50 hp johnson backfire

[upfish123]

I have a 50 hp Johnson that backfires. Run one cylinder at a time and they run fine put both together and that's when the trouble starts. Good spark, new VRO and no junk in the carbs.There is no vents on the tanks is that how it is supposed to be? Help!

 

[WillyBWright]

Re: 50 hp johnson backfire

Welcome to iboats.<br /><br />You must have original OMC tanks, either metal or plastic. They're made to contain a certain amount of internal pressure, but vent off the excess. That helps keep the fuel fresher longer by limiting the amount of air that enters and leaves the tank. Less air means less oxidation of the gas (oxidation causes varnishing). It also keeps more of the fumes inside the tank. Fumes can collect in the tank area and bilge creating an explosion hazard and that is greatly reduced with those tanks.<br /><br />Are you sure the motor is backfiring and not lean-popping. Backfires are exploding volumes of fuel usually, in the exhaust, and is usually an ignition timing problem. Lean popping comes out of the carbs and sounds more like a sneeze. To stop the sneezing, open the idle mixture needles 1/4 turn on older models, a full turn on newer models...I don't know what the year cutoff would be, but the older ones are set out about 1 1/4 turns and the newer ones require 2 1/2 to 3 1/2 turns. Both carbs should be set the same (if yours has more than one). Since it has a VRO, I'm betting it has a pair of the newer style carbs. What year is your motor?

 

[Kenneth Duke]

Re: 50 hp johnson backfire

Make sure your wires aren't crossed.Try Switching the spark plug leads and see if that makes a difference.If so then its possible the wires from the power pack to the coils are crossed.usually orange with blue stripe is up and orange with green stripe bottom.

 

[upfish123]

Re: 50 hp johnson backfire

Thanks for the advice. It's an 86 Johnson and it's definitely backfiring through the exhaust. Seems to do it more at low idle than when running faster rpm. I'll check the wires on the power pack anything is possible. Also there is no adjustment on the carbs. Just a low idle jet and a fast one buried inside.

 

[upfish123]

Re: 50 hp johnson backfire

I forgot to mention that when primed it runs great. That's why I thought it was starved for fuel and was wondering how the fuel system breaths without the vent cap on the tank.

 

[upfish123]

Re: 50 hp johnson backfire

Well much to my delight and dismay the problem was trying to run the motor with earmuffs and not submerged in water. If you have a motor that exhausts through the propeller do yourself a favor and buy a barrel and cut it to size. Seems the engine needs the back pressure from the water otherwise it sounds like it's missing. <br /><br />Live and learn... but sometimes I think Im learning more than living...!

 

[Solittle]

Re: 50 hp johnson backfire

upfish - Don't put the tools away just yet. You do have something going wrong with your motor. A properly tuned good running motor does not hiss, miss, sniffle, or backfire when running on the muffs.

 

[upfish123]

Re: 50 hp johnson backfire

Any suggestions I'll take them. Runs great in the lake. Still need to check the timing. Too bad you can't set it at idle...? That would be way too easy. Need to find a dock post I'm not going to rip off at 3500 RPM. Pushes my 16 foot fiberglass Starcraft to 32 knots according to my GPS.

 

[DJ]

Re: 50 hp johnson backfire

I think there may be some confusion as to what constitutes a backfire.<br /><br />That engine, on the muffs, will really "bark".

 

[upfish123]

Re: 50 hp johnson backfire

Well it would kind of backfire and die at low idle.Then I put it in the lake and it was fine but not perfect. So my manual from the dealer arrived in the mail(I had been using one of those fake manuals from someone other than the dealer) and I went through the throttle adjustments and synch check now she's purrin like a kitten at low idle and no backfire on the muffs. But you're right the neighbors really appreciate the "bark"