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is 18 in.Hg. normal for a stock smallblock chevy? i was setting the timing with a light and my balancer must have slipped because it wouldnt run at its recomended setting so i used a vacuum gauge and it was 15 so i advanced it up.18 was the most i could get other than that it runs great now. i have no vacuum leak either.
 

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Adjusting the idle mixture with a vacuum gage works well but not sure using vacuum for the timing is a good idea.

Yes, 18 is about right. 19 would be better. But you gage could be off a little.

My 396 with a mild rv cam (206/212 on 111 lsa) makes 16inhg at 15 degrees of timing and 18inhg at 35 degrees of initial timing (vacuum advance on manifold) at 850rpms.

I think you need a new balancer or at least make or buy a piston stop that screws into the spark plug and remark tdc.
 

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If you do not have a timeing light or if you harmonic vibration damper has creeped ( outer ring slipped on inner ring) you can set your timing using this method. Adust your carburetor, air/fuel mixture and idle. Then hook a vacuum guage to a vacuum port on your intake and slowly turn distributer back and forth ( retarded and advanced untill you achieve the highest vacuum reading then turn distributer and reduce reading by two increments and lock distributer down. For example if your highest vacuum reading is 18 turn distributer untill it reads 16. This will get your timing very close. I still reccomend the use of a timing light but if one is not available or your damper has slipped this will do the job.
 
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