Author Topic: Power Valves  (Read 12844 times)

rszmjt

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Re: Power Valves
« Reply #15 on: October 17, 2020, 02:09:16 PM »
I Hope the lapping works for you, it’s a good idea, I’ve never tried that. When I worked at the Corvette/Camaro Restoration shop for 20 years we tried many sets of stainless lines brake & fuel from different vendors, but never had a lot of success with them. They always seemed to leak or sweat. We finally quit buying them and only ordered and Recomended original type lines. Most of the time they were nut & bolt restorations and stainless looked wrong anyway. Fuel is bad enough but brake fluid takes paint off, a lot of customers were not very happy when that happened. Good Luck & keep us updated.
Cheers.
Mike

Henry Cloutier

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Re: Power Valves
« Reply #16 on: October 17, 2020, 08:12:39 PM »
Thanks Mike,

Will do on the updates. I need her running again. I thought the stainless would look a little more attractive which is why I picked it. I knew when I bought the car she needed a little TLC so here I am. It's been awhile since I owned the 68 and for sure have not kept up with the classics. Trial by fire as they say so I learn from you all every chance I get. I had a timeline in my head what should come first, second etc, so now with the fuel leaks experienced needed to change the order. Much to do yet, 1 step at a time!

Thanks for all info provided and look forward to much more!

Hank

Henry Cloutier

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Re: Power Valves
« Reply #17 on: October 22, 2020, 03:51:06 PM »
Hi everyone,

First thanks for all the info already provided! The Z is back together and I have issues I didn't have before. The idle is more stable and the engine seems a little cleaner in its response except the is now a stumble every time gas is applied. Its fine right off idle but at any cruise speed if the gas is applied the is a stumble. Cruise is about 2200-2300 rpm. I had one backfire occur during the test drive. I checked for vacuum leaks and could not find any. I'm wondering if the power valve is incorrect now as the primary one taken out was an 8.5 and I put a 6.5. The jetting is currently 69's primary and 73's secondary. I also think the engine doesn't pull as hard as before the carb was taken off.
Any ideas are greatly appreciated!

Hank

z28z11

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Re: Power Valves
« Reply #18 on: October 22, 2020, 04:17:25 PM »
Starving for gas off-idle, open up the accelerator pump squirters as much as you can. and/or change the pump cam to a more aggressive ramp. 30cc standard pump is sufficient, it just needs to deliver more gas at the initial opening of the throttle. As I mentioned before, a more common problem with the 4053/302 combo than you would think.

Have MacNeish rebuild/recurve your distributor while you're at it - timing should keep up with the carb. If you're running an original 1111480, advance springs are likely shot or may have been changed, bushings wear, things do get sloppy with age -

Regards,
Steve
1968 Z28 M21/U17 BRG/W 1967 Chevy ll Nova SS 
1969 Z28 X77/M20/VE3 LeMans/W
1969 L78 X66/N66 Cortez/BVT
1969 Z11 L48/M35/C60/C06  1949 3100 5wd 235/6

Henry Cloutier

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Re: Power Valves
« Reply #19