MS 390 broke from bad gas??

[NWwoodsman]

Hi all,

I'm new here but have been lurking for awhile. Some real good info on here, with lots of activetly. So I've got a problem with my fairly new (1 -1/2 year old) MS 390 saw. I've owned it since new and cut 25 cord or more a year. Also cut some cedar block (3 cord) and saw logs for my Mobil D. sawmill. I run a 24 inch forester bar with sthil full comp chains, use quailty mix in per measured containers with regular pump gas from Washington, Montana and North Dakota. Saw stopped running at the end of a tank of gas. Toke it to my sthil dealer and the motor shoot, needs new jug and piston. Dealer say that this happens now with the gas we have, something about the alchol or etyhol in it. Anyone else have this problem?

[Rocky_J]

Lean seize. One or more of three causes for lean seize.
1- carb set too lean causing extra heat (friction)
2- cutting with a dull chain causing extra heat (friction)
3- an air leak resulting in not enough fuel/oil getting in the motor causing extra heat (friction)

In other words, the inside of the motor needs a protective coating of oil whenever it is running in order to reduce friction and heat. Two cycle motors carry the oil in the fuel, so the only way to kill one is starving it of fuel.

A bad habit that some users have that exacerbates the above mentioned causes is to continue trying to finish a cut after the saw starts stumbling, indicating it is about out of fuel. Those few seconds of running lean multiplied by a couple hundred tanks of fuel = lean seize/ broke motor.

[Rocky_J]

And yes, ethanol in fuel will cause it to run leaner because ethanol is not as efficient as a fuel. Old fuel loses its potency very quickly as well. If the saw is not tuned/adjusted for running on ethanol then it will burn up. Some people can tell by listening to the motor run that it is running too lean, others are blissfully ignorant and can run a saw to death at 17,000 rpm without ever having a clue that something is wrong.

You are not the first, nor will you be the last. But the explanation has to be repeated every single time and it is always exactly the same thing. Sorry.

[welderskelter]

I blew up my 066 after about 10 cords of wood. Had to put in new piston and cylinder. I found out later on the newer saws they are putting smaller holes in muffler to make them more ecologically perfect and it causes heat to build up in engine. Drill a couple new holes and let her talk. I got about 25% more horsepower now. She really goes. Harold

[ladylake]

 Only 17000 RPM  Rocky, I know what you mean, just keep running it untill it won't turn. The most important thing for saws is enough fuel which keeps it cool and lubercates them. A lot of these burn downs come after leaving the saw sit a couple of months with ethonal fuel that partially clogs the carb.  Use Stabil or Seafoam, if it sounds at all funny after storage clean the carb, if it doesn't respond to the adjuters clean it again. When milling they need even more fuel.   Steve

[windthrown]

Washington gas is far better than E-10 Oregon gas (less ethanol) and I buy my premium gas in Washington state now. I have run my saws just fine on E-10 Oregon ethanol gas as well, but after re-tuning them with a tach. One thing is to use super and not regular gas. Stihl saws are made to run on a minimum of US mid grade gasoline (and ethanol gasoline at that). Note that premix oil will drop octane, so you want a higher grade gasoline to compensate for that. Lower octane gas can lead to pre-detonation and top end piston damage (the gas ignites unevenly and the piston wobbles from uneven pressure, or gas flares will burn small holes in the piston crown). Mid grade is good, but premium is better. A second thing here is to use Sta-Bil in your fresh gas to: 1) keep the octane high and 2) keep the gas from varnishing and 3) to keep the ethanol in gas from phase separating (the ethanol absorbs water and then the ethanol and water separate from the gasoline, creating a separate solution layer in the gas tank below the gasoline layer).

StaBil will solve the ethanol issues in your gas tank and carb, but not in the engine. Ethanol has a secondary effect of leaning out your gas/air mix. Reason being is that ethanol has 60% of the burn energy of gasoline. While this difference is small, 10% ethanol will increase the lean ratio of your fuel-air mix by 4%. This will cause your engine to run hotter. Runnign hotter can casue your engine to score, usually on the cylinder just below the exhaust port where it gets the hottest. That scoring in turn will score your piston on the exhaust side, and in a rather short time, kill your engine. It is similar to a straight gas scoring, caused by no oil in the mix, but in the case of straight gassing, the entire cylinder and piston are more likely to be scored. StaBil will not fix this issue. Re-tuning your carburator will. In many cases with the new Stihl carbs, they have limiter caps in them to prevent them from being tuned richer. This is for EPA clean air reaons, and as a result most saws sold now are right on the edge of running too lean. The 390 is the same setup as the 290 and the 310, both saws that I have owned and modified. To adjust the carb richer on a 290, 310 or 390 saw:

You need to pull the carb, and pull out the limiter cap on the H screw side of the carb. You can also pull the limiter cap on the L side, but that is usually not needed. To pull the limiter cap (a red plastic cap that covers the carb adjusting screw):

Use a magnifier and good light, and turn the limiter cap until the tab can be seen in the slot
Get a small brass picture hanger eye screw, and twist it into the hole in the limiter cap
Twist the eye screw until the limiter tab is in the slot again, and pull or pry the eye screw and the limiter cap out
Now, you will have a red limiter cap with a small long narrow slot on it in your hands. It needs to be trimmed.
Take a box cutter or exacto knife or one-edge razor blade and cut off the limiter tab. You are done.
Press the limiter cap back into the H cab hole, aligning the tab that you cut with the slot.
You need to re-set the limiter tab on the jet screw so that the carb does not vibrate out of tune.

