I have an MS290 that is now out of warrenty. It has been a good saw, but I have been frusterated with the lack of power when tackling big wood. Today I modded the muffler in hope of correcting some of this. I know that I need to retune the carb, but am not sure where to start because I assume that the manual starting points will no longer be valid. Im I correct? Please help me get started.
Post muffler mod tuning
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watsonr
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fearofpavement
Trying them all
Assuming the saw was correctly tuned when you started, the H will need to be turned slightly counterclockwise. And then tune from there. If you don't give it the additional fuel it will be running too lean.
Chris-PA
Where the Wild Things Are
I've been thinking about this issues of needing to enrich the mixture after a muffler mod. Now, first off, one should be prepared to adjust the mixture at any time with these carbs, and certainly any time you know you made a change of any kind.
BUT......
More airflow means a richer mixture, that's what "4-stroking" is all about. So all other things being the same, a modified muffler should increase the airflow, and increasing the airflow gives you a richer mixture.
BUT......
More airflow means a richer mixture, that's what "4-stroking" is all about. So all other things being the same, a modified muffler should increase the airflow, and increasing the airflow gives you a richer mixture.
Nitroman
Addicted to ArboristSite
Exactly backwards. The increase in airflow without an increase in fuel leaves the saw lean. Must increase the fuel to take advantage of the increased air.
GASoline71
Mr. Nice Guy
To correct the lack of power properly... you need to buy a bigger saw for bigger wood.
Gary
Let as assume that a new saw is not in the budget. This saw is used for firewood and a little ranch work so having a bunch of saws just does not make good sence. Before the muffler mod, the saw was starting and running pretty good. I did the mod, cleaned the air filter and set H and L one turn out to set up for a new carberator tune. Now the silly thing will not even pop. Absolutely no life at all. I am going to open L and H another turn and see if that makes a diference. I'll let you know. Any other suggestions?
Saw got wet
I forgot to mention. Last week, this saw was used to cut firewood for a camping trip. It was left on the wood pile over night and we had an unforcasted rain storm that was unreal. No one remembered to bring the saw in and it got a significant soaking. I have not tried to use the saw since then so I do not know if the water was a roblem. I drained and replaced the fuel just in case, but it did not help. Can you think of anything else that I should try?
I forgot to mention. Last week, this saw was used to cut firewood for a camping trip. It was left on the wood pile over night and we had an unforcasted rain storm that was unreal. No one remembered to bring the saw in and it got a significant soaking. I have not tried to use the saw since then so I do not know if the water was a roblem. I drained and replaced the fuel just in case, but it did not help. Can you think of anything else that I should try?
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Chris-PA
Where the Wild Things Are
You need to study how carbs work - this is the point I've been making lately, that these carbs cannot provide a constant fuel/air ratio with varying load. They lack an air bleed on the jet, so the fuel flow is the square of the air flow. That's what 4-stroking is - missfire due to a massively rich mixture.
You reduce the load, rpms and airflow increase, the fuel flow increases much more than proportional, and it misfires.
The effect should be the same here - the increased flow increases airflow through the carb, and it picks up more than a proportional amount of fuel.
I've done 6 muffler mods now to all of my saws, and I've adjusted the mixture afterwards - but it's never really been too lean. And it shouldn't be either, it's just another old wives tale.
madpogue
ArboristSite Member
Have you checked for spark?
Not yet. I wanted to but I can not find any metal portion of the saw that I can hold the plug against to see a spark. Instead, I dribbled a small amount of fuel into the cylander and turned it over. No life at all. So, at this point, it appears that there is no spark but I do not know that for sure. What is the easiest way to pull the plug, attach the wire and see if the plug works? Again, the only metal that the plug can reach is the plug hole but if I put it back there, I cannot see if there is a spark.
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kodiakyardboy
ArboristSite Operative
Have a friend stick a screwdriver on the spark plug wire and give it a good pull. If your friend jumps a foot off the ground it has good spark.
