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Random Quote: I know I fish too much so from now on I will only fish on days ending with Y
Hello,
I'm new to this website and thought i'd introduce myself. My name is Demitri and I live in San Francisco. I've wanted a boat for years and i've finally bought one. Since I am strapped for cash I bought one that needs work of course. I bought a 1974 Tahiti ski boat with a 4CYL 140HRS GM OMC motor. I picked the boat up for $350.00 which included a trailer. Figured it was a good deal. At any rate, the boat needs work. I've done a diagnostic on it and found water in the 3rd and 4th cylinders. I've removed the head and exhaust manifold. Both the gaskets look to be in okay shape. I've looked for cracks in the head and the exhaust manifold but can't seem to find a crack. The block has no water mixed in with the oil. Engine oil is clean as a whistle! A little about the boat; I bought the boat from a kid that had it parked in the backyard for about a year uncovered. Maybe water entered the exhaust manifold through the carb? I am wondering if there is a way to test the manifold for leaks/cracks? Any suggestions/comments are taken in stride.
Do a compression check of each cylinder. I don't know the spec, but they all should read about the same psi with no great differences. Water entering the cylinder only happens a couple of ways, through the intake, or a bad gasket, or could be condensation buildup over the year. Was there a lot of water or just trace amounts? There is one other possibility....frozen and cracked cylinder walls.
Thanks for the responce. I will most likley do the compression test after I put the head and exhaust manifold back on with the new gaskets. I set the motor at TDC when I took the head off. I don't belive it is a good idea to move the cylinders while the head is off. The motor was frozen into place, but I was able to break it free, turns over just fine now. There wasn't a lot of water built up, and not a trace amount either. Just a little amount of water that was really dirty, almost like mud. I believe it was salt water. The tops of the pistons are not corroded.
You may need to provide a little more info. Is ANY rust present on the cylinders/head compustion chamber area? If so, is there any pitting? If there is pitting, the motor will probably need to come out and be bored. If there is only light rust on the cylinders this can be cleaned-up with 200 grit sandpaper.
If the cylinders look good then maybe the water came in after you unbolted the head, which is generally what will happen. The "muddy" looking substance may be nothing more than old head gasket that fell apart.
What happens is the side walls of the riser and or manifolds rust through inside. The cooling water gets by the exhaust valve. Also, you may have to have the head magnafluxed, sometimes visual doesn't do it looking for head cracks!
Wow, lots of good information here. There is only little surface rust building up in the cumbustion chamber. The cylinders have very light corrosion(sp) on them (3 and 4 cylinders only). There is no pitting. What does it mean to have the head magnafluxed?
I worked on the 120 HP version of this several times, the water problems I saw was a hole in the head, corroded through the intake side passage, also had a case where the exhaust riser and exhaust manifold gasket didn't seal, due to warped mating surfaces, had to do a blue check to find it and had the two surfaces machined to cure it. You can take the head to a machine shop and they can do a pressure test of the head to check for leaks. If that is OK, then I would do some checks of the exhaust manifold and riser, make sure the gasket surfaces are true, and the gaskets are sealing. If this is the same setup, you have an obsolete outdrive and engine, can be difficult to get parts for. When I did this NAPA was still able to get parts at a reasonable cost.
This one may sound weird. I had an OMC 260 that on more than one occasion, started to kick & sputter before dying. Upon checking it out from the dealer, they found water in the pistons. I had one dealer tell me that I had a cracked head, but all testing proved negative. I had several dealers take a look at it with no resolution other than to get rid of the water and get is started again. I finally found one guy who was able to determine the problem. On the OMC, when shifting from Forward to reverse (or ******, the motor shuts down ever so slightly in order to facilitate this shift. When the shift cable becomes somewhat worn, it sticks in this limbo-mode that sounds much like the engine is dieseling. If you allow the motor to continue to run in this mode, it actually runs in reverse and sucks water back into the engine filling the cylinders. If you wiggle the shifter at this point, it will kick it out of this mode and continue to run normally. The dealer was able to duplicate the problem. The resolution was to replace the shift cable and make sure it is replaced when ever it this re-occurs.
If after getting rid of the water and starting it up, if all else seems to be fine, this is a possible cause.
It took me about 5 years worth of problems to finally find out what was causing it on my boat.
also, this can be breakdown of the "flappers"
which sit just before the exhaust goes out...they
are there to prevent backflooding when you back
down hard or launch deeply...they are inexpensive
and do disintegrate...when they do, water will be
in the cylinders, but without a crack in the head
dan
One last possibility for you to check since you have the head off anyway.
From memory (and you can check this out yourself to confirm), the GM blocks didn't have blind head bolt holes. I think you'll find the head bolt holes in the block go all the way thru to the water cooling jacket in the block.
It's important to seal the threads on the head bolts with gasket goo when re installing and torquing down the head bolts.
If not - water under pressure can find it's way UP the head boplt hole bye passing the thread, and then if it gets past the head gasket - it's straight into the cylinder. Even if it leaks to the outside of the head / block joint - you still have a pressure leak meaning she will boil at 212 degrees f not 220+ as it should with the header tank 'radiator' cap in place.
It's cheaper to manufacture blocks with thru head bolt holes as all the drilling and threading swarf goes thru into the water jacket to be flushed out during acid dip cleaning before assembly.
Ford blocks however have blind head bolt holes, which are more labor intensive to clean out of swarf during & after manufacture - BUT - at least you don't ever have to worry about water ever finding it's way up the head bolts and then past the gasket into a cylinder.
Maybe somone before you, had the head off at some stage and didn't re-seal the head bolt threads when re-installing.
Thats the difference between GM and Ford engines, ones designed and built to a budget and the others designed and built to a specification.
Engine runs in reverse, I don't think so! It sounds like you were sold a bill of goods. What you actually were expierencing is the shift interrupt function, all it does it makes the ignition system miss, which takes the load off the shifting. It does not run in reverse. What a lot of OMC engines expierienced was the flapper valves in the exhaust would not shut off the exhaust fully, so when the boat was in reverse and the shift interrupt was stuck in the on postion, the engine missing, the exhaust did not provide enough pressure to keep the water out of the exhaust system. Boat backing pushing water up the exhgaust system, not enough pressure, you get water ingestion.
I'm with you Kevin. The only engine I have ever seen spin backwards (Late model Stock Car)wound up with a total rebuild. Engines aren't made to spin backwards from normal rotation.
On the topic of engines running backwards, I have a little glow engine on one of my model airplanes that when I flip start it, will run backwards about 50% of the time. I know an outboard is more complex than my little .53 CID model engine, but some engines can run backwards
Water in my cylinders problem 
