my MS290 rebuild, help with port timing please

i bought it last week on craigs "freshly rebuilt" by joeshmo. 1 day of cutting later and it was down on power and i could hear a sucking noise at idle. 90psi compression test, tore it down, found this.






i picked up a new piston and ring, honed the cylinder and started thinking more power couldnt hurt.



ive been doing heads for 4 or 5 years now


problem is ive been studying these for the last decade and 2 strokes are so difficult for me to visualize.




so i sat down with a notebook and jug/rotating assembly today trying to figure out timing events in order to hop this saw up a bit. im hoping someone can critique my conclusions because im not sure how much 4 stroke behavior will carry over.







my understanding is:


1. lowering intake floor or cutting skirt will both retard intake closing and advance intake open.

2. raising the exhaust roof or trimming the piston crown edge will advance the exhaust opening and extend total duration.

3. raising transfer ports or cutting piston crown edge advances "intake" timing on the downstroke, but also ******* it on the upstroke.

4. BDC occurs when piston is level with exhaust floor height. lowering exhaust floor shouldnt really effect much besides port area?

with 4 strokes, the primary component to placing the powerband where you want it is intake valve closing period. close early (advance) and you get lowend, close later (retard) and you get top end.

what i cant figure out is if a 2 strokes top end comes from focusing on getting your crankcase filled or getting your transfer ports to really flow.. or most likely, both. all i seem to read about is raising the exhaust ports, hence starting blowdown sooner. id guess this would take advantage of the higher cylinder pressure to evacuate the cylinder better?

Being that's a clam shell motor, you don't want to raise the exhaust or you can kiss compression bye bye. Given you are not raising exhaust, you don't want to raise transfers as you would then be hurting top end with insufficient blowdown. And as you are not raising the other two ports and increasing RPM, you likely don't want to increase the intake durration any signifficant amount either or you will loose base compression.

Just widen out the exhaust, maybe aim the transfers a little more back and try to improve the top corner at the outside of the transfer duct a little. then widen out the intake, match everything up and with a good muffler mod, that about all that can be had from a 290 as a work saw. Might get away advancing the timing a couple degs to make up for not being able to increase compression.

But myself, I would get it running stock with a new piston an make sure there are no problems before getting into porting, The idea of doing some porting as the saw is disassembled sounds good, but it can bite you in the a$$ if there are pre-existing problems with car, air leasks or whatever. Might only take an hour to put the saw together test it out and then pull it down to mess with porting.

thanks alot TW! this saw is a pain in the balls to tear down. today i did just what you said.. very mildly cleaned up intake and exhaust, gave the base of the transfers a little smoothing and very conservatively widened both ports to gain volume without changing timing more than a few thousandths.

im gonna leave the tops of the transfers alone altogether, since one little ****oo will cost a cylinder.

thanks alot for the advice!

ive been up to my ears in other stuff and havent got it back together yet, but heres the ports on the 290.





ive been using the jonsered and my 041av in unison to not overwork either saw, and the difference in RPM is amazing. if the 041 would rev that saw would be the cats ass but it doesnt. so maybe that will be my next project.

you might want to check the specks on the old 41 I think only 7000 !!!!!

yeah, its something like that. so what port mods raise a saws RPM band?

Wider ports and longer port durations favor higher RPM. There are limits to how wide dictated by piston skirts and ring life and as mentioned there are limits to how high you can run the other ports a lot of it based on not being easily able to increase compression. If you raise the exhaust compression goes down. If you raise the transfers without raising the exhaust you get insufficient blowdown and the transfers simply won't breath at higher RPM. If you increase intake without changing the other ports then the intake is trying to run at one RPM yet the other ports want to run at a lower RPM and the saw is fighting it's self. End result is poor low speed, difficult tuning and fuel spitting back out the carb.

As far as porting goes on this saw, it's all a limitation of the design of the 290 and other consumer type saws. that have the upper half of the crank case built into the cylinder. there is more you can easily do with the 041.

Those ports look good!

thanks for the info once again. the 290 isnt back together yet but im not doing any other mods.. was asking in regards to the 041. when i find the time ill tear it down and post pictures of whatever i find and go from there. thanks alot!

Hate to see that stuff ,my ms310 bought at auction done the same after about 10mins hard work figure the fuel mix wasn't rich enough for that job.

mcb--- you left us hanging! We've been waiting a year for a full report! What was the verdict? How'd it perform?

But for all your work, you get rep!!!

