Sportfury70
ArboristSite Operative
I don't need a Cummins either, but WANT is a powerful motivator..
Sportfury's DCS6401 Build
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Sportfury70
ArboristSite Operative
Was there any run of these that didn't come with a limited coil?
I ask because I finally had time to fine tune it today after the HD kit while noodling some big maple rounds I've been meaning to split. WOT it just seemed to keep climbing as I closed the H screw; it just never seemed to "faux 4stroke". I ended up backing the screw out till it 4stroked under load (in the wood) and then tightened it till it juuuust stopped burbling in the wood.
Another thing, although it's only about 5ccs larger than the CS590, it's MUCH more powerful.
I ask because I finally had time to fine tune it today after the HD kit while noodling some big maple rounds I've been meaning to split. WOT it just seemed to keep climbing as I closed the H screw; it just never seemed to "faux 4stroke". I ended up backing the screw out till it 4stroked under load (in the wood) and then tightened it till it juuuust stopped burbling in the wood.
Another thing, although it's only about 5ccs larger than the CS590, it's MUCH more powerful.
Sportfury70
ArboristSite Operative
I need some advice.
I just ordered the 7900 P&C kit and plan on doing some mild porting. I've got a dremel with plenty of stones and milling bits for working on 1911s, and a full kit of gunsmithing files and chain files. What I'd like to verify is:
Unshroud transfers and just polish the rest?
Widen intake and exhaust, but not raise or lower?
Safe to lose the base gasket?
Is it worth advancing the timing and if it is, how much do I file off the key?
And last, any tips and tricks (or key things to avoid) for this particular saw?
I just ordered the 7900 P&C kit and plan on doing some mild porting. I've got a dremel with plenty of stones and milling bits for working on 1911s, and a full kit of gunsmithing files and chain files. What I'd like to verify is:
Unshroud transfers and just polish the rest?
Widen intake and exhaust, but not raise or lower?
Safe to lose the base gasket?
Is it worth advancing the timing and if it is, how much do I file off the key?
And last, any tips and tricks (or key things to avoid) for this particular saw?
Tenderfoot
ArboristSite Guru
I would not fuss with it as a first go. Try learning on some AM cheapy cylinders, not a $200 one.
Sportfury70
ArboristSite Operative
I understand why you would say this, but I'm no noob. I promise I won't ruin it LOL. I've polished my share of cylinder heads and fitted more beaver tails than I can count. Most days I do some form of metal work either as a hobby or through my job.
I'll probably give the existing cylinder a massage just to get a feel for it.
I'm looking to do a moderate port job on this saw, pretty much by just widening and polishing the ports a bit, and smoothing the transfers. Nothing extreme.
Just looking for tips from folks who have been in these saws so I have an idea of how far I want to take a moderate woods port.
Tenderfoot
ArboristSite Guru
Trust me, a first timer will make a 7900 run worse. Brad Snelling has a bunch of info on them. Part of why they run so well is because the transfers are narrow keeping the fuel atomized. Learn on a cheap aftermarket cylinder before you go to work on a $200 one you will probably make run worse. My 6401/7910 hybrid has a .22 thou squish with the base gasket. Do a muffler mod and research a lot more before hacking them up. They are so tight from the factory there is not a lot of room for the first timer to improve outside of the muffler.
Sportfury70
ArboristSite Operative
Really Tenderfoot, I appreciate your concern, but I didn't ask whether I should do it or not. I asked what to look out for while porting this saw.
Are you saying that a mild port job isn't worth it? I've done lots of research, and all the posts (here and elsewhere) show decking the cylinder, removing the squishband, raising and lowering ports, and other advanced things. I'm not interested in that, just some basic port work.
Even if I don't enlarge the ports, I have a hard time imagining that simply polishing and gasket matching won't be of some benefit.
If I wreck a new cylinder it's on me, not you, so feel free to impart some wisdom that will make this just a bit stronger than stock.
Tenderfoot
ArboristSite Guru
The problem with theses saws if you want to do that is they are made so exactly to begin with. I am telling you to go read up on what has been posted specifically on the 7900 family of saws. The base gasket delete usually means you do not have enough clearance inside the cylinder and IME and from what I read smoothing the ports and intake does little or nothing and is a waste of time, like trying to explain what not to do with a perfectly good cylinder. Do a muffler mod, learn how to do the advanced stuff, then go fuss with a 7900. They have changed a little going to the 7910. My 100% stock saw is 195 psi of comp and will out run the ported 372s I have put it against.
