At idle, I can hear the ball in the PCV valve rattling around. Is this normal? The valve is original. I do have another to replace it, if it isn't working properly.
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Re: '66 small block PCV valve
Mine rattles at idle 66 L-79 and it is not blocked. I thought it was a rocker until I found the culprit. -
Re: '66 small block PCV valve
This can happen on some engines that don't generate enough manifold vacuum to keep the valve seated. The metering rod pistons on my AFB and PCV valve rattled until I installed a proper vacuum can on my '63 327/340 that stabilized the idle speed and vacuum.
So what engine do you have? Not all "small blocks" are the same. OE? Modified?
What vacuum can? Post the number on the mounting bracket.
What's your current idle speed and manifold vacuum?
It's all in the details!
DukeComment
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I'LL POST ALL THE INFO WHEN I HAVE IT... *NM*
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Re: '66 small block PCV valve
Duke,
I removed the vacuum can, which was marked B1, and installed a VC 1765 from NAPA. So far, the idle speed (about 700RPM) has remained steady. What engine is the B1 for?Comment
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Re: '66 small block PCV valve
Rob -
The "B1" can is for base engines that pull 18" Hg. manifold vacuum at idle; it isn't fully deployed until 16" Hg.Comment
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Re: '66 small block PCV valve
The "B1" can provides maximum advance at 16-18", and I no longer recommend it, even on base engines, which should idle at about 18" (500 RPM idle in neutral with MT)because it may not have sufficient margin to keep the diaphragm pulled to the limit at idle.
I now recommend the "B22" can for 250/300 HP engines with MT, which provides full advance at 14-16". This should provide a minimum of 1-2" idle vacuum margin to ensure that the diaphragm remains "locked" at the limit of travel at idle.
L-79s were originally equipped with a 8" can, which is really more aggressive than necessary. I consider the "B20" can (VC1765, 11-13") as "optional" for L-79 as long as the idle speed is set to pull at least 14", which is probably no less than 750. I also recommend this can for all BB including SHP that are converted from ported vacuum advance and also recommend it for 250/300 HP with PG and optional on those engines with MT; 250/300 HP engine with PG should idle at 450-475 in Drive. I'm not sure how much vacuum they pull idling in Drive but it's less than MT at 500 in neutral, so a more aggressive can is required, and these specs were OE on '66 300 HP with either MT or PG.
The above is reflected in an article just published in our local chapter newsletter, and these three cans essentially cover all single point/TI distributor pre-emission engines or engines converted from ported to full time vacuum advance.
They may not have identical specs to OE, and they won't have the correct OE Delco manufactured stamped codes, but their overall geometry is the same as OE, and they are functionally correct for the referenced applications.
The reason for your previous idle instability was because the B1 can is not suited to the manifold vacuum characteristics of L-79. Because L-79 generates insufficient idle vacuum to pull the diaphragm to the limit, it would "dither" and vary the idle timing. Any loss of idle advance will cause loss of idle speed, which looses vacuum, which causes additional loss of idle speed. It's a positive feedback loop that causes idle instability.
The loss and variation of idle vacuum due to a poorly matched vacuum can can also set the PCV valve to "dithering", which adds to the instability and if often noticeable as a "rattle".
The key to idle instability is having a vacuum can and PCV valve that are pulled to their limit of travel under prevailing idle manifold vacuum.
DukeComment
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Correction!!!
The last sentence should read:
"The key to idle STABILITY is having a vacuum can and PCV valve that are pulled to their limits of travel under prevailing idle manifold vacuum."
DukeComment
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Re: '66 small block PCV valve
Duke,
With the VC 1765 advance can installed, the idle has remained steady and the PCV quiet. Hopefully, it is a permanent fix. Thanks for the recommendation.Comment
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