Stihl FS 45-Z String Trimmer Not Working – Only Occasional Backfire

[evh]

I was given a Stihl FS 45-Z 281859012 DOM 12.09 (Dec-2009). It is in great shape but does not run. The owner said he replaced the coil with an aftermarket, and it only occasionally will backfire, that is it.

I tried it and yep, an occasional weak backfire. I replaced the coil with a used OEM and same results. Here is a list of things I have tried to no avail:

- Used starting fluid in both carb and spark plug hole
- Confirmed spark with spark checker
- Used 3 different spark plugs (2 – NGK’s and one Champion)
- Cleaned spark arrestor (it was hardly dirty, but cleaned it anyhow)
- When installing the used OEM coil, I used a business card to gap it
- Compression is fantastic
- Fuel is new REC gas with high quality 2-Cycle oil at 50:1

One thing I noted when I had it apart was that the flywheel does not have a separate key but used a small bump built into the flywheel. When I put it on, it has slight movement left and right. I assume this is ok because reassembly tightens it down. I examined the key/bump on the flywheel with a magnifying glass and I don’t see any damage (i.e. checking if I have a sheard key – a common backfire cause on small engines).

So, having said all of this, what would you check next? What am I missing???

Thx.

 

[StarTech]

Verify you are using the correct ignition coil.

 

[Tiger Small Engine]

I was given a Stihl FS 45-Z 281859012 DOM 12.09 (Dec-2009). It is in great shape but does not run. The owner said he replaced the coil with an aftermarket, and it only occasionally will backfire, that is it.

I tried it and yep, an occasional weak backfire. I replaced the coil with a used OEM and same results. Here is a list of things I have tried to no avail:

- Used starting fluid in both carb and spark plug hole
- Confirmed spark with spark checker
- Used 3 different spark plugs (2 – NGK’s and one Champion)
- Cleaned spark arrestor (it was hardly dirty, but cleaned it anyhow)
- When installing the used OEM coil, I used a business card to gap it
- Compression is fantastic
- Fuel is new REC gas with high quality 2-Cycle oil at 50:1

One thing I noted when I had it apart was that the flywheel does not have a separate key but used a small bump built into the flywheel. When I put it on, it has slight movement left and right. I assume this is ok because reassembly tightens it down. I examined the key/bump on the flywheel with a magnifying glass and I don’t see any damage (i.e. checking if I have a sheard key – a common backfire cause on small engines).

So, having said all of this, what would you check next? What am I missing???

Thx.

Remove muffler and visually look at the condition of piston, rings, and cylinder for scoring.

 

[evh]

Thank you both for the feedback. I will remove the muffler and check for scoring. Note, it feels as though it has great compression.

StarTech, what you describe seems to match what the previous owner was telling me. I don’t remember all the conversation, but it was very similar.

I took the unit apart (again) and I have verified my crankshaft matches the one on the right. So that would seem to need the …1303 coil. The first pic below shows the coil that was in it. The previous owner said they replaced the coil. There are no markings on it at all.

The second photo shows the used coil I bought and installed (same results). Note it is a …1300A. According to the post below, that is the wrong one, thus, same results.

I will find a …1303 and see if that resolves the issue.

StarTech, can you point me to the resource you took that information out of?

Finally, see the last photo and the red sealant on the housing. Is that factory, or did the prior owner do more than change the coil? I googled it and it seems to indicated Stihl uses red sealant.

 

[Auto Doc's]

Remove and torch the muffler red hot to burn the carbon buildup out of it. I have had many of these with a choked muffler due to much oil being mixed with the gas with DIY types. "If a little is good, then more is better" type of folks.

 

[StarTech]

From IPL