Saw Dr.
Junk Collector
I wonder if the crank end could twist? Sounds like retarded timing to me.
Let's play diagnose the farm boss chainsaw.
- Thread starter fearofpavement
- Start date
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towingace
ArboristSite Guru
I need a 390 p/c for my 290! Seriously, that's a dirty bugger. I'm sure you'll keep us posted when you get back on it. Kinda' takes the wind out of your sails when you think you finally got it figured out and get whacked back to the start. At least there's a lot of things you've already eliminated, so a little sleep on it might help for a fresh start. Feel for ya' man, but you'll git 'er. If not, remember I need a 390 p/c.
KUBOTA OWNER
ArboristSite Operative
This sounds like a echo blower that I fixed for a friend a few weeks ago, he even had it to a dealer
and ended up buying a new husky blower when the dealer told him it was to expensive to repair. I also
did everything I could to try to figure it out. different carb ect. Unit would start good and would never
make high rpm. It had a rotary style carb on it with no mixture screws. It always seems to be rich. I even
clamped of the fuel line to see if it would at least run for a little bit until it ran out of fuel. No help. My conclusion
was that it was out of time. I even thought about trying to put a timing light on it. --- I kept thinking that
the coil might be screwing up the timing even though it had a great spark. I din't have another coil but saw a complete seized unit on ebay for 20 bucks and 3 days later it was here. 10 minutes to swap the coil and ran perfect.
First time I every found a bad coil with a good spark??
and ended up buying a new husky blower when the dealer told him it was to expensive to repair. I also
did everything I could to try to figure it out. different carb ect. Unit would start good and would never
make high rpm. It had a rotary style carb on it with no mixture screws. It always seems to be rich. I even
clamped of the fuel line to see if it would at least run for a little bit until it ran out of fuel. No help. My conclusion
was that it was out of time. I even thought about trying to put a timing light on it. --- I kept thinking that
the coil might be screwing up the timing even though it had a great spark. I din't have another coil but saw a complete seized unit on ebay for 20 bucks and 3 days later it was here. 10 minutes to swap the coil and ran perfect.
First time I every found a bad coil with a good spark??
fearofpavement
Trying them all
I'll think about this issue in the middle of the night. I've had times I've wakened in the wee hours with a thought that has led to a solution. Is it physically possible for the coil to be installed upside down? I put it on like the one I took off and the one I installed had previously been removed... Hmmm, have to check that out. Another thought I have is to shear the key on the flywheel and advance the spark a bit and see what happens...
I will eventually figure this out and get the saw running but it has sure kicked my butt.
I had a weird problem on a chevy small block once and it finally turned out that the rotor had a hole in it and the spark was shorting to the distributor shaft at above idle speeds. (the truck would start and idle fine but everytime you gave it gas it died)
I will eventually figure this out and get the saw running but it has sure kicked my butt.
I had a weird problem on a chevy small block once and it finally turned out that the rotor had a hole in it and the spark was shorting to the distributor shaft at above idle speeds. (the truck would start and idle fine but everytime you gave it gas it died)
Reading the thread again, I saw you already did the coil swap experiment. My mistake on that. At this point, it seems that you have pretty well eliminated the fuel system. You got some effect by working on the electrical system and found a problem with the coil wire. The saw ran better for a little while, but then the problem re-appeared. Like you say, it may be time to set it down for a day or two.
fearofpavement
Trying them all
I think that the previous coil (plug wire) issue was exacerbated by me while trying to troubleshoot the saw. ie, it was running bad, then it ran really bad, then when I changed the coil it ran better but when I tested it, it went back to the original bad running mode. I have run through every possible troubleshooting process I know of so the ignition timing issue is interesting to me. It is something that wasn't affected by changing the coil however... I can try a different flywheel, I can try advancing the timing, etc. I'll sleep on it and decide where to go from here...
Can you check to make sure the flywheel and coil "match" and are made for this saw.
Please excuse me if this is not a possibility, I havent worked on a ton of the saws in this series.
Please excuse me if this is not a possibility, I havent worked on a ton of the saws in this series.
Adirondackstihl
Flyz, Fuel & Fotos
I need a 49mm piston 
fearofpavement
Trying them all
Huztl.I need a 49mm piston
towingace
ArboristSite Guru
Hey, I thought I already spoke for that! All kidding aside, good used oem p/c's for that saw are tough to come by. I saw one on evil-bay, IIRC it was a "buy it now" deal. I got busy and spaced it for about 3 days. Of course, when I remembered, it was long gone and I've been kicking myself ever since.I need a 49mm piston
Well, since we're throwing everything at this problem, I think I would take another long LONG look at that ignition switch. I have been tripped up by what looked like a very normal switch only to find something out of alignment that was shorting part of the time and giving me fits! Probably not the issue, but sounds like everything else has been tried!
fearofpavement
Trying them all
I'm done searching aircraft parts inventories for today so am going to head to the shop to check out the timing of this saw. What I really need to be doing is fixing the tire of my Bobcat so I can load some logs but...
