MS390 piston and transfer port question

[Adamski]

So I am not really one to mess with clam shell saws. I see there uses and I loved my MS170 when I had it but when it comes to the bigger ones I usually fix and move them on.

But see here's the issue, this 390 I have become fond of as it was dirt cheap to buy and it is now cleaner than a freshly mopped floor.

So whilst tearing it down I started to look at the piston and cylinder and noticed that the piston skirt restricts the lower transfer a tad. It becomes quite apparent when the crank bearings are in place. See the below image. I have highlighted in red where I suspect I can trim the piston back to improve flow into the port. I really want to wake this saw up, coupling this with a muffler mod and 0.5-1.00 mm exhaust port widening either end of the port then I think I am half way there.

Any views?

 

[Terry Syd]

I can't remember if I trimmed that bit on the piston, but it looks like something I would do.

The ignition timing on these saws is all over the place - from 23.5-28.5 degrees from the factory. Dialing in the timing really helps to wake them up.

The air filter is a joke. To call it an 'intelicarb' is an insult to intelligence. The reason it has the 'compensator' on the carb into the filter is because the filter is already too small from the factory. I built a bigger, hi-flow filter for mine. I used the saw three times without cleaning the hi-flow filter and then ran a WOT test to check the RPM. I then put on a clean, never used, stock filter and the engine dropped 75 RPM at WOT. I didn't bother testing a dirty stock filter.

The factory powerband is supposedly from 7,000 to 10,000 rpm. Mine likes to cut at 11,000 using stock port timing. I expect that the muffler mod, air filter and timing change are what shifted the power up higher.

 

[Adamski]

So I will trim this off the piston then. Next I will look at the air filter, so many filters are restrictive, whether it be too much material on the flange or poor filtration etc! Damn OEM designs.
The muffler has two 10mm holes drilled under the spark arrestor cover. The cover has been opened up around the standard factory opening and also I have dremeled a neat slot on the underside of the spark arrestor cover. I'll see what these mods give me before I meddle with the timing. Thanks for the info Terry

 

[Terry Syd]

Yeah, then if you want to get really crazy, you can make up a 'sleeper' like I did and put a Husky 20" narrow kerf bar on the saw. It takes a heap of work, but not only does it cut 15% faster, it also is a lot lighter and makes a more agile saw to work with.

I run an 8-pin with a cutting angle of more than 6 degrees, cuts like a mofo. I was surprised that I could take a POS saw like this and turn it into my go-to saw (it was supposed to be my 'loaner saw')

I'm waiting for the 'high compression' piston to be manufactured. I might have to drop the ignition timing back a degree, but the additional torque will be an added bonus.

http://opeforum.com/threads/stihl-ms290-390-pop-up-piston-kit-to-be-sold-soon.6027/

 

[Adamski]

Yeah, then if you want to get really crazy, you can make up a 'sleeper' like I did and put a Husky 20" narrow kerf bar on the saw. It takes a heap of work, but not only does it cut 15% faster, it also is a lot lighter and makes a more agile saw to work with.

I run an 8-pin with a cutting angle of more than 6 degrees, cuts like a mofo. I was surprised that I could take a POS saw like this and turn it into my go-to saw (it was supposed to be my 'loaner saw')

I'm waiting for the 'high compression' piston to be manufactured. I might have to drop the ignition timing back a degree, but the additional torque will be an added bonus.

This is what I like to focus my mods on.. I have my 372xp for most things (stock bar muffler and base gasket changes)..... but I have pimped the F**k out of my 038 mag without touching the transfer ports. It now has gone from a redundant heavy saw to a saw that out cuts my 372xp and my old (now sold) 044 in the bigger stuff. It is a beast and anyone who says a 28" bar is too big for an 038 mag needs to mod theirs.

I guess I just have a soft spot for those saws that from factory have been stunted somewhat. If you know a ms390/039 piston equivalent with more metal from wrist pin to piston top then let me know. I looked at a 268 piston for a pop up on the 372xp but thought why mess with a saw that starts well, runs great and doesn't miss a beat in any situation. But with this ms390 I think if there is opportunity then why not.

This is what I am currently at with the 390, full strip clean and rebuild to follow. I know it is a clam shell but hey I will be posting images of the build.

 

[Dub11]

This is what I like to focus my mods on.. I have my 372xp for most things (stock bar muffler and base gasket changes)..... but I have pimped the F**k out of my 038 mag without touching the transfer ports. It now has gone from a redundant heavy saw to a saw that out cuts my 372xp and my old (now sold) 044 in the bigger stuff. It is a beast and anyone who says a 28" bar is too big for an 038 mag needs to mod theirs.

I guess I just have a soft spot for those saws that from factory have been stunted somewhat. If you know a ms390/039 piston equivalent with more metal from wrist pin to piston top then let me know. I looked at a 268 piston for a pop up on the 372xp but thought why mess with a saw that starts well, runs great and doesn't miss a beat in any situation. But with this ms390 I think if there is opportunity then why not.

This is what I am currently at with the 390, full strip clean and rebuild to follow. I know it is a clam shell but hey I will be posting images of the build.

View attachment 69612

Theres a ms390 pop up in the works that will be available on HL Supply. But its been delayed.

 

[Adamski]

Theres a ms390 pop up in the works that will be available on HL Supply. But its been delayed.

ooooo, with a set of caber rings or the current ones that are OEM off this one and a set of decent spring clips that are actually made of spring steel then that sounds like I could be on to a winner!

What sort of delay are we looking at?

 

[Dub11]

ooooo, with a set of caber rings or the current ones that are OEM off this one and a set of decent spring clips that are actually made of spring steel then that sounds like I could be on to a winner!

What sort of delay are we looking at?

They'll come with cabers. And a member here just needs to put some saw together to check fitment. I volunteered 2 saw a oem 390 and am 390 so I'll know when hes done playing. As for timeline Iam not sure.

 

[bikemike]

I'm following this. I did a poulan up and before the pipe went on it it was a good strong runner. I would personally grind out some of the divider for the transfers to get it away from the bearings and round the edges to split up the flow to primary and secondary transfer it will breath much better not having to zig zag around the crank bearings

 

[Adamski]

I'm following this. I did a poulan up and before the pipe went on it it was a good strong runner. I would personally grind out some of the divider for the transfers to get it away from the bearings and round the edges to split up the flow to primary and secondary transfer it will breath much better not having to zig zag around the crank bearings

I like this idea and with it being an open port this will be a doddle.

 

[bikemike]

Keep us posted with progress pics