High Speed Carb Tuning.

[redunshee]

Looked at threads re carb tuning and have the following question. Tuned my saws starting at 1 turn open for low and high speed. As a general rule, my saws seem to cut best, with power, with high speed @ 3/4 open(or leaned 1/4turn). Am I safe at this setting or am I flirting with disaster? I guess the question is" How do you know when you've leaned it too much?" Thanks for your thoughts

 

[timberwolf]

You are flirting with disaster. One saw to the next, one carb to the next the setting in terms of turns is going to be different.

Tune by sound, read the plugs, or keep the saw bone stock and put a tach on it.

If you pull the plug right after a good run and quick shut down it should be nice and tan, white and it's lean. Should hear a nice burble at WOT no load that will clear once the saw is in the cut.

 

[DMShaver]

Depending on the saw model and manufacturer, 3/4 of a turn out may not be too lean. You need to obtain the correct RPM from a service manual and use a tach to set the high speed needle. Turn the needle out until the RPM's start to drop below desired, and turn back in just until the desired RPM is reached. If the the saw has a limited coil, it is easy to lean it out too much because once the RPM's get to the preset limit, no matter how lean you make the carb, they will never increase. If the saw does not have a limited coil, then leaning the carb at high speed will show an increase in RPM until the saw is so starved for fuel that the RPM will start to drop back off. Running the saw in this lean condition will cause engine failure.

 

[Jredsjeep]

so is the only way to properly do it with a tac? i know on other 4 stroke engines you turn in till it sputters and back till it sputters then put it right in the middle.

havent had much luck reading plugs as more than an indicator

 

[ShoerFast]

Want to be safe, the very best 'H' setting will be the fastest timed , full-bar cut in good solid wood.

Cutting cookies, have someone time the cuts from what yours very sure is too rich. turnning the 'H' screw in a little each cut, perfect will be when the saw dose not gain from leaning. (you might want a very slight adjustment out (same time in a cut) from making now further gain)

Running too fat (rich) will only cost you a plug every now and again, some saws will carbon up, but long after crapping the plug. Running too lean will take out the piston and jug (cylinder)

a good rule of thumb,

"Better the plug then the jug"​

 

[timberwolf]

No a tach is not the only way, in a lot of ways it is better to tune by ear and reading the plug. Example if you tune by tach on a warm day with 3 foot bar then run the next day in cold weather with a short bar RPM will be very different.

Tachs for tuning to a known reference point once you know where your saw runs best and are good too if you stricly follow manufacture specs and make no modifications saw or changes to fuel/oil ect.

Like is pointed out 3/4 of a turn might be fine on some saws, but without knowing what saws or even if they are stock it could well be too lean. Also if a jet is even slightly blocked 3/4 of a turn could be a disaster, much beter to not depend on estimating what fraction of a turn you are at.

 

[stihl only]

Tuning to max RPM isn't the way to go. If you must do it by ear, start lean, and as has been mentioned, open it up till is is running a bit rough. It should sound like a slight miss at WOT no load but when it is in the cut, it will run smooth.

A saw adjusted on the fat side won't sound as mean when you are running it WOT and not cutting, but fuel = power so fat is better and also much safer!

When I tach a saw, I almost always set it at least 500 RPM below MAX RPM.

 

[DMShaver]

so is the only way to properly do it with a tac? i know on other 4 stroke engines you turn in till it sputters and back till it sputters then put it right in the middle.

havent had much luck reading plugs as more than an indicator

With experience, you can get it close by ear, but to properly tune a carb you need a tach.

 

[timberwolf]

With experience, you can get it close by ear, but to properly tune a carb you need a tach.

Sorry I have to fully disagree, tuning to a set RPM plain makes no sense. Every bar and chain is a little different, be it 16 inch 20 or 25 it puts a different WOT load on the saw and that affects the RPM. Prove it to yourself, put a 16 inch bar and chain on, better yet make it a .325 or a lightened race chain then put a 20 inch bar on with 3/8 sprocket, then throw on a three footer with some .404 take note of the WOT RPM each time, don't touch the mixture.

Get the filter a little dusty or some carbon on the exhaust and the saw will turn a different RPM even if it was possible to give it the same air fuel mixture. Then no two saws are the same even same make and model will have slight variations. Keeping to factory spec with a aboslutly stock saw in A1 condition and you will be in the ball park for mixture. But that does not the ideal mixture.

