HM100 Generator running rough.

Have a HM 100 that's become my project.... to be a dedicated to run my oxygen generator. I've got most the kinks out it, but it just will not run smooth as it should. I get a constant "background" poof poof puff mixed in with the normal sounds. I've looked and vids and done some reading and it seemed to point at exhaust valve not closing all the way, causing it to backfire. Opened it up today checking the clearances and they are within spec... so THATS not the problem. Next guess for me is maybe the ignition coil? I have verified the clearances on this was correct... can they cause an engine to do like mine is doing?? Thanks..... any advice is appreciated.