Car Fix DIY Videos

I did this about a year ago, so I’m trying to remember all of the points of interest. The engine would over-heat if taken above 2500 rpm. The previous owner tried replacing the elbow (exhaust riser), but the problem remained. Eventually, I took off the leg, and I found the old flapper split into the rubber and fibreglass halves lodged into each exhaust port of the leg. Past 2500 rpm, the exhaust back pressure pushed water out of the cooling circuit and the engine over-heated.
I read quite a bit about the flapper valves used on OMC 3.0 cobra drives (OMC was bought by VP but they just used the OMC design for years). There was a service bulletin on this era of drive by VP recommending the deletion of the flapper. Somewhere I read that it was recommended the flappers were replaced about every 100 hours. People didn’t do that. I’m assuming that the risk of destroying the engine from overheat (by inevitable flapper failure) was greater than the risk of damage from the deletion. So, if you can even find someone to work on these old boats (lots of mechanics will not touch them), they will just remove the flapper if they find one. Their rubber around a fibreglass core construction is the problem. One over-heat from a failed impeller or thermostat usually damages them. They will stick and obstruct the exit of exhaust, and at the very least decrease performance of the engine. If they fall off, they can restrict exhaust flow so much they cause a disastrous over-heat or the engine may not even be able to start. So why did the OMC engineers include them?
As you likely know, the whole point of the riser elbow off the manifold is to prevent accidental reverse flow of water up the exhaust ports, into the manifold and potentially into a cylinder where the exhaust valve is open when the engine isn’t running. That would hydrolock the engine as water isn’t compressible. If it happened to you out on the lake, you might be able to take the spark plugs out and blow the water out by turning the engine over. Water from the block and manifold combines with the exhaust after the elbow where it goes straight down and out. The few inches higher that the riser elbow provides puts it high enough above the waterline that it’s almost impossible for this to happen. Sometimes guys put a higher riser if the boat design allows it just to be safe. However, it depends on the design of the boat, and how much weight is at the stern, or how much reverse pressure there is (no engine running). An aggressive trip down the boat ramp could do it on some boats. Large waves pounding the stern might also do it. It depends on how high the riser is relative to the waterline.
My brother and I have a combined weight of almost 500 lbs and we sit on the rear platform. You add a few beers and a couple of other people + a trailing sea: the riser elbow is now too close to the waterline (from all the weight) and the pulses of water pressure from waves hitting the stern could push water into the head. The flapper valve is there to prevent this from happening. It opens with exhaust pressure and snaps closed with reverse pressure. In the forums, there were a number of guys that removed their flapper valves and then hydro-locked their engines – particularly some low profile hull designs. So, I decided to put a new one in. It wasn’t easy to find. I found one “new old stock” on ebay, but I also found someone in Europe that was making them. The key is to always maintain the impeller and thermostat so that you don’t overheat. Once that happens, you really have to pay attention as you may have fried your flapper and the only way to fix it is to remove the riser, or the leg (if it’s fallen down).
I think I bought my gasket set from ebay. You don’t need a new stainless baffle plate (riser restrictor plate), but pay attention to the orientation of the slot (take a picture). It can affect the water flow rate from the cooling system if you place it in the wrong position (it may overheat, or never reach optimal operating temperature). On my engine, the opening on the plate was on the side facing the valve cover (you can just make out my drawing in the video).
I ordered Permatex Copper Spray gasket maker from Amazon and it was here in a day. Enough for a lifetime of riser gaskets now! Wear a respirator. It’s spicy stuff and probably very unhealthy to breathe. I lightly coated all surfaces that would be in contact. The nerve wracking part of the job is that you need to come directly down on those rear studs while inserting the other end in the rubber exhaust hose. Two sets of eyes and hands would make this far easier and it is a little heavy and awkward. I managed, but I was really hopeful!
I torqued the riser to 25 ft/lbs in a criss-cross pattern gradually. The rear port side was difficult to torque because it’s so inaccessible on my boat. I had to torque by feel relative to the others.