Modding The MS261
- Thread starter Mastermind
- Start date
Help Support Arborist Forum:
Metals406
Granfodder Runningsaw
I can't be showing all my tricks. I will say that it's been Mooberized.
Like a Boss!
Terry Syd
Addicted to ArboristSite
Yeah, it's unfortunate that the Stihl design doesn't have the dual intake/strato function, the velocity in a bigger carb could be low for the idle and low speed circuit. Although I recall members modding 346s with 357 carbs which have a 17mm venturi.
The advantage of the Husky with the dual system is that they can take the extra area quite easily, heck, I am running the equivalent of a 19mm carb on my little 50cc strato. Perhaps the 261 could take the extra area with some tweaking of the metering lever height and POP.
You've already increased the intake/strato duration by dropping the jug, you may want to give a bigger carb a try before you increase the timing anymore. Of course, it's the owner's call if he was willing to pay for another carb.
The advantage of the Husky with the dual system is that they can take the extra area quite easily, heck, I am running the equivalent of a 19mm carb on my little 50cc strato. Perhaps the 261 could take the extra area with some tweaking of the metering lever height and POP.
You've already increased the intake/strato duration by dropping the jug, you may want to give a bigger carb a try before you increase the timing anymore. Of course, it's the owner's call if he was willing to pay for another carb.
Last edited:
Mastermind
Work Saw Specialist
The carb's linkage is a bit different.......a simple swap it would not be. I'll be looking at all my options though.
Bowtie
Gearhead
The carb's linkage is a bit different.......a simple swap it would not be. I'll be looking at all my options though.
I not trying to butt in, but I make my own linkages out of what I think they call "piano rod" from the local Ace Hardware. It is sold in different sizes and is very strong. Maybe I am late to that trick, but it has worked for me very well.
Mastermind
Work Saw Specialist
The 261 has a linkage that swivels and pushes the throttle around. I would have to swap the throttle shafts and also adapt the intellicarb metering cover from the 261 carb to the C3M.
Terry Syd
Addicted to ArboristSite
"I'll be looking at all my options though ."
That sounds cryptic, perhaps you may be looking at a bit more transfer time/area to make up for the bit you lost when you dropped the jug?
That sounds cryptic, perhaps you may be looking at a bit more transfer time/area to make up for the bit you lost when you dropped the jug?
chopperfreak2k1
Addicted to ArboristSite
this thread is a prime example of why AS is so great. amazing info in this thread so far!!
Mastermind
Work Saw Specialist
You lost me Terry?
The transfers seem to be very large in area compared to say an 026.
Terry Syd
Addicted to ArboristSite
You had to loose some time/area when you dropped the jug, the piston now covers up more of the port opening at BDC.
The engineers at Stihl designed the engine with a certain time/area of the transfers. If you were just dropping the rpm and going for torque, then a slight reduction in transfer time/area could work. However, if you are bumping up the cutting speed then you need to increase the time/area.
Here's what I wrote some time ago for those that missed it.
When you're looking at the maximum delivery ratio of the engine (max torque), the optimum time/area for a given RPM is when the flow from the crankcase just finishes getting into the cylinder and the transfer port closes.
Above that RPM there is less time available, so the flow doesn't have enough time and the port closes before all the mixture can get into the cylinder. That's when we see the torque start dropping, but the revs are still increasing so we still keep getting more work (horsepower) from the engine.
The opposite occurs when the revs drop below the maximum delivery point, the port stays open too long. The crankcase charge flows into the cylinder, but some of it gets sucked back out of the cylinder and down into the crankcase because the transfer port is still open while the piston is rising. We see that as a drop in torque as the revs drop.
So what I'm saying is that if you want to increase the cutting speed, you will have to increase the transfer time/area at least back to the stock time/area or even add a bit more.
As an aside, look what Husky has done to both the 555 and the 365XT, they have crimped the flow of the transfers to detune the engines. All they've done is moved the point of max torque (maximum delivery ratio of the transfers) lower in the RPM range.
The engineers at Stihl designed the engine with a certain time/area of the transfers. If you were just dropping the rpm and going for torque, then a slight reduction in transfer time/area could work. However, if you are bumping up the cutting speed then you need to increase the time/area.
Here's what I wrote some time ago for those that missed it.
When you're looking at the maximum delivery ratio of the engine (max torque), the optimum time/area for a given RPM is when the flow from the crankcase just finishes getting into the cylinder and the transfer port closes.
Above that RPM there is less time available, so the flow doesn't have enough time and the port closes before all the mixture can get into the cylinder. That's when we see the torque start dropping, but the revs are still increasing so we still keep getting more work (horsepower) from the engine.
The opposite occurs when the revs drop below the maximum delivery point, the port stays open too long. The crankcase charge flows into the cylinder, but some of it gets sucked back out of the cylinder and down into the crankcase because the transfer port is still open while the piston is rising. We see that as a drop in torque as the revs drop.
So what I'm saying is that if you want to increase the cutting speed, you will have to increase the transfer time/area at least back to the stock time/area or even add a bit more.
As an aside, look what Husky has done to both the 555 and the 365XT, they have crimped the flow of the transfers to detune the engines. All they've done is moved the point of max torque (maximum delivery ratio of the transfers) lower in the RPM range.
bcorradi
Arboristsite.com Sponsor
Your the same guy that was trying to index a flywheel without a key
...imagine that.Surely u can do it without a key, but it will take u a lot of trial and error.
Bowtie
Gearhead
Your the same guy that was trying to index a flywheel without a key
Surely u can do it without a key, but it will take u a lot of trial and error.
Aww gee, thanks for that.
I am well aware how to advance timing. I was asking the question to people who have actually done it on a 261!
Bowtie
Gearhead
Gotta love stratos! Got me curious, Im gonna have another look at my 261.
So far in this thread I've heard that this saw is no longer a strato due to mods...... So the gotta love stratos comments are kind of moot at this point when that feature needs to be disabled in order to get this power :hmm3grin2orange:
Mastermind
Work Saw Specialist
Good stuff as always Terry, I see what you mean now.
The comment I made about "all my options" was just that......I'll be looking at this saw now as a non-strato and raising the transfer ports is definitely on the table. I'll also most likely raise the exhaust a few more degrees.
I'll be felling another pine so I can get consistent cuts though. The hardwood is too inconsistent to make measurable timed cuts and gauge my progress in.
This saw will probably set my schedule back a few days but I'm committed to learning the proper way to build the 261 as a woodsported saw. :msp_mellow:
Go ahead and say what you really want to Tom............
When it comes to stratos......Husky rules.
Mastermind
Work Saw Specialist
Longer than I wanted to run it. :msp_thumbup:
Without changing the carb I doubt we'll lose too much of the fuel economy.
deye223
Addicted to ArboristSite
Go ahead and say what you really want to Tom............
When it comes to stratos......Husky rules.
:taped: Not me man, me gon nevah dun said them word right thurrr...
