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Old 05-09-2008, 08:43 AM
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Blower Carb Boost Referenced power valves

Just a general question regarding a boost-referenced power valve setup. I have two Mighty Demon blower carbs that are set up to have the power valves relying on intake manifold vacuum (or boost) for actuation. If I connect a vacuum source, in this case a Mighty-Vac hand pump with gauge, on the power valve hose connection and draw a vacuum, I can draw a vacuum and close the power valve. No problem there. The question I have is, should it hold the vacuum with no leak down, seeing that there should be no air leakage past the valve, of is it "normal" for the gauge to show a leak down?

I tried this with my carbs thinking that I had a leaking power valve since I'm still working an issue of over-richness at idle. When I draw a vacuum on the valve, the gauge will show that I am holding a vacuum against the power valve only if I keep on pumping. As soon as I stop pumping, the gauge will slowly leak down to zero.

I'm thinking that the system should keep a vacuum. If I take the power valve out and test it by itself, if holds a vacuum with no leakage. It seems that with the engine running and pulling a constant vacuum, that the valve would remain closed until the vacuum drops low enough to open the valve.
Anyone else try this? Is this leak-down a non-issue, or could there be leakage at the metering valve gaskets or somewhere else that need to be fixed.
I spoke with a tech at BG, and he told me it was okay, but i'm not convinced. Just interested in someone else's take....

Thanks,
Steve

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Old 05-09-2008, 09:48 AM
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Steve,

It should hold a vacuum. If it doesn't there may be a small vacuum leak where the vacuum fitting goes through the baseplate.

Now with that being said it's most likely not enough of a leak for it to change the way your engine is running... You could check the vacuum at idle going directly to a gauge and then to your carburetor to see if it's causing you a problem, but I would bet it's not.
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Old 05-09-2008, 10:57 AM
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CarbGuy addressed your power valve question. Are these BG BC's or modified NA carbs?

I have two Mighty Demon 750 BC's on an 8-71 383, and experienced similar rich idle issues. I spoke to the BG tech at the Vegas Nats, he recommended an extra set of primary idle air bleeds and opening them up. I did, opened them on all four corners to .080, it helped a lot. I still get the 'blower roll' somewhat, but I can breath better along with the guy next to me at the light.
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Old 05-10-2008, 09:21 AM
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Thanks for the replies, guys. These carbs are the BC type made for the boost reference. There is one fitting in the baseplate that leads directly into the power valve chamber in the main body.
I checked the fitting in the baseplate and it's not leaking there. The last time I had the engine running, I had a vacuum gauge connected to the same hoses going to the carbs, and I was pulling up-wards of 16" Hg. I know that the power valves are seeing a constant vacuum, so I'll start it back up and see how it is. I have also enlarged the idle air bleeds. First time was to .078, and now I'm trying .081. This was told to me by the BG tech also.
I probably have nothing to be concerned about, I have replaced every gasket just to make sure.

Thanks again,
Steve
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Old 05-11-2008, 09:01 AM
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Try putting some pressure to it and using soap and water to find the leak.

It's probably a non-issue but peace of mind is worth quite a lot.
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Old 06-10-2008, 11:37 AM
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Been over this again and again. I am being told that I need to run vacuum lines from each carb to the blower manifold to "boost reference" the carbs. The Blower Shop manifolds do not have any vacuum ports on them to do this, much less any room. What have y'all done for this, or do you not have any vacuum lines from the carb to manifold?

Jason
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Old 06-10-2008, 12:15 PM
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Quote:
Originally Posted by 41willys
Been over this again and again. I am being told that I need to run vacuum lines from each carb to the blower manifold to "boost reference" the carbs. The Blower Shop manifolds do not have any vacuum ports on them to do this, much less any room. What have y'all done for this, or do you not have any vacuum lines from the carb to manifold?

Jason

You can either drill and tap the intake, or just "T" off from where you have your Boost Vacuum gauge right now.
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Old 06-10-2008, 12:24 PM
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Quote:
Originally Posted by CarbGuy
You can either drill and tap the intake, or just "T" off from where you have your Boost Vacuum gauge right now.
I do not have anything like that plumbed yet. Where in the manifold do I drill and tap? This is my first blower, and beginning to get a little irritated with it. It is a Blower Shop 8-71 and their manifold, and the carbs are Mighty Demon 850's.

Jason
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Old 06-10-2008, 12:41 PM
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There should be some sort of fitting in the base of intake...
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Old 06-10-2008, 01:18 PM
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Jumping the gun, I did not look hard enough, there is an 1/8th" port by the pop off valves. Thanks for the help, I am calm now...

Best Regards,

Jason
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Old 06-11-2008, 04:58 AM
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Jason,
The Weiand intake manifold that I have didn't have any connections for a vacuum fitting, so I drilled and tapped the manifold back near the #8 runner. Here's a picture of my motor when I had it running on the test stand. You can see the two vacuum lines running from the base of each carb to the tap on the manifold, with the other part of the hose going to a vacuum gauge. Also shown in the picture is my PCV setup that runs from the valve cover breather to the carb base adapter.
Good luck with your blower setup.

Steve
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Old 01-18-2012, 12:54 PM
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I now this post is.2.5years old but need.help os anyone on thos thread still around.of so look my my other.post in engines or click mt name.and veiw my post only got 3 post at this time.thanks
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Old 01-19-2012, 08:59 PM
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So on rhe.boost ref. Port it should be tied into the bottom of the.blower so it can get.vacuum and boost to open and close.the power valve?

And what to do about my trans mod. Valve? Tie it into the carbs.base.plate?

Thanks
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Old 01-20-2012, 12:56 PM
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Boost reference is to the manifold below the blower, doesn't matter just where.
Don't worry about the modulator for the trans and hook it where you hook the boost reference. Do not use any check valves!
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Old 01-22-2012, 06:15 AM
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Quote:
Originally Posted by Hotrodrobert
Boost reference is to the manifold below the blower, doesn't matter just where.
Don't worry about the modulator for the trans and hook it where you hook the boost reference. Do not use any check valves!
thanks
so the.trans.mod.line.gose into.to boost lone also.?
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