B31 Loss of power

[DartmoorStar]

Hi
Since getting the B31 (1955) up and running again, hoping to put in some more longer runs and feel confident to get out on some VMCC Club runs and the like, but I have hit an issue losing power after a short while on some local test runs which arn't overly taxing. I thought at first the Clutch was overheating as there was a slight burning smell, the clutch rod was adjusted, this seems to have cured that particular issue , I have since discovered that the choke was sticking, this is now rectified, New Spark Plug of two grades higher is on order (N3C fitted). I have ensured the fuel cap vent is clear, along with fuel reaching the carb, as i did originally have a few rust particles from the tank standing for 20 plus years. Other suggestions include Magdyno coil failing and the bore being too tight? Is there anything else I should check?
There is what i describe as a pinking noise in the minute or two before I lose Power. It will then kick over again after a 10/15 minute cool down.
I BSa Owner called ROGER has witnessed one of these restarts after a breakdown, as he was kind enough to stop and offer assistance, he didn't believe the engine was over hot, so suggested posting here for your thoughts.
Today I ran for about 45 minutes covering 26 miles.

Spark Plug looks good I believe (attachment)

A short Video is posted of one test ride here - Vimeo
https://vimeo.com/346589412

another here in a facebook thread if a member, this shows both videos as Power is lost.
https://www.facebook.com/groups/britishmotorcyclemechanics/2419028874826402/?comment_id=2424240434305246&reply_comment_id=2420474111348545&notif_id=1562704242912486&notif_t=group_comment

same link here for Vimeo ( see 06:00 and 11:50 minutes, or enjoy the full video)
https://vimeo.com/347347004

I HAVE ORDERED AN INLINE FUEL FILTER FROM TANK TO CARB

Appreciate the groups helpful advice as this is the third time in a week.Perhaps there is some local knowledge who could take a look with me and guide me in troubleshooting this.

[JulianS]

N3C is a long reach plug.

Iron head B31 takes a short reach L82C.

I would try a short reach plug and see if that sorts it.

[DartmoorStar]

N3C is a long reach plug.

Iron head B31 takes a short reach L82C.

I would try a short reach plug and see if that sorts it.

Thanks Julian - I have already ordered the L82C, should be here tomorrow, so will see where that takes me.

[chaz]

similar to another posting recently..
maybe engine hot and heat transferring to carb and boiling or vapourising fuel in carb.
do you have a tufnol spacer fitted?

[MGILBERT]

When you have fitted your new plug, recheck carb/mixture, sounds like its running to weak/hot. plug should be a light brown/biscuit colour.
bfn mike

[berniej]

The pinking definitely suggests an overheating engine that may be starting to seize when hot.

Did you fit new pistons and/or rings during a rebuild? If so, the piston ring gaps might be too small and causing a hot seize.

It might also be that you're running with the ignition too far advanced - is it manual or auto-advance on your mag? Suggest check the static timing again either way.

Cheers,
Bernie