Stihl ts400 backfires

[spudulike]

I have just read the first post....the engine runs 100% fine without an air filter and then backfires with the air filter fitted...is this correct?

If it is, when you blip the throttle without the air filter fitted, do you get a puff of fuel vapour emitted from the carb?

[adw]

Not sure if this helps but a power cutters have some way of governing the engine speed, this to control the wheel speed, the Husqvarna has an ignition governor, I guess the Stihl also has a governor like this, they will be set at 9800 to 10000rpm, when the engine hits the ignition governor they do back fire and even spit flames from the muffler, if something has gone wrong with the ignition this may be the issue with it coming in too early, just a thought.

[adw]

Another thought, it will have a compensator carb, is the hose blocked or trapped, if so this will weaken the mixture causing it to run lean, a lean mixture will bring the governor in too early.

[spudulike]

2 hours ago, spudulike said:

I have just read the first post....the engine runs 100% fine without an air filter and then backfires with the air filter fitted...is this correct?

If it is, when you blip the throttle without the air filter fitted, do you get a puff of fuel vapour emitted from the carb?

My thoughts in asking this question was that if the piston skirt on the inlet side is badly worn, it won't seal the inlet port and you will get significant "free-porting" where the fuel vapour isn't controlled within the crankcase and cylinder but is pushed back out of the carb hence watching it without the filter and blipping the throttle and watching for puffs of vapour from the carb.

How this impacts on the engine may be dictated if the air filter is fitted or not but from my experience, you just get a poor pickup and unstable idle and have never had backfiring from it but can't understand how fitting an air filter will change the running that dramatically unless it is completely blocked.

Fitting an air filter will drop the revs and from ADWs thoughts, it should be less likely to hit a rev limiter or governor etc 

Any back firing I have ever had has been caused by ignition/flywheel issues causing a shift in ignition timing. 

I would check the flywheel part number on the other machine or try the complete coil and flywheel from the other machine and see if it cures the issue.

[ChrisNewport]

Coils and flywheels on the ts400, two flywheels and two coils. XXXX XXX 1201 flywheel fits the coils with 2 fixing holes and XXXX XXX 1200 fits the coils with 3 fixing holes. You cant mix and match.

[spudulike]

10 minutes ago, ChrisNewport said:

Coils and flywheels on the ts400, two flywheels and two coils. XXXX XXX 1201 flywheel fits the coils with 2 fixing holes and XXXX XXX 1200 fits the coils with 3 fixing holes. You cant mix and match.

Is that 100% as I think the 660 has three mounting holes but both the earlier three hole and later two hole coils fit.

Just trying to totally discount the mix of parts scenario!

[ChrisNewport]

5 hours ago, spudulike said:

Is that 100% as I think the 660 has three mounting holes but both the earlier three hole and later two hole coils fit.

Just trying to totally discount the mix of parts scenario!

As far as im aware its correct, my head scratching moment was with a flywheel from a 3 hole coil on a saw with a 2 hole coil, just wouldnt have it, it popped now and then but just wouldnt fire. Owner said it just stopped working. I had suspicions someone else had been at it because the saw was a state but the flywheel looked cleaner that it should have been. I put the correct flywheel on and away she went. 

[spudulike]

There do appear to be two types of crankcase that may be for the two different types of coil.

If the OP can give us a serial number, I can tell what flywheel and coil he should have!

I still don't get this air filter on....backfires and air filter off.....runs fine thing I think I would start it with the filter off and then put it on with the engine running....all seems a bit weird!!

Edited November 5, 2021 by spudulike

[spudulike]

I wonder what happened!

[adw]

I think he has seen the light and purchased a Husqvarna K770 oil guard.