Chainsaw bar size vrs rpm

[Timbercruiser]

I was told by my local jonsered dealer that the rpm ratings are with a 16 inch bar . And when you put a longer bar lenght you reduce your rpms ?  Like a 266 husqvarna was rated at 12 500 rpm and he said to put it to 11 800 rpm with a 18 inch bar.  Any thoughts from chainsaw guys out there ?

[bigbuckhughes]

I bought a Husky rancher and put a 20 inch bar on it.  OEM was an 18 incher.  Was not told anything about rpm reduction, but it makes sense.  But then, to a rookie wood cutter, does it matter?

[ZeroJunk]

700 RPM difference in no load tuning between bars that are two inches different in length is curious to me. Are you cutting the same size wood ? Those no load RPM settings are a place to start or a place to set a saw you are going to sell and don't know the operating habits of the end user or his elevation, gas choice, etc. I usually set them a little richer than called for. If you are going to use the saw yourself make some timed cuts with it and wherever it cuts the fastest will be as close to in tune as you are going to get. No load RPM doesn't tell you a whole lot.

[JohnG28]

It would make some sense that with more load, in this case additional chain to pull around the bar, that some rpm would be lost if not retuned. As was said though, this will make no real difference cutting. Get it running well in the cut is key.

[Andyshine77]

There will be no noticeable difference in rpm between a 16" and 18" bar and chain. The factory listed rpm is a basic starting point, not a hard number. Each saw will reach proper air fuel mixture at a different rpm, and that's what we're looking for, A proper air fuel ratio, not a specific rpm number.

Tuning a saw for the fastest cutting time, is not something I would advise. It will work fine for a few cuts, but would likely be too lean for all day work. The longer a saw is worked, the hotter it will get. Which means the engine will need more fuel to keep it cool.

[ZeroJunk]

Yep, you will get in trouble anytime you tune a saw before it gets up to operating temperature. Plus it takes a little time for the crankcase mixture to settle down from whatever you dumped in there starting it. Everybody I know is going to fudge some on the rich side unless they are making a youtube video or  in a cookie cutting contest. Otherwise there is not much point in running on the edge.

[ladylake]

 Mine are tuned a bit on the rich side also as they don't bog as easy as when tuned on the edge of being lean. Steve

[Timbercruiser]

well see now im having a hard time trying to figure out if its set right . I use the tach to check them out now in confused . :D

[ZeroJunk]

If it was mine I would cut with it for a while to make sure it was good and warm, set it at 12,000 and use it with 16 or 18 inch bar. It will probably 4 stroke no load at 12,500 but you would be hard pressed to notice any difference working and you don't have to worry about it creeping up on you.

[Andyshine77]

Once again, I'll go ahead and repeat myself. Every saw will reach proper air fuel mixture at a different rpm. The manufacture recommend max rpm is merely a basic reference. Ideally the saw will four stroke a little without load. I have a couple videos demonstrating how how to do just that.

[ZeroJunk]

The saw manufacturer is not going to recommend a RPM that will burn their saws. Trying to learn what a proper tuned saw sounds like on a video may not work out so well, or at best not get you any closer than just using a tach to start with. Timed cuts and tachs are objective. What a saw sounds like to you is subjective. It won't make a hill of beans in the scheme of things if the saw is not tuned perfectly as long as it is not on the lean side.

[CTYank]

I've seen it happen TWICE that dealer "techs" adjusted the carb on my 40 cc RedMax only with a tach. Their results were consistent at least. Way off, stinking rich, enough to make your eyes water in seconds, able to empty the fuel tank in half the original time. Cranked (H) adjuster in about a full turn, and now getting clean 2-stroking morphing into light 4-stroking on lifting. Arrogant Idiots.

As others have said here, adjusting mixture by tach is a kludge, at best a quick, cheap & dirty to get an approimation of a high-speed mixture. Check out how "auto-tune" etc. do it- constant incremental mixture changes seeking max speed under load. That should work out to an AFR of about 12.5/1.

To insist that setting by tach is the only way is nonsense, IMHO. Keep your spark-arrester screen in place for the glowing chunks of carbon you may broadcast if you go that route.

[ZeroJunk]

Nobody said it was the only way, but just because you ran in to trouble doesn't mean that the other million people who had their saw tuned with a tach did.

