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The crankcase fumes go from very little at idle (when there is a vacuum in the intake manifold) to a lot at full throttle (when there is practically no vacuum in the intake manifold). So the crankcase venting sort of goes both directions if you have a PCV valve hooked up. At idle, the valve lets the engine apply a little active scavenging by sucking air in through wherever you have that tube routed. With the pedal down, the vacuum decreases and the volume of gas to vent goes up, so it starts going back the other way.
So you have to be aware of that two way flow, and plan accordingly. If you have an oilcap with a fitting on it, and you run a hose from that to the PCV valve, and you are using the dump tube, be aware that at idle the engine will be pulling fumes out of the valve cover (good), but unfiltered air in through the dump tube (could be bad - if it's dusty).
And if you run the PCV valve hose to the breather on the box, and use a vented oil cap with a filter - you're goo at idle because the filter will prevent dust from entering the motor, but at full throttle you'll have the blow-by venting out the cap - which is bad only to the extent it spreads oily fumes from that point in the engine bay. Not so much of an issue when the motor is newer, gets messy on an older engine with more blow-by.
So if you're going to use the dump tube, you might be better off just using an unvented cap. That way uniltered air is never drawn into the engine.
I'm running a pair of DCOE's on my PV's motor - never had an issue with the weight being unsupported. I'm using soft mounts as well, and it is tricky to get the tension on those just right. Too loose, and they can allow air leaks, too tight and the carbs get a little too buzzed and the fuel mixtures suffer. I was using a wideband O2 sensor when setting mine up, and I could see on the gauge how erratic they got when over tightened.
I never seemed to get the set of soft mount gaskets that came with my carbs to work properly, I just got a new set and it all seemed to work better. I think the rubber hardens with age and exposure to gas and can't conform to movements of the carb very well. It's fairly easy to check - just push a carb up or down, see if the idle suffers noticeably.
Love the DCOE's, the throttle response is so much sharper. Just almost brutally instantaneous. And while it can be a bit of a struggle to get a nice smooth idel, the top end power over the SU's makes it worth it. And as an added benefit, once you get that idle set right, you won't need to do it again for a long time. Mine have been in 'set it and forget it' mode for a good 5 years now. Unlike the SU's which need periodic tweaking, the DCOE's don't seem to drift off tune over time.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)
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