Weird clunking noise when kickstarting a MK2a

Hi All,
When kickstarting my nearly finished MK2 there is a weird clunking noise coming from near the kickstart but only when under load with the plugs in.
Without the spark plugs in kicking over, no noise.
It has a new kick start shaft, pawl and spring and RGM belt drive plus a new 1st gear on the layshaft and mainshaft which I have just put in but it is still the same. The new k/start shaft was fitted before I just swapped the 1st gears.
I have just removed the primary belt etc and can find no rubbing marks on anything.
I have turned the engine over with plugs in and no primary attached and no noise.
I am considering fitting the old k/start shaft to rule that out.
I did see a 3 part youtube video with exactly the same problem on a MK2a but they did not seem to solve the problem as far as I know.
Any advice welcome.

You're on the right track.
Backtrack.... find the problem

Belt could be slipping and skipping a tooth on the crank pulley kicking it over with the plugs in especially if you used the twist belt to 90 degrees adjustment advice often given for other belt systems. That advice never worked for my RGM belt clutch that uses the AT10 belt. Near catastrophic results in my case. Somebody is bound to say well I use the twist to 90 degrees advice and have never had a problem.

Reread the instructions that came with the RGM belt clutch. My belt is shorter than it would be on a Commando, but total movement up and down in the middle of my upper belt run is about 1 1/8".

Also consider a search using RGM belt clutch, but don't read anything I've posted about it. My RGM belt clutch is fit in a P11 primary and connected to a TTi gearbox.

Final note: my RGM clutch works, but is noisy at idle if the idle RPM is low. Off idle and above 1200 RPM it starts to quiet down.