Fuel pours out of bottom cylinder (no 4)

[F456M]

Hello! First post.

I have a problem with my Mercury 120XR (the one with the Force/Chrysler four cylinder block). It pours gasoline from the lowest cylinder when the spark plugs are out. The engine also lock as if it had a water lock in the bottom cylinder. The starter motor is just a few years old. The starter stops because the fuel does not compress like air/fuel normally do. If I remove the spark plug on the lowest cylinder and leave the three other sparkplugs installed (and leaving the other three ignition wires off of the three other plugs), the starter turns the engine and gasoline is pouring out of that lowest cylinder. It can seem like this problem has gradually been more and more present. We have previously had problems with fuel igniting in the jet exhaust chamber so the neoprene gaskets blow out. Have installed new gaskets and even glued them in with 5200. They still blow out! Now, we can't start the engine anymore. I have checked spark and all plugs work. It is just a mystery why this and only this cylinder get so much fuel. Both bottom and the second cylinder from the bottom is fed by the same carburator and I can't see how the carb can feed only the lower with so much fuel...?!?! I have also checked the fuel pump diaphragm, and it looks fine without holes or cracks. What should I do? The complete engine was bored and all internal parts new (including all electronics) just four years ago. Please help! We want to sell the boat and engine too as we have too many boats. But can't sell it as long as it is not running...

Thanks a lot for some good advice.

 

[racerone]

edited

 

[Scott Danforth]

you have carb issues and probably power head issues with cylinder #4.

fuel leaks run down the intake manifold to the lowest cylinder

 

[jerryjerry05]

Check the diaphragm in the fuel pump.

 

[racerone]

Post #1 says that diaphragm was checked.

 

[Redbarron%%]

Check the fuel pump again!. Either there is a leak in the fuel pump or the bottom carb is overflowing. No fuel will flow to any extent from the top to the bottom on the intake manifold as I understand it.
The issue with the ride plate gasket blowing out is probably trying to start it without the water running into the hose from the pump with a service adapter - Not the flush hose provided. That is just for flushing, not running.
If you do not have the water running before you try to crank the engine the collected fuel and vapors will explode and blow the ride plate gasket and possibly bending the plate.
If you have problems with water getting into the bottom cylinder you probably have problems with the base gasket letting water go from the front of the exhaust water jacket under the base and into the exhaust. A common problem especially if the engine has been off the pump as many times the wrong base gasket is installed and the jacket is not sealed.

 

[F456M]

Check the fuel pump again!. Either there is a leak in the fuel pump or the bottom carb is overflowing. No fuel will flow to any extent from the top to the bottom on the intake manifold as I understand it.
The issue with the ride plate gasket blowing out is probably trying to start it without the water running into the hose from the pump with a service adapter - Not the flush hose provided. That is just for flushing, not running.
If you do not have the water running before you try to crank the engine the collected fuel and vapors will explode and blow the ride plate gasket and possibly bending the plate.
If you have problems with water getting into the bottom cylinder you probably have problems with the base gasket letting water go from the front of the exhaust water jacket under the base and into the exhaust. A common problem especially if the engine has been off the pump as many times the wrong base gasket is installed and the jacket is not sealed.

Thanks for the advices. I bend the little tap on the bottom carb float so it shuts the needle valve properly. Now the engine runs quite good! Yes the explotion bent the plate and we had to buy a new one a couple of years ago. Now the reverse cover over the jet exit is stuck. Cable broken I suppose. Probably really fun to replace... One question: -What is the yellow small hose between the carbs for? And which setting do you have the idle mixture screw? Is this screw adjusting the amount of fuel or is it the amount of pre-mixed air and fuel (like on somme Weber carbs which has a fixed idle nozzle. And then you just adjust the amount of pre-mixed idle air/fuel mix.

Thanks.

 

[racerone]

Yellow hose appears to be something to do with this being an outboard powerhead that is now an inboard installation.--Prevents carburetor overflow from turning boat into a small bomb , is the intent.-----Coast guard regulation ??----Not sure how affective that is ??

 

[F456M]

Yellow hose appears to be something to do with this being an outboard powerhead that is now an inboard installation.--Prevents carburetor overflow from turning boat into a small bomb , is the intent.-----Coast guard regulation ??----Not sure how affective that is ??

It is vented i to the air filter box. Engine runs fin without the bose being connected to the air filter box. But I was cusious if the hose was there to balance out the pulses in the manifold or something.

 

[racerone]

??----I guess it does whatever you like to think.

 

[The Force power]

??----I guess it does whatever you like to think.

I'm still stuck on the orange; bungee cord ??

 

[Redbarron%%]

The orange tube is the carb vent line and it connects to a port on the fire arrestor ("air cleaner") When running the engine pulls in any fumes from the carb vent(s).
The idle screws are set per the manual. I have a 90 hp with three carbs and I run them at 1 1/2 out. I think that the further out the richer the mixture. They should not affect WOT, but may affect the mixture at lower throttle settings.
I can't speak for the 120, but the 90 seems to accelerate more with timing than the throttle plate opening. The plates don't open very quickly off idle, but open fast as they get closer to WOT.
The timing advance is fairly linear where the throttle is not. I think that this allows for some "fuel economy" by backing off WOT a little at speed. How is that for a joke with these older 2 strokes?