No rep without pics.

funny this gets bumped the day after i tear down the 041av for some hop-up work.

well, its been back together a while but i didnt have a tach until recently so it was set to the factory H/L settings and was a big disappointment. complete dog and worse than before.
so i drilled a 1/2 hole in the muffler between chambers but still under the spark arrestor, felt like a happy medium between performance and noise, since i wanted to climb with it. really not loud at all. finally got the tach and dialed in the carb, 10x improvement. its got a new 20" arbormaster bar with full chisel chain, square filed.

i havent made time for a shootout, but before the rebuild, the jonsered 2155 and farmboss both with 16" semi chisels were dead even in several races. now, the jonsered with 18" round chisel, very revvy and peaky, works great in its rpm but bogs quick if you lean on it, especially in hardwood. the farmboss pulls the 20" with more authority than the jonsky's 18 and has a wider powerband with better low end.

then again its hard to consider the farmboss anything to talk about after you put down a stock 395xp. like a camry vs a funny car.

fiddled with the saw some more. cant seem to get it uploaded, but i took video of two cookie cuts per saw, the 290 with 20" round chisel vs the jonsered (its actually a 2150 i just noticed) using an 18" round chisel. both saws were within a second, which was a bit disappointing.

previously i bored a .500" diameter hole between the two muffler chambers under the deflector and screen, but i hadnt enlarged the deflector. the other day i filed that hole into a soft square, and opened up the deflector. it was obvious that this effected the tune right away. did a bit of carb tweaking and square filed the chain, then used it to buck a pine i climbed yesterday and it rips for a 290. i would have guessed it were a 62-65 cc saw. the torque isnt amazing since i went to the 8 tooth powermate rim but its pretty fast in pine when the revs stay up. they were 12-16 inch rounds.

come spring i'll try to set up a video and figure out what sprocket works better between soft and hardwoods. im gonna keep the 20" bar on this one since it fills the gap between my limber/jonsered and the 85/95cc saws pretty well for now. when i find a 65cc saw to hop up with a 24" i will call my stable complete.

heres the thread for the 041av im currently building now that the farmboss is done.

http://www.arboristsite.com/showthread.php?t=123252

Want to know more

I garbage picked an 029 Super 1998. While riding my bike in Central Long Island. I saw just the grip and had to look, no shame. So I hooked it on my bars and headed back to my sisters. It had lean seized but was still on its original chain. I like the size and weight of the saw but its not pro quality saw. I am not a pro, but I cut and burn my own wood and fix my own saws. Originally I was was going to fix it a sell it to support my hobbies, competative mountain biking, but I saw this thread. The cylinder cleaned up nice and I have a 46mm aftermarket piston kit coming. I would like to know how much widening you would do to the intake and exhaust. I understand from that thead this is the only thing you can do. I have a die grinder. What are the do's and don'ts. Also, the muffler hole is roughly 1/2 soft corner square? I don't have a tachometer, and tune my saws warmed up in wood and make sure they almost 4 stroke just a tad at the full rev. I run fresh gas 91 or better with Stihl Ultra synthetic. Any how-to in detail would be great. I apprenticed as a machinist, but also don want to throw the saw away with a mistake. Is just polishing the ports a better idea?

well, first thing is to make sure you can get the cylinder cleaned up, a scratch or two isnt the end of the world but you definitely cant have big gouges or piston material stuck on it. run a ballbrush hone with a bunch of oil through it for a bit of crosshatch to seat the rings on after you do whatever porting, and always make sure you deburr and radius the edges of the ports where they meet the cylinder or a ring will catch on the material there.

as for port width, measure the piston diameter, you can go about 70% portwidth of the piston diameter maximum from what i gather. make sure that if you do widen it out that much, none of your ring gaps will land in that region and that the piston skirt will still cover the ports. look at the wear pattern in this cylinder (from my 041.) your ports need to stay well enough within that wear region to have adequate sealing also.


use honda-bond on the cylinder base and if you have to surface it, do so very sparingly. if you take material off the jug it wont seal because the bearing races wont let it seat any deeper.

theres not really much need to "polish" your ports to a mirror shine, i use cartridge rolls personally. 60 grit, then 100 and finish with a 120 grit loose sandpaper flap and some transmission fluid. been porting heads for years and never any "carbon buildup" issues to my finish.

a half inch round hole under the muffler screen + opened deflector + retune + square file make the saw pretty capable and still quiet. it will drink a lot more fuel after, especially idling so shut off when not in the cut. on a 20 inch bar, i use 7tooth for hardwood, 8 tooth for soft.

Thanks and a question more

Good info thanks. The cylinder clean up and it is in very good cond. So if I am going to open the intake and exhaust ports up, I assume you do it symmetrically on the left and right sides only not the top and bottom? Would .5mm on each side make a difference (widening it by a total of 1mm) or is that too conservative. What is Honda bond used for?

70 percent of piston width

I did the math and with a 46mm piston you would max open up the ports to 32.2 mm. Sound correct? I am not sure what they are now, but I will look when I am down in the Barn.

The actual measurement

I measured with a dial caliper the exhaust port width to be 1.181" which translates to 29.99 mm or 30 mm. I could go 31mm conservatively and that is what I think I will do.