Stihl 660 heavy backfire no start

[Treemendous123]

Hi folks, 

 

Have got a stihl 660 which wasn’t running brilliant. Swapped out the carb to another I new was running well. Ran fine for a minute or two and then the flywheel spun off shearing the key. Luckily I had another so swapped that out and refitted checking the coil gap while I was at it. Now it won’t fire and every third pull or so I get a heavy backfire. Any suggestions? Possibly timing out now?

 

thanks connor  

[Stubby]

3 minutes ago, Treemendous123 said:

Hi folks, 

 

Have got a stihl 660 which wasn’t running brilliant. Swapped out the carb to another I new was running well. Ran fine for a minute or two and then the flywheel spun off shearing the key. Luckily I had another so swapped that out and refitted checking the coil gap while I was at it. Now it won’t fire and every third pull or so I get a heavy backfire. Any suggestions? Possibly timing out now?

 

thanks connor  

Wrong flywheel for that coil ?

[Treemendous123]

37 minutes ago, Stubby said:

Wrong flywheel for that coil ?

Well it’s the same flywheel that was on just the key that I swapped out. And the saw was running pretty well before it flew off. I reckon it just had not been torqued up properly. Think it could be the coil going bad? 

[spudulike]

The 660 coil and flywheel are pretty robust and rarely go wrong. The flywheel has a steel taper and it is rock solid and can take a lot of torque on the location nut. 

Backfiring is a sign that the ignition timing has shifted which is strange. Best to pull off the flywheel, check the key and then reset the flywheel again and tighten - not sure how you locked the crank in place to do this but you should use the Stihl double ended plastic stop for this IMO. 

Not sure how the original flywheel "spun off"????? They usually need a damn good whack once the puller is fitted, to get them off! Are you in the repair game??

If the key is OK - I suspect it has sheared again as I don't reckon you are tightening it enough and am guessing you had removed it before......it is possible the flywheel coming loose has hit the coil and either damaged the internals or the flywheel impact has damaged the magnetism in the flywheel. I have seen this before - the magnetism loss in a MS461 so very much like the 660 flywheel but this one lost its spark!

In order, I would say Key, Coil, flywheel would be the most likely issues.

Is there any other history to this machine?

 

[Stubby]

51 minutes ago, Treemendous123 said:

Well it’s the same flywheel that was on just the key that I swapped out. And the saw was running pretty well before it flew off. I reckon it just had not been torqued up properly. Think it could be the coil going bad? 

Sorry , I thought you said youi fitted another flywheel I can see now you meant a new key . Still sounds like timimng thouigh .   edit . Spud has suggested the key sheared again . i am thinking the same .

Edited September 26, 2020 by Stubby

[openspaceman]

2 hours ago, spudulike said:

Best to pull off the flywheel, check the key and then reset the flywheel again and tighten -

Do you not lap the taper before fitting a new key in case the keyway has raised a lip?

[Treemendous123]

6 hours ago, spudulike said:

The 660 coil and flywheel are pretty robust and rarely go wrong. The flywheel has a steel taper and it is rock solid and can take a lot of torque on the location nut. 

Backfiring is a sign that the ignition timing has shifted which is strange. Best to pull off the flywheel, check the key and then reset the flywheel again and tighten - not sure how you locked the crank in place to do this but you should use the Stihl double ended plastic stop for this IMO. 

Not sure how the original flywheel "spun off"????? They usually need a damn good whack once the puller is fitted, to get them off! Are you in the repair game??

If the key is OK - I suspect it has sheared again as I don't reckon you are tightening it enough and am guessing you had removed it before......it is possible the flywheel coming loose has hit the coil and either damaged the internals or the flywheel impact has damaged the magnetism in the flywheel. I have seen this before - the magnetism loss in a MS461 so very much like the 660 flywheel but this one lost its spark!

In order, I would say Key, Coil, flywheel would be the most likely issues.

Is there any other history to this machine?

 

Hi spud, thanks for the in-depth reply. Took the flywheel back off and as predicted the key has sheared again. Is this down to not torquing up the flywheel to the correct setting. Just recently bought the saw and when I say the flywheel spun off I meant it was spinning with the key sheared the first time round. Still attached but the bolt was barely hand tight. I’ll get another key and get it all put back together see what it’s like after. I just picked up the saw and have used it a handful of times. Once heated up it was stalling at idle. Made me think the coil was on the way out and then by chance this happened also. I’ll try the new key setting the coil gap again and report back. 

 

Thank very much for the advice 

[Treemendous123]

6 hours ago, Stubby said:

Sorry , I thought you said youi fitted another flywheel I can see now you meant a new key . Still sounds like timimng thouigh .   edit . Spud has suggested the key sheared again . i am thinking the same .

No probs, thanks stubby as mentioned have seen the key is sheared again so will investigate that first and go from there thanks for the help! 

[spudulike]

8 hours ago, openspaceman said:

Do you not lap the taper before fitting a new key in case the keyway has raised a lip?

The crank is manufactured out of damn hard steel and is very unlikely to get damaged or form any sort of "lip" etc. I just fit a new key and then crank the flywheel down pretty damn hard - the exception being some of these lighter modern flywheels that can crack if you torque them too hard. The 660 is fine, it has a steel boss!