Greetings all,
I have a 1976 Merc 200 serial number 4353XXX. I have owned and maintained this motor for more than 20 years, and have managed to keep it going with help from multiple Merc outboard related sites, so let me begin by saying thank you!
The current problem began earlier this year when I noticed that the motor was not reaching it's full power and speed, topping out around 4400 RPM. With my boat and normal load it should get a bit above 5000 at WOT and plane nicely. On that same fishing trip (4 days) I also noticed it was using more fuel than usual.
I suspected the float bowl inlet needle was not closing fully so I pulled the carb when I got home and replaced the needle. I also notice a wing broken off of the plastic venturi in the carb to I replaced the venturi with one from a spare parts carb I had (I have accumulated a lot of spare parts over the years). I also put in new plugs before the next fishing trip.
However, the same two primary symptoms persisted on this more recent trip. When I got home I did a compression test and got 105 psi in each cylinder (cold, cranking with the electric starter, throttle and choke open). This is the first time I've done a compression test on this motor so I don't have a baseline, unfortunately.
I also noted that the upper plug was clean and shiny with normal black traces from the spark path while the lower plug was all black and fouled. After dark I went back out and hooked up an inline spark tester and both seem to be getting the same intensity of spark when I crank the motor.
I'm a little stumped on my diagnosis at this point. Could the broken wing of the venturi be lodged somewhere that it would cause this? Could it be a bad crank seal? I don't see a Merc spec for the compression, and I think 105 may be a bit low, but that is cold cranking compression and I expect it might be higher if I tested it warmed up. I also think the fact that compression is the same on both cylinders but the brand new plugs look very different after 8 gallons of gas may be telling me something, I just don't know what
BTW, Quicksilver oil at 50:1. And normally 6 gallons does a whole five-day trip with some left over, it's been a very efficient motor. We motor up river, then float back down fishing. This time I needed more gas on the last day, never had to do that before. No change in boat set up or load.
Update: I originally posted this on another site and got the suggestion that there might be a hole in the fuel pump diaphragm that was causing the lower cylinder to flood. So I pulled the carb today, opened the fuel pump, but I don't see a hole. I suppose it could be a pinhole that doesn't show up except under pressure, but it looks ok. Other thoughts on what would flood one cylinder and not the other?
Thanks!
I have a 1976 Merc 200 serial number 4353XXX. I have owned and maintained this motor for more than 20 years, and have managed to keep it going with help from multiple Merc outboard related sites, so let me begin by saying thank you!
The current problem began earlier this year when I noticed that the motor was not reaching it's full power and speed, topping out around 4400 RPM. With my boat and normal load it should get a bit above 5000 at WOT and plane nicely. On that same fishing trip (4 days) I also noticed it was using more fuel than usual.
I suspected the float bowl inlet needle was not closing fully so I pulled the carb when I got home and replaced the needle. I also notice a wing broken off of the plastic venturi in the carb to I replaced the venturi with one from a spare parts carb I had (I have accumulated a lot of spare parts over the years). I also put in new plugs before the next fishing trip.
However, the same two primary symptoms persisted on this more recent trip. When I got home I did a compression test and got 105 psi in each cylinder (cold, cranking with the electric starter, throttle and choke open). This is the first time I've done a compression test on this motor so I don't have a baseline, unfortunately.
I also noted that the upper plug was clean and shiny with normal black traces from the spark path while the lower plug was all black and fouled. After dark I went back out and hooked up an inline spark tester and both seem to be getting the same intensity of spark when I crank the motor.
I'm a little stumped on my diagnosis at this point. Could the broken wing of the venturi be lodged somewhere that it would cause this? Could it be a bad crank seal? I don't see a Merc spec for the compression, and I think 105 may be a bit low, but that is cold cranking compression and I expect it might be higher if I tested it warmed up. I also think the fact that compression is the same on both cylinders but the brand new plugs look very different after 8 gallons of gas may be telling me something, I just don't know what
BTW, Quicksilver oil at 50:1. And normally 6 gallons does a whole five-day trip with some left over, it's been a very efficient motor. We motor up river, then float back down fishing. This time I needed more gas on the last day, never had to do that before. No change in boat set up or load.
Update: I originally posted this on another site and got the suggestion that there might be a hole in the fuel pump diaphragm that was causing the lower cylinder to flood. So I pulled the carb today, opened the fuel pump, but I don't see a hole. I suppose it could be a pinhole that doesn't show up except under pressure, but it looks ok. Other thoughts on what would flood one cylinder and not the other?
Thanks!