And, if you give the same saw to several different individuals to tune by ear you will get tuning all over the place.

[Andyshine77]

I've seen it happen TWICE that dealer "techs" adjusted the carb on my 40 cc RedMax only with a tach. Their results were consistent at least. Way off, stinking rich, enough to make your eyes water in seconds, able to empty the fuel tank in half the original time. Cranked (H) adjuster in about a full turn, and now getting clean 2-stroking morphing into light 4-stroking on lifting. Arrogant Idiots.

As others have said here, adjusting mixture by tach is a kludge, at best a quick, cheap & dirty to get an approimation of a high-speed mixture. Check out how "auto-tune" etc. do it- constant incremental mixture changes seeking max speed under load. That should work out to an AFR of about 12.5/1.

To insist that setting by tach is the only way is nonsense, IMHO. Keep your spark-arrester screen in place for the glowing chunks of carbon you may broadcast if you go that route.

Correct. With the EPA involved the factory settings are now normally too lean.

The sound of a saw four stroking, really isn't subjective, it's a distinctive sound. On non rev limited saws, myself and most any experienced Sawyer or two cycle mechanic will be able to tune a warm saw, spot on without the aid of a tac. rev limited saws are a bit tricky. When the limiter kicks in, it can sound similar to four stroking, so tuning under load is advised. But what do I know.😉

[ZeroJunk]

myself and most any experienced Sawyer or two cycle mechanic will be able to tune a warm saw, spot on

Yep. And normally the person asking the question ain't that or he wouldn't be asking.

[JohnG28]

Ctyank, I know you are saying that they tuned your saw way rich, but I would take that over a way lean tuned saw out of the shop. Not best for operating long term, but less chance of burning up at least.

[Andyshine77]

myself and most any experienced Sawyer or two cycle mechanic will be able to tune a warm saw, spot on

Yep. And normally the person asking the question ain't that or he wouldn't be asking.

My philosophy is to learn how to do things correctly, instead of sticking my head in the sand and never learning anything new. With our help, the aid of video and sound clips, one can at least start practicing and learning, how a saw sounds and behaves with different air/fuel ratios. It's a learning process no doubt. Yes tuning rich if you're still unsure is the safest option.   

[Ianab]

Most important is probably to realise what a lean saw sounds / acts like. Then when it eventually happens to you, you know to STOP, and work out what's wrong. Then a hole in a fuel line is a minor repair. You don't try and finish that job, and melt the saw.

Ian

[JohnG28]

http://www.madsens1.com/saw_carb_tune.htm

This is where I got started learning what to listen for. Wear some good headphones or listen on good speakers and you can get a good idea of what to listen for tuning a saw.

[Al_Smith]

FWIW I know a good many really good builders but I don't know a one who use a tachometer as the final tuning of a saw engine .They all use the the good old ears .Now they might use a tach as a reference.

RPM's in air is just noise.RPM's under load is what counts . ;)

[ladylake]

Good advice on learning how to tune yourself, I've got saws in here just tuned by  dealers  both pig rich and burn up lean.   Steve

[ZeroJunk]

FWIW I know a good many really good builders but I don't know a one who use a tachometer as the final tuning of a saw engine .They all use the the good old ears .Now they might use a tach as a reference.

RPM's in air is just noise.RPM's under load is what counts . ;)

That may well be the case. And, that is sort of the way some others are looking at this. But, 99+ % of the people out there have never even rebuilt a stock saw, or even a carburetor. And, they are not tree service or loggers.  IMO telling everybody who asks an RPM question on a forum that they need to ditch the tack and tune by ear because that is what saw builders do is hazardous to their saw.

[mad murdock]

If you really want to get down to the minutia of the matter, the EGT is a better indicator of if the saw is too rich or lean, rather than just living or dying by the tach. Mechanical things are never exactly the same, you take 2 saws of the exact same brand and model, and they won't be exactly the same in how they run. Manufacturing tolerances just aren't that exact, too many variables. They will be close but not the same. If one is so worried about tuning too lean, read the exhaust temp. The proper fuel/air mixture will help keep things cooled down. FWIW, I vote for ear tuning as well. Same way I adjust timing on a distributor type ignition on a car.  Never use a timing light.