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backfiring engine

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briang0

Regular Contributor
I have a 1983 Mercruiser 898 Chevy 305 We went out for a cruise today on the way out she ran beautifully. The were out for about an hour and when I turned to head home I I brought the RPMs up to about 3000 from about 2000 and after about 3 minutes the engine started to back fire through the carburetor. I had to back down the RPMs to about 2500 and then after about 5 minutes the engine started to do the same thing again so I had to back it down again to 2000 it ran for a little while longer and then I had to back it down again 1500. I ended up getting towed back to the dock but I kept the engine running the whole time all the gauges were in the normal range. New plugs and tune up and wires. Gas is suspect. When I got back to the dock the engine ran fine. ????? Any ideas? I did check the times and it was good.
 
If the engine is backfiring through the carburetor and the timing is correct can I assume the the engine is running in a lean condition? The fuel in the tank is from last year it is Valve Tech marine fuel and I did add stabilizer I put in a couple of bottles of dry gas before we went out, first cruise of the season for me. I'm considering hooking up a fuel pressure gauge to see if I'm loosing pressure. I did about 30 minutes of no wake boating before we starting having the problem. I also thinking about looking in the tank to make sure there isn't anything floating around in there that might be getting sucked up into the draw tube.
 
Well, if it ran good going out and then the backfiring I check the fuel sep for water. dry gas dosent help.
Check the sep first .betting its full of water
 
if the fuel in the tank is suspect, the first step is running on a portable fuel tank. If the engine runs fine on the remote you will know where you have to focus.

I have pumped out more than 1000 gallons of treated fuel do to phase separation this spring.

Your engine will run on Phase separated fuel and it will run lean and backfire. It will eventually ruin your engine.

Do not add dry gas AKA(ethanol) to your already ethanol mixed fuel.
 
I pumped the tank dry this morning. I took an air line and blew out the fuel line from the fuel pump to the tank to make sure there was nothing clogging it. I was pleasantly surprised how clean the tank was inside. I changed the fuel filter/separator and the filter on the carburetor. Then I put 20 gal of fresh fuel in and took the boat for a spin. No Joy :( I checked under the distributor and found that the contact on the rotor the brass one that touches each plug was crooked and the plastic was broken so I replaced it with a spare, I inspected the cap and everything else inside looked fine so I and took the boat out again. Still no Joy. The engine will come up to speed 3000 rpm cruising and purr for a minute or two then backfire and start to stumble. Then I have to back off the throttle to about 1500 rpms for a few minutes and i can repeat the it again. The engine idle beautifully at the dock. I have new wires and new plugs the cap is new and when the engine is not under load it well rev fine. The engine is an 83 898 with a Thunderbolt IV ignition system. The wires on the coil and to and from the distributor are sealed or coated with something to prevent corrosion and everything is tight. I'm not sure where to look next. Could the coil be breaking down under load? I don't think it's fuel anymore.
 
briang0 said:
I pumped the tank dry this morning. I took an air line and blew out the fuel line from the fuel pump to the tank to make sure there was nothing clogging it. I was pleasantly surprised how clean the tank was inside. I changed the fuel filter/separator and the filter on the carburetor. Then I put 20 gal of fresh fuel in and took the boat for a spin. No Joy :( I checked under the distributor and found that the contact on the rotor the brass one that touches each plug was crooked and the plastic was broken so I replaced it with a spare, I inspected the cap and everything else inside looked fine so I and took the boat out again. Still no Joy. The engine will come up to speed 3000 rpm cruising and purr for a minute or two then backfire and start to stumble. Then I have to back off the throttle to about 1500 rpms for a few minutes and i can repeat the it again. The engine idle beautifully at the dock. I have new wires and new plugs the cap is new and when the engine is not under load it well rev fine. The engine is an 83 898 with a Thunderbolt IV ignition system. The wires on the coil and to and from the distributor are sealed or coated with something to prevent corrosion and everything is tight. I'm not sure where to look next. Could the coil be breaking down under load? I don't think it's fuel anymore.
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When is the last time you gave the carb a good cleaning...and checked the float level?
 
I always thought to look at first was ignition when backfiring thru the carb but I would also check the vent it might be plugged.....
 
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