G'day Bushy,

Could be coil , condenser , timing or spark plug (ignition) problem.

The points should fully close ,in the image with the rubbing block on the high part on the lobe the gap is set then rotate
the coil plate so the points just start to open when the piston is at 1/8 of an inch before TDC.

The rubbing block on the points that rides on the cam lobe can wear down. This will affect when the points open and thus the timing. If it's significantly worn, the points may not open at the correct time or for the correct duration.

Even a clean gap on pitted or burned points can lead to a weak or inconsistent spark. Inspect the contact surfaces closely. If they are not smooth and clean, the spark will be unreliable.

I usually check the magnets on the flywheel with a screw driver to make sure they are not weak.

The small restrictor in the air cleaner housing is important as they won't run properly if another air filter is used.


Worn crankshaft seals can allow air to enter the crankcase, leading to a lean mixture and affecting scavenging, which could indirectly contribute to poor running. However, this usually manifests as erratic idle and poor overall performance .

One way I test the crank seals on these mowers to see if the mixture is lean is start the mower and while it's running you
crimp the intake tube with 2 fingers and while the intake tube is about 50 to 80 percent restricted closed with your fingers see
if the motor revs out fully and if it does you know the seals are leaking.

There are a few ways to check crank seals ,sometimes I use a feeler gauge that is a couple thou thick and slide that between the crank
shaft and the seal and when the seals are old the feeler gauge slides in without resistance.


In essence, leaking crankcase seals create a chaotic environment in the crankcase and cylinder that disrupts the normal flow of the fuel-air mixture and combustion process, leading to the spitting back you're observing.

So, while ignition timing is a very common cause of backfiring through the carburetor, leaking crankcase seals are definitely another significant possibility, especially if you're experiencing lean running symptoms at higher RPM. The "finger test" or restricting the intake tube is a good way to further investigate this possibility.

Cheers
Max.