1973 Ossa Explorer - issues with engine transplant

[konrad]

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!

[Triad]

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.

[konrad]

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.

[Triad]

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.