15 hours ago, Triad said:

...bike hard to start, then starts, works for a short period (1-5 minutes) and then starts bogging.

The hard starting may or may not be related to the bogging.

When I'm trying to find thermal problems with electrical components, I use a heat gun https://en.wikipedia.org/wiki/Heat_gun and/or freeze spray https://en.wikipedia.org/wiki/Freeze_spray  

Freeze spray is fairly expensive.  A lower-cost alternative is a compressed air duster (like for computer keyboards) turned upside down.

BTW, I must complement your English, it's very good!

Thanks! I was a guitar builder and my customers base was always in the US, so I had time to practice 😂

I noticed the spark plugs fitted on the head have a SHORT threaded portion... I'd say 12mm or so, while in pure theory they should be the long version. Could it be the cause of the problem?

I doubt it, but you never know.

59 minutes ago, Triad said:

I noticed the spark plugs fitted on the head have a SHORT threaded portion... I'd say 12mm or so, while in pure theory they should be the long version. Could it be the cause of the problem?

That could cause hard starting (and poor running in general).  But probably not bogging after 1 – 5 minutes.

Yeah, that's what I thought. Weird they used a short spark plug, makes no sense.

I think I'll try to make it run with the stock Amal, if possible, with a loooong transparent hose carrying the premix to the bowl and with the end vented to atmosphere... so I can rule the carb out.

But as I said, I'm afraid it's an electrical issue.

21 hours ago, Triad said:

Thanks! I was a guitar builder and my customers base was always in the US, so I had time to practice 😂

I noticed the spark plugs fitted on the head have a SHORT threaded portion... I'd say 12mm or so, while in pure theory they should be the long version. Could it be the cause of the problem?

I doubt it, but you never know.

Pure theory dictates you should use the correct spark plug, wire and cap 👍  So did that fix it?  

...short plug in a deep hole in a 2-stroke might well run like a plug that is 12 ranges too cold depending on what wrong plug was installed.

Didn't want to hunt the correct spark plug so I ordered one on Amazon, should receive it tomorrow.

In the meantime I sent the PWK carb back and plan to buy one made by Polini (don't know if you guys know the brand). It's an italian brand that makes big bore kits for 2 strokes, exhausts, variators, automatic clutches for scooters etc. - I already fitted one of their PWK carbs on a Beta Rev and they're obviously much better than the generic chinese clones.

Sorry for the lack of updates... got a bad flu that kept me inside for the whole week.

Received the spark plug. Tried to buy an used Polini PWK carb but most sellers are asking €80 when they cost €100 brand new. I was a bit in hurry as I wanted to diagnose this issue (still pretty sure it's due to the stator or the coil, though!) so in the meantime I ordered another PWK (Koso clone), this time it's a 30 (the other one was a 28mm).

I'm pretty sure the bike will still have the same issue as it's not carb-related. I was considering the idea of removing the flywheel to replace the crankseal on that side too, and while it's out, to put the other engine's stator/flywheel and see if the bike behaves differently. Not sure they have the same flywheel as one is an Explorer engine and the other is from a Pioneer... in pure theory I'd expect a lighter flywheel on the enduro, but I guess the stator itself will be exactly the same.

 

Uh, got another "idea": might the bogging be caused by a bad flywheel side crank seal? I don't think so, as the bike idles regularly 'til it starts bogging and if it was getting air I think idle should go up erratically.

Should be close! 

So, just as a reminder: 1973 Ossa Explorer with late 70s Pioneer engine, Koso PWK30 carb, PVL coil. Still needs a decent silencer as that chinese crap touches the rear tyre.

Just fitted the tank, air filter, tank+petcock, fuel hoses, clutch cable.