Dolmar PS-420 owners: tune & tach question on your saw

[themaddhatter]

Background:

Last week I was running my PS420. Earlier this year I swapped out the cat muffler for the non-cat factory one, and did a tach-based retune.

At that time, I set the RPM ~ 2,800 rpm, adjusted the low speed based on chainsawr instructions (turn CW until runs bad; CCW until runs bad; set in midpoint between two). I also set the high speed at 12,500 rpm. Used it off & on in the spring for short periods.

Last week, was limbing some blow downs, and it was running smokey at idle. Then, took a break (~ half tank left) & sat in the shade for a while to cool down (was in the upper 90s & high humidity). Came back, and had hard start. Usually when it is warm, you flip it on, one tug and off you go. Couple pulls and it wouldn't catch, so I set the throttle lock, gave it a couple pulls, and it would start on high, then die when the throttle was blipped. Didn't want to spend the rest of the time futzing with it, so I finished limbing with it's big brother (6401).

Did some research on AS, and went to the garage today to get it squared up:

- Pulled the plug, and one part of the plug was oily (telling me it was loading up correct?)
- Opened the muffler, and it was black sooty inside (again, loading up)
- Piston looked fine
- Changed air filter
- Changed plug
- Swapped out gas for fresh blended batch (91 no ETOH bought 3 weeks ago) with synthetic 2-stroke oil w/ stabilizer (50:1)

I had read comments from bsnelling about tuning in wood, and a comment from jus2fat on tuning more so by "feel". Following a hybrid of the two, here is what I did to the carb:

- Brought saw up to temp
- Found idle where saw seemed to run smooth w/ a tiny amount of chain creep (tach said ~ 2,700 rpm)
- Turned the low speed screw in quite a bit (about half turn) while checking throttle response (blip throttle). Found spot where saw didn't smoke & was not bogging on throttle blip.
- Mash the throttle, and it makes the "gravelly 4-stroke" coil limiting sound (at least as it sounds to me, as I have hearing issues)
- Buried in wood, was leaning it out, and it kept losing power. After futzing with it, decided to go richer than starting point. Noted power picking up. Kept going up to where the saw seemed to pull well (much better than earlier)
- Checking it with tach, saw hits ~ 12,600 unloaded; ~ 10,800 while pulling.
- Shut it down, easily restarts with 1 pull and idles well (tried several times after idle periods and WOT periods)

My questions are this:

-Does my carb adjustments seem reasonable? Or am I looking for trouble following the described practice?
-Dolmar specs idle @ 2,800 rpm. I am assuming me being at 2,700 isn't that big of deal, correct?
-Dolmar also specs 13,500 RPM for top RPM. With me being @ 12,600, should I be pushing to get that number closer? Would I just richen it up further?
-Does ~ 2,000 RPM drop in the wood seem right? I was letting the saw pull, not leaning on it, so it was doing the work on it's own.

Any feedback is appreciated.

 

[Fedaburger]

All sounds good to me. I have to tune by ear because I have no tach. If the tach is saying your rpm's are low according to specs but your tunings making the saw run right. I would stick with what u got better a tad rich than a tad lean atleast thats what I've learned from these guys (AS members are pretty fart smellers) and my own trials and errors.

 

[battlefordguy]

opcorn:
Sounds like a good practice to me, but i am no expert.
I completed a muffler mod on my 420, set the carb with a tach around 13000.
havent touched it since or had any issues.
However learning a better tuning method is always good.

 

[blsnelling]

Why so rich? You're not doing your saw a favor by running it so rich. Replace the plug and retune to the factory spec.

 

[themaddhatter]

Why so rich? You're not doing your saw a favor by running it so rich. Replace the plug and retune to the factory spec.

That's what I am asking here :msp_wink:

Plug is brand new (just swapped it and new filter at beginning of tune job)

By "rich" are you saying to lean out the high? Just turn it CW some more? The spec is 13.5K, so should I try to hit that?

Am happy with what the throttle response is doing, so it "feels" as if the low is doing what it should (plus no blue smoke puffs during blips)

I was going in more, just didn't know how close it should get to the 13,500 mark. Saw has about 6 or 7 tanks though it, so it should be broken in.

Is the 100 rpm on the idle any issue? The tach does do a little dancing around, so you eyeball the high/low and split the difference to get the 2700 value.

I really wish I could distinguish the rev limiter & 4-stroking sound better. Damn sensorineural hearing loss mixed with tinnitus from various reasons (manufacturing equipment/living with a band in college/firearms) means I can "hear" just not "process" what I am getting very well. Between that and my wife having conductive hearing loss & not wearing her hearing aides EVER, we sound like an old couple to the outside observer "heh? What? huh?" We joke about it when we flake out on each other, and one of us will look at the other and say "THE BLESSING" (love Christmas Vacation).

Will dink around some more tomorrow in a cut.

 

[Steve NW WI]

My 420 can be finicky at times, it's the only one of my 3 Dolmars that needs adjusting often. I shoot for 13K even on the tach at WOT, low idle speed just below clutch cut in (no creep), and low screw set for best throttle response. Sometimes I need to go back and forth between h & l jets as I'm tuning to get where I want to be.

Guys like Brad who are looking for every ounce of perfomance will probably still think my 13,000 is too low, but I'm a firewood cutter who could care less about 1/2 second difference in the cut time, and I like to stay a little on the cautious side.

I have mild to medium hearing issues from mostly the same stuff as you, working in metal stamping, guns, lots of loud toys, etc. For those of ya out there without hearing loss, use hearing protection, every time. Once it's gone, it ain't coming back.

 

[SawTroll]

12,500/12,600 sounds awfully rich to me.....

 

[blsnelling]

13,500 is where the saw was designed to operate, as a work saw. Just check it regularly and make sure it doesn't go much higher. If you're on the limiter, it'll read erratically and lower. Start rich and bring it up.

 

[Fitchnhunt]

Thinking about cutting some wood this weekend......still haven't used this new PS-420. Anyone have some good "instructions" on what I need to do to remove the cat from the muffler? Also, if I do go about this, will the saw need to be re-tuned? I have no tools or know-how for tuning. Thanks!

 

[166]

The 420's run best around the 12,800 - 13,000 rpm range. When the rev limiter kicks in on a 420 you will know it because it cuts the ignition off for a longer time than the 7900/5100 style rev limiters.