A short introduction and some advice on a 120si

[Crsracing]

Recently come to the conclusion I needed a new saw. I work for a rental management company so its mostly storm clean up some pruning and occasionally taking a tree down. I wanted a good all around saw something not to heavy for small stuff but powerful enough for anything I would encounter normally. I settled on a ms261c w/ 20" bar and have been really enjoying it, but in the course of researching found this site. I had never really thought of modifying chainsaw even though I can't leave anything else alone. So the first thing I did was muffler mod (first 2 pics) my cs280e echo as I felt it was lacking. after that and what I'm hoping is a proper tune it seems more responsive and a bit quicker in the cut

Which leads me to my questions, I have a dolmar 120si that from what I've read seems to be a great saw it needs a new bar and chain (Planning on a 24"). so I dug it out started cleaning it off and the muffler was lose but I wanted to pull it anyways. Of course my luck both bolts snapped off flush in the head so I had to pull it off and throw it in the drill press. But now I have the head off I would like to clean it up/port it was hoping for some advice. I know @Mastermind has done one of these before, I was reading the thread but the pics no longer work. I have 4 main qestions,
1 is it safe to pull the gasket for better squish
2 how much should I widen the ports
3 can I remove this weird protrusion (3rd pic)
4th is it supposed to have a gasket on the muffler to cylinder?
I have already modified the muffler for the 120si it looked very restricted so I put another set of stock style vents below (4th pic)

 

[Wonkydonkey]

Good morning and welcome @Crsracing

I will quickly attempt to answer your questions.

the squish band needs to be about 20thou, there are ways to make it about this, but first you need to get an accurate measurement, then work out where you need to go.
Use the thinnest solder you can get if it’s not thick enough to get it to flatten, then twist two bits together.
Then if it’s less then 20 thou you can use an assortment of materials to make it up to 20 thou.
The other way is machine (lathe) work... I,m sure this can be gone in more depth if it’s what your looking at.

you can widen the ports up to almost the edge of the piston but leave a little more for rounding off (bevelling ) the ports . The saying goes you can take it off but you can’t put it back on.

the weird thing in the exhaust, is to stop the rings from catching in the port. Tinman on you tube ported a homlite and took out this, why I will never know, I posted the rings will snag. And not long after in another vid he showed the thing die, and the tore it apart and sure enough.

i,m sure someone will add more about fixing up your saw, ie your exhaust and everything else.

 

[Wonkydonkey]

I will add, after reading the. Thread that you were probably referring to about the pics not working [as lotta data when it crashed etc].
I'm wondering if the thing in the exhaust port is for the ring end gaps. Someone was talking about running a saw with the piston in backwards ?

but like I say someone will else will help in your quest to fix up your saw.

btw. The vid of Randy where he it’s the nail

 

[Mastermind]

I barely remember doing a 120si so I'm not much help aside from generalizations. And can't add much more than what's been posted already.

Just be sure to actually check before doing anything. Check squish clearance. Check piston skirt width, and ring end position. Then make your decisions about alterations.

That "tonsil" in the intake port....you can probably thin it, but I wouldn't completely remove it. It supports the rings at bottom dead center.

It's the little things that bring a saw alive. Timing advance, muffler mod, thinning of throttle and choke shafts, etc.

Take your time, look for gains.....

 

[Crsracing]

I appreciate the quick responses! The "tonsil" <-great name for it lol is actually in the intake port like mastermind said. I assumed it was for ring support, but had never seen that before. And yes that must be the thread I was reading he started talking about piston in backwards and something offset and with no pics I was lost. I will toss the head back on and get a measurement on squish. Little things are really what I was looking to do anyhow I don't want to do to much and make it a dog or unresponsive.

 

[Crsracing]

Checked the squish it was just over .040 pulled the gasket and rechecked its .030 now. I've previously read about people doing this but what no one has really seemed to mention, is it advisable to raise the ports the same amount they where "lowered" or is it so little as not to worry about it ?

 

[Mastermind]

Checked the squish it was just over .040 pulled the gasket and rechecked its .030 now. I've previously read about people doing this but what no one has really seemed to mention, is it advisable to raise the ports the same amount they where "lowered" or is it so little as not to worry about it ?

Don't alter the heights of the ports without a plan.

Look at the width of the piston skirts, and see how much you can widen the intake and exhaust ports. Just be aware that once you start cutting on the jug, you have to be able to bevel the port edges and smooth it all out so it doesn't eat the piston and rings.

 

[Crsracing]

I guess my plan was to put the top of the port back exactly where it was before it dropped with the cylinder.

Just took measurements these are as measured straight across not on a curve not including the chamfer
Skirt width 1.350"
Exhaust port 1.1"
Intake port 1.150"
Already noticed the exhaust port tapers narrower towards the muffler as I can't pull my id mic straight out.

 

[davidwyby]

Welcome!

Degree wheel time ;-)

 

[Mastermind]

I guess my plan was to put the top of the port back exactly where it was before it dropped with the cylinder.

You will have a hell of a time just raising it a really small amount unless you grind it rather than cut it with a burr. The plating tends to grab the bit and cause it to dig in. Like David said....a degree wheel is a good idea here. See what the timing numbers are now, and make a plan.

 

[davidwyby]

Some rummaging on the internets revealed this quote by Mastermind :

120SI Porting numbers? | Arboristsite.com

I hope it's OK to link over there.

 

[Mastermind]

Some rummaging on the internets revealed this quote by Mastermind :

120SI Porting numbers? | Arboristsite.com

I hope it's OK to link over there.

Good sleuth work !!!!!

And yes....it's fine.

 

[Crsracing]

I'm definitely saving those numbers! Due to the growing list of things that are arguably more important then hoping up my saw i think I will have to be happy with just pulling the gasket, cleaning up the castings, smoothing out the ports/portmatching, and removing the taper on the exhaust port. Even with just that im excited to see how it runs. I may also thin the throttle shaft/screw that's a interesting idea I hadn't heard before.

I have to say I really appreciate the quick detailed responses. I'm a member of lots of forums due to lots of interests, and this is probably the best results I've got.

 

[davidwyby]

I'm with you. I have a few saws I'd like ported and I have the background, the shop, and have played with it a bit...but it's easier to pay one of these guys, and probably get better results.