Thanks for this thread. This weekend my 44 had the same symptoms. After swapping carbs, verifying fuel and impulse del every and reading this thread I found the connection at the wire and spring under the boot to be bad.
old-cat
Fir Man
I found that same problem, same place on another STIHL saw. That terminal had a tiny burr on the barb that sticks through the insulation so it refused to go in far enough.
fearofpavement
Trying them all
Well, today I disconnected the kill switch wire and tried the saw. no joy.
Tried a completely different flywheel, regapped the coil to it, confirmed the coil can only be installed one way, tried yet another carburetor (since it seems so fuel related) and it is still acting up.
The current symptoms are:
Saw readily starts and will idle indefinitely.
When applying throttle, if done quickly it will just bog. If you milk it, the saw will come up to rpm. I was actually able to cut some wood with it today but the saw is low on power, bogs very easily and won't perform at all in big wood. I was cutting some 4" pine and it did ok on that as long as the rpm's were kept up. If you let the saw go to idle, it takes about 1.5 to 2 seconds to come back to high rpm.
Another interesting note and I should have mentioned it previously but the saw seems like it can't be tuned. ie, it seems to 4 stroke at all carb adjustments and I can completely close the H screw (with limiters removed) and the saw runs about the same. ie, when turning the H, it does affect rpm but not like normal. I tried various L settings as well and no difference on the acceleration rate.
I am just completely out of ideas on this saw and am thinking I may just exchange the engine to see if that could be the cause.
Meanwhile, I finished building up another MS390 with better success. That one needed a lot of parts (oil line, clutch cover, sprocket, bar and chain, impulse nipple in cylinder, muffler, misc hardware and isolators, and so on). It runs good though. I haven't had it in wood yet. I put it together and then had to pull the carb and pull the limiters as I couldn't get it rich enough. So I'll do final tuning at the wood pile.
Tried a completely different flywheel, regapped the coil to it, confirmed the coil can only be installed one way, tried yet another carburetor (since it seems so fuel related) and it is still acting up.
The current symptoms are:
Saw readily starts and will idle indefinitely.
When applying throttle, if done quickly it will just bog. If you milk it, the saw will come up to rpm. I was actually able to cut some wood with it today but the saw is low on power, bogs very easily and won't perform at all in big wood. I was cutting some 4" pine and it did ok on that as long as the rpm's were kept up. If you let the saw go to idle, it takes about 1.5 to 2 seconds to come back to high rpm.
Another interesting note and I should have mentioned it previously but the saw seems like it can't be tuned. ie, it seems to 4 stroke at all carb adjustments and I can completely close the H screw (with limiters removed) and the saw runs about the same. ie, when turning the H, it does affect rpm but not like normal. I tried various L settings as well and no difference on the acceleration rate.
I am just completely out of ideas on this saw and am thinking I may just exchange the engine to see if that could be the cause.
Meanwhile, I finished building up another MS390 with better success. That one needed a lot of parts (oil line, clutch cover, sprocket, bar and chain, impulse nipple in cylinder, muffler, misc hardware and isolators, and so on). It runs good though. I haven't had it in wood yet. I put it together and then had to pull the carb and pull the limiters as I couldn't get it rich enough. So I'll do final tuning at the wood pile.
fearofpavement
Trying them all
I'll add that the tuning issue was the same on all the three carbs I tried on this saw, so it's not a carb issue but something else...
When you pressure tested the crank case did you turn the crank slowly by hand a few revolutions?
I know its shouldn't matter but I was chasing my tail a while again with an air leak and that is the only time it lost vacuum was when I turned the crank slow by hand.
How does the saw idle on its side? Clutch side down.
I know its shouldn't matter but I was chasing my tail a while again with an air leak and that is the only time it lost vacuum was when I turned the crank slow by hand.
How does the saw idle on its side? Clutch side down.
fearofpavement
Trying them all
I did not turn the engine over while doing the tests. I wiggle it around a bit but did not make revolutions...
VinceGU05
PSA vitim.
the linkages are ok? getting full throttle? or just a lack of compression, glazed bore and cant hold the compression when it warms up? . motor swap sounds like a great idea. that would put to bed the fuel and ignition/electrical system once and for all.
Did you vac test also?