If you learn to tune by ear and read the plug and do some homework on cut times you will do just fine and will consistently cut faster than someone who just sets it and forgets it. Where a tach comes in handy is once you know where a saw cuts best with the set up you use, then it's easy to keep track of it and set it to a known point.

Factory RPM applies only to stock saws in good condition and it is a conservative simplification. Likely done to keep dealer techs from blowing saws up.

If you hand a customer who knows nothing about tuning a saw one day with one bar and chain you would want to be sure it's still ok the next day with different bar and chain, fuel... ect. Thats where a conservative factory spec works on a stock saw. It's all fine and good if your not dealing with a modified saw or really don't care that much about performance.

 

[Termite]

Timber Wolf

I agree with you Timber Wolf. Two comments on plug reading. First, the saw should be shut down when under full load and wide open throttle. And the WOT. reading is the color at the base of the electrode. The color at the tip is the rich- lean condition at idle. This is the way I understand it. It is very difficult to see the electrode color at the base without a special optical tool.

 

[DMShaver]

Sorry I have to fully disagree, tuning to a set RPM plain makes no sense. Every bar and chain is a little different, be it 16 inch 20 or 25 it puts a different WOT load on the saw and that affects the RPM. Prove it to yourself, put a 16 inch bar and chain on, better yet make it a .325 or a lightened race chain then put a 20 inch bar on with 3/8 sprocket, then throw on a three footer with some .404 take note of the WOT RPM each time, don't touch the mixture.

Get the filter a little dusty or some carbon on the exhaust and the saw will turn a different RPM even if it was possible to give it the same air fuel mixture. Then no two saws are the same even same make and model will have slight variations. Keeping to factory spec with a aboslutly stock saw in A1 condition and you will be in the ball park for mixture. But that does not the ideal mixture.

If you learn to tune by ear and read the plug and do some homework on cut times you will do just fine and will consistently cut faster than someone who just sets it and forgets it. Where a tach comes in handy is once you know where a saw cuts best with the set up you use, then it's easy to keep track of it and set it to a known point.

Factory RPM applies only to stock saws in good condition and it is a conservative simplification. Likely done to keep dealer techs from blowing saws up.

If you hand a customer who knows nothing about tuning a saw one day with one bar and chain you would want to be sure it's still ok the next day with different bar and chain, fuel... ect. Thats where a conservative factory spec works on a stock saw. It's all fine and good if your not dealing with a modified saw or really don't care that much about performance.

I agree that drastic differences in bar lengths and chain sizes will SLIGHTLY affect WOT RPM. However, I believe most people on this forum are not concerned with setting speed records while cutting. Most are concerned with setting a carburetor so that their saw performs well, and doesn't burn up. I cut competitively and understand how to read a plug and tune for speed (I still routienly use a tach), but for the other 99% on this site that don't, using a tach for tuning is the BEST way to achieve optimum engine performance.

 

[timberwolf]

With experience, you can get it close by ear, but to properly tune a carb you need a tach.

Could be the way I read it, but to me sounded as if a tach was needed to properly tune a carb, which is not really true. It might be the best and easiest way for someone with limited experience I do agree. It is not the best way to achieve optimum engine performance though in any given senario or conditions.

You would be surprised just how much change bar length, gearing and chain makes on WOT RPM, esp on smaller higher reving saws.

 

[mattmc2003]

I work for the county road dept, and all our saws are adjusted pretty lean. We don't have a tach. We have old 032av stihls. 034, 039, 55 husky, 365, 262xp, 570. And some old homies. Never had any problems. I think if you are on borderline, you won't hurt it unless you just absolutley run the piss out of it for a long long time. And all kinds of folks run our stuff, and it ain't hurt. My 55 is pretty lean, and i cut with it friday, solid wide open cutting for a whole tank of gas, and it was fine. I did it by ear. And it didn't get overheated. I gassed it up and started right back at it, no problems.

 

[Nailsbeats]

Sorry I have to fully disagree, tuning to a set RPM plain makes no sense. Every bar and chain is a little different, be it 16 inch 20 or 25 it puts a different WOT load on the saw and that affects the RPM. Prove it to yourself, put a 16 inch bar and chain on, better yet make it a .325 or a lightened race chain then put a 20 inch bar on with 3/8 sprocket, then throw on a three footer with some .404 take note of the WOT RPM each time, don't touch the mixture.

