Re: Removing power valve
[Re: varunner]
#2649197
04/26/19 09:54 AM
04/26/19 09:54 AM
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Joined: Jan 2003
Posts: 14,493 So. Burlington, Vt.
fast68plymouth
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I Live Here
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I’d check the sizes of the pri and sec ifr’s and iab’s....... to see if their sizes seem to make sense.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Removing power valve
[Re: varunner]
#2649316
04/26/19 02:36 PM
04/26/19 02:36 PM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Taking time off to work on my car
Joined: Jan 2003
Posts: 15,439
Val-haul-ass... eventually
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On the secondary metering block, my carb guy asked if I wanted one, he said some guys do it some don't , so I said sure do it. Supposedly only advantage is changing jets. If it doesn't fly I'll remove and put a plate on it. I have another carb getting done without it. So with a plate how do you richen or lean it out ? different plates or can you modify it ? Aftermarket plates can use standard Holley jets, but you'll need to be sure the jets don't interfere with the float's range of motion. Check Quick Fuel and AED and see what they offer.
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Re: Removing power valve
[Re: varunner]
#2660997
05/31/19 05:02 PM
05/31/19 05:02 PM
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Joined: Jan 2003
Posts: 14,493 So. Burlington, Vt.
fast68plymouth
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I Live Here
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Imo, this a classic case of where substituting a “known good” part would help shed some light of where to look.
My suggestion is to see if maybe one of your racing buddies has a known good working bracket race carb with 4 corner idle you could put on the motor........ run it for a while, get the motor up to operating temp, etc...... then pull it off and see if the manifold is wet like with the other carb or not.
If the plugs aren’t wet, and the manifold looks normal....... you know it’s simply a carb issue, and can stop looking at other items as being where the problem is.
From this side of the keyboard...... I’m betting it’s a carb issue.
My 383, which had a bigger cam than yours, and blocked heat crossover didn’t have a wet manifold plenum, and the same set of plugs were used for well over 100 dyno pulls and probably 200 passes in the car(3 seasons)....... and still looked fine. Also...... with regards to the blocked heat crossover....... my guess is virtually every good running stocker has a blocked heat crossover(not to mention the countless other race car motors that use “air gap” style manifolds without crossover heat).
Of course, I had a good working carb with low speed calibration suited towards poor carb signal at lower rpms.
I won’t get into a debate over it, but the 2.5pv isn’t what you want.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Removing power valve
[Re: varunner]
#2661026
05/31/19 07:21 PM
05/31/19 07:21 PM
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Joined: Jan 2003
Posts: 14,493 So. Burlington, Vt.
fast68plymouth
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I Live Here
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The only thing a PV will do for that application is help keep the plugs cleaner when you’re putting around the pits.
I like the rating to be pretty high so it opens early on in the throttle opening at the starting line.
I’d also prefer it not to be real close to the vacuum reading at idle, if possible, so it doesn’t fluctuate open/closed during idle.
I’d be using something around 6.5-8.5........ and if it’s just open all the time at idle, that’s fine(usually helps with throttle response in that type of application).
Without seeing the carb myself and what’s been done to the low speed circuits, my suspicion is that the conversion from a “reverse” idle circuit to a “normal” idle circuit isn’t quite right for the application. I’d want the primary idle/transition circuit(including ifr/air bleed sizing and placement) to be laid out just like a normal non-emmisions calbrated carb is.
In having messed with this stuff for almost 40 years........ my experience is that nothing is better for isolating a potential carb problem...... to the carb............than swapping it for a known good carb.
Basically, my viewpoint is........ if you put something like a QFT Q-750 on it......., assuming that carb was working as it was intended to........ the wet plenum and fouled plugs would go away. And if that happened, you’d know the problem was strictly a carb issue.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Removing power valve
[Re: fast68plymouth]
#2661029
05/31/19 07:35 PM
05/31/19 07:35 PM
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Joined: Jan 2003
Posts: 14,493 So. Burlington, Vt.
fast68plymouth
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Here’s a post I made recently on another board about the value of the known good carb: As a slightly off topic example of the value of “known good”.....
I tested a motor for a customer who had several new carbs to try, and one other which was supposed to be known good. It was a new motor combo, so no previous history to judge by. Started with the known good carb........ carb seems to need an abnormal amount of jet to get a/f ratios where they should be, and then starts to become somewhat numb to increases in jet. We’ve got lots of carbs to try......so, install another one....... and then one or two more. All were different Venturi diameter sizes, along with different booster configurations.
They all exhibited the same two qualities....... really really rich at part throttle...... all took way more jet than they should have to get the a/f ratios where they should be...... and were still kinda lean.......with a gradual leaning out throughout the pull.
Well, as it turned out, the known good carb had the main body substituted....... so it wasn’t exactly the known good carb anymore.
