Number three cylinder not exploding or firing, but has fire.

[ronnyd8719]

Hi I am new to the forum and old and dumb. Well not that bad. I need some help. I have a 1981 mercury 70 hp. I have been in and out of the hospitals in the last couple of years. To get started. A couple of years ago I was trying to get my motor running again. I had clean the carbs. and I was putting it all back together and I fell out the boat trying to get out. Well I went to hospital AGAIN. Now it is present day. 4-052009. I had the boat sitting with out the carbs on right and sitting for over a year that way. I have clean and ajusted the carbs again. I get the motor to start by the hardest, but I get no fuel to number three cylinder. I have fire, but the plug is clean and new when I pull it after starting the motor. The number one cylinder has a little carbon build up and number two cylinder has more than one and three is clean. I checked compression in number three cylinder and the compress is about 124 GOOD. I thought about the reed valves, but the fuel is not being forced out the front of the carb. One thing I have thought of but did not check at this time is a dirt dobbler making a nest on the reed valve. So this is still possible.
The motor is strange it has two carbs and three cylinders. and in this case I find it strange for one and two cylinder to get fuel and not three. Can someone help a old, dumb , and crippled man with a motor that has a cylinder that will not get fuel.
Thanks Ron

 

[CharlieB]

Re: Number three cylinder not exploding or firing, but has fire.

You said that #3 plug is still new, but you didn't say if it is wet or dry.

Wet would indicate that tho you have spark it could be weak and not firing under compression. Will the spark jump a 7/16 gap with a bright blue crack? If so it is fine. You could have a defective spark plug.

Dry would indicate either leaky reed valves or a poor ring seal near the BOTTOM of the cylinder, not pulling a vacuum is the piston starts its up travel.

Remove the exhaust cover and inspect the cylinder walls for scoring. You can have scoring low on the cylinder wall and still have good compression in the upper cylinder.