ms 390 bashing

[THALL10326]

I ran my 390 today with the 25" bar, ran great!!! Then....... I had to drop one tree, it had to fall, forest fire burnt the core and it was going to fall, I just guided it's direction. I cut maybe 1 1/2 cord (3 rick in my neck of the woods) but did not run it out of gas. Went to start it to drop the last tree would not start, for anything, would not start. Filled it up with gas, stgarted right up, then continued to die!!!!! Finally got it to run, but ran like crap the rest of the time!!!!!!! Temp was low 70's with 50% humidity, any thoughts?
Andy

How old is that saw? Old 290's-310-390's all have one very common problem. If you look in the crack between the carb housing and the top of the gas tank you can see the fuel hose. If its wet theres a hole in it right there. These saws have a nack of breaking the fuel line or the line rotting out in that one spot after they get some age on them. Its a 10.00 repair if thats what it is. If you got a fairly new saw then something else is going on....

 

[sawinredneck]

3 Months old.....

Its only 3 months old, maybe 5 gallons of gas, 2 1/2 after the muffler mod., vapor lock is very common in Kansas, but.... at 70 deg. ?
Andy:jawdrop:

 

[THALL10326]

Its only 3 months old, maybe 5 gallons of gas, 2 1/2 after the muffler mod., vapor lock is very common in Kansas, but.... at 70 deg. ?
Andy:jawdrop:

Hmmmmmmmm something is going on. 70 degrees shouldn't bother it at all. When you say it was running crappy what do you mean, won't open up and run , bogging in the cut, leanig out??????

 

[sawinredneck]

Ran great wide open, just had a hard time getting it there! It started, would idle then die, start again idle fine give it gas, die, try to load it in a cut, die, get in a cut, ackted like it was missing ( for lack of a better word) I am a mechanic worked at a John deere dealership for 2 years, and worked on const. equip. (bobcats) for 2 years, never seen anything like this before though!!!!
Andy

 

[THALL10326]

Ran great wide open, just had a hard time getting it there! It started, would idle then die, start again idle fine give it gas, die, try to load it in a cut, die, get in a cut, ackted like it was missing ( for lack of a better word) I am a mechanic worked at a John deere dealership for 2 years, and worked on const. equip. (bobcats) for 2 years, never seen anything like this before though!!!!
Andy

hmmmmmmmmmmm,have you looked in the muffler and checked the piston?

 

[sawinredneck]

hmmmmmmmmmmm,have you looked in the muffler and checked the piston?

Honestly..... Kinda scared to, my first modded saw, not feeling real good about it right now!!
Andy

 

[TimberPig]

We won't know for sure until you take a peek in there, but it does sound like a piston seizure. That's why when it's not running right, it's best to shut it down and find out why. Two stroke engines are more prone to damage when you continue to run them than 4 strokes because of the possibility of lubrication failures since the lube is delivered to the engine in the fuel mix. Continuing to run them often results in more serious problems and damage. Hopefully it is something simple, and not that serious, but I wouldn't bet on it from what you have said.

 

[THALL10326]

Honestly..... Kinda scared to, my first modded saw, not feeling real good about it right now!!
Andy

Take the muffler off and look at the piston for scuffing, sounds like a start to me...........................

 

[sawinredneck]

Looks beautiful!!!!!!!!!!

Pulled the muffler, nice and smooth, no scuffs, no grooves, the top of the piston is clean, nice and oily, plug has the nice light brown tint to it, don't see any wetness on the fuel line, air filter is not plugged, I dunno on this one.
Stupid newbie question, do I need to start another thread so as not to hijack this one?
Andy

 

[THALL10326]

Pulled the muffler, nice and smooth, no scuffs, no grooves, the top of the piston is clean, nice and oily, plug has the nice light brown tint to it, don't see any wetness on the fuel line, air filter is not plugged, I dunno on this one.
Stupid newbie question, do I need to start another thread so as not to hijack this one?
Andy

Welp thats good thing. Now you got three other things to check. Air leak, fuel system and ignition. One of the three is going to expose your problem. Have you got the gauges to performed a air leak test???

