Fuel:oil ratio

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I don’t say this to be argumentative at all...but I have run into a couple of small two-strokes with fixed high-speed jets which seemed to run too rich on the top end. One was an old Homelite ST-200 trimmer (engine at the top, big fat curved shaft, single-line auto-feed head...and I think the same engine as one of the little red plastic saws. XL-2, maybe?) That was at my Midwest 725’ elevation in the summer. 32:1 helped a little, and a couple of additional holes drilled in the exhaust outlet finished the job. Ran great and cut really well for a couple summers...although my hearing range may now top out at about 12 kHz instead of 20kHz! LOUD trimmer.

The other is an old 18cc Weedeater Featherlite (square housing, not the newer jellybean-shaped housing) my Stepmom uses around her garden. She lives west of Denver CO at about 8,500-9,000 feet, and runs 50:1 in that and her saws (MS170, Husqvarna 50). When I visit I get weedeater detail. I mixed up a few quarts of fuel at different ratios last time as an experiment. It seemed to work about right on 20:1, a holdover from my dirt bike days as a kid. Smoked a
bit on that fuel, but ran pretty well considering the altitude.

I wonder how her MS170 does on 50:1 up there. I would ask but I don’t think she speaks rich/lean.
Not argumentative at all Mex , it,s the trade-off between lubricity and cooling epa. requirements .It all has to do with the molecules of oil being larger than that of fuel molecules though a fixed orifice (jet) in conjunction of fuel as a cooling agent and oil for lubrication . Most new premium syn oils have finer molecules that burn with the fuel charge (energy gain) and leave little residue or carbon fouling . Older thicker viscosity oils would give more horsepower gain from better ring sealing and higher compression , however could cause overheating (hot spots) issues from carbon fouling due to less technology and additives . The fact is that yes more oil means less fuel to air ratio , potential for a leaner running engine . P.S. As you are likely aware the dirty air filter would cause a rich fuel \ air ratio potential that Steve was trying to determine ;)
 
What I was saying for the 2 saws I mentioned, that I was going to run what the spec said.
32:1 in the old Homelites and 50:1 in the Shindaiwas.
With the shindaiwas I agree . The Homelite that's a ancient oil ratio due to poor oil technology of the time , ensure to use quality oil to reduce carbon issues !
 
Yep, I’m aware of the dirty filter thing...but that Stihl exhibited rich running from brand new. Fully warmed up, bucking a log with the bar at full cutting depth and it was blubbering like I’d left it on half choke. It was slightly better on 50:1 gas/oil than on 50:1 Moto-Mix, I thought...but I’ve been using 32:1 gas/oil since early last fall and it starts right up, idles steadily without loading up, four-strokes out of the cut and cleans up in the cut. Sorted! At least until the carb accumulated some crap in it.
 
That must be an oddball 170. In my experience these are almost all borderline too lean but then I am only 250' above sea level. Can you post a photo of the silver engine family label on the handle so that we can know when it was manufactured? Also what elevation and temperatures are you cutting at? Both of these will affect the tuning.
I agree majority of the 1st generation epa regulated Stihls were set quite lean from the factory ! I had to rejet a few 2010 , MS261 & MS361 .
 
I'm starting to understand this a little more now. So with the better oils made today, less is needed to provide lubrication, but the equipment HAS to be set leaner. If you run a richer oil mix, then the equipment has to be set a little fatter. Correct? I've always just run 50:1 in my string trimmers and 40:1 in my saws, but the fixed jet saws never ran quite right.
Very few of today s epa fixed jet saws run properly , that is why limiters are removed or welch plugs are drilled out to achieve proper tuning . To many variables , relative humidity , barometric pressure etc. I do not have any Electronic or Computerized saws . However have friends that do and many have had their saws reflashed due to erratic idle and high rpm miss,s . They have a way to go with their onboard micro processor controls . P.S. You understanding of current saw fuel & oil mix technology is correct !
 
My observation was that 40:1 in the Homelite was too rich so I suspected 40:1 would be too lean in the Shindaiwa.
40:1 in the Homelite with todays modern oil would be optimium , unless viscosity is paramount (engine wear issue) since it was recommended for 32:1 with old crude mineral oils . The Shinny in retrospect should remain @ manufacturer 50:1 unless you understand engine performance requirements . Just ensure you buy quality oil !
 