Now, you are free of any EPA tyranny, and you can tune your saw correctly. Adjust the LA (idle) screw to 3300 RPM and L screw so that you get the highest revs. Reset the LA screw to 3300 again and re-adjust the L screw so that you get the highest revs. It may take a few times to get the peak L setting. Then reduce the idle to just below where the chain stops running. Now...  Adjust the H screw so that the saw burples or 4-strokes at WOT. Then test the saw with a tach and in the case of the 290, set the WOT to 12.5k, +/- 500 RPM. In the case of the 310 and 390, set the WOT to 13km +/- 500 RPM. Do some test cuts and tweek the H screw so that the revs do not fall off too fast or hard in the cut. A 1/16 turn of the H screw is about the most that you should need. If the saw sounds clean and burple-free at WOT out of the wood, it is set to lean. Richen it up until it pops and burples, or 4-strokes. 

[windthrown]

You can also easilly open up the American 290/310/390 factory muffler to the EU muffler and get about a half HP more out of your saw. Take off the cover plate and fire screen of the muffler. On the front of the muffler, you will see two horizontal slots below a rectangular dimple. Open up the smaller slot so that is it ths same size of the larger slot. Now above the rectangular dimple drill 2 more horizontal slots in the flat area below where the cover plate and fire screen set. Or drill three or four x 3/16 holes in a row in the same area. Now file open the over plate to enlarde the exhaust port by 2-3 times. I usually grind the slanted raised area to the angled area of the cover plate to good effect. This will give you a 'standard' muffler equivalent used in Europe, that give you about 1/2 HP more. After you do this, replace the muffler and re-tun your H screw on the carb as I described above (after removing the limiter cap tab). I have found that after a muffler mod of this type, the H screw will need about 1/2 turn more than factory setting (1/1-4 turn total, or a tad more).   

Here is a photo of the finished muffler mod on my 310:



This mod will give you pleanty of more power, and not be that much louder. I have tried more holes in the muffler, but this size and type seem to give about the best results with the stock carburetor jetting. This muffler is about the same as the 'standard' one used in older model 029 Supers, and the 1127 saws in Europe. Imagine that? I wonder where I got the ide of where to drill these holes...

[NWwoodsman]

Thanks alot windthrown, that was the info I needed. I just don't want to have to fix the saw so soon again. I had wanted a 361 when I brought the 390 but the dealer talked me out of it.
Jason

[windthrown]

Thanks alot windthrown, that was the info I needed. I just don't want to have to fix the saw so soon again. I had wanted a 361 when I brought the 390 but the dealer talked me out of it.
Jason

Bad dealer. The 361 is the better saw, for several reasons. Main difference for power is that the 361 max rev is 14k RPM. Also the 361 has more metal and better plastic in it and the engine is a lot easier to work on. The 390 has a clamshell engine, and they cannot be worked on, ported or modified as easy. The 361 also has better AV. The 390 is cheaper though, and the exhaust on the 390 is a lot easier to modify than the 361. Also the bearings are very easy to replace in the 390, and very hard to replace in the 361. I once considered them both when my Olympic died (coil fried) and opted for the 361. I like my 310, and with the modded exhaust it will cut close to the 361. The modified exhaust 390 should beat a stock 361. BOTH saws are prone to this lean running problem though, not just the 390. Actually all Stihl saws are prone to lean failure and can be tuned too lean with the new limiter caps and should be tuned correctly to prevent lean failures from happening. I have read about similar lean failures in the 361, and dealers that claim that 'bad gas' is the reason (when many times they tuned the carb too lean). You should use higher octane fresh gas and a good premix oil*. Both saws are going away now. The 362 is out here in the states to replace the 361, and the 391 is out in Europe to replace the 390.

