Stihl backfires and won’t start... how to fix!!! (Works for Stihl FS, KM, MM, HT, BR equipment)

Hey everyone!!!
This is the second time I have ran into the issue of a Stihl trimmer / yard boss / blower / Kombi / pole saw backfiring when trying to start.
It took me a while to diagnose it the first go around, but I knew exactly what to look for when it popped up again!!!
The backfiring is caused by a failed ignition coil.
This video is a step by step how to replace the ignition coil on a Stihl FS / KM / Yard Boss system. It will work to fix the other types of Stihl equipment, but the process for removing the coil will be different for other items.
Once I had it swapped out, it fired up first pull!
Take care out there,
-Dave

[@THEMOWERMEDIC1]

Awesome Dave! My Fc55 did that about 4 years in! Coil fixed it but mine was under the lifetime ignition warranty

[@DavesSmallEngines]

Thank you Sir! Good to know I’m not the only one!

[@jimwesselman]

Thank You Dave! I just purchased a new MM56C, bought a non-OEM cylinder and going to try to rebuild my MM55 and this video gives me a good place to start! I have had many cultivators including a Honda FS110 and by far the Stihl is the best! These should not be confused with a tiller, such as the Honda 220, unless you have some great soil. My Kentucky clay is too hard and rocky. The Stihl does incorporate soil amendments very well!

[@DavesSmallEngines]

That’s awesome Jim. Keep me posted if you have any questions! Cheers!

[@raynaldgodin3314]

Good job Dave, it's nice to see a different king of machine.

[@DavesSmallEngines]

Thanks 👍

[@benjaminkolsom1466]

Thankyu so much that's very impressive Sir and a good day to you tomorrow Sir....

[@Trent777howsit]

Awesome got the same issue, thanks very much for posting 😊👍

[@DavesSmallEngines]

You’re welcome! Thanks for watching

[@HobbyMotorDK]

Great fix Dave. Thanks for sharing

[@DavesSmallEngines]

Cheers!

[@puchmanwal1685]

great i have a McCulloch Gladiator 550 with the exact same problem so hope fully the coil replacement will work for me thank you ,either way enjoyed your video

[@jeremybrain5774]

Nice fix Dave. Keep up the good work.

[@DavesSmallEngines]

Thanks Jeremy!

[@sylvanuseastman8489]

Thank you this is of great help.

[@nashguy207]

Thanks for the video and the great tips!

[@DavesSmallEngines]

Thanks for coming back, Nash!

[@petefroud5308]

Nice work . Learned something today.

[@DavesSmallEngines]

Thanks Pete!

[@ebfromwyoming4118]

wow great fix sir and a very clean shop sir

[@DavesSmallEngines]

Thanks EB! Great to hear from you again.

[@josephwalsh5486]

Thanks for the video! My weed wacker did this to me today lol.. I know what i have to buy! Great video

[@tc3603]

Great vid Dave. Currently have same problem with a husky 61. Your garage is very organised.

[@DavesSmallEngines]

Hey TC! Check the coil! It’s kind of messy right now actually... I’ve been super busy lately!

[@tc3603]

@@DavesSmallEngines will do Dave as everything else is good👍

[@Bungi81]

Hey Dave. Good job👍👍 Greetings, Stefan

[@DavesSmallEngines]

Hey Stefan! Thanks!

[@donyboy73]

nice job dave!

[@DavesSmallEngines]

Thanks Don!

[@gasperajdnik5627]

Good work dave 👍🤝

[@DavesSmallEngines]

Thanks!

[@pentaxk110d]

Thanks for the video. What else could cause a backfire? My trimmer backfires but only at full throttle. Otherwise it runs well and is in good condition. Would you suspect a faulty ignition coil also?

[@stevanperic9080]

Nice job. 👍

[@DavesSmallEngines]

Thank you!

