Flames up when starting

[craig newton]

96 Mercruiser 4.3 L 4bb V-6 Vortec with Alpha One outdrive.
Boat hadn't been used for about a year. Stored without gas. So two days ago I filled the tank, bought a new battery and it started up. Went to idle for a few minutes with the ears on and I shut it down. Today I cranked it over for about 5 secs and it wouldn't fire so I took off the air filter to spray some starter fluid in it but I tried pumping the gas 3 or 4 times instead. This time it started with a flame shooting out of the carburetor about a foot high so I immediately turned it off. My wife was manning the ears and thought she saw a small flame shoot out from the drain hole. There was no plug in it at the time. I checked the gas lines for any leaks and couldn't find one but still not sure about trying to start it

 

[Scott06]

96 Mercruiser 4.3 L 4bb V-6 Vortec with Alpha One outdrive.
Boat hadn't been used for about a year. Stored without gas. So two days ago I filled the tank, bought a new battery and it started up. Went to idle for a few minutes with the ears on and I shut it down. Today I cranked it over for about 5 secs and it wouldn't fire so I took off the air filter to spray some starter fluid in it but I tried pumping the gas 3 or 4 times instead. This time it started with a flame shooting out of the carburetor about a foot high so I immediately turned it off. My wife was manning the ears and thought she saw a small flame shoot out from the drain hole. There was no plug in it at the time. I checked the gas lines for any leaks and couldn't find one but still not sure about trying to start it

If it was running prior and no one moved the distributor I would think you have a valve hanging open. Normally a back fire like this is timing being so off the cylinder fires when a valve is open. Common when starting a new engine or if the distributor is removed and engine is being retimed. I would take the plugs out. Disconnect the two wires to distributor, spin it on starter, check you have compression in all cylinders. Wouldn’t surprise me sitting if a valve was hanging up. Will probably loosen up if this is the case.

if there are valbes not seating this will show as low or no compression on a cylinder

 

[Grub54891]

You pumped it 4 times. That is a fair amount of fuel. Probably slightly flooded it. The air filter is called a spark arrester. It's to help stop the flaming thing you had. If the hood was closed it makes a potential for blowing up! I've seen it happen. If any of mine don't start with one pump I stop and diagnose the issue. Take the advice as stated above on the other posts.

 

[Lou C]

Agree with the theory that an intake valve hanging open can cause a backfire thru the intake/carb this can be caused by a bit of corrosion on valve stems. When troubleshooting a non start ESPECIALLY when using starter fluid ALWAYS REPLACE THE FLAME ARRESTOR before starting!!!! That’s what it is for! You are very lucky that could have ended quite badly!