At the end of the day, we installed one of my “known good” dyno carbs. No problems with the overly rich part throttle operation, and it didn’t require any jet changes at all to have normal wot a/f ratios. In fact it was the richest of all the carbs tried, with way way smaller jets in it. This carb has a tendency to slightly richen as the rpm increases, which it did on this motor as well....... the exact opposite of the tendency of the other carbs.
Prior to installing that last carb, there was speculation that perhaps the new combo was presenting some sort of unique fuel delivery issue that the carbs were having a hard time coping with. In the end, that wasn’t the issue at all........ it was just the other carbs tunes were all that far off. Which was made instantly clear once the motor was run with the “known good” carb.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Removing power valve
[Re: fast68plymouth]
#2661037
05/31/19 08:31 PM
05/31/19 08:31 PM
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Joined: Jun 2006
Posts: 660 Virginia
varunner
OP
mopar
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OP
mopar
Joined: Jun 2006
Posts: 660
Virginia
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Agreed, it's the first thing I'd try if I had one. When I first got this carb back, I couldn't get it to idle under 2K, in order to diagnosis that I borrowed a carb that was just "re-built", it was in bad shape, coughing and back-firing. So I had an old 600 that I put on, it had multiple leaks. Nothing else local to pick from. So if I had a carb that I knew was good I'd put it on, but doing so wouldn't tell me much about the one I'm trying to fix. I'm all for swapping parts, but I'm sure my current carb is the issue. I have another carb getting done and will have it a few weeks. Looking forward to that. But I do think I'm on the right track, at least I hope so. I re-finished the carb body tonight and will try it tomorrow, although I don't have a new gasket yet.
Thanks for the tip on the PV
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Re: Removing power valve
[Re: varunner]
#2663388
06/07/19 09:35 AM
06/07/19 09:35 AM
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Joined: Jan 2003
Posts: 14,493 So. Burlington, Vt.
fast68plymouth
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I’m not sure what I’m supposed to be looking at that would make the light bulb turn on......... but I doubt from the factory they came with an open gasket installed between the carb and manifold. Not that it shouldn’t work fine....... just that they also worked fine without it.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Removing power valve
[Re: varunner]
#2663397
06/07/19 09:50 AM
06/07/19 09:50 AM
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Joined: Sep 2014
Posts: 4,457 Washington
madscientist
master
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master
Joined: Sep 2014
Posts: 4,457
Washington
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The first gasket blocked off the passage that delivers manifold vacuum to the back side of the power valve. Manifold vacuum is what keeps it closed. The original gasket and insulator were open, not 4 hole. I don't think the gasket blocked anything off. Those channels expose the power valve port to manifold vacuum regardless of the gasket sitting on top of it.
Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
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Re: Removing power valve
[Re: varunner]
#2663399
06/07/19 09:51 AM
06/07/19 09:51 AM
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Joined: Jan 2003
Posts: 14,493 So. Burlington, Vt.
fast68plymouth
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I Live Here
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The first gasket does not block that cavity. That’s what the “x” channel cut into the baseplate is for.
Furthermore....... the PV is not part of the idle circuit....... so open or closed....... does not affect the idle A/F ratio. So, if the motor is loading up the plugs while idling...... the PV is not the source of the extra fuel...... unless it’s leaking fuel past the gasket or is torn.
But....... you’ll know shortly after you’ve got it running again if you’ve solved the problem or not.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: Removing power valve
[Re: varunner]
#2663413
06/07/19 10:19 AM
06/07/19 10:19 AM
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Joined: Jan 2003
Posts: 14,493 So. Burlington, Vt.
fast68plymouth
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I Live Here
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So. Burlington, Vt.
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I’ve seen things solve problems before, when they shouldn’t have. But as has been said, the x channel that’s connected to that PV signal cavity in the carb is only there to provide access to vacuum for that cavity when a 4 hole gasket is employed. Did you try the carb with the 4 hole gasket after you milled the baseplate? The open gasket looks a lot thicker and more compliant than the 4 hole you’re using. My guess is if you installed a std 4 hole gasket made out of the same material, and the same thickness as the open gasket you have...... it would run the same. I’ll add this...... all Holley 4bbl carbs use that cavity for the PV vacuum...... and they all seem to retain PV functionality just fine when used with a 4 hole gasket. The only reason I thought it had the backwards idle circuit was because of the info provided in the link I posted, which seemed like it was coming from someone well versed in that particular carb. I’ve never had one of that list number carbs in my hands. 9” of idle vacuum is surprisingly good for that combo. And if the power valve is open, that fuel would be coming out of the booster, correct ? The fuel that gets added to the main circuit by the PV would only be coming out of the booster(along with the fuel from the main jet) if enough air is flowing through the Venturi to activate the main circuit.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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