2-Stroke Fuel Requirements Vs. Engine RPM?

[SteveSr]

Hello,

Here is something for your thinking cap... Before that first beer!

There was recently a poster who caused an overheating / partial seizure issue with an 028 AV while he was "noodling" (i.e. bar buried) rounds. This was confirmed with IR temperature measurements and solved by opening up the high-end screw another 1/4-1/2 turn.

I was recently rebuilding an FS-120 with a brand new carb set to the Stihl Specs for that particular carb. With a polycut head it ran fine at these settings but ran into the ignition limiter making it impossible to tune the high-speed screw on the carburetor. To get it off the limiter I removed the blade guard and installed a mowing (line) head with the line set about 2" longer than the cutter on the guard would normally allow.

Under these conditions the trimmer was not on the ignition limiter and sounded pretty normal. Unfortunately, I don't have a tach to measure. The interesting thing is that at WOT the power output and head speed increased rather noticeably when I opened up the H-screw. This would tend to tell me that the engine was running lean before I made the adjustment. In fact, this is how Stihl tells you to set the carb in one of their manuals (I forget where I read it) minus the extra string length.

Both of these cases would tend to indicate that for optimum operation under load the carb should be set richer than the stock unloaded "burble" sound.

This sort of makes sense because at lower RPM the exhaust port is open longer which allows more of the incoming charge to escape before the piston closes the port thereby leaning out the mixture.

So at what RPM below the power band does this effect occur? Was noodling the 028 and longer string on the FS-120 too severe a test?

Would setting the high end richer help with horsepower and saw longevity on the smaller saws? I am not sure it would matter much on the larger saws as it would be much harder to overload them and get them out of the power band.

Your thoughts?


Thanks,

Steve

 

[Stihlbillie]

Ive been told its better to be a little on the rich side. Just like life....



-chimpboy

 

[tickhound93]

Tune a machine to the parameters you run it. Pull any brand new machine off the showroom and it will be lean. Same goes with 4 stroke. If it is not tuneable, something is wrong.

 

[Rockjock]

Steve running a longer line will lower the RPMs, increase the engine temperature and increase fuel consumption. The added temperature will cause the oil in the fuel to coke and carbon up. We see it often with landscapers.

an FS85 with the correct length of line will run at 8500 RPM's and has a spark plug temp of 431F
Same unit with a line just 2 inches longer runs at 8200 RPM's and has a spark plug temp of 504F

Tickhound we sell a great many trimmers and it would seem they are tuned correctly. ( Stihl ) randomly we will tach then and they are in spec and run well. If they were lean the 8 hour 6 day a week run times would kill far more then we see in for service.

* Info came from my Master Tech manual.

 

[Andyshine77]

The burble as you called it, is the engine misfiring, it has to do with the engine not being able to properly combust the charge properly. The settings listed in the manual are a basic guideline, as every engine runs differently. What you want is a slight 4 stroking or burble under light or no load, which should clear up as soon as you put any real load on the saw. With rev limiters things are a bit harder, a tach can be helpful, but tuning under load with a rev limited enginesis still the best method. Always start really rich and slowly lean the mixture out.

 

[tickhound93]

Steve running a longer line will lower the RPMs, increase the engine temperature and increase fuel consumption. The added temperature will cause the oil in the fuel to coke and carbon up. We see it often with landscapers.

an FS85 with the correct length of line will run at 8500 RPM's and has a spark plug temp of 431F
Same unit with a line just 2 inches longer runs at 8200 RPM's and has a spark plug temp of 504F

Tickhound we sell a great many trimmers and it would seem they are tuned correctly. ( Stihl ) randomly we will tach then and they are in spec and run well. If they were lean the 8 hour 6 day a week run times would kill far more then we see in for service.

* Info came from my Master Tech manual.

Steve running a longer line will lower the RPMs, increase the engine temperature and increase fuel consumption. The added temperature will cause the oil in the fuel to coke and carbon up. We see it often with landscapers.

an FS85 with the correct length of line will run at 8500 RPM's and has a spark plug temp of 431F
Same unit with a line just 2 inches longer runs at 8200 RPM's and has a spark plug temp of 504F

Tickhound we sell a great many trimmers and it would seem they are tuned correctly. ( Stihl ) randomly we will tach then and they are in spec and run well. If they were lean the 8 hour 6 day a week run times would kill far more then we see in for service.

* Info came from my Master Tech manual.

