Hey guys first time posting. I was wondering if anybody has had the same problem Im having. I have a set of Imco Powerflow manifolds and risers on my 502 BBC in my 2000 Baja 272. The problem Im having is that I had it out for the first time last weekend and noticed that the risers were getting a little hot so I brought it back to the dock and started checking it out. The engine never got above 160 degrees and was running perfect. I noticed I have some water flow though the thru hull exhaust when I first put her in but when I was coming back to the dock not as much as normal so when I got back I pulled the cooling water feed hoses off while it was running and noticed exhaust gases were coming through the hose barbs where the water hoses on both manifolds connect also out of the bungs when I took the plugs out on the risers of both sides. I took the risers off and noticed the gaskets looked corroded so now im waiting for the new gaskets to come in the mail. I guess what my question is, is there a chance that a riser gasket is causing this problem or does it sound like I need to empty my wallet and buy new manifolds. The boat runs and starts flawlessly so Im thinking there is no water leaking into the exhaust ports due to the inside of the manifolds looking all black from carbon and I dont see any visible cracks with a flash light. I was told the boat was only in brackish water a few times and by the looks of the boat I would say the same its very clean with only 150 hours on it when I bought it.

Sounds like you have a water flow issue. You never mentioned when the last time you serviced the impellor. Sounds like you had some water fow at first then lost it as you ran it longer? Corroded gaskets? What gasket material corrodes? If theres that much rust in your manifolds you might consider having them checked. Salt water boat?

I guess My last post when I stated the gasket was corroded was confusing,What I should have typed was that the manifold flange that the riser bolts to looked a little corroded on the manifold side where the gasket lays and looked like a little water seepage on the outside of the gasket had happened. The raw water pump unit was brand new at the end of the season last year and now has an hour of use on it. Im not thinking its a cooling system or flow problem. I think it is hopefully a riser gasket issue letting exhaust gasses pass into the cooling water passage and not a manifold issue since the motor runs perfect and there is no hydro locking condition but I didnt know if anybody else has experienced this problem from just bad riser gaskets. I cant say 100% that it wasnt in saltwater but from a title search on the boat and being as clean as it is that it has lived its whole life in Ohio and I can only verify 45hours of the now 210 that are on it now. There is no rust on or anywhere around the manifold and riser due to manifolds being cast aluminum. The riser is chrome plated so those look brand new. The water flow has stayed the same from when I first launched this year and did the initial check out to where I am now. I should have stated that the water flow exiting the boat seemed to be stronger last year but in saying that I believe the problem im having now is pressure from the exhaust gases leaking into the cooling lines is not letting the cooling water exit through the risers properly this year

I'm no expert but I doubt seriously that exhaust gases are overcoming water pressure. I have never heard of that. The reverse is always heard of as reversion so if you had a bad riser gasket seal, water would be leaking into your exhaust, not the other way around. Just because the raw water pump was new last season doesn't mean the impellor didn't frag. It takes about 3 seconds after which pieces of it can clog passages and prevent water flow. IMO the diminished water flow out of your tips is a sign that there is a flow problem. Some builders will also use restrictors in the system usually at the crossover to slow water down allowing it to absorb more heat before it exits the exhaust. There are many small and narrow passages in the cooling system and especially the manifolds that can become clogged easily. I cap my exhaust off when not in use. I've seen wasps build hives up there in boats not being used, birds etc. Sand, silt and weeds can also be passed thru the pumps and lodge in passages. I still think you have a flow issue though. IMO