the manifolds and risers...I am just paranoid about the whole merc design on these things, OSCO's look good, just not in the budget right now, I have no Idea if these have ever been checked and I have a couple of water hoses underneath that are pretty spongy, I will check it all then...at least I don't have to pull the intake!!<br /> Thanx again Don