'71 Merc. Runs at full throttle only.

[JaySquareHead]

Hi, I have a 1971 7.5hp Merc.
I had it running with a new impeller in a bucket of water. Seemed to have no issues, started several times ran without to many issues. I took it to the lake 10 min later and it wouldn't start. Coughed a bunch of water out the exhaust a couple times but that's it. Took it home and it will start and run in forward gear at full throttle but that's it. Can't slow the motor at all or it quits. I'm thinking fuel delivery? I cleaned the carb, but I am a novice.

 

[Capt'n Caution]

Could be some debris from cleaning is caught in the slow speed needle valve. If you had the the carb apart to clean it, possibly a seal was damaged or lost. This could allow extra air to get into the carb and screw up the mixture. So check to see if there is an O ring missing from the top of your fuel filter. Try pumping the squeeze bulb to see if any fuel dribbles out of joints in the fuel system to indicate a defective seal. Most 2 cycle engine problems are with the carburetor. You must have spark to get it running at full throttle. A suggestion for the future: once you get it running well, use premium gasoline not regular. It probably is allowed, but check to see if it is suitable for this engine. I am assuming that premium gas in your area is free of alcohol. This is especially important when you store the motor, as alcohol, ethanol or methanol, can leave a sticky residue as any remaining fuel evaporates. Lots of luck!

 

[JaySquareHead]

I pumped the bulb and no fuel came out anywhere.
I use premium that is indeed alcohol free in everything but my truck. I used a carb gasket kit when I cleaned it. I could pull it off again and check it. I had it at a nice low idle while it was in the bucket. The only thing I can think is something got knocked loose in the 2 min drive to the water while it was on the boat on the trailer.
Is it normal to cough water out of the exhaust? It wasn't a small amount.

 

[Capt'n Caution]

I wonder where the water exited the engine. The exhaust is through the prop, so you would not usually see extra water there as it is hidden. Is it possible that something leaked after you replaced the impeller? I wonder about the tube carrying the water up to the power head. If the engine was vertical when trailering to the lake, all water should have drained out. If it was tilted high, possibly some water could be retained in the power head.
The running problem is probably unrelated. I recently traded in a 1996 Mariner 20 hp as it had the same problem. There were 2 carb kits for it, and I knew a diaphragm for the primer was defective, so I bought that kit. After cleaning and the new kit, it would not run unless at high throttle settings. I needed a smaller engine for a new boat so I gave up and traded it in. Their mechanic checked it over, also could not run it except at high throttle settings and said even though I had done careful work I had missed or caused another problem. He said they would boil out the carb with cleaners, change all the gaskets etc, and he said he knew this would fix it. He was pretty confident. Maybe you just have to take the carb apart and clean and blow it out again. You might find carb parts diagrams at boats.net. You need the serial number to get accurate info. I'm afraid that is the extent of my ideas.

 

[JaySquareHead]

There are 3 Square holes at the back of the motor where the exhaust exits. That is where I believe the water came from. I took a picture but I can't seem to upload it. There is a video on YouTube that shows it clearly if you search "1979 mercury 7.5hp". It's the same set up on mine just a different year.

I'm going to try working on the carb again, frustrating when it ran so well at home and totally crapped out in the bay. Don't really get how that happens?

 

[Capt'n Caution]

My '96 20 hp Mariner had the same holes. It spit some water out to start, but after running it stopped. I have no idea what the holes are for. I never paid much attention to it.
Is it possible the carb float and valve system jammed open letting too much fuel into the system? The needle valve drops only by gravity. Just a guess.

 

[JaySquareHead]

I pulled the carb apart again. Ripped the gasket that goes between the carb and motor when putting it Back on. Won't run at all now but I did notice the water spray is coming from the water pee hole. I was obviously way off about that. It comes out with some serious pressure, thinking that may be a whole different issue. Possible water or air where it shouldn't be or a blockage.
My next step is total rebuild on the carb. I'm also noticing a fair amount of oil in the bucket. Pulled the bottom up and there's gear oil coming from behind the prop. Going to need to get that seal too.
I'm getting close to putting my evinrude back on the boat and forgetting this piece of crap. Summer is too short here. My evinrude may be slower but it's reliable and gets me fishing.

 

[60sboater]

Maybe fix the Merc in the off season if you live in the north areas with short summers. Down here you can boat year round,,,not much of a winter. The rbld kit for those is $$ but the seal kit is not...no pump diaphragms.