Good to know dealers are checking stuff over thoroughly. Most new saws I should say, seem to run lean out of the box. I can't speak of trimmers

 

[Chris-PA]

These carbs don't respond directly to load at all, they don't know anything about load. When the throttle is held at a fixed position, the fuel delivered is a function of air velocity only (they do not hold a set fuel/air mixture like other carbs). As the air velocity goes up, the mixture gets (much) richer, and eventually the engine misfires (4-strokes).

With a chainsaw running no load WOT the only resistance is the friction of the chain on the bar and the cooling fan, and these do not represent much of a load. When you tune at WOT the main thing that determines rpm is the terminally rich fuel mixture.

With a string trimmer or blower the spinning string or blower fan is a load that increases non-linearly, and eventually that load becomes the rpm limiting factor. It's a pretty different situation from a chainsaw, unless you can remove the trimmer head or fan.

 

[Rockjock]

Good to know dealers are checking stuff over thoroughly. Most new saws I should say, seem to run lean out of the box. I can't speak of trimmers

We start everything thing we sell before it goes out and will randomly tach them. I do not recall having anything running lean that we have sold. Now saying that just on Saturday there were a few units that were very hard to start up. but they are kept in a non heated room so it took a few extra pulls to get those units started. The only time I have ever heard about lean out of a box was on here about Echo products. Talking to our Echo rep he dismisses that notion as brand bashing.

 

[SteveSr]

Running a longer line will lower the RPMs, increase the engine temperature and increase fuel consumption. The added temperature will cause the oil in the fuel to coke and carbon up. We see it often with landscapers.

I bet the landscapers keep finding new and innovative ways to kill equipment!

An FS85 with the correct length of line will run at 8500 RPM's and has a spark plug temp of 431F
Same unit with a line just 2 inches longer runs at 8200 RPM's and has a spark plug temp of 504F

Thanks for the data points! Is this an experiment that you performed or was it Stihl? 300 RPM for a 17% increase in temperature. That curve must be as steep as a double black diamond!

We run a lot of 4137 series (FS-85) doing trail maintenance. We normally use polycut heads which are aero enough to hit the rev limiter. Should these carbs be set to Stihl factory spec considering that we are at sea level? Or should we temporarily install a line head with the correct line length and tune them for this?

Thanks,

Steve

 

[Rockjock]

I bet the landscapers keep finding new and innovative ways to kill equipment!


Thanks for the data points! Is this an experiment that you performed or was it Stihl? 300 RPM for a 17% increase in temperature. That curve must be as steep as a double black diamond!

We run a lot of 4137 series (FS-85) doing trail maintenance. We normally use polycut heads which are aero enough to hit the rev limiter. Should these carbs be set to Stihl factory spec considering that we are at sea level? Or should we temporarily install a line head with the correct line length and tune them for this?

Thanks,

Steve

It was not done by me but by Stihl, we get many trimmers in without the guard and with blocked spark screens. So I suppose they looked into it. From what I know we set the carbs with the line heads and make sure it is up to spec. Then if the customer uses a polycut head they should be good to go. 1 trick we do tell them is to soak the polycut inserts in water for a few days. ( or put them in a ziplock back with some water ) and it keeps them more supple and they last much longer. But I do think it a good idea to set them up with the line head. Pull the muffler and make sure the exhaust port is not clogged up with carbon and you are good to go. In the class they showed us pictures of the transfers coked up with carbon from a unit that was run with line that was too long. Good fuel, good mix and proper operation and I am sure they will perform well.

 

[bikemike]

We start everything thing we sell before it goes out and will randomly tach them. I do not recall having anything running lean that we have sold. Now saying that just on Saturday there were a few units that were very hard to start up. but they are kept in a non heated room so it took a few extra pulls to get those units started. The only time I have ever heard about lean out of a box was on here about Echo products. Talking to our Echo rep he dismisses that notion as brand bashing.

Echo is horrible about that but I line the product and price and engine quality. Factory tuning is not the product it's self but can be delt with easily

 

[SteveSr]

It was not done by me but by Stihl, we get many trimmers in without the guard and with blocked spark screens. So I suppose they looked into it.

So what do the spark screens look like under these conditions? Ash gray or coal black? I would think that the higher exhaust temperatures would burn the screens clean. I think I have actually cleaned a screen that wasn't too bad in an 1123 saw this way by running it WOT in a cut but not overloading it (kept the RPMs up).

