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Discussion Starter · #1 ·
Ford 351W/385HP crate motor with 650 Speed Demon and mechanical secondaries.

What vacuum should I get at idle? I am only getting 5 to 9 inches of Hg.

I can get it to idle down to 800 rpms, but I get a popping through the exhaust so I have been making further adjustemnts. My mixture screws are only out 1/2 a turn, otherwise it is too rich at idle. I think the power valve is opening because of the low vacuum at idle and this is causing an excessively rich mixture. Should I switch to a power valve with a lower rating? The stock is 6.5 inches of Hg.

As far as I know I have no vaccum leaks.

thanks for any help,

craig
 

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My guess is that something is amiss...

Not sure what the vac should be on the crate motor...My 302 makes 15 inches at idle..800 rpm

Where are you hooking up the Vac gauge? Is the reading steady or is it fluctuating?

If your Vac is down under the power valve rating then the valve will open causing a real rich condition.

Mike
 

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its good to get a power valve that opens 2-3" lowwer than your base idle vac reading. but your idle vac is very low for a manual trans car.if you use a 3.5-4.0 power valve it will rarely open and could cause lean problems. auto trans cars in gear will have readings that low with a really big cam or too tight of a converter.your power valve is probably opening or "blown if the car ever backfired" causing an uncontrollable rich idle mixture.are there 2 power valves on that carb? if so check/change them both. one last thought. if your vacuum is fluctuating you may have a valve adjusted to tight or burnt.
good luck,chris
 

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Discussion Starter · #4 ·
It is a "fair to rough idle" cam with a Victor Jr. Single plane manifold so the vacuum at idle could be on the low side and still be normal. The vacuum stays steady until I throttle it. At higher RPMs it will peak up to 22 inches of Hg. The gauge is hooked up to the manifold vacuum fitting on the carb.

Can a power valve blow from a back-fire even if the manufacture says it is blow proof?

Should I hook the distributer's vacuum advance to the ported or manifold fitting on the carb? I have tried both.

thanks,

craig
 

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Discussion Starter · #5 ·
This carb has only one power valve. Do you know if the Holley power valves will work with the Speed Demons?

What makes a power valve "blow-proof"?

thanks again,

craig
 

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Craig,
It's been awhile but I seem to recall that my 351C only had about 9-10" of vacumn at idle. I was using a Holley 750DP carb. I used a 6.5 power valve. I also had a fairly "lopping' cam and a duel plane intake.

What makes a power valve "blow proof" is a check ball in the carb that, in the event of a backfire, would be pushed up into the power valves metering slot, blocking the pressure from the backfire. At least that's what Holley used.

Cheers
Jim

[ September 25, 2002, 08:03 PM: Message edited by: Jcolman ]
 

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Discussion Starter · #7 ·
Thanks for the replies, here's yet another question.

Can you tell if a power valve is blown by looking at it?

thanks,

craig
 

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How big of a cam are you running...The longer duration the lower the vacuum....But bare in mind with a decent cam and an idle set at 800rpm 5"-9" of vacuum isn't bad..But if you are running a fairly stock cam I would look for a vacuum leak...Also are your reading steady....Need more info..Good luck
 

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Craig,
If you put the big end of the power valve in your mouth (after cleaning it a little by wiping it on your sleeve) and suck on it, you should be able to make the brass valve move in and out if the valve is still good. It's been a few years since I gave a BJ to a power valve but that's how I remember checking my valves.

Cheers
Jim

[ September 25, 2002, 09:01 PM: Message edited by: Jcolman ]
 

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Discussion Starter · #10 ·
Valve lift is .520" intake and .538" exhaust. Duration at .050" is 236 degrees intake and 246 degrees exhaust.

thanks,

craig
 

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With that kind of duration you are fine at 5" of vacuum....But idle is low...Are you sure the idle mixture screws are working..I would think with that kind of duration..You would want lots of advance and a higher idle..1100-1200 rpm would be good to start..But that much duration likes alot of advance..

I run a similiar setup on a sbc 355 [email protected] and the best I can get for Vacuum 5" with a 1200 rpm idle...

You will have to change the power valve..It is probably open all the time with that kind of vacuum..Good luck
 

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craig,

Valve lift is .520" intake and .538" exhaust. Duration at .050" is 236 degrees intake and 246 degrees exhaust.
I've got a 390 running a .572 lift/296 duration cam. My vacuum at idle (1000 rpm) is ~ 8-9". This is normal for this cam. Because of the low vacuum, I run a 4.5 power valve in my holley. Before I made the change, I had severe eye burn going on.

Now keep in mind, I'm no expert at this but here's what I would do. Assuming no vacuum leaks, set the idle to 1000 rpm minimum and change the power valve to a 35 (3.5") or 45 (4.5"). The rule of thumb is 1/2 of the idle vacuum. I would start with the 45 first.

If you really want to dial it in, you need to monitor your vacuum at idle and cruise. Use a power valve that is about 2-3 inches below the lowest vacuum reading you get on cruise and idle (in gear for automatics). If the power valve opens at idle, it can act as a pump and create a overly rich condition. This is why you can't adjust your idle circuit properly. Now, if the power valve opens too much while you're just cruising, your gas mileage will suffer. It's a balancing act. The whole purpose of the valve is to add extra fuel during period of low vacuum (going up grades, towing, jumping on the pedal to pass, etc).

One thing to keep in mind is that with a big cam, you may never get a "steady" reading at idle. Usually the bigger cams are made to come at higher rpm's. The 'loping' you hear at idle is really the engine missing.

Good luck,

Gene

ps: I don't know if the Speed Demons use power valves or metering rods. Either way the concept is the same.
 

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Ford 351W/385HP crate motor with a Holley 670 Street Avenger and vacuum secondaries.

11 inches HG at 800 rpm.


Hope this helps
 
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