Time to get familiar with Dolkitas... picked up a 6401 today.

See, that's what I want to hear!

As to what they're worth, this one with an aftermarket big bore kit went for $525 (i.e. the seller cleared about $457), and mine's probably in better shape.

 

Jon any chance you sanded the high spots off the piston - even though it is useless - just to see if there is any evidence of a chip missing in the path of the snowballed aluminum?

 

I haven't yet, but I can try that.

 

This is probably fairly meaningless, but I found several IPL's spanning the many years of the 6401's production, with three different part numbers for the OEM 7900 decomp cylinder kit. 038-130-030 seems to be the oldest. That one seems to have been superseded by 038 130 031, which is the PN most often mentioned on AS. Most recently (on an IPL dated 2012) the number is 038 130 070. I've found suppliers offering all three versions, but the cheapest (by a small margin) is the last one, which came from an IPL produced about 4 years after my saw was built. I'll be amazed if anyone here has solid info about this, but feel free if you'd care to venture a guess as to what the differences might be.

 

Not sure on the cylinder, but the latest upgrade from Dolmar was a slab sided piston.

 

Okay... why is that an upgrade? I know nothing about piston design.

 

They produced more power... As far as why? Lotsa theory out there,(less turbulence, better Case compression, better transfer velocity) so don't get me lying to ya... Mastermind may be able to shed some "intelligent" light on this particular piston...

 

It's HEEEeeeere. A general-purpose tool parts dealer in Utah had a good price (about $205 shipped) on the most recent version of the OEM 7900 piston and cylinder. It arrived with this morning's mail. Can you guess what I'll be doing this weekend?

Besides being bigger, the 7900 cylinder is closed port whereas the 6401 is open. Also, this being the first OEM top end kit I've ever purchased, I was happily surprised to find that the piston ring and one of the wrist pin spring clips were already installed in the piston for me, and there were two other clips in the package just in case I send one shooting across the room.

 

Started fiddling with it last night, thinking maybe I'd just bolt the top end on, but I kept going until the saw was fully assembled. I figured I'd better take a few pics while it's still clean, because I'm likely to get it a bit dusty when I get home from work this afternoon unless the carb is clogged up somehow. Here it is -- my DCS7901, née 6401.



As another indicator of how many miles this thing has on it, here's a shot of the original starter cord:

 

Well, the saw started right up and I did some initial tuning on Friday afternoon. I spent a couple of minutes fiddling around with the little orange screwdriver and wondering why I couldn't get the saw to stop 4-stroking even with the H screw all the way in, and then I realized I'd neglected to fully open the choke. Where's the facepalm smiley?

Yesterday I answered a nearby free firewood ad from someone who had big chunks of black locust trunk that still needed to be cut into manageable pieces, and that gave me the chance to take the saw for a spin. During the first cut it kept 4-stroking under load so I leaned it out a bit, and away we went. I can't make a comparison to the 64cc version since I never got to run it that way, but this is really nice as a 79cc saw. I only brought home a little of the locust because my stacks are essentially full for the season, but the homeowner was appreciative anyhow and invited me to come back next time I need wood because he has several downed smaller locusts in the wooded area just off the back yard.

Later in the day I restarted the saw to cut a couple pieces of wood for a neighbor's bonfire. Having cooled off for an hour or so, it started just as easily this time, but until it warmed up for several seconds it wanted to die when I grabbed the throttle, so there's still some fine-tuning to be done. Does anyone know of a troubleshooting chart that might explain the way saws (or 2-stroke motors in general) respond to various slight carburetor maladjuststments? I'm getting to the point where I can tune the high speed reasonably well by ear, but the low-speed adjustment is still sometimes a bit of a challenge. I'm wondering what happens, and why, when the low-speed is set too lean or too rich in various degrees. Adjusting for good throttle response seems to get me in the ballpark, but some saws (like this one) seem to have good response over a surprisingly wide range of L screw positions and I'd like to be able to dial things in more precisely. I'm assuming there's a 'just right' setting that affects more than throttle response, e.g. makes starting a little easier.

 

Looks like you did an awesome job on that saw! Wish you would have had it at the GTG I would have loved to try running it!

 

Thanks! I can't take too much credit here; aside from the P&C and some accumulated surface dirt the saw was in good shape to begin with, and the conversion to 7900 could hardly be simpler. Makita/Dolmar uses the same parts list for the 64, 73 and 79cc versions of this saw. I scanned through the whole IPL, and the only differences I found between the models were the piston, the cylinder, and the sticker on the recoil cover.

 

You do this just to ensure that it won't accumulate unburnt fuel inside the engine and stall?

 

[quote="MasterMech, post: 106279, member: 4"A little rich on the L will not hurt it at all so long as it... doesn't die when you release the trigger.[/quote]

Missed this earlier. Why would having the low end set too rich cause it to die when releasing the trigger? Does it cause a sort of instantaneous flooding as the throttle butterfly slams shut?

 

Does it "fall on it's face" in exactly the same way whether the L adjustment is rich or lean?

I want to go rev up a saw and start playing with the adjustments, but it's Sunday in the suburbs. My neighbors would hate me.