When do you need adjustable rockers on a 383?

Excuse my engine building ignorance upfront. I have a stock freshly rebuilt 383 (factory stock rocker arms) with a new stock cam that is only pulling 12 inches of vacuum. An engine guy told me that if the heads and/or deck have been milled too much in the past, it could be preloading the lifters too much, resulting in the loss of vacuum. The fixes I assume would be adjustable rockers or custom length pushrods, right? Forgetting any other possible causes for low vacuum, does this explanation sound plausible to you guys?

Yes you need either adjustable rockers , adjusatble pushrods or the correct length custom pushrods .

Has the block and heads been cut a lot ?

Hydraulic lifters as a rule have a lot of forgiveness it them. with a relatively mild application and normal usage you will be fine as long as the lifter plunger is not bottoming out in the lifter. If the lifter plunger bottoms out and is tight it will hold the valve open much the same as adjusting a solid lifter with no lash. If this is happening there is a no start or the least a very poor running condition.

JohnRR, unfortunately I don't know how much milling has been done.

Mopartforlife, that's what I always thought about hydraulics, that they "took up the slack" unless a huge amount of milling was done. The car starts/idles great, and runs smoothly at any constant rpm. The only performance problem is a major hestitaion at all rpms when accelerating, which may be due to other factors I suppose. Right now I am just trying to figure out the low vacuum.

I have heard there is a device made by Snap-On that is essentially a smoke generator. This device seals over the top of the carb and blows smoke into the engine. I assume you have to plug off all the vacuum ports, etc. Any leak produces a puff of whie smoke. My engine building friend used this device and found a grain-of-sand size hole casting defect in the body of one of the carbs on a six-pack set-up that nobody could get to run right. We're going to try it on this problem.

The hesitation could be a bog caused by the carb - too much gas or too much air. Possibly the accelerator pump or the secondaries.

I think if the issue was with the valves not closing you'd be having major starting issues, you'd be getting reversion, backfiring through the carb and severe dieseling when you shut the motor off.

As for that "smoke generator" idea, you can do the same with a propane torch. Start your engine and open the valve on the torch (DON'T LIGHT IT). Move the torch nozzle around any areas that could potentially be a leak. If there is a leak you will notice an immediate increase in RPM. Don't worry, this is a safe method, just don't forget to turn the torch off when you're done!

12 inches isn't that low, chances are good if it's got a cam with much duration it's normal... Adding more base timing may help increase the vacuum & eliminate the bog.. there may be other issues that need looking at but I'd start with checking base timing.... Knowing cam specs would help...

Rockers are NOT your problem. You would not be running/idling smoothly if a valve was not closing all the way.

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The only performance problem is a major hestitaion at all rpms when accelerating,

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Stock rebuild/cam? What year/carb? Hesitation at part or WOT. timing/lean (jets,pv,float levels)/vac leak. been a prob since rebuilt but not b 4?