Normal Blowby for a BBC?

[Gary]

Howdy, I have a 1974 Chevy C30 with a big block 454 that has a mild cam. My timing is set to 4 degrees before TDC. It has 16 inches of mercury for a vacuum reading at idle when at operating temp. The spark plugs are all burning evenly and with no signs of oil, lean condition, rich condition, etc. The motor has 89,000 miles on it and was rebuilt some time ago before I owned the truck. My question comes to play when I removed pcv hose from the carburetor to take the vacuum reading. all of a sudden some slight wisps of white smoke…more like steam came from the valve cover breather filter on the other side. As i remove the pcv valve to see if its the same on the drivers side same thing. When I rev the motor up from idle, this steam/smoke puff increases and then let off throttle it dies back down to a trickle. I put the pcv hose back in the carb and it goes away completely WOT or not. Motor makes no bad sounds and does not overheat, has great oil pressure(25-60 depending on rpm), and does not stall out at lights or bog down at all(this is a manual trans Muncie 465). Is this normal blowby?

Topic

[college man]

Is the vacuum reading steady? At idle advance your timing to 8 or
10 degrees capping off the vacuum advance. what is your idle rpm?
should be about 800-825 for manual. see if the vacuum changes. you may
need to reset the idle after the advance.

[Gary]

The vacuum reading is quite steady it has a very very small flicker(less than 1hg of vacuum) due to slight valve overlap from the cam. When I rev the motor the vacuum goes from 16 down to 3-5 and then quickly shoots up to 24-25 with a snap of the throttle and then goes back down to 16. I have an infrared tachometer only is that accurate enough?

[college man]

I would trust the tach. at 3000 rpm you want to see 36deg advance

[Gary]

Ok. Tommorow morning or afternoon after work I will try that. So you suspect its a timing issue?

[college man]

[quote=”Chevyman21″ post=100715]Ok. Tommorow morning or afternoon after work I will try that. So you suspect its a timing issue?[/quote]

I would like to rule the timing out or see if an improvement
happens with checking initial timing and advance timing. are
you burning oil? also is the vacuum advance line hooked to
manifold vacuum or ported vacuum?

[Gary]

Ah ok I see. I don’t think im burning any more oil than i should be with a BBC 454 bout 1 quart every 1k miles/100 gallons of gas. The distributor is hooked up to ported vacuum I believe. This is a roch quadrajet carb if it helps. Pre-smog

[college man]

You may want to switch to manifold vacuum.

[Gary]

This may be a stupid question but how do I know for sure its on ported vacuum? It’s connected to the carb via a small hose so that’s why I believe its ported vacuum as well as the fact I can remove the line and the engine won’t even sputter, not to mention the orifice diameter is very very small. How would live manifold vacuum make a difference?

[college man]

[quote=”Chevyman21″ post=100721]This may be a stupid question but how do I know for sure its on ported vacuum?[/quote]

Ported vacuum is going to be off a carb port.
Manifold vacuum will go to the manifold. if you
pull the line off at idle and it sucks your finger
its manifold. if no sucking ported.

[Gary]

It is most certainly ported in that case as I thought. Above the throttle plate and such. Now if I tee’d it off to a live manifold vacuum source what would be the goal of that? I should also add the make and type of distributor, it is a Delco Remy HEI Distributor

[college man]

see if this helps.

http://www.hotrodders.com/forum/454-ignition-timing-question-99643.html

[Gary]

Wow that is quite a bit of info. I see why full manifold makes more sense now and that I should probably slightly advance the initial timing like you suggested. But, how will this all affect the “blowby” that I’m experiencing with the PCV system disconnected(it ain’t disconnected now it was only disconnected for the vacuum gauge)?

[college man]

when timing is changed or not set right it changes vacuum draw
which may be sucking more blow by. what I would do is check your
timing and see if there is a difference. if not it may just be blow by.

[Gary]

Ok thank you very much. I will check the timing tomorrow and post my findings here. I just assumed the old 454s normally just have some visible blowby in high humidity environments with the pcv system disconnected and it aint like im blowing the pcv valve out of the valve cover 🙂 .

[Lorrin Barth]

What makes you sick is when you get the motor back from the rebuilder, have paid the big bill and tons of blowby. It happens too often.

For the number of miles you have on the rebuild of that big motor I’d say less it better but what you describe sounds normal to me.

[Author]

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