Stihl MS 290 Lemon

I'm going to throw this out there and see what it stirs up.Newer saws recommend a 50:1 fuel to oil ratio. This is one of the biggest problems , along with the ethanol in the gas now a days. The reason for a 50:1 ratio is to cut down on 2 stroke emissions , an environmental issue Using a 32:1 ratio will extend the life of your saw 10X over. Your saw may not have the peak rpm that the saw maker states , but it will cut quite fast as long as you keep a sharp chain on the saw. . Most people may never have a problem with 50:1 , if you always use fresh gas , but the few times you buy cheaper quality oil or use old gas in a saw , you run into trouble. Once the cylinder gets a little scratch , its down hill from there.
Ethanol in the fuel will destroy a saw and or carb faster than any other factor. Never , never leave gas in a saw even if its for a few days . The ethanol is hydroscopic, means that it absorbs water . Ever see the white film or corrosion on aluminum? That is what the little passages in the carb , collect over time . You can't flush it out, its the breakdown of the aluminum. If you can find , non -ethanol gas use it and always use 93 octane and quality oil at 32:1 up to 40:1
The average homeowner needs a reliable saw , not a high performance racing saw , like you see at the timber cutting shows.

Great points Mark Let me elaborate a bit, if I may. Running your mixture 'oil rich' will NEVER hurt your saw. The worst thing that can happen is that you will foul a plug, which would be a result of other pre-existing issues. A rich mixture has a lot of benefits. More oil means more lubrication, longer piston, ring and cylinder life. It also creates power. The added oil in the mix can help create a better ring to cylinder wall seal which helps boost compression. Two stroke engines are typically loose, as the piston to cylinder wall clearance is greater than in a four stroke engine. I raced moto-cross for 12 years, semi-professionally, and we tried just about everything to get more zip out of a big chainsaw engine. Best bet in a chainsaw, or any high revving two stroke: Good Gas, Good oil, (not off the Home depot shelf stuff) and rich oil mix, open the plug gap up a bit and don't let it sit there idling like a taxi.
We had a 82cc Kawasaki engine dyno'd: 23hp@12900 rpm, 112 octane VP fuel, Bel-Ray MC1+ oil @ 25-1. That SOB woulda cut some wood!

The Weimar

I mix up fuel 1 gallon at a time using those 1 gallon mix oil bottles (stihl synthetic). I typically stop short when filling, around .9 gallons of gas, which if my math is right, leave me with a 45:1, for a little margin.

More oil means more heat...? Is there a danger in going too high with the mix? I have a pro saw, with the magnesium case, so I assume it dissipates heat better...homeowner saws maybe not as well?

Fiziksgeek said:

More oil means more heat...?

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Not sure where you got that idea, but no. More oil does mean less gas, and the mixture screws might need to be tweaked ever so slightly to compensate for that.

Fiziksgeek said:

I have a pro saw, with the magnesium case, so I assume it dissipates heat better

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Not much heat ever makes it to the case. The combustion happens up at the top of the cylinder, and the heat dissipates through the cylinder's cooling fins while cool gas and air is constantly being sucked through the crankcase.

Your comment:

It never seemed like anyone could get the carb on the 290 adjusted to where it would keep running if you set it down for 30 seconds, unless it was set so the chain ran all the time. Plus it would stall if I didn't take it real easy with the throttle.

Are classic signs of an air leak. DO NOT RUN a saw with these signs.

Check the clutch you might have weak springs or one that is broke.

Jon1270 said:

Not sure where you got that idea, but no. More oil does mean less gas, and the mixture screws might need to be tweaked ever so slightly to compensate for that.

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Not sure where I got that idea either, haha. I distinctly recall someone telling me that the mix burns a little hotter with more oil in it...but I do admit that they may not have known anything at all!

I know...or I think I know, that if you have too much oil you can fowl your plugs with carbon build up...do I know that? haha

Fiziksgeek said:

I distinctly recall someone telling me that the mix burns a little hotter with more oil

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The person who told you that wasn't wrong, exactly; they were presumably saying it would burn hotter than a mix with less oil would if the carb weren't adjusted to compensate. That's probably true, but it only matters if you intend to change the fuelil ratio without adjusting the carb, which you shouldn't do.

Ok, the engine will not run hotter because there is more oil in the mix....nope, not even a little bit. Too much oil won't foul the plug, too much oil/gas mixture going through the carb will foul the plug. You would have to be so oil rich to foul the plug that, well, you just couldn't do it. If you use modern, high performance synthetic pre-mix oil, you will never foul a plug because of oil, and quite frankly the engine will barely smoke. Once again, don't let it sit there idling like a cop car on traffic detail, they were designed to run WFO...
ya know what? Try it out, 20-1 with some high quality oil and at least 93 octane, if it doesn't run better, call the Dr.

The Weimar

If you did indeed see mild scoring on the cylinder after pulling the muffler, it might be prudent to get that cleaned up before you do more damage. Stihl's cylinders are Nikasil plated, so they can't be bored out to the next sized piston. But the great thing about Nikasil is it's hard as a rock.

When the aluminum piston rubs on the Nikasil, often from a lean running issue, that's called transfer. The aluminum transfer on the cylinder walls can be sanded and buffed off. A lot of times you can get it back to looking like new.

If you go to the Arboristsite forum, in the chainsaw section, you will learn pretty quickly how to clean up the cylinder and install new piston and rings. I have done it to many saws I have found on Craigslist. Pretty easy to do if you are mechanically inclined.

Most will suggest you buy an after market Meteor piston with Caber rings. Quality stuff. But recently Stihl dramatically lowered their prices on certain parts, including pistons. So at this point for a few dollars more, you could install factory parts back on the saw. Oh and if your impulse line was bad, it's possible your fuel line could go soon as well. Might as well install one of those with a new fuel filter. Once again pretty easy to do and cheap insurance.

MarylandGuy said:

But recently Stihl dramatically lowered their prices on certain parts, including pistons.

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This is very interesting. Could it be that the aftermarket competition is finally getting to them?

That is the general consensus within the chainsaw community. Apparently the high price of replacement parts has been a major complaint for the dealers. Think about all the sales they lose to aftermarket chainsaw parts on ebay. Prices started falling around the first of the year. It remains to be seen how long they stay low.

Sadly, there are still some dealers who are using the old price structure and pocketing the extra money. So you may want to shop a few dealers if say you need a new cylinder and piston.