Sorry If I missed it, but have you looked at the piston ?
Stihl MS-261C M-tronic with weak spark
- Thread starter scottieamn
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scottieamn
ArboristSite Lurker
I have not. Does m tronic sense compression? If so my spark test with plug out would not spark. If not I don't see the connection between no/weak spark and compression. Low compression and not firing I understand the connection.
Also, I have an open question earlier in the thread asking if compression testers work on my saw. What psi should I observe if healthy?
Well, if you are sure it has no spark, isn't the only thing left the flywheel? Can you borrow one of someone there? You disconnected it at the coil end right, so a shorted wire wouldn't kill the spark?
Take the kill wire off the coil try a new plug and if no spark get a flywheel. Just make sure there is no spark, try it in a dark room .
Didn’t know it had changed that much.
MT has nothing to do with comp. If your comp is low the piston/cyl has issues. But, the saw should “run” with low compression to a point.
How often do flywheels go bad? I’ve never heard of that other than some sort of physical damage to it or a defective one.
If the piston is scored it does not matter whether it is Mtronic or not. It will not run.
Compression testers on small displacement can be way off. Should run OK at 120. 150 or higher would be great. Get down much lower than than 115 or 120 and it probably won't idle reliably.
But, you are not likely to go to Advance or similar and get a tester that will work on small displacement. Snap On has some with a low threshold schrader valve in both ends that work OK. Get your pocketbook out.
You can take that muffler off in 30 seconds. Simplest thing to do. If the horizontal tooling marks and all look good then keep going the way you are going. But, if the piston got scored somehow you are barking up the wrong tree.
Rarely, but I have seen it happen twice. It will give you fits, no one expects this.
Definitely a good idea to check that. I was going on the no spark diagnosis .
scottieamn
ArboristSite Lurker
Does low compression effect spark on a m tronic?
I understand compression effects if it runs.
But would a m tronic saw with low compression spark the same as a m tronic saw with 150psi? I know on a standard saw the ignition has no clue if the head is removed, it sparks the same.
I'm unsure what sensors the saw has.
I'm well versed in controls for my day job.
Yes, it should spark regardless of compression. Did you pull it over in the dark or touch the electrode after swapping a new plug yet?
scottieamn
ArboristSite Lurker
Thanks for confirming. I'm confused why people keep asking about the piston when it has spark issues. Yes, I have done in dark and touch test.
2nd this.
Learned this using starting fluid trying to mount tires Not starting a tire mounting procedure war here. Only stating my experience with "new" starting fluid. Have been told John Deere starting fluid has highest percentage of ether?
Because over 50% of the people who bring in equipment saying it has ignition problems dont have ignition issues-it’s something else-piston or carb or?
Only focusing on one part of the diagnosis will get you in trouble every time. Test compression by holding the starter handle. The weight of the saw should drop slowly or hold. Pull it over. Does it spin a few revolutions or maybe just one? (looking for internal binding)
Dump the fuel into a glass jar. Is it a color other than jut clear or yellow? (To show it is mixed fuel) Does it have water? Does it have a strong GAS smell.
Check spark with a plug that has a gap of a 3/16 to 1/4 inch. It should jump it. Bright and blue.
PULL THE MUFFLER!!!!! Check the piston condition. Machine marks around the piston should be visible. Marks up and down show scoring.
Remove carburetor. Look inside to piston. Push on rings. Should have slight movement.
Check carburetor with a pressure tester. Should hold.
Disassemble carburetor. Inlet screen clean? Diaphragms soft and pliable? Any corrosion? Inlet needle and lever free?
Check fuel hose for cracks and mushiness. Replace fuel filter.
VACUUM AND PRESSURE UNIT. Yes it is that important.
Now we can talk about the issue.
Dump the fuel into a glass jar. Is it a color other than jut clear or yellow? (To show it is mixed fuel) Does it have water? Does it have a strong GAS smell.
Check spark with a plug that has a gap of a 3/16 to 1/4 inch. It should jump it. Bright and blue.
PULL THE MUFFLER!!!!! Check the piston condition. Machine marks around the piston should be visible. Marks up and down show scoring.
Remove carburetor. Look inside to piston. Push on rings. Should have slight movement.
Check carburetor with a pressure tester. Should hold.
Disassemble carburetor. Inlet screen clean? Diaphragms soft and pliable? Any corrosion? Inlet needle and lever free?
Check fuel hose for cracks and mushiness. Replace fuel filter.
VACUUM AND PRESSURE UNIT. Yes it is that important.
Now we can talk about the issue.
scottieamn
ArboristSite Lurker
I'm not part of that 50%. As stated above, I have never brought in a small engine for service. J/k. Good advice!
With a new plug? If you are positive no spark, this points to the flywheel . Did you gap the new coil ?
Pull it over faster and see if you have spark
And that would be about everything to check i believe.
