Inliners International Forums General Discussion Engines vacuum not steady at idle

vacuum bounces quickly from around 17-21 +/- at idle, smoothes out with the slightest increase off idle. less than 20k on motor but never ran as good as I thought it should. would this be a manifold leak, valve issue or normal for a 206@50 duration cam?

May be a valve not closing securely, or a vacuum leak. Try propane around intake joints - if it speeds up that's probably contributing.
What's your cranking compression (hot, WOT)?

haven't checked compression yet. was not able to find a vacuum leak but did not check the bottom of intake, maybe this weekend. was setting the fast idle and when choking the rear carb, it would run rough as expected but front made no difference at all. engine has a slight miss that I've chased since installed! not bad, but its there. second set of carbs but same intake, replaced distributor, have tried different advance curves and jetting in both extremes and just wont smooth out and have crisp throttle response. I am ready to pull the head and send it off, just hate to do that if the problem is elsewhere.

compression ranges between 188 and 196. found very minor leaks at rear of intake, bottom and top of adapter on rear carb. assuming all 3 (all at rear) is enough to matter. intake and head looked good so I cleaned it up and installed a new gasket. old one had a small ding near the leak, probably happened during install. adapters are very crude. sanded tops and bottoms of both along with the carbs. I was surprised how poorly they were made. maybe reassemble tonight, tomorrow maybe.

IME vacuum leaks are a small fixed volume (or it wouldn't idle), representing an inconvenience when they're a big % of CFM at idle, but almost zero when cruising etc.
You're probably safe.

vacuum leaks fixed but needle still bounced 18-21. did run better though. adjusting valves and lifters seemed too week and couldn't adjust enough to stop the clattering. replaced them and found the ball from one of the pushrods smashed and laying there! new lifters, an old spare push rod and adjusted valves. rock steady vacuum just above 20 psi. now to check my jetting WITHOUT vacuum leaks and mess with the advance curve as both are surely off.