 

[sawinredneck]

Nope

Thinking I need to buy a new muffler and take it in for warranty work!!!!!
Andy

 

[THALL10326]

Thinking I need to buy a new muffler and take it in for warranty work!!!!!
Andy

To tell you the truth I would do exactly that. That modded muffler is messing up your warranty but if your willing to buy a new muffler and take it back it is under warranty and they would solve the problems with it. From what your describing, this missing, does it get hot and then its hard to start and then after you let it set abit it starts up ok, thats a coil going bad. Unusual but thats how a saw reacts to a coil going bad. Some coils work fine cold and dy when they heat up. This may explain your engine missing as it warms up or in the cut. From dead cold it runs fine and only screws up when hot and starting missing it tends to sound like a coil going bad.

 

[sawinredneck]

I'll buy that!

For about 30 bones, I can get a new muffler, and yes, that sounds very close, just don't understand if I let it sit ( at least today, allways ran fine before) and cool of... It runs like crap! A coil usually takes 15 min. to get hot enough to act up?
Andy

 

[THALL10326]

For about 30 bones, I can get a new muffler, and yes, that sounds very close, just don't understand if I let it sit ( at least today, allways ran fine before) and cool of... It runs like crap! A coil usually takes 15 min. to get hot enough to act up?
Andy

Well yes and no with saw coils. At 12,000 rpms its firing 200 times a second so it dont take long to heat up. A coil going bad is a major pain to figure out. I had a MS180 that would idle but nothing else. Come to find out the coil was putting out just enough fire to run at idle. When you threw the gas to it it would flood out.This engine is missing usually points to a igniton problem. Fuel problems seem to be more apparent to the ear as it runs, others words you can pretty well tell its a fuel problem by how it sounds. Fuel problems don't usually make a engine miss. Let me ask you , when this thing misses does it act like your flipping the on/off level back and forth??????

 

[sawinredneck]

yeah, took me a minute to figure that one out, you got "fat fingerd" on the keyboard, as I often do! It is similer to fliping the on/off switch, its real good, then nothin, then real good again, kinda make sense?

 

[THALL10326]

yeah, took me a minute to figure that one out, you got "fat fingerd" on the keyboard, as I often do! It is similer to fliping the on/off switch, its real good, then nothin, then real good again, kinda make sense?

Makes perfect sense now. It appears you could possibly have a shorted coil problem cause by the ground wires going to it. I've seen this only a few time and each time drove me nuts. Check all your ground wires from the on/off switch all the way back to the coil. Make sure there are no coating breaks or cracks anywhere in the wiring. Make sure your ground wires are firmly screwed to the engine. Just take the pull rope assembly off and you'll have full view of all the wires. I'm sure since your a mechanic you've done the old run the engine in the dark to see if sparks are flying. On a saw that is very hard and dangerous to do because you would have to pull the rope assembly off while it is running and thats a major no no. So check those ground wires closely. If you see any cracks in the wire coatings, any pinches in the wiring, loose connection to the engine block you have most likely found your problem.

 

[sawinredneck]

Thank you kindly, I will look into that tomorow and see what I see. That sounds a lot more realistic to me. Again, thank you very much for you're time, I will let you know if it works tomorow.
Andy

 

[THALL10326]

Thank you kindly, I will look into that tomorow and see what I see. That sounds a lot more realistic to me. Again, thank you very much for you're time, I will let you know if it works tomorow.
Andy

Anytime. From what your describing your saw is cutting off and on and only the ignition system does that. May want to change the plug too, they do go bad. Check those wires close. Its agervating but it sounds like its your best best. A fuel system problem would not make your saw cut off and on so you've narrowed it down alot. Good luck with it, hope you getter going.....

 

[Lakeside53]

Thal has it nailed... one more thing (there always is..) - make sure your plug has a solid end connector, not the screwed on type - these tend to unscrew and if used are supposed to be crimped tight with a pair of vice grips. Definitely run it in dark location - I've been caught a number of times with shorting ignition wire and bad plug connectors that would have easily been seen at night. one I caught only 'cos it was mid winter and I started the saw before going home in the dark to see if it was still a problem , and sure enough, sparking to the cylinder fins...

My bet is coil... and it has a lifetime warranty... if your dealer is "o.k." he might look beyond your muffler mod... maybe...

 

[sawinredneck]

Finally got time to mess with it again.

I have some weather moving in today so I have some time, I pulled the recoil and checked the wiring, I gave it maybe an 1/8 turn with a 1/4 drive ratchet, the "two fingers tight" tork spec., the coil is gapped at a very snug .014". No loose insulation or cuts in the wires, that I noticed? I have some wood on the trailer yet so I will try running it in a bit to see if it acts up again. But.... everything seems to lead to the coil.
Andy