If it was me Norm and the bottom seal has been replaced and the old seal if the rubber had gone hard I'd be wanting to change the top seal,
I know it's a bit of a pain with the later 160 cc FC when the motor has to come apart to hit the seal in from the crank counterweight end,
It's just that we know when there's a problem like this kickback you need to know where the problem is and to change the seal will
rule out that as a possible cause but you can check the rest and see how you go and do the top seal last if the other checks and remedies don't fix the kickback.
I've always found most old 2 strokes run and start better with new crank seals also it's interesting when I have a 2 stroke
that I know the seals need replacing and it's hard to start and keep running and all I do is convert them to electronic ignition
and they run and start a lot better but I still know the seals need replacing.
The way I look at it is every old 2 stroke motorbike I've ever owned ran and started better when I changed the crank seals
As we know crank seals aren't made to last
A leaking top seal allows air to enter the crankcase during the intake phase.
This dilutes the fuel-air mixture, making it too lean or inconsistent.
It can also reduce the vacuum needed to draw in fuel from the carburetor properly.
.
Condition Expected Lifespan
Regular use, well-maintained 5–10 years
Infrequent use, dry storage 8–15+ years (less wear, but seals may still go hard over time)
Poor storage (heat, sun, fuel exposure) 2–5 years
Cheers
Max.