[@dgmenace73]

Thanks for this video. I have a Husqvarna chainsaw that has only been used to drop one tree then it sat for nearly a year. It did the same as this ....while pulling to start it it backfires loud. I already replaced the carburetor then the coil....same exact thing. It backfires while cranking it. Will not run. Do u have any other suggestions? The key on flywheel looks fine.

[@peteramirez50]

I purchased a stihl bg56c hand held blower it started back firing I did a maintenance on it Changed spark Cleaned arrestor changed fuel and fuel filter along with air filter and still back fired I’ll order ignition coil hope it works !!

[@seanmcgillicuddy956]

Nice video..Thanks

[@DavesSmallEngines]

Hey Sean! Cheers!

[@brianking1138]

I knew you would get it when you turned the hat around . Great job I would have thought it was a sheared key.

[@DavesSmallEngines]

Haha thank you Brian!

[@stellarproductions8888]

If you own a spark tester, you can also confirm weather your getting spark from the coil that way too. And yes, the coil was fried, and while that is not necessarily a common occourance in small engines, it does happen from time to time. Good work as always Dave.

[@DavesSmallEngines]

Thanks Stellar! I’ll look into the tester.

[@RestoreMore]

@@DavesSmallEnginesI’m struggling with a HS45 right now. It won’t start but it will backfire just like this engine. I pulled the plug and confirmed get a spark when pulling the cord. Will a new coil fix my issue too?

[@henerker]

Nice job, Dave! Great instructions for us. Why is the new ignition coil so rusty? Was it a “used” part?

[@DavesSmallEngines]

Yes - I picked it up used for $20. I have the contact points a wire wheel job and tossed it on! Cheers!

[@ferrellcat5860]

I have a fs 38 trimmer , it starts right up and may run for 2 seconds or 30 seconds, it backfires out the exhaust and dies . Starts right back up and does the same thing runs for a few seconds or so and then backfires and dies . I thought it was water in the gas causing it to run bad . Changed it all out and still does this . Thoughts on a fix ? Could it be the coil as well on mine or ? It might be 4 years old .

[@sempreame]

Hello, Geetings from Rio! Cool tips. What if it backfires while throttle is open and engine is under load?

[@jonathanranguy9115]

Is the placement of d piston have any thing to do with d backfire too

[@K-ug5is]

I was just wondering that I have a FS 38 start for a little bit and it will die and sometimes it will backfire I bought a brand new carburetor and I looked at the the fuel filter that's okay and I don't know what else it would be I just wanted some suggestions

[@SwiftBolt17]

I have the same issue on my FS55R, I thought the flywheel was out of time but the woodruff key is still intact so now I'm trying to source an OEM coil without breaking the bank, most amazon ones say it backfires

[@xmanb8193]

Nice vid

[@DavesSmallEngines]

Thanks!

[@jonathanfierberg3807]

Impressive

[@DavesSmallEngines]

Thanks Jonathan!

[@adnacraigo6590]

Nice job. Good diagnostics.

[@DavesSmallEngines]

Thanks!

[@EzRay11]

Hey Dave It looks like the key to getting the casing off is to turn your hat backwards. Great video Bud. Cheers

[@DavesSmallEngines]

That could be the trick!!!

[@carlmiller1034]

"Over the Top" Stallone-style!!!

[@SheikShak]

Thank u

[@DavesSmallEngines]

Cheers!

[@sprinterdiscovery]

Good job! Does a new coil help with first-pull starts? I maintain my KM90 myself at home, it runs very well, but does take maybe 5 pulls after priming to get it to start. It's from 2011, and I use it weekly from February to November here in Vancouver.

[@DavesSmallEngines]

Have you adjusted the valves yet? That’s a service item on the 4mix engines.

[@sprinterdiscovery]

@@DavesSmallEngines Yes definitely, a couple times a year, and a new gasket once in a while.

[@DavesSmallEngines]

@@sprinterdiscovery I have an FS90 and it likes to start first or second pull. How many times we you purging? Choke on and throttle locked in start position?