Nitroman
Addicted to ArboristSite
I won't argue. You may want to go to Wiki and study the Venturi effect to learn just how this is all occurring.
I used a jumper cable, one end on the spark plug and the other to a piece of exposed metal. Spark plug is fine. The plug was wet so I think I am flooding the thing.
Chris-PA
Where the Wild Things Are
Umm, yeah - there are better sources but that will give you the basics. You might want to start with the Wikipedia entry on carburettors.
"The main disadvantage of basing a carburetor's operation on Bernoulli's principle is that, being a fluid dynamic device, the pressure reduction in a venturi tends to be proportional to the square of the intake air speed. The fuel jets are much smaller and limited mainly by viscosity, so that the fuel flow tends to be proportional to the pressure difference. So jets sized for full power tend to starve the engine at lower speed and part throttle. Most commonly this has been corrected by using multiple jets. In SU and other movable jet carburetors, it was corrected by varying the jet size."
This is not entirely correct, as air bleeds are what is typically used to correct this problem in fixed jet carbs. I can't be the only one here who's tuned a Weber on a race motor.
Anyway, '4-stroking" is a terrible term to describe a simple rich mixture misfire caused by an increase in airflow when the load is reduced. So we all know that increased flow through the carb causes a rich mixture, because we've all experienced it every time we hear a saw "4-stroke". It is purely a function of the carburettor. Increasing the flow through the engine and carb will also cause a richer mixture - it has to.
fearofpavement
Trying them all
carbs pick up fuel based on the velocity of air going through the carb venturi. That is why aircraft have mixture controls. You have to reduce the amount of fuel going in at high altitudes or the engine will be running very rich. It will also make only a fraction of its horsepower rating at high altitudes.
That said, when a chainsaw gets muffler modded, it is hopefully making more horsepower. That more horsepower requires more fuel. If it was just an rpm increase, the carb would pick up more fuel because the air velocity would be increased. But RPM doesn't need to rise to develop more power. I guess I will make notes next time this situation arises.
That said, when a chainsaw gets muffler modded, it is hopefully making more horsepower. That more horsepower requires more fuel. If it was just an rpm increase, the carb would pick up more fuel because the air velocity would be increased. But RPM doesn't need to rise to develop more power. I guess I will make notes next time this situation arises.
Chris-PA
Where the Wild Things Are
Forget the rpm for a moment then - it's an increase in airflow, or there would be no point in opening up the muffler because that's all it accomplishes. And fuel flow is based on airflow squared. So more airflow means a richer fuel/air mixture.
Now as to rpm, well no one expects to run at a lower rpm after a MM, and it's at WOT and the displacement has not changed, so at least it's flowing more air at the same rpm.
Beat me to it!
I wouldn't run one stock and expect to produce efficiently with anything over an 18" bar.
one.man.band
ArboristSite Guru
glad to hear you got the problem fixed.
agree!!
don't have a stihl, but the walbro carb on my husqvarna has an air bleed. air bleeds, just adjust the point where idle/main transition take place is all i know. this is based on air velocity. air flow is a relative term i guess, based on total volume.
is the richer (fuel) adjustment necessary based on?
a. air velocity
b. air flow (volume)
c. the altering (opening up) of the muffler which lessens the 'supercharging' effect of the back-wave as shown at the bottom of page here:
2 STROKE Guide to Wisdom
i say "c".....and a very sharp chain.
-OMB
agree!!
don't have a stihl, but the walbro carb on my husqvarna has an air bleed. air bleeds, just adjust the point where idle/main transition take place is all i know. this is based on air velocity. air flow is a relative term i guess, based on total volume.
is the richer (fuel) adjustment necessary based on?
a. air velocity
b. air flow (volume)
c. the altering (opening up) of the muffler which lessens the 'supercharging' effect of the back-wave as shown at the bottom of page here:
2 STROKE Guide to Wisdom
i say "c".....and a very sharp chain.
-OMB
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