I'm new to the forum, but have been a pro lurker for a little while. I purchased a 590 last year to take down a problem tree. I've run saws in the past for firewood gathering, but they were all homeowner type, smaller displacement saws. This 590 was a big step up and I'm glad I did it.
Over time, I've modded it, using forum info and you tube videos, and the performance gains are very noticeable. Up to yesterday, the mods were:
1. Completely gutted muffler, but retained the top deflector and spark screen.
2. Plug high jet rev limiter hole w/JB Weld (I have a replacement solid jet from an 036? Stihl carb).
3. Compression release delete (netted 5 psi of compression).
On Friday, I decided to try my hand at porting. After getting my stock numbers (111* exh, 79* intake, 128ish* transfers, .037" squish. Exh duration: 138*, int duration: 158*, transfers: 102ish*). This is with a base gasket.
I pulled the saw apart yesterday, removed the BG, and rechecked my numbers. I lost 1*across the board, and squish came in at .026".
I'd planned on raising the exhaust roof to 107* (I'm sure you all can see where this is heading) , while leaving everything else alone, with the exception of the bottom of the transfer divider, which I angled to a peak to help with flow between transfers. I ended up getting a bit carried away on the exhaust roof, blowing right past 107* and landing at 102*after gluing the cylinder down with Motoseal. I also was very careful in my chamfer on the exhaust, as I've read horror stories of junking rings and pistons if you don't. Lastly, I widened the exhaust port to 1.15", which is 65% of bore, a well as polishing the runner.
So, at this point, I have the following numbers:
Exhaust duration:158*
Intake duration: 160*
Blowdown: 28*
I only know for sure what the exhaust timing is because it's the easiest one to check with the saw assembled (Oops, forgot to check the rest of the numbers post-work), and it was really the only one I was concerned about, since that was the only port I'd worked on. I just let the rest ride and hoped for the best.
I say all of this because I'd watched a popular channel of an experienced porter who was having all kinds of problems with the 590 he was working on. His numbers weren't anywhere close to mine, so it caused me concern when I saw my final numbers, especially the exhaust.
So, yesterday, I sent it. I filled the gas tank half way, pulled the choke on, yanked on the pull rope 4 or 5 times, and it sputtered. I took off the choke and chain brake, yanked it once more and it came to life! I ran a couple of heat cycles, still thinking something bad was about to happen, but it never did.
I've since run it in some wood, nothing big mind you, but it runs great! When I pull the trigger, it boogies. It's easy to tune and it seems to have quite a bit more pull than it used to have. At max rpm, and when I lean it out, the thing screams. I don't have a tach yet, but I'm thinking it in the upper 13k range, and definitely higher rpm than stock.
Lastly, I'll post some pictures of my "handiwork".