I will look for the other label when I get back home later today. I don’t suppose the long number etched into the chassis below the muffler, visible in the one photo, can be decoded for any date info...?
That is the machine serial number. If the saw was registered by the selling dealer any dealer could look up the number and tell you when it was sold and to whom.
 
The other is an old 18cc Weedeater Featherlite (square housing, not the newer jellybean-shaped housing) my Stepmom uses around her garden. She lives west of Denver CO at about 8,500-9,000 feet, and runs 50:1 in that and her saws (MS170, Husqvarna 50). When I visit I get weedeater detail. I mixed up a few quarts of fuel at different ratios last time as an experiment. It seemed to work about right on 20:1, a holdover from my dirt bike days as a kid. Smoked a
bit on that fuel, but ran pretty well considering the altitude.

I wonder how her MS170 does on 50:1 up there. I would ask but I don’t think she speaks rich/lean.
The MS170 would probably have a very long life and not run very well on 50:1 at that elevation. This is one of the compromises of a fixed jet carb.

BTW, Elevation changes (primarily) and temperature changes is the reason behind adjustable carbs with limiter caps. The limiters allow the saw to be tuned leaner for high elevation. This is why the stock setting is full CCW at sea level (or close to it).
 
AC044AC6-5D4E-46F5-A2E7-FB154C7D714D.jpegOkay, here’s the label from the mysteriously running-rich-from-new Stihl MS170. If I am understanding that sticker, it was manufactured in May of 2018...?
 
What I was saying for the 2 saws I mentioned, that I was going to run what the spec said.
32:1 in the old Homelites and 50:1 in the Shindaiwas.
Oils have gotten MUCH better in the years since the Homies were designed. They should be fine on 50:1 with high quality oil. However, they will need to be retuned for the richer (gas) mixture. This is where the issues come with the non-adjustable carbs. They can't be re-tuned if the mix ratio changes.
 
Oils have gotten MUCH better in the years since the Homies were designed. They should be fine on 50:1 with high quality oil. However, they will need to be retuned for the richer (gas) mixture. This is where the issues come with the non-adjustable carbs. They can't be re-tuned if the mix ratio changes.
I'll make a small batch and try it at least.
 
I'll make a small batch and try it at least.
If you feel confident in your ability to identify lean running symptoms, that should work.

One thing to keep in mind is, chainsaws are either idling or WOT. So the margin for detecting a too-lean condition is narrower than it would be on a trimmer (idling or low-load part-throttle most of the time). Trying the different mix ratios in my stepmom’s trimmer was pretty low-risk; it’s not a very highly-stressed engine, it’s at high altitude on top of that, and it’s a Featherlite facrissake. Blew up a featherlite? No big whoop. Go find three more in your neighbor’s dumpster.

Aside from idle mixture settings, Most of your “this thing is too lean” cues are easiest to detect as flat spots in your response and acceleration, or slow return to idle...and a saw will give you less opportunity to observe and react to those cues since it spends very little time doing throttle transitions from one carb circuit to another. So you have to pay pretty close attention to that idle-to-WOT transition, how promptly it comes back to idle, any hint of flat spots in the acceleration response, exhaust smell...thus the “tune it to blubber a little out of the cut and pull clean in the cut” thinking. Gives you a margin of error, since a saw is less likely to survive running on the razor edge of tip-top mixture accuracy.

I read a pretty cool book about GP road race bikes in the early 60s, when two-strokes were starting to eat four strokes alive. Those guys were riding three-figure speeds into corners on the Isle of Man, next to stone walls and big trees, trying to hear signs of impending seizure so they could hopefully pull the clutch in fast enough to not be turned into impact sausage. A lot of them were not quick enough, apparently.
 
These oil threads crack me up. Pour some gas in the can, pour in amount of oil you want to use, finish filling can to desired amount. Shake, pour into saw and start cutting. It's your saw mix it how you want. It's my saw I will do the same.
I think I am a bigger two-stroke nerd than you are.
 
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