Reassembled my son’s B21F engine (265k mi) with a reconditioned cyl head, and had some trouble seeing the crankshaft index mark to align the timing belt marks. Started the engine and it does not run well. I may have set the belt up one tooth off, BUT: it actually starts right up, runs moderately smoothly at 2000+ RPM, but has a rough idle that deteriorates (lumpy, shuddering) and won’t rev to the throttle very well. Further info – vacuum reading at idle is low – only about 7inches. Compression (cold engine, dry test) is quite even at about 140psi on all cylinders.

I don’t see how the belt can be one tooth off on the crankshaft pulley with that compression, but what do you think? Anyone got a suggestion for finding TDC accurately without using the standard index marks or removing the head again? (Finding TDC will tell me if the belt is aligned, and also let me mark the front pulley accurately so I can check ignition timing.) I realize that the above symptoms could also result from a bad vacuum leak, but I can’t find one. Thanks, all.