Stihl ts400 backfires

[Ian Brand]

Why does a stihl ts400 run perfect without filter but backfires through exhaust wit new filter fitted

[openspaceman]

17 minutes ago, Ian Brand said:

Why does a stihl ts400 run perfect without filter but backfires through exhaust wit new filter fitted

A backfire is normally from a premature detonation, before the piston has reach the top of the compression stroke and the piston is driven down in reverse. If you have an unfired mixture  detonating in a hot exhaust the it could be an over rich mixture.

[AHPP]

Four stroking?

[Ian Brand]

52 minutes ago, openspaceman said:

A backfire is normally from a premature detonation, before the piston has reach the top of the compression stroke and the piston is driven down in reverse. If you have an unfired mixture  detonating in a hot exhaust the it could be an over rich mixture.

I've rebuilt hundreds of saws, never seen this before, fitted new piston rings, put the carb in an ultrasonic cleaner, fitted new carb kit, new plug, tried another coil, problem is I got it in for repairs in bits along with a few other saws for repair, help before I go nuts

[Ian Brand]

Just now, Ian Brand said:

I've rebuilt hundreds of saws, never seen this before, fitted new piston rings, put the carb in an ultrasonic cleaner, fitted new carb kit, new plug, tried another coil, problem is I got it in for repairs in bits along with a few other saws for repair, help before I go nuts

Forgot to mention it is a single screw carb

[Stubby]

38 minutes ago, Ian Brand said:

Forgot to mention it is a single screw carb

Has the flywheel key sheared putting it out of time ?

[spudulike]

I am surprised you have never had a backfiring engine if you have rebuilt that many but the primary cause is the flywheel key shearing putting the ignition timing in to a position where ignition occurs where it shouldn't causing a backfire - usually retarded. The other possibility is that the coil has failed or been replaced with an incorrect type - coils have smart electronics often with advance ramps and if they fail, it may give bad ignition timing. Also, I have seen two machines of different years have different coil/flywheel types and if mixed up, this can give incorrect ignition timing.

It will be an ignition timing issue....you can try everything else but a backfire is always where the ignition has retarded that much that ignition happens after TDC causing a loud backfire. If this was too far advanced, the saw would kickback like a mule!!

[Ian Brand]

Changed coil with same part no. from running saw, flywheel is perfect

2 hours ago, Stubby said:

Has the flywheel key sheared putting it out of time ?

No. It's perfect

[Stubby]

9 minutes ago, Ian Brand said:

Changed coil with same part no. from running saw, flywheel is perfect

No. It's perfect

O.K. That is eliminated then . Maybe a coil problem ? Edit that came up as I was typing 🙂 So where does that leave us ?

Edited November 4, 2021 by Stubby

[spudulike]

OK, if the key is intact and the coil is good...you have a mix of parts.

This TS400 has two different flywheels and three different coils.

The flywheel has to match the coil and the good news is that there is only one crank so if you shift the flywheel AND coil from the good machine, the faulty one will run and you then just need to get the correct flywheel...simple!