Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties lean front cylinder... help!

lean front cylinder... help!

Moderators: Curt!, Pa

Post Thu Nov 24, 2011 4:16 am

Posts: 10
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

Post Thu Nov 24, 2011 9:59 am

Posts: 607
Location: Menomonie, Wisconsin, USA
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

Post Thu Nov 24, 2011 10:00 am
Pa Site Admin

Posts: 5551
Location: Ohio USA

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

Post Thu Nov 24, 2011 12:19 pm

Posts: 10
Much appreciated Jerry and Pa.

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


Post Thu Nov 24, 2011 5:39 pm

Posts: 159
Tight tappets?

Post Fri Nov 25, 2011 9:18 am

Posts: 10
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?

Post Sat Nov 26, 2011 5:53 am

Posts: 3011
Location: Central Illinois, USA

Please follow Jerry's advice, but use from 12 to 15 psi max:

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

Good luck!


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