 

[mountainlake]

Talking to our Echo rep he dismisses that notion as brand bashing

Your Echo rep is full of BS most ECHO saws come set lean to get by the EPA and so do other brands, I'd never run any brand 2 stroke without proper tuning before using it and when tuned even a little lean they make more heat and are gutless. Steve

 

[tickhound93]

We start everything thing we sell before it goes out and will randomly tach them. I do not recall having anything running lean that we have sold. Now saying that just on Saturday there were a few units that were very hard to start up. but they are kept in a non heated room so it took a few extra pulls to get those units started. The only time I have ever heard about lean out of a box was on here about Echo products. Talking to our Echo rep he dismisses that notion as brand bashing.

Maybe its just how I tune saws, I don't use a tach, just my two earballs. I tend to tune just a fuzz on thevrich side, my uncles brand new 311 was in my shop the other day for sharpening, and it sounded plenty lean, so I fattened it up a tad. Personal preference probably

 

[Rockjock]

So what do the spark screens look like under these conditions? Ash gray or coal black? I would think that the higher exhaust temperatures would burn the screens clean. I think I have actually cleaned a screen that wasn't too bad in an 1123 saw this way by running it WOT in a cut but not overloading it (kept the RPMs up).

Black, oily totally blocked. It reminds me tar.. Cleans up with a propane torch but that sludge is also in the engine as well.

 

[Rockjock]

Maybe its just how I tune saws, I don't use a tach, just my two earballs. I tend to tune just a fuzz on thevrich side, my uncles brand new 311 was in my shop the other day for sharpening, and it sounded plenty lean, so I fattened it up a tad. Personal preference probably

For us it is for Stihls benefit, they do run to fail tests and they deem those settings to be the best. I know it keeps the customers happy when the units are in tune, getting them to keep the deflectors on is another story. I know I can tune a pro saw much closer to spec than I can do with a homeowner saw.. Something about the harmonics for me.

 

[Rockjock]

Your Echo rep is full of BS most ECHO saws come set lean to get by the EPA and so do other brands, I'd never run any brand 2 stroke without proper tuning before using it and when tuned even a little lean they make more heat and are gutless. Steve

MM That is what he said to me when I asked about the 590.. he even called the lead tech and he poo pooed the idea that they are lean. We are a Echo dealer, but never get asked to bring in a saw so I can not say for sure. BUT I have seen echo saws come in, never touched and plenty of hours on the saws and they still run. Are the tuned perfectly no probably not Too many factors, temperature, humidity, air density.. .. are they going to blow up if run as is? Doubtful IMO.

 

[mountainlake]

are they going to blow up if run as is?

Yes they are going to blow, I've bought many off Ebay that were lean seized due to lean factory setting. After fixing the P@C pull the caps and adjust richer and they run good. Some might by with those lean settings in light use but the first long hard cut will melt them down. I believe your Echo reps told you that but they are full of BS. Steve

 

[Rockjock]

Yes they are going to blow, I've bought many off Ebay that were lean seized due to lean factory setting. After fixing the P@C pull the caps and adjust richer and they run good. Some might by with those lean settings in light use but the first long hard cut will melt them down. I believe your Echo reps told you that but they are full of BS. Steve

Steve I do not want to start a fight but I simply can not agree with you on this. WHY would they purposely tune them that way so they blow up and offer a 5 year warranty on them? I can understand 1 or 2 that slipped by the QC guy on a friday.. but to paint the whole line as such i think is just wrong. As I said i have repaired plenty of Echo saws and they all were well used, and ran just fine. I am in agreement they could run better ( that can be said with pretty much all the OPE out there ) with proper tuning but to purposely set them up for failure I can not believe.

 

[Chris-PA]

Steve I do not want to start a fight but I simply can not agree with you on this. WHY would they purposely tune them that way so they blow up and offer a 5 year warranty on them? I can understand 1 or 2 that slipped by the QC guy on a friday.. but to paint the whole line as such i think is just wrong. As I said i have repaired plenty of Echo saws and they all were well used, and ran just fine. I am in agreement they could run better ( that can be said with pretty much all the *** out there ) with proper tuning but to purposely set them up for failure I can not believe.

They are trying to meet emissions without the benefit of strato or feedback carbs. They do their best to make conventional porting work with careful transfer designs and port timing, but they're still left with the extremely non-linear fuel delivery of conventional all position carbs. All they can do about that is set them as lean as they can get away with, but that means no safety margin. That's a disaster waiting to happen because these carbs are totally incapable of providing a fuel/air mixture as air velocity (i.e. rpm) changes, not to mention temperature, altitude, etc. They were intended to run very rich at all points except max load WOT, where they would be merely just a little rich.

They didn't make the investment in technology development, or pay to use what others developed, and attempts to push the old technology further than it is capable of going result in reliability issues.