lean front cylinder... help!

My '46 U has started running very lean on the front cylinder. The plug colour is totally different to the rear. The rear is nice and dark with soot around the edges, the front is clean with no deposits and almost pinkish colour electrode. A cold compression test came up at 70psi. There's no noticeable signs of leaks from the head gasket area and no visible cracks. She's running on both cylinders but not smoothly, lots of popping and banging.
My first guess was inlet manifold. I tried spraying carb cleaner around the inlet to see if it drew it in and started to race but it made no difference.
Anything else that I've missed? My thoughts are a leak on the inlet at the front manifold nut. If it is the inlet do I need new parts or just a clean and reassemble? Do you use a sealant?

Many thanks, Steve

Spraying carb cleaner does not always detect a leak. I would pressure test to about 6psi and use soap and water. Sure sounds like an intake leak. Jerry

[Pa]

Replace seals with Peek seals. Available from two different vendors on this forum. Search "Peek" ...you will find the vendors. Pa

Much appreciated Jerry and Pa.

I've found the Peek seals, thanks for that tip.

Steve

Tight tappets?

I'm going to check the tappets as the bike is new to me and they're on my to-do list but if they were tight enough to let air in wouldn't it loose compression?

41WLD45!

Please follow Jerry's advice, but use from 12 to 15 psi max:
http://virtualindian.org/11techleaktest/html

Beware that you must have an un-blemished manifold for Moldthreads's seals to fit properly.

Good luck!

....Cotten