Can a duff magneto cause engine seizure?

[stevejs]

Keep it simple....I'm with the fuel starvation mob. Check the filter on the top of the carb. I had similar symptoms when using a bike after a long lay-off. The filter was full of cack. It let enough petrol through for low speeds, but not enough for sustained higher speed. Stop, wait a few seconds, and it would start again fine. Up the speed, engine splutters and dies. Not a seizure, just a lack of fuel. Clean filter and problem solved - in my case at least...

[Greybeard]

Also is possible to remove the rockers/cylinder head with the engine in frame? (plunger model) I haven't tried before as I went straight for a total restore last time.

Carb off. Rearmost rocker cover stud out. Head is held by bolts so all will come out.

[Rex]

I'm going to remove the header (seeing as it's winter anyway) and have a look for any signs of engine seizure. I am a little concerned about whether the newly bored engine is out of spec for the new +.60" hepolite pistons fitted. I assume that I'm looking for scoring of some kind on the cylinder?

If seizure was the problem then it wouldn't start again until the piston/cylinder had cooled enough to "unseize" and while I'm no graduate of Thermo-Dynamics I think it a little unlikely that such a large lump of cast iron would cool "enough" in mere seconds.
Given the sudden lack of a cooling breeze it would be more likely to indicate a rise in the temp of the block than a fall...

[Triton Thrasher]

A seized engine can free off before you coast to a stop, sometimes.

Which does not mean his engine has seized!

[RichardL]

TT,

Right with you regarding a seize letting go. If the issue was the expnansion and contraction of the cast iron it might be different, but aluminum changes size much faster.

Regarding scoring on the piston skirts, I think owain is hoping to make a judgement based on removing the head, only. In that case he might see some faint vertical scoring on the bore walls. Might be very hard to detect for quick-release  seizures.  Uuuh, not saying the problem is seizures.

Richard L.

[muskrat]

I prefer to call them a nip up which will release in just a few seconds of coasting. If riding on at a less frenzied pace it may not happen again till the worry has abated and speed increased. Slight scoring of the bore will be evident but I'd put money on the piston skirts would look a bit worse.
Cast pistons with split skirts can run at 1&1/2 thou clearance, solid skirts need 3 to 5 thou and forged pistons need 5 thou and a good warm up before riding.
Excessive heat is what causes a seizure so the three things that cause it are not enough piston/bore clearance, incorrect ignition timing (retarded usually) or lean fuel mixture. To be sure of the ignition timing you would need to strobe light it (degree wheel attached under the cush nut). To check fuel try closing the choke, if it goes better your jets are too small. Or use a colour tune.
It may also be as simple as changing to a colder spark plug.
Cheers