*BTW: Use a good premix oil. Do not use TWC rated outboard motor oil, which is the most common type of 2-stroke oil available. That oil is designed to run in cooler running water cooled engines, and it will foul your air cooled chainsaw engine. Some years ago I used Castrol Jaso FB rated dyno 2-stroke oil, but it smokes and smells, and it will foul your muffler spark screens. I flipped to Jaso FC/FD rated 100% synthetic oil, and never looked back. Cleaner burning, higher revs from my saws, and no problems. I used Mobil 1 synthetic until they stopped selling it here in the states, and so I use Elf 100% synthetic oil now. I get it at the local motorcycle shop. 50:1 mix rate with StaBil in Washington state premium gas. I never thought that I would have to drive to Washington to get better gas... &^%$ing Oregon greenies and Ethanol. California had bad gas too with that MTBE crap. They have since stopped using it, and have about the same ethanol mix as Washington state does now (about 5% ethanol). Until last year, Oregon had a limited amount of 100% gasoline available, but it was all changed over to E-10 by law to the maximum 10% ethanol here by March, 2009. Supposedly they passed a law that went into effect Jan 1, 2010 that 100% premium gas can be sold for marine and off-road use here in Oregon state again (after Salem got snowed with complaints), but I have yet to see any available in Oregon. Oregon gets all its gas from Washington, as there are no refineries in this state, but it is blended differently for sale in Oregon. Washington has higher octane premium available than we do in Oregon... 92 vs 91.

My advice is that if you run gas from a variety of western states (I run a variely of CA, OR and WA gas myself), either get a tach and retune your saws each time you switch to a different gas, or tune them to the crappiest gas available, that being Oregon E-10 with 10% ethanol in it. That way, if you run better (lower ethanol) gas, your saw will just run a tad richer. If you flip back to lean E-10 later, the saw will not burn up as it did in your case before. Also err on tuning a saw a tad rich and you will not have these types of problems. This typically cannot be done with the original factory limiter caps left in place though, so you have to remove the limiter caps on the H side of the carb. I am sure that the EPA would like to lock me up for telling people about this

[JohnG28]

Windthrown, thanks a lot for all of that info, its very informative...I live in NY state, and all we have available is E-10 gas here, and I have been worried about killing my new 361 with the same problem...I have read a lot on here about this issue, that is most certainly one of the best, thanks

[D._Frederick]

Windthrow,

I get alcohol free gasoline from Marc Nelson Oil Products in Salem Oregon, 800 523 7676. They have regular and Premium and will sell you any amount.

It has cost  me to much to have carburators re build every year using Oregon gas. I now use alcohol free gas or LL 100.  Hope this helps.

[windthrown]

Thanks for the lead. As in "leed," not "led". I can get LL100 here (AvGas) but it has lead in it, and I would rather avoid breathing lead fumes when I am sawing. Also 100 octane gas is overkill, and it is expensive. I have not seen the non-ethanol gas for sale in Salem. I rarely get down there any more though, and for my use, the Washington premie gas runs OK (and I am up there a couple times a month).

[John Mc]

I'm a pilot, and have easy access to 100LL avgas, but I don't burn it in my chainsaws. The lead compound in that stuff is particularly nasty (breathing it, or even getting it on your skin just ain't good for you - there's a good reason it's the only leaded fuel left in the world).

Some states have exceptions in their requirements for ethanol in the gas for special cases. A couple of places to look: marinas are one possible (ethanol can attack the fiberglass some of the powerboats' fuel tanks). Smaller airports are another - the larger ones will typically have 100LL, but smaller airports that serve older, single engine prop planes may have ethanol-free autogas (some aircraft engines - typically older, smaller ones - don't need 100+ octane, and have problems with the amount of lead in 100LL.) Ethanol is prohibited in the fuel of production aircraft

[sharkey]

The real problem with ethanol laced fuel is that it will hold water.  Gasoline was developed not to ever hold water.  You would be surprised how much water can build up in a gas can, especially in the winter.  Think about how dew builds up on the grass at dusk when the temps fall.  Water absorbed into the alcohol fuel mix will force the two stroke oil out of suspension and ruin an engine. 

Keep your gas can tightly capped and out of the sun if you can.  Most important is to not use mixed fuel that has been sitting for longer than a couple of weeks.  Mix your gas the day you plan to use it, and when your done drain your saw if it will sit for awhile until used again.  If the gas cans have been sitting for some time, dump them into your truck or mower and buy fresh. 

[JohnG28]

So is it ok to run old mix fuel through a regular 4 cycle engine, such as mower or car/truck without damaging the engine?

[flapjack23]

So is it ok to run old mix fuel through a regular 4 cycle engine, such as mower or car/truck without damaging the engine?

Should be no problem at all. I mix 1 gallon at a time for weed eater and saw, more if I'm doing a fair amount of cutting and usually put in my truck to mix fresh (sometimes it goes in the sled). Won't hurt anything.

[jteneyck]

I would not put 2-cycle fuel in any vehicle with a catalytic converter not designed for it; that means a car/truck.  The converter as well as the oxygen sensors are sensitive to many metal compounds which may be in some oils.  Also, if the oil is not burned in the cylinders it will burn in the converter, and could result in over heating/melting of the converter internals.  It's not worth the risk of having to replace the converter and oxygen sensors (easily $1000). 

Put it in your lawn mower, tractor, etc. that doesn't have a converter. 

[JohnG28]

Sounds simple enough, ill run it through my snowblower if we get enough snow rest of winter, mix the rest with more gas and run through the mower, have about 3/4 gal left from the fall that I dont want to use in my saws, especially my new 361...thanks guys