Get the filter a little dusty or some carbon on the exhaust and the saw will turn a different RPM even if it was possible to give it the same air fuel mixture. Then no two saws are the same even same make and model will have slight variations. Keeping to factory spec with a aboslutly stock saw in A1 condition and you will be in the ball park for mixture. But that does not the ideal mixture.

If you learn to tune by ear and read the plug and do some homework on cut times you will do just fine and will consistently cut faster than someone who just sets it and forgets it. Where a tach comes in handy is once you know where a saw cuts best with the set up you use, then it's easy to keep track of it and set it to a known point.

Factory RPM applies only to stock saws in good condition and it is a conservative simplification. Likely done to keep dealer techs from blowing saws up.

If you hand a customer who knows nothing about tuning a saw one day with one bar and chain you would want to be sure it's still ok the next day with different bar and chain, fuel... ect. Thats where a conservative factory spec works on a stock saw. It's all fine and good if your not dealing with a modified saw or really don't care that much about performance.

+1

 

[MoreCowbell]

I stumbled onto this interesting thread from Nov. 2008 and not sure if it’s active or monitored anymore. I will see if this posts, and then come back with a couple questions/comments.

 

[MoreCowbell]

I see my post was made once I registered. New to this site …

What I get from this thread is that for virtually all chainsaws, the L & H jets meter fuel and turn CCW to increase flow.

I have a PoulonPro PR4016 which I bought in 2018. It has never run right on the high end. Used very little when I took it to a saw shop, they rebuilt the carb and supposedly reset the tuning. It’s still never been quite right. I can get it started but have to tickle the throttle trigger to get the RPMs up. Once I get there, it soon wants to die, as though (my impression) it’s starved for fuel.

Most unfortunately, the Owner’s Manual gives scant information on how to adjust the L jet, and nothing on the H jet !! I since got a splined tool for adjusting those jets. That shop told me that I should start with these settings:

L: 1½ turns out
H: 1 turn out

The comments on this thread mostly talk about keeping the H jet ≥ ¾ turn out.

When the saw was returned from the shop, they had set the H jet out at least 2½ turns !! (The L jet was opened 2¼ turns.) Right now, I have the H jet at 2 turns out and it runs fair at high RPM but not what I’d call ideal. I still have to tickle the trigger to maintain high no-load RPMs.

I realize it’s almost impossible for anyone to assess this on a thread without handling the saw but I thought I’d post this and see if there are any impressions, especially wrt the PR4016.

Are PoulanPro’s really hard to tune right?

Any useful comments are welcome.

 

[NSEric]

Those poulan pros are fussy to tune.
About 2 turns out is a starting point on those saws not 1 turn out like older saws.

 

[Vintage Engine Repairs]

It’s probably got a dependant carburetor. All the fuel flows through the H drilling before going to the L. In instances like this, you tune the H screw first, then the L. Often the H needs to be about 4 or more turns out.

 

[MoreCowbell]

Thanks for both responses.

To NSEric, I appreciate your confirming that the PoulanPros are tricky to tune right. I also appreciate your comment that the starting point is about 2 turns out. That fits with what I am seeing.

To Vintage Engine Repairs, I’m not sure about the “dependent” carb. Your comment that “all fuel flows through the H [jet] before going to the L” is interesting and actually makes sense. My shop guy advised that I try to set the H jet first, then move on to the L, which matches with what you said. It’s also interesting that you claim the H jet sometimes needs to be opened 4 turns.

I’m very familiar with engines - maintained and rebuild several small engines (4-cycle & 2-cycle). My experience with this Poulan is that to get it running right – especially on eth high end – is quirky. I appreciate any tips I can get here because a couple years ago, Husqvarna bought out Poulan and I just discovered the hard way that they no longer accept phone calls for customer support !!

 

[Vintage Engine Repairs]

If you want to test if it’s dependant, close the H screw fully while the machine is idling, then gently open the throttle a bit (not fully) and it will bog or die. It may even die at idle just closing the H. If it’s an independent carb, it won’t be affected. It’s not something many know. Don’t be mistaken either they are most often genuine walbro carbs on those machines. That doesn’t make them a good concept, but it does make them decently made.