Stihl MS390 decomp vs. non-decomp head?

[hal9000]

Hi all, I'm new to the site, but have been lurking around here occasionally picking up tips while I shopped for a new saw. That ended the other day when a friend handed me a stihl MS390 that needed a new top end and said, "It's yours if you want it". Since this is my first saw rebuild (I've got a few dozen assorted other stock and modded engine builds under my belt), I'm hoping to get a couple questions answered.

1) Are there any "must do" or "must check" items that I should address while the engine is apart? The bottom end looks and feels fine, but I'm not sure if there are any critical measurements or inspection points.

2) This saw is a non-decompression model. I live at 7K ft altitude, and cut wood anywhere from about 6K to 9K ft, so obviously the performance is going to take a hit. It appears that nobody makes a high compression piston, but since I need a new cylinder anyway, I'd like to find out if there's any compression or performance advantage to either the decompression vs. non-decompression head so I can shop appropriately.

3) Considering the altitude, is there anything outside of the muff mod that you all would recommend to help the performance. I'm not building a hot saw by any means, but 25% power loss makes one think twice about modding, and it's not like I have anything to lose in this case....

Thanks everyone

 

[splitpost]

This thread might helphttp://www.arboristsite.com/community/index.php?threads/191477/

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[Roll Tide]

A good muffler mod and advancing the timing by shaving the flywheel key .020 would be a great start. Make sure to remove the limiters on the tuning screws

 

[hal9000]

Thanks for the link splitpost. Good stuff. So did they ever resolve the TS400 piston question? Until I reached the point that they were questioning the stock dimensions that seemed like a good upgrade for my circumstances.

Roll Tide, Can you give me any more info on the timing tweak? Are you doing anything to compensate for the thickness of the flywheel key (IE filling in the empty space on the back side of the key) to ensure the timing doesn't wander?

Still wondering about the decomp. vs. non-head.... performance differences aside, I'm wondering if there's a way to use a modified decomp plug to reduce the volume of the head a little bit. Maybe machine a bolt head into a nice low radius and install from the inside to form a "bump" in the head. It would need to be externally nutted or staked to prevent it from backing out internally with catastrophic results, I don't know enough to decide if that's a brilliant idea or pure foolishness.... At leas half my ideas fall into the second category!

 

[Jon1212]

I have an early MS390(bought new 2002), and it is from the changeover fromnthe 039, so no decompression valve. I lived at a much lower elevation at the time, and it definitely had some snappiness to the starter. Now that I live at 4700', it doesn't seem to have quite the same resistance. It was worked over some by Mastermind Worksaws back in 2011, and I'm sure @El Moobs can give you a good idea of how to increase gains from this much maligned model.

 

[Roll Tide]

Thanks for the link splitpost. Good stuff. So did they ever resolve the TS400 piston question? Until I reached the point that they were questioning the stock dimensions that seemed like a good upgrade for my circumstances.

Roll Tide, Can you give me any more info on the timing tweak? Are you doing anything to compensate for the thickness of the flywheel key (IE filling in the empty space on the back side of the key) to ensure the timing doesn't wander?

Still wondering about the decomp. vs. non-head.... performance differences aside, I'm wondering if there's a way to use a modified decomp plug to reduce the volume of the head a little bit. Maybe machine a bolt head into a nice low radius and install from the inside to form a "bump" in the head. It would need to be externally nutted or staked to prevent it from backing out internally with catastrophic results, I don't know enough to decide if that's a brilliant idea or pure foolishness.... At leas half my ideas fall into the second category!

Once its tightened down it will not budge due to the taper

 

[splitpost]

Thanks for the link splitpost. Good stuff. So did they ever resolve the TS400 piston question? Until I reached the point that they were questioning the stock dimensions that seemed like a good upgrade for my circumstances.

Roll Tide, Can you give me any more info on the timing tweak? Are you doing anything to compensate for the thickness of the flywheel key (IE filling in the empty space on the back side of the key) to ensure the timing doesn't wander?

Still wondering about the decomp. vs. non-head.... performance differences aside, I'm wondering if there's a way to use a modified decomp plug to reduce the volume of the head a little bit. Maybe machine a bolt head into a nice low radius and install from the inside to form a "bump" in the head. It would need to be externally nutted or staked to prevent it from backing out internally with catastrophic results, I don't know enough to decide if that's a brilliant idea or pure foolishness.... At leas half my ideas fall into the second category!

If the stock ts400 piston was measured I don't recall


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[Homelitexl903]

Been drinking today but pretty sure decomp vs non decomp is decomp is for wuss bags or older folk with less strength. Decomp just means it's easier to start if given small weak arms. Pretty sure same design just with a hole for decomp which means something else to fail down the road. Some people need/love decomp. Im 6'2 and 220lbs and can start a homelite 750 with no decomp. My non decomp ms390 is easy to start and just needed a muff mod and correct tune to become a real saw. If you want an insane clamshell do what stumpbroke did on his build. But dont waste your money and have someone else do his style build on that saw

 

[hal9000]

All right... thanks again for the ongoing replies guys. I do appreciate it. Next question: You're removing .020 from the key... That's from the entire surface of the key, right? Meaning that you're advancing the flywheel by a total of .040? Just want to be sure I'm picturing things correctly...

Wouldn't it be easier to groove the coil mounts and move the coil?

