redunshee
Addicted to ArboristSite
Wow, maybe we should be running 100/1. that would be 2 times richer than 50/1.
Craftsman -Poulan saws trashed by new owner
- Thread starter cranman1951
- Start date
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I have a box full of dead top ends from saws that were tach tuned to a manufacturers magic number.
Hey Old2stroke, Could you expand some on your findings? ( purely a academic curiosity )
arto_wa
ArboristSite Operative
I see, who tuned them - was that you or someone else?
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The problem with tach tuning to a manufacturers number at WOT is it isn't a reliable way to get to the sweet spot where it 4-strokes at WOT out of the cut and cleans out as soon as the cut starts. Manufacturers number is selected to make the saw comply with government mandated emission standards as a priority and not necessarily the longevity of the saw. Depending on the make and model of the saw, it works some of the time but for others it is often on the lean side of optimum. Repair shop mechanics love to use this method as it's fast and they don't have a log out back to test with and a lot of them never actually use a chainsaw anyway. Many will set the WOT rpm a little lower then recommended just to be on the safe side with the mix. Many conditions, such as a change in elevation, will require a change in tuning and being sensitive to the sound of the saw is a good way to achieve this.
Thanks for taking the time to write out your thoughts. As noted it is a good starting point using a tachometer and to adjust the "H' richer slightly drops the RPM to be safe.
There are a whole list of environment and saw mechanical that effect a chainsaw running. That could be the mix, temperature, elevation, condition of the chainsaw and even what bar length and wood being cut. Lots of factors...
I take care of multiple chainsaws that are used by different users and that brings different users styles and abuse. Elevation can range from 5000 ft to 10,000 ft with a wide temperature range.
Yes I use a tachometer trying to tune in the middle mean of all the conditions ( elevation, temperature etc...), then back out the "H" to give the saw some protection on the rich side. Doing this the saw can withstand and work satisfactory to the changes. Multiple users do not know or understand how to make a saw run at it's best, the skill set is not there, They just want it to run and cut wood.
So your experience using the manufacturers "H" RPM values has shown you that saws are running lean leading to a risk of top end damage. Thanks for expanding on your first comment.
Cheers...
cranman1951
ArboristSite Operative
I double checked what was left in the saws....looked like a good mix bright blue
These were stashed older new saws that you sold. Curious if you ran a tank of fuel cutting wood with them before you sold them? If not, wonder if what happened to the person might have happened to you?
SteveSr
Addicted to ArboristSite
I do the same thing for our state wide (NC) trail club. Increasing either temperature or elevation helps richen the mixture. Since I am at about 250 ft ASL I try to tune saws during the winter at the coldest likely use temperature. Even then I'll leave them a little rich. I would rather deal with a "poor performance" complaint than a burned up saw.
I also like to send the non-adjustable carb MS170/180s to the mountains so they will richen up and slow down a bit. I do have a MS180 nearby that will run 14-14.5K WOT @45 degrees F but it hasn't burned up yet.
SteveSr
Addicted to ArboristSite
Looks like you hit the quadruple jackpot :-(
1. Cold temperatures = leaner mixture.
2. Borderline lean out of the box.
3. Made leaner by 40:1 instead of 50:1
4. Novice user with a dull chain.
Steve since you seem like a good guy I am just going to tell you this. Take one of your good running saws at 50:1. Now change it to 40:1 and see if it blows up. Then try 32:1 then 20:1. Keep going until it blows from lean mixture. I have challenged several to this test and no one has done it. I have Steve THEY DON"T RUN LEANER!
cranman1951
ArboristSite Operative
More oil is better I thought......
I guess Steve thinks less oil is better. Maybe 100:1 or 300:1 is good. You need lube. Try to tune your saw to run on 300:1 Steve.
I see this a lot on the boards. Technically more oil is slightly less fuel, but when you factor in part of the oil burns and the fact that we are only talking a half an ounce per gallon it’s like a 1% change in mixture. I can’t see that most people have their saws tuned that closely.
i’m sure there are some cases, but not in my limited experience
i’m sure there are some cases, but not in my limited experience
If they were straight gassed the piston will be scored on both sides if so he should pay you for all 3 doubt he will though. Allway check the cylinder on a no start on a saw( especially new)
SteveSr
Addicted to ArboristSite
Well, according to the laws of physics they DO run leaner and to prevent the spread of mis-information this is how it works:
A carburetor maintains a specific ratio between intake air and the fuel mix. The optimum air/fuel ratio for complete combustion is called the stoichiometric ratio and is about 14.7:1 for 4-stroke engines. Now a 2-stroke engine won't run for long at this ratio as some extra fuel is needed to cool the engine (by evaporation). I have seen numbers somewhere around 13:1 for 2-strokes.
So now with a 2-stroke you introduce lubricating oil into the fuel. So now you have the fixed ratio set by carburetor:
Carb fuel ratio (which is constant) = Lubricating oil + Gas
So if the amount of Oil goes up the amount of Gas goes down to keep the carb fuel ratio constant. You can also think of this as the oil is diluting the gas. Lean/Rich has only to do with the amount of GAS/AIR ratio. The oil is just along for the "ride" to lubricate the engine.
So yes, 20:1 has less gas than 40:1 which has less gas than 50:1 and so the more oil the leaner the mix.
REFERENCES:
https://x-engineer.org/automotive-e...nce/air-fuel-ratio-lambda-engine-performance/
OK stick with that. I get it. Scientifically/Technically you are correct. However you used your example to blame seizure of a saw. This is where I take maximum exception. You can not run without oil but richer and richer oil will NOT cause a lean seizure. Prove me wrong in the real world by taking my challenge.
This is a valid point and I sympathize with the static you are getting. It all depends on how close to the "sweet spot" the saw is tuned and how sensitive the operator is to whether it is at the sweet spot. If the saw is tuned slightly rich (hopefully most are) a moderate change in mix ratio would probably not be noticable and could hardly be blamed for seizure. However, from experience I have a few OLD saws with fixed main jets and with a 40:1 mix they run too rich but if I feed them the 25:1 that is called for, they run much closer to optimum. Another case, a friend came with a brand new Stihl set to specs (meaning slightly lean) and fresh fuel with Stihl oil mixed at 50:1. The saw would bog down in the cut and act like it was fuel starved for power. Turns out he wasn't good at volumes and had used twice as much oil as he should have and the saw was using 25:1. After correcting the mix, the saw ran like it should have.
I think the op said he tuned it rich on 40:1 so I don’t see how it was a lean seizure. The best thing to do is have the op pull the engine apart so we can see the piston.
cranman1951
ArboristSite Operative
I pulled the exhausts off both saws, and they were badly scored....worse then some straight gassed saws I've replaced cylinders and pistons on. when I poured out the leftover gas from the two saws, it had good blue color that matched what I put into them. The chains were dulled and stretched badly, my only guess is the OP never let off the throttle and just pushed the saws to death with the dull chain
