old-cat
Fir Man
DON"T run gas in a diesel engine!
MS250-Too Much Compression?
- Thread starter drf255
- Start date
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l don't have heaps of ported saws....only three
, but l do have quite a few stock saws. The saw that impresses me the most each time l run it is a MS261 standard carb model that Brad did. Its been strato gutted and has the cutting personality similar to a 70cc saw...its awesome!! For what its worth it blows 170ish psi. I have other saws with over 200 but they don't run like this MS261.
, but l do have quite a few stock saws. The saw that impresses me the most each time l run it is a MS261 standard carb model that Brad did. Its been strato gutted and has the cutting personality similar to a 70cc saw...its awesome!! For what its worth it blows 170ish psi. I have other saws with over 200 but they don't run like this MS261.
super3
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Come on guys, ya gotta love that 3rd line.
old-cat
Fir Man
If you run the squish real tight then you can go much higher on compression than if your squish is sloppy.
I think it's the other way around.
hseII
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nmurph
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I think the problem is going to be the bearings and rod. What we don't know is the true compression because of the way he has gone about assembling the saw. Oiling the cylinder will give a falsely high number, but he oiled the rings and not the cylinder??? 230 should run fine on 93 octane.
drf255
BAD CAD
Ok guys.
I believe I oiled the cylinder. I definitely doused the rings in oil, I mean I poured it all around the groove through the gaps, the rotated the rings all around. Oil was dripping everywhere.
I turned the saw up on its face last night with the piston down. I got a lot of the oil out of the cylinder.
I'm down to 240 psi, and that's with a still oiled cylinder.
Realize the these numbers are with no muffler or carb connected. I don't know of those attached and the saw warming up will lower compression. I know that most of my saws lose 30-40 psi when hot.
If you guys think this is just a grenade with zero chance of any life expectancy, it will come back apart tonight. If you think it may do ok on some race gas (yeah, I'll go get some just for this saw) then I'll give it a whirl. Adding toluene is another old racers trick to bump octane.
It's a clamshell, so hours of work to disassemble. But the work will be worth it if I'll be disassembling it after it explodes anyway.
If you guys think I'm dissembling either way, then I'm doing it now before there's tons of damage.
Fire in the hole!!!!
I think the crank pan lets go...I've broken several of them on other stihl clamshells without really trying. (65cc blowers trying to get fan wheel off) and this piston has additional mass that none of the components were designed for.
I think the crank pan lets go...I've broken several of them on other stihl clamshells without really trying. (65cc blowers trying to get fan wheel off) and this piston has additional mass that none of the components were designed for.
drf255
BAD CAD
Sounds like I'm gonna disassemble tonight and take some of the dome off. That will improve flame travel as well.
My understanding on squish was that less squish causes less detonation. Is that wrong?
I had no ability to adjust squish on this motor. I just reduced combustion chamber volume with the dome.
The piston weight is exactly the same as the AM piston was before. I added 1.2 grams of aluminum, and the stock pin and circlips were 1.2 grams lighter than the AM one. So a net weight gain of ZERO.
drf255
BAD CAD
One more question.
With this much compression, would I make any gains from raising the exhaust port roof a mm or 2? That would lower compression and give me more exhaust duration.
With this much compression, would I make any gains from raising the exhaust port roof a mm or 2? That would lower compression and give me more exhaust duration.
drf255
BAD CAD
THANKS TO ALL THAT POSTED.
Saw coming apart tonight. I gained about 90 psi over the stock 150. I'm going to try to visually grind about half the dome volume off. Mostly grind the top down.
Will post pics
Saw coming apart tonight. I gained about 90 psi over the stock 150. I'm going to try to visually grind about half the dome volume off. Mostly grind the top down.
Will post pics
wigglesworth
Booned
I had a 288XP he built that showed 235psi and ran just fine on 87 octane...
Y'all worry too much...
komatsuvarna
Arboristsite MVP
yuppy.
Ive ran a couple at 230 before on pump gas, no problems.
I didnt notice any real difference over 190-200psi except on the pull rope though.
old-cat
Fir Man
You're right about the squish gap. If it was .020" or even a little less, your 230psi would probably work.
drf255
BAD CAD
A few differences.
He knows what he's doing, I don't.
He runs a flat top with lots of squish, I'm not.
So what's the consensus?
Premium with octane boost and run it, or dismantle and shoot for 190 psi?
old-cat
Fir Man
Put the premium and octane boost and try it , short time. Then reduce the octane boost, see what happens.
komatsuvarna
Arboristsite MVP
Couple short thoughts from a hillbilly hack.
The 30lb difference from 150psi to 180psi will be a lot greater than the 30lb difference from 200psi to 230psi. IMO 230psi isn't needed to mske a great running saw.
Ive run some big popups before and lots of compression to "play around"
...and in my opinion its not the way to go.
The 30lb difference from 150psi to 180psi will be a lot greater than the 30lb difference from 200psi to 230psi. IMO 230psi isn't needed to mske a great running saw.
Ive run some big popups before and lots of compression to "play around"
...and in my opinion its not the way to go.
wigglesworth
Booned
That 288 was actually a pop-up saw. One of his early ones before he "wised up".
I'd fire it off, get it warm and check comp after its cooled off, then go from there.
Did u take any timing numbers?