Try reading this thread start to finish. The 7910 piston is lighter and similar to what was done to this saw AFAIK. There is not a lot to be gained with basic mods to a 7910 with a 7900 (6401) muffler and 13.5k coil. I know a few things were done that made things faster.
http://www.arboristsite.com/community/threads/progressive-modding-of-a-dolmar-7900.107027/
Sportfury70
ArboristSite Operative
Ahhh now I'm starting to smell what your cooking. Kinda like putting an intake and exhaust on a bimmer. Doesn't really do much but sound different and waste money.
I haven't seen that thread, I'll give it a read. Thanks man.
I haven't seen that thread, I'll give it a read. Thanks man.
Tenderfoot
ArboristSite Guru
Exactly. They made a lot of changes over production that took away the 'easy' mods. The most recent cylinder run is the best for power apprarently, as is the old style muffler the 6401s have. From what I gather that combination runs stronger then the original 7900s due to the updates to try and offset the SLR mufflers.
Sportfury70
ArboristSite Operative
Sounds like I'm already starting with the winning combination then. 
So MM, and I'll measure squish to see about the gasket. Hopefully it's like yours and has just the right squish with the gasket.
Thanks for your patience buddy, hope to have some test and tune vids after the swap in the next few weeks.
So MM, and I'll measure squish to see about the gasket. Hopefully it's like yours and has just the right squish with the gasket.
Thanks for your patience buddy, hope to have some test and tune vids after the swap in the next few weeks.
jmssaws
Addicted to ArboristSite
Without machine with and a degree wheel you will find some gains but just enough to make you want more and that's how it begins.
Everybody has to start somewhere and everybody,no matter how experienced was a rookie at some point.
Parts are cheap and the only way to learn is to do it.
Read all you can and I'll do the best I can to answer your questions if you have any.
Everybody has to start somewhere and everybody,no matter how experienced was a rookie at some point.
Parts are cheap and the only way to learn is to do it.
Read all you can and I'll do the best I can to answer your questions if you have any.
Tenderfoot
ArboristSite Guru
Good luck. I re-read that thread and apparently with a stock 7900 a muffler mod only makes it louder. I fiddled with a few saws over my time, but realized in the real world, filing makes me cut faster. I would bet money that your saw will be pretty close to mine without doing anything. I was considering all the porting and fussing, but once I actually ran it I was happy. Without doing anything it can keep up with a husky 390 until it has a buried 28 in bar, and then you only notice with a stop watch.Sounds like I'm already starting with the winning combination then.
So MM, and I'll measure squish to see about the gasket. Hopefully it's like yours and has just the right squish with the gasket.
Thanks for your patience buddy, hope to have some test and tune vids after the swap in the next few weeks.
Maybe to you. I think a $30 part is too much to ruin with my own incompetence!
jmssaws
Addicted to ArboristSite
If you can't afford to replace it if you mess it up then you shouldn't mess with it.
A 7900 ports very good and the only way to be happy with a stock one Is to never have run a stout ported one.
They need a little machine work and a little grinding to make one night and day different.
A 7900 ports very good and the only way to be happy with a stock one Is to never have run a stout ported one.
They need a little machine work and a little grinding to make one night and day different.
Tenderfoot
ArboristSite Guru
Being able to afford is not the same as wanting to spend the money.
Sportfury70
ArboristSite Operative
I'm not concerned about the money, even with the kit I'm still only right at $400 into this saw. However, from what it sounds like this saw needs a solid recipe and without a lathe and some magic timing numbers a simple port job becomes moot. I will probably still do a bit of polishing and gasket matching just to satisfy the tinkerer in me, but I may hold off on dedicated port work until I have access to the machinery (and know-how) to do the full work up.
Nonetheless, my results will be posted here and Im always open to suggestions regardless.
Nonetheless, my results will be posted here and Im always open to suggestions regardless.
Sportfury70
ArboristSite Operative
Considering it was YOUR saw that started my obsession with this blue monster, this seems like one of those things you just don't pass up.
I'm 5 beers deep in a 8% IPA, so I'm gonna have to double check this post in the morning
You will be PM'd tomorrow!
Tenderfoot
ArboristSite Guru
I found this today, it was a pretty interesting comparison of the saws. It does match up with my experience, but I have found the chip management on the Dolmar plenty good for my uses, even in the brush. I do also disagree that the Dolmar is more of an Owner-op saw while the Husky is more of a fleet saw. Other then the AV mounts (which were upgraded over their run) and the chain tensioner they really do not have many issues. On the whole it was a really good video other then a couple nitpicks.
Sportfury70
ArboristSite Operative
Yeah that was a good one, seemed pretty unbiased to me too which is nice. Cool to see how similar and also completely different the three manufacturers have designed things when theyre sitting side by side. Don't think he took a breath the whole vid tho lol, man can that guy ramble!
