hello all.
i have a 1988-89ish mercury 70 OB motor. its a three cylinder, tri-carb, 2-stroke. i have replaced the plugs, and gone through the FSM setup for setting timing, tip in, and fuel enrichment. the engine runs great at everything but idle. at idle though, it has this random popping, almost like a back fire, accompanied by a quick puff of smoke out of the starboard side of the motor, down low, close to the upper section of the lower unit. The RPMs drop with this pop, but this rarely causes a stall. im trying to locate the source and possibly cause of this pop.
any insight you can give would be great. thanks!
i have a video posted to the fecesbook. im trying to link it here somehow.
-J0N
moderator attention: if this violates some kind of rule, feel free to remove.
heres the link to the FB video.
https://www.facebook.com/jonathan.moller.100/videos/1701479176530753/?l=1376678811948317264
Since this is a 2 stroke, does it have reed valves? Check them, they are a wear item.
Did you clean out the low speed jets?
i did not replace the reeds, nor did i clean those jets. im a bit leary of removing the carbs, TBH.
do you guys know what this puff is, and where its coming from? i've read through a few dozen threads, and i've seen 'lean' come up often.
-J0N
Commonly known as a sneeze. It's part of life with a 2 smoke outboard. Doubly true when cold. What's your idle speed. You may need to increase it a little. As mentioned above, it could also be a little lean on the idle jets.
HappyAndy said:
Since this is a 2 stroke, does it have reed valves? Check them, they are a wear item.
I have never had to replace them on a outboard and my oldest one is 59 years old. That said, all of my outboard experience is with OMCs or Yamaha, so it might be worth looking into. When they fail, it's usually catastrophic.
In reply to jmthunderbirdturbo:
It's a backfire. Usually caused by ignition before the intake ports close, so it burns the fuel in the crankcase. Lean will cause it as will low idle speeds.
Ah, that helps immensely. i went out a few hours ago and redid the last portion of the FSM fuel enrichment setup, and i was basically super lean. it purrs like a kitten. later, i waited for the engine to cool to dead cold, and it only took two tries to start it, and only a bump of the electronic choke. usually much harder to get running. thanks for the info fellas! i love this board...
https://www.facebook.com/jonathan.moller.100/videos/1701934586485212/?l=6708694300222790535
-J0N
In reply to Toyman01:
They are a common failure point on 2t motorcycle engines. Because the reed is a check valve in the intake, when they don't seal the mixture becomes unpredictable. Usually rich, because the air will double pass through the carb venturi, which makes sense, but I've also seen a reed failure act like it was lean, for some reason it seemed like fuel fuel wasn't getting to the combustion chamber.
I'm not questioning your personal experiences, but marine 2t reeds can and do break. If you do an image search for broken or damaged reed valve you get plenty of hits for marine engines. I just fact checked that statement. My search turned up an even mixture of bikes, sleds and boats, with a few air compressors in the mix.
In reply to HappyAndy:
You are correct, they can and do break. Usually caused by exceeding red line, or corrosion. For the general use outboard, they should last pretty much forever with minimal care.
I just spent the last three years dealing with this exact motor (88 M70)
Absolute first things to do: Pull the carbs and clean them looking for black specks. Replace the fuel lines. Notorious problem. The fuel lines degrade and let specks come off which get lodged in jets and needles. It causes exactly what you're describing.
The two-stroke "sneeze" comes from a lean condition. What is likely happening is you have one carb with a speck in the jet and another with a speck in the needle. That's what happened to mine. I would get one carb lean causing a sneeze, then one that would dump raw fuel because the needle didn't seat. Sneeze and smoke.