Changing the subject, are the OE stihl cylinders chrome or nikasil lined? I've finally gotten around to cleaning the cylinder, and I think it's going to be saveable.

And just because I hate not knowing things, can somebody please explain what's going on in this photo from stumpys build thread? I understand porting intake/ehxaust for flow or to change the port timing, but I'm not that well versed with two strokes. what's the reason/effect of raising/widening the top of the transfer ports (that's what they are right?) as appears to have been done here? If nothing else, knowing will deepen my knowledge base a little bit

Thanks again

Here's the port work.

 

[hal9000]

Been drinking today but pretty sure decomp vs non decomp is decomp is for wuss bags or older folk with less strength. ... Im 6'2 and 220lbs and can start a homelite 750 with no decomp.

Snort! At 5'9" and about 155 I'd put myself in the wuss bag category if it wasn't for the fact that I tend to outwork most mere mortals. Not worried about the decomp for starting. I just didn't know if maybe they'd changed the head design. Thanks for the info on that.

Since it looks like I'm keeping the OG cylinder and my rebuild costs are going to be minimal anyway, I may do a little mild porting. Worst case, I have to buy a cylinder (which I was counting on anyway) to undo the damage.... No need to go crazy though. If I can get it to perform at altitude like a stock saw would most other places. I'll be more than happy.

 

[Stihl 041S]

Been drinking today but pretty sure decomp vs non decomp is decomp is for wuss bags or older folk with less strength. Decomp just means it's easier to start if given small weak arms. Pretty sure same design just with a hole for decomp which means something else to fail down the road. Some people need/love decomp. Im 6'2 and 220lbs and can start a homelite 750 with no decomp. My non decomp ms390 is easy to start and just needed a muff mod and correct tune to become a real saw. If you want an insane clamshell do what stumpbroke did on his build. But dont waste your money and have someone else do his style build on that saw

Ahhhhh.......no.
The decomp makes it a lot easier on the recoil parts.

 

[Jon1212]

Been drinking today but pretty sure decomp vs non decomp is decomp is for wuss bags or older folk with less strength. Decomp just means it's easier to start if given small weak arms. Pretty sure same design just with a hole for decomp which means something else to fail down the road. Some people need/love decomp. Im 6'2 and 220lbs and can start a homelite 750 with no decomp. My non decomp ms390 is easy to start and just needed a muff mod and correct tune to become a real saw. If you want an insane clamshell do what stumpbroke did on his build. But dont waste your money and have someone else do his style build on that saw

Are you Bluto from Popeye? For some reason when I read your post, I could hear Bluto's voice. I'm 6' and 215lbs, so I don't generally need the decomp, even on my ported saws, though by using it when the saw is "cold" I save wear, and tear on the starter. Nothing like getting ready to cut, and giving an honest effort on the starter handle, only to have the rope break.......grrr!

 

[hseII]

I have an early MS390(bought new 2002), and it is from the changeover fromnthe 039, so no decompression valve. I lived at a much lower elevation at the time, and it definitely had some snappiness to the starter. Now that I live at 4700', it doesn't seem to have quite the same resistance. It was worked over some by Mastermind Worksaws back in 2011, and I'm sure @El Moobs can give you a good idea of how to increase gains from this much maligned model.

Wasn't there a really long weight time for that one and a Pull On?

 

[hseII]

Are you Bluto from Popeye? For some reason when I read your post, I could hear Bluto's voice. I'm 6' and 215lbs, so I don't generally need the decomp, even on my ported saws, though by using it when the saw is "cold" I save wear, and tear on the starter. Nothing like getting ready to cut, and giving an honest effort on the starter handle, only to have the rope break.......grrr!

Yes Ron.

 

[Jon1212]

Wasn't there a really long weight time for that one and a Pull On?

Just the Poo Lawn.

 

[Jon1212]

Yes Ron.

 

[ssm1699]

The 039/390 isn't a hard pulling saw. I rebuilt my 029 Super and put the 039/390 kit in it. I had the option to put a decomp valve on it, as the new cylinder had the port for it. I didn't want to replace or cut my case to accommodate for the decomp. That saw pulls the same way my 029 super pulled, for starting. I have never had a problem with my starter assembly and I am hard on my saw. I wouldn't worry about needing the decomp on that saw.

 

[hal9000]

I wouldn't worry about needing the decomp on that saw.

Nope, just asked about performance differences....

Along those lines, does anybody know the wristpin size or pin to crown height (at the edge) of a 49mm ts350/ts360 piston. I'm still interested in a high compression option and with the domed top I'm wondering if that one would make a good parent to machine to the appropriate height (I need an excuse to fire up my lathe anyway). I'm waiting to find out for sure, but it seems like forester might not be stocking the TS400 piston anymore.

Not being able to leave well enough alone can be such an awful burden sometimes

Thanks all

 

[a. palmer jr.]

Actually, de-comp is mainly to protect the starter and it's plastic parts. If you're really weak you probably shouldn't be using a chainsaw. I'm a senior citizen and still can start my non de-comp 390 without much difficulty. Actually my new cylinder has a hole for a de-comp and I plugged it. I own 2 390s and neither has a de-comp.

 

[Gugi47]

I have one with decompression....never use it......I use it ones just to see the different.
Suggestion: If you have that baby a part, I would change the bearings and the seals.
That would not be expensive and give you a pace of mind.
Good luck.