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What's causing a "soft" miss or backfire at low RPM's? 1999 Mercury 15hp 2 stroke

mgray

New member
Brought the engine from the lake(3500ft), down to the ocean for the summer. Probably put maybe 5hrs on it since. Had it serviced before using it down here. Wasn't too happy with them, as they charged me almost $400 to change 2 plugs, gear oil and lube the nipples. Engine was covered in rust dust too when it came back. Wasn't impressed with them at all. Thought it would have been a good idea to get a professional to take a look at it since it's been over 10years, but I have many mechanical family members who help take care of all the engine toys at the lake. Still, I though it couldn't hurt to get these guys to service and give a once over. They said everything seemed fine and gave it back (covered in rust dust!)

Never had any problems with this engine, and she still looks brand new, especially under the cover!

She starts fine. Pull the choke and crank a couple times and she starts right up. A little rough idle when cold, but never dies. Once she warms up, she seems to have a rhythmic miss when idling or trolling. At high rpm's she seems to smooth out completely.

So now today and the other day I noticed a weird thing happening when trolling around at low rpm's. There would be a quick, "soft" miss or backfire and a puff of smoke, then back to normal. Almost made me think I hit something and broke a prop pin, but it was so quick and she never dies. Would only seem to happen when in gear and under a little throttle, but still slow and no where near planing. Also notice that it kinda lags and feels "rough" in the first half when accelerating to full, but then becomes quick and smooth once rpm's come up past half way or so.

I pulled the plugs today and they seem normal(they only have about 5hrs on them) They are dry and brown, with little to no deposits. The only sign of oil is around the plug threads

So this has me worried. Does this sound like symptoms of a lean engine? Maybe there is a high altitude jet installed? I'm hoping it's maybe a fuel issue? I try to run my fuel/oil about 75:1. I think 100:1 is recommended and 50:1 for commercial use, so I figure 75:1 is probably good, especially in the salt water.

Any ideas?

Thanks
 
Re: What's causing a "soft" miss or backfire at low RPM's? 1999 Mercury 15 2 stroke

Run 50:1 at all times.-----------------Motors will run on less oil ( nicely in fact ) but they wear out quicker.----------------Sounds like it has a " lean miss " so yes open up the low speed adjustment.
 
Ok thanks guys, I will adjust that next time I'm out.

And just for the record, I'll copy what is said in the Mercury Owners Manual.

"After the break-in fuel mixture is used up, use a 100:1 (1%) gasoline/oil mixture
in engines used for pleasure applications. To provide additional protection for
commercially used products, a 50:1 (2%) gasoline/oil mixture is recommended"

Regardless, I think I'll use 50:1 from now on.

Thanks again
 
Ah yes, of course that makes sense...any idea how quicker an engine will wear out at 100:1?? Would it be twice as fast as 50:1? Given everything else equal obviously
 
It might depend slightly how you use it.
If mainly using it for slow trolling, I would not run the 50:1, but more closer to 75;1. If mainly running full throttle to your fishing spots with high load and in addition do not use the engine regularly, stay with 50:1.
Carbon build-up at low rpm might be well as damaging to the engine.
What is the most damaging to engines are storage with 'no oil'. 100:1 does not leave much oil. I used to stop my smaller engines by pulling the choke if not using it same day.
 
So I looked into it further and checked out more recent Mercury manuals(2005) and they have deleted the 100:1 recommendation. It's now just 25:1 for break in, and 50:1 for everything else. Interesting, I wonder why??

And good idea Haff about shutting down by choking, never thought of that.

Some great advice here guys, thanks a bunch
 
At 100:1 outboards are DAMAGED and it DAMAGES the reputation of a manufacturer.----------------Johnson and Evinrude tried 100:1 for 2 years I believe and it cost them plenty.
 
The real problem was not 'when running' but when NOT running.
In most cases when leaving the engine a week or more, not much oil-film left.
Had some professional fishermen as customers using both Yamahas speced at 100:1 and Evinrude/Johnson speced at 50:1.
During eel season ran 8-12 hours a day.
They mostly ran around 75:1 changing engines at around 1000-1500 hours. Never saw any wear except seals on those engines.
At the same time I've had a lot of 'pleasure' boats been running constantly on both 50:1 and 33:1 rattling around after just a few hundreds hours.
 
Just a side question...if my mixture is too lean, would that cause my primer/idle speed knob to not do anything to the idle speed? The primer part works, but when it's idling, if I rotate the knob nothing changes. But at the lake (3500ft) the knob would change the idle speed
 
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