Breathing Room

I opened up my chainsaw muffler ports today.  The MS290 has always been an OK saw, but it bogs down in full-bar cuts.  When doing heavy limbing and bucking work, speed is everything.  Long, slow cuts wear my arms out, especially on big undercuts.  Even with a brand new, nicely sharpened chain, I end up having to back off frequently as the saw bogs down.  I’m not ready to shell out $800-$1100 to buy a 70-75 CC class chainsaw that I’d like, so I have been looking around for an easy upgrade on my existing machine.

It seems that the only low cost and reasonably guaranteed upgrade is a muffler modification, so after reading a few writeups, I took advantage of an afternoon rain storm to spend some time in the garage today reworking the saw.

The factory muffler has 0.77 square inches of inlet area, but only 0.16 square inches of outlet area, about 15% of the inlet.  That’s a severe amount of restriction.

Original Muffler

Kinda pathetic, just those two itty bitty slots…

I used a drill and files to expand the two small oblong ports into one large port and I added a second exhaust at the top of the muffler.  The combined area of these two outlets is 85% of the inlet.  I left the internal baffle box in place since there were plenty of perforations for air flow.  The new vents are positioned to accommodate the factory spark arrestor screen.

Modified Muffler

Same muffler as above, but with the bigger ports and cleaned up with a wire brush.

I wear ear muffs, so the saw doesn’t seem much louder, but it does feel far more lively in my hands.  That makes sense, since it is no longer trying to exhale through a sippy straw.  I had to retune the high side of the carburetor to richen the fuel/air mixture because the engine is now pumping more air.  I really should pick up an inductive tachometer.  That would help me do a better tuning job by looking at RPMs rather than guessing by ear, but it feels (and smells) like I have a reasonably good tune on the high side.

The saw still bogs down with a bar buried in hickory, but it wears me out a little less than it used to.  There are anecdotes online of dyno testing yielding 20% horsepower gains.  I’m not sure what kind of power increase I’m getting, but if I feel like my saw isn’t letting me down, it won’t be as much of a discouragement to use it. The challenges from wrestling with inadequate tools often transform straightforward jobs into toil and trouble.  If I feel confident in a task, I usually do a better at it.  Even if my saw wouldn’t show any power gain on a dyno, if it sounds like it is doing a better job, I’ll do a better job.  Let me enjoy my fiction.

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