Backfire

[high life man]

I was using my old Jonsered 520SP on the job today. Just to keep the cobwebs out I guess.

Anyway, we were cutting some big chunks down to size so that we could lift them in the truck. I was in the middle of a cut with the saw buried. The thing acted like it was running out of gas. I pulled it out of the cut and it backfired. I mean small flame out the muffler and all! (It was dusk.)

I made one more small cut just to see. Yep, starving for fuel. I filled 'er up, made several cuts without any problem. I ran it full throttle unloaded for a few seconds, and it's slightly four-stroking like it should. I will say that I put on a new muffler about a month ago but haven't used the saw much. The saw before and after ran excellent. Never happened to me before on any saw.

Questions:

What causes backfire?

Should I be concerned ?

 

[JimL]

back fire is a result of unburnt fuel...

 

[Jumper]

GAS?

Was the gas initially in the saw month plus old? If so it might not have been igniting properly, thus the backfireIt could have been "summer formulation" as well, which might have an effect depending on how cold it is there.

 

[high life man]

Jumper,

The last time I used and fueled the saw was on 12/15/01. I remember because I use it so rarely.

Now that you mention it, we ran out of gas that day, and had to get more at lunch. My routine is to add two to three capfuls of Seafoam to the mix at time of mixing. Did'nt have any along, so I added it to the gas can when I got home. What was in the saw must have been left untreated.

Good point, Jumper, and it makes sense. Although it ran well this afternoon, after the backfire and subsequent refueling it seemed to perform a little better -- as if it coughed up a hairball.

Wasn't too bad today. +8F @ 8:00AM, warmed up to +20. Felt good to work outside. It was extaordinarily warm on 12/15 though. We worked in shirtsleeves on that job.

Thanks.

High life man

 

[Walt Galer]

Howdy High life man,

Your case was a good testimonial for being very careful about what gas you put in a high performance engine! You were very lucky not to have destroyed your chainsaw. Continued operation could have given you a real disaster.

Gas these days is so unstable that it should never be left in a saw for more than a week. Myself, I recommend (here locally) Connoco 92 octane, and with fuel stabilizer added per direction immediately, not later. The major brand two-cycle mix oils now have stabilizer included in the oil, and if the gas is mixed at the time, (at the pump), this is adequate. Myself, I usually buy 5 gallon at a time, and use some for lawn and garden, or mabe a different mix for outboard, in otherwords i'm never sure where it is going, so putting the stabilizer in at the pump is most adviseable. I avoid mixing more than I am apt to use immediately. Best deal around these parts is on Golden Eagle fluel stabilizer from the Carquest auto parts house.

Yes, it is higher price for the gas, but you use so little, it is very false economy to go cheap.

The brand recommendation on the gas would be different for every different locallity according to which brand uses the least oxygenated additives and reprocessed lighter factions put back in at the refinery. You have no way of knowing this without asking around with the professional servicing saw dealers. They quickly figure it out, and it changes seemingly every year and/or season.

As much as 80% of the service work at a large dealership, will be found to be bad gas related.
(Carburetor, fuel lines, stuck pistons, dissolved antivibe mounts, busted internal parts, etc.)

Regards,
Walt Galer

 

[HUSKYMAN]

I think you guys missed an abvious one here. I agree with Walt on the fuel issue but go thru the ignition system just to be sure. A saw backfires because the fuel does not burn in the cylinder and gets into the hot muffler and the explodes. The reason it did not burn is the spark in the cylinder did not light it. You could have a slightly fouled plug or a weak coil. Just something to think about...

 

[homelite360]

i don't know those old macs would take any age gas you could throw at them. my first mac was a 200 and it was sitting on the guys shelf for 15 years un touched with gas in it. got it home and got the thing going on 15 year old gas in about 6 or 7 pulls. but another thing that could cause your saw to back fire would be mis set carb adjestments

 

[Walt Galer]

Howdy,

Here is another scenario for you to consider: Suppose the fuel is leaving a residue inside the cylinder (often on the dome of the head) and this deposit then begins to glow. Won't this result in some oddball ignition at some random time in the stroke? (Possibly, late and during the beginning of the exhaust port opening, as unburned debris finally decides to ignite the fuel that now has conscentrated octane boosting additives).

Normally, an "odd ball" ignition occurs as the piston is comming up for correct firing, and this is known as preignition, or even detonation (if premature fire is way early and the gas really volitile.)

With deteriorated unstable gas, containing so many strange additives that are SUPPOSED to retard burning, I say almost anything is possible- especially in a piston ported engine.

I don't know if it is just me or what, but I seem to have noticed that some of the Jonserd's saws in the series from 2149 to 520 seem to be quite sensitive to mixed gas quality considerations. Plug fouling and carbon up problems seem to happen very quickly with too much oil, or with poor gas. Perhaps more so than one would expect. I also believe that the higher quality the engine (highperformance) the more likely the problems. May be those little johnny's are too good.

Regards,
Walt Galer

 

[high life man]

Walt, JimL, Huskyman,Jumper, and JimL,

I've read the accumulated posts this afternoon. I took out the sparkplug and mufffler. Wall and piston look clean and undamaged. No funny noises. All looks kosher to me. The plug was a little wet, but I ran it this morning w/o load for only about 90 secs after a thorough cleaning. I cleaned the plug, about 5 hours on it, and reassembled the saw. Runs t??s up. That's tit mice if you're wondering.

I don't see any sign of ignition problem. I tend to agree with JimL and Walt's diagnosis. I fueled up outside of my routine.

I'm sort of a Stihl fan, but ???? if I don't love that old 520. My dad gave it to me about ten years ago, barely used, when I bought my house. My first saw. My favorite saw. I like the way it snorts.

I've got a '93 15hp Johnson outboard, but enjoy my '70 9.8 Merc much more. Same kind of thing I guess.

Thank gentlemen. Now back to the high life.