Finally got working on my 147 that was backfiring.
 

Finally working on 147. I found the problem, stuck open exhaust valve. I've removed keepers. Cam will open valve OK, very tight. Tried washing down stem with blaster, etc. Still tight. I only can turn valve with vise grips,watching not to bend or Nick. No movement up. I'm thinking get in on stem,valve spring side, with carb cleaner and use shoe string soaked and try to remove gum. Any other ideas would be helpful. This is new to me. Thanks.

Pull the head, pull the valve, clean the valve or replace, clean the valve guide or replace. Once all this falls back into Factory Specs, then re-assemble.

:IH Trusted Hand:

Valve will not come out. Stuck at normal lift. Tap back down closed. It's hard to even turn. Valve looks excellent otherwise. Im thinking oil residue build up on stem. Soaking but still no movement. Keepers are out.

The exhaust valve was really tight! Finally used carb and choke cleaner,some string on stem ,valve spring side,soaked pulling back and forth,spray exhaust side. Used vise grips to turn back and forth. Finally 2hrs later got it out. Used automotive manual valve spring compressor with wedges moving an .250" at time. Valve looks good,now really clean stem and ream guide. Thanks.