[@sprinterdiscovery]

@@DavesSmallEngines I have tried purging 5 times, and also 10 times (probably to much). I'll record a video later and post it for you to see. It would be incredible if we can diagnose over KZbin! 👍

[@bracaarboristi]

What if the unit is brand new? And it happens once at the first pull and then turns on and works nicely. Why then does it pop? Too rich?

[@mikesiefkas3873]

Got a stihl 017 doing that after putting a new coil on it. Won't start, backfires through carb once in a while. Don't know what is going on.

[@shaunsykes96]

I have changed my ignition coil and mine is still back firing? Any suggestions?

[@c0ldw1nd27]

Same problem for me, swapped the coil and the same thing keeps happening, can the new coil also be a faulty? Are there other factors that can cause this?

[@DavesSmallEngines]

I had swapped another coil and had the same issue in the past. If you have access to one off of another trimmer, give that a try!

[@nikolavincek2347]

this coil you put works but it have rev limiter to low it is not good, i had similar problems and i put coil from fs 44 and adjust timing on magnet for correct time

[@DavesSmallEngines]

Hey that is good to know!

[@jerimykelley2559]

Great video.... can the module provide spark and still be bad? My Stihl weedeater has spark but won't start and is backfiring.

[@DavesSmallEngines]

Check the flywheel key isn’t sheered.

[@jerimykelley2559]

Ok, Thanks and keep up the great videos!

[@jerimykelley2559]

I'm really stumped. Flywheel checked out and compression was 140. Any other ideas?

[@fredderf4075]

Hi Dave. My FS55r does exactly the same thing, but there is still spark at the plug. Is this possible?

[@miatalife94]

I've replaced the carburetor and cleaned the exhaust on my mm55 and it also was getting spark and having these symptoms, I just ordered a new coil from stihl

[@fidelcantu5735]

Hello dave I have a question, when an engine does not have compression or is damaged as you say, the first thing is to check the muffler to see if the piston is not scratched, I would like to know why when the piston is scratched most of the time it is scratched on the muffler side it will be because it is where there is more heat ? or what will the reason?

[@DavesSmallEngines]

Correct. It’s almost always more damaged on the exhaust side from the heat.

[@philsmith5829]

I have also thought about this and from what i can figure the engine is trying to force exhaust OUT so also whatever is being chewed up wether piston or cylinder it goes along fir the ride and causes most of the damage on that side

[@SuperJamster1]

Nice video. The real question is "Why does a bad ignition coil cause backfire?"

[@DavesSmallEngines]

Hey Super! I’m not an electrical expert but my understanding is an internal failure causing a mistiming in the spark

[@thelumberjack5218]

I’ve replaced the ignition coil,now when I go to start the whipper snipper it makes a cracking sound and there’s smoke coming from spark plug and boot 🤷‍♂️

[@rustipants]

But how had the coil failed?? Any more information on how it failed? I have just replaced the coil as I didn't have a spark, but now it is back firing with a new coil.....

[@mikiedunn9263]

I just did the same thing with a mm55c .had no spark, tested it and found the cool to be bad, replaced with a new one and now it backfires. Could the coil be bad or the wrong one?

[@fidelcantu5735]

Nice video dave , but why a coil that is damaged cause a backfire?

[@DavesSmallEngines]

Good question - I don’t know any electrical engineers but I am assuming it’s because of an internal issue causing weak or mistimed spark!

[@mariovillalobos1234]

Iam running into thi issue with my stihl MM55 ,I installed a brand new coil but no change, could it be the flywheel, I also installed a new car too ,

[@DavesSmallEngines]

Could be a sheared flywheel key.

[@donaldbower4097]

Could it be timing, I just put a brand new coil on my fs55r, and it is doing this. I find it hard to believe that the brand new ignition coil which has great spark is bad.

[@DavesSmallEngines]

Non adjustable timing.

[@donaldbower4097]

@@DavesSmallEngines I'm aware, I've seen some act like that and it was a sheared flywheel key, don't know how, but the flywheel spun.

[@Huero805]

mine does the same thing but it has a spark so???

