mixing elbow
Try the easiest fixes first: the air filter, the fuel-water separator, fuel filter(s). But it sounds like your mixing elbow is clogged. We have had the same engine since 1999 on our 1986 31': we have replaced the elbow two times! I honestly think the best solution is to replace the whole system, from manifold to rubber exhaust hose. It makes no sense to try to 'clean' that elbow, just bite the bullet, pay the tourist price to Yanmar supplier for your zip code and get sailing. The learning curve on the job is steep: all day the first time (just the elbow), then 4 hours for the modular replacement of all three pipes. It is dirty, finger-nubbing, tight, work prone to iatragenic damage, but worth the peace of mind.Joe