Chainsaw Carb Tuning Question - Max RPM vs 4 Stroke Sound

[Zack R]

For those of you who tune and adjust your own chainsaws how close are you setting the high side to max RPM? What I'm finding is that the blubber sound (4 stroke) occurs way below the specified maximum RPM.

For example on my Husqvarna 365 Special (older style with the Zama carb) the maximum RPM specified is 12,500. I could hear an obvious blubber at 10,500 but at the current tune of 11,500 I can no longer hear it. Saw started with one pull (choked, fully cold 42F outside), idles correctly at 2700rpm and accelerates well. Spark plug is brand new, air filter just cleaned, carb just disassembled and cleaned, using ethanol free premium fuel with Stihl Ultra 50:1 mix). Some blue smoke on startup but after warmed up it clears out.

Should I be tuning closer to where I hear just a little blubber (full throttle no load) or closer to the maximum specified RPM. I don't want to leave any power on the table but also don't want to run too lean up top.

 

[VirginiaIron]

How are you determining your rpms? Do you have a tachometer indicating the actual rpms?

 

[DodgyNomad]

I tune by ear pretty much the same way. Saw warmed up, then set idle, then the low speed fuel adjustment, then idle again, then high adjust until she just screams then back it off quickly until it just starts to break up-(4 strokes), then I give it one more itty bitty 1/32 or so turn backing it off a touch more additionally for safety. I find that gets me great power in the cut, but doesn't run it too lean. Then I will check the idle, low and high transition again, then try it in wood. I think that's a safe way to do it.

I feel I'm better at it by ear than I am using a tach, but maybe because I'm just comfortable at it.

I will check it in the wood and make a final adjust that way too, but it's usually a very tiny adjust if any, and I'd rather have a buddy running the saw in the wood while I do the final check, as I don't like running the saw one-handed. Then I check it again out of the wood.

I also try not to free rev it any longer than necessary, and try to just touch the maximum RPM's momentarily before backing it off to 4 stroking of course.

 

[Zack R]

I tune by ear pretty much the same way. Saw warmed up, then set idle, then the low speed fuel adjustment, then idle again, then high adjust until she just screams then back it off quickly until it just starts to break up-(4 strokes), then I give it one more itty bitty 1/32 or so turn backing it off a touch more additionally for safety. I find that gets me great power in the cut, but doesn't run it too lean. Then I will check the idle, low and high transition again, then try it in wood. I think that's a safe way to do it.

I feel I'm better at it by ear than I am using a tach, but maybe because I'm just comfortable at it.

I will check it in the wood and make a final adjust that way too, but it's usually a very tiny adjust if any, and I'd rather have a buddy running the saw in the wood while I do the final check, as I don't like running the saw one-handed. Then I check it again out of the wood.

I also try not to free rev it any longer than necessary, and try to just touch the maximum RPM's momentarily before backing it off to 4 stroking of course.

This sounds like a good approach. Since I'm new to this I have a tachometer to help match sound to RPM and to see how things change with each minor adjustment of the settings. The reason behind my question is that the manual specifically recommends the 4 stroke method in the carb tuning section, however it also lists a maximum RPM. I thought these would land a bit closer together than they currently do but to play it safe I'll wind it out until it just starts to scream (assuming its below the max RPM) and then richen it up just enough to barely hear the 4 stroke. Test cut in the wood to see if the 4 stroke cleans up and see how the power feels.

 

[DodgyNomad]

You're doing it the right way. I Can't improve on the scientific and proven tuning you've mentioned.

My only suggestion is when I'm tuning a saw, I make darn sure that I'm a touch rich on oil, and using good stuff, (44:1) and like yourself, using very good fuel, with a clean muffler screen, and clean air filter.