[@lewishaslam]

Would this be the same on a sthil chainsaw?

[@DavesSmallEngines]

Yep! Any two stroke engine really.

[@welshmanbob3889]

Great video as always. I did cringe when you started it up and the blades rotated. You with your shorts on 😧

[@DavesSmallEngines]

Safety first! In al honesty, I had pulled it so many times (probably 100) I knew it would not start.

[@JS-km6ol]

Hello I have a fs38, it used to work before but then all of a sudden when I was using it, it randomly turned off and when I’d try to turn it on it wouldn’t turn on I checked the spark plug and it wasn’t generating any sparks so then I replaced the ignition coil, after that it started generating sparks but when I try to start it, it starts backfiring but it still doesn’t start. When it backfires it either comes out of the carburetor or the exhaust Can u please help me out because I don’t know what to do

[@DavesSmallEngines]

I had that happen too. It wasn’t the right coil or it was a faulty coil. I found a used one and it fired right up! Good luck!

[@JS-km6ol]

@@DavesSmallEngines Hey Dave! Thank you so much for your suggestion, I really appreciate it. I replaced the aftermarket coil with the original one, like you had said. And now it works perfectly! Once again, thank you. I also needed help with something else, there is something rattling where we insert the shaft, near the engine. This didn’t happen before but it’s happening now, is there something that can be done about it? Please let me know.

[@DavesSmallEngines]

@@JS-km6ol hey JS! I’m glad to have helped. What do you mean by rattling? Put up a video of it and let me know. I’m sure I’ve heard it before!

[@JS-km6ol]

@@DavesSmallEngines I recorded a video but where’s should I send it?

[@DavesSmallEngines]

@@JS-km6ol upload it to your KZbin and then let me know here. I’ll check it out

[@harrisonkatzz7090]

Dave what is the gap ?

[@DavesSmallEngines]

I use a business card!

[@harrisonkatzz7090]

@@DavesSmallEngines The gap in numbers.

[@DavesSmallEngines]

@@harrisonkatzz7090 .010” from my quick research

[@bobgreen3116]

I have the same problem and I put on a brand new coil from Stihl and it still backfires. Any ideas?

[@DavesSmallEngines]

Wrong coil potentially?

[@bobgreen3116]

@@DavesSmallEngines I bought the coil from the stihl dealer. It looked the same as the one I took off. That had no spark at all.

[@DavesSmallEngines]

@@bobgreen3116 is it sparking appropriately now? Have you tested it? Is it grounding correctly? Are the magnets clean?

[@bobgreen3116]

@@DavesSmallEngines it ran for about a minute then started to backfire. I rebuilt the carb. New fuel lines and filter. Than I just bought a new carb. Oh and a new plug tapped at .030

[@DavesSmallEngines]

@@bobgreen3116 hey bob. Take a good video and send it to me on my site davessmallengines.ca/

[@LJ-ow2ph]

Iv just replaced the coil and now I'm having this issue

[@DavesSmallEngines]

Did you get it sorted LJ?

[@LJ-ow2ph]

@@DavesSmallEngines yes the oil seals where gone and causing it to backfire

[@DavesSmallEngines]

@@LJ-ow2ph good find!

[@2THPWR]

You’re a yard boss.

[@DavesSmallEngines]

:)

[@TheAuzzieTools]

It didn’t fix mine

[@shermankearns200]

Milwaukee not Dewalt lol

[@DavesSmallEngines]

Hey Sherman!!! I totally agree. I'm kicking myself for not going Milwaukee from the start. I may make the jump soon!

[@shermankearns200]

t27 lol

[@DavesSmallEngines]

Yep!

[@gallagherkevin4]

Lucky it didn’t start at 0:45

[@DavesSmallEngines]

I had pulled it about 100 times before that - I knew it wasn’t going to start! Good call though!

[@gallagherkevin4]

@@DavesSmallEngines really enjoy your videos! Keep it up!

[@shermankearns200]

LOLOLOL

[@DavesSmallEngines]

Hey!