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Old 01-09-2014, 09:00 PM   #21
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Blowers on these engines have a shear coupling. Its a shaft that is replaceable by removing the cover behind the blower drive. But, that is not the problem here, if the blower spins its ok, if not the engine wont run. Fuel bleed back seems to be more likely.
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Old 01-10-2014, 01:03 AM   #22
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Old 01-10-2014, 12:01 PM   #23
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I've got a 73 Buick that I need to prime to start quickly. Leave it for a day or two and it starts fine but after a week it needs to be primed. Had the carburetor rebuilt thinking that would solve the problem but it didn't. I'm thinking a hairline crack in the carburetor leaks the fuel either into the intake manifold or out on the outside of the manifold. I ruled that out in the past because I couldn't see the fuel but now think it leaks so slow it evaporates and is not visible.

I mention this as I've had this car for 14 years or so and still have the leaking problem. Don't drive the car often so priming it is less trouble than fixing it.

But w the DD in question here I think it's most likely a leak of fuel or air most likely fuel. Mechanics usually have "leak down" testers or pressure testers that may identify whatever seal is not sealing completely. Perhaps very small differences between the two engines could compared to localize the difference and thus the problem. I'm practically convinced from the text so far that it's a fuel or air leak problem. If a good mechanic can find it on your boat then maybe a good mechanic can find a similar problem on my boat. Looking fwd to good news.
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Old 01-10-2014, 12:52 PM   #24
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Your Buick probably has a Quadrajet carb. These carbs have small plugs, kinda like little freeze plugs, in the fuel wells. They are notorious for tiny leaks. It leeaks into the intake manifold. Kinda ruff on the cylinder walls and rings. Rebuilders dont pull and replace them. It's any easy fix. On the DD, put a 12 volt fuel pump inline and plug the return at the tank. Pressurize everything and the leak will show up, if its an air leak. To determine If its a leakdown problem unplug the return.
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Old 01-10-2014, 03:39 PM   #25
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if you have a lift muffler and it is almost full of water that might be your problem too, happened to me on a delivery, putting fuel in the port tank and the boat leaned way over and the exuast was under water to much and filled up the lift muffler and the engine would not start, to much back pressure,took the plug out of the muff and drained the water and oit started right up.A LESSON LEARNED one of many I incountered over the tears by the way the starboard tank was dry 500 gals on one side is heavey?
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Old 01-10-2014, 05:12 PM   #26
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I've not seen any DDs fitted with water lift mufflers, but if so and the outlet gets far enough underwater to fill the muffler I question the use of the water lift to begin with, poor design, but possible. Detroits dont do well with to much exhaust back pressure and trying to push water uphill would be plenty.
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Old 01-10-2014, 06:28 PM   #27
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Quote:
Originally Posted by kulas44 View Post
Detroits dont do well with to much exhaust back pressure and trying to push water uphill would be plenty.
Considering the 6V92 natural is good for over 54 inH2O backpressure it would take quite a waterlift to create problems.

GM Cleveland 2 strokes started quite well with up to 15 psi backpressure on submarines. They had to be able to build up that much to blow out the snorkel exhaust mast.
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Old 01-10-2014, 07:36 PM   #28
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My 6v92 service manuals call for .6 in of mercury at no load and 1200 rpm as a max. for exhaust backpressure. 1 inch of merc equals 13.6 inches of water. so about 7 or 8 inches water max at 1200 rpm. But, that is at 1200 rpm. That blower may not move 8 inches of water up the exhaust outlet, especially if its a little worn. I still dont think this is the reason for the slow starting.
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Old 01-10-2014, 07:44 PM   #29
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it wasn't a poor design, the boat was tilted way over and the ex was way to far under, I have seen many DDs with lift muff,on many yachts, and I have been delv. them for 35 years
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Old 01-10-2014, 08:11 PM   #30
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also the reason they use them is a boat with a aft cabin they save a lot of space and can make it wider without a big muff behind the walls so they put it in the engine room. makes sence???
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Old 01-10-2014, 08:34 PM   #31
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most of my experience has been with sportfishers and oil field supply boats. They usually try to wring out all they can from an engine. And big mufflers do require a lot of space. I dont know what the maximum startup pressure is for the exhaust on a 6v92, thats not addressed in any of my service manuals, and I've never thought about it. A service bulletin on 2-71 powered gensets cautioned to avoid long cranking intervals because of possible water intrusion. The crank timer was set to preclude that when on auto start. I assumed it was because of using a waterlift muffler. I was "told" by an old mechanic that some DDs wont start with any back pressure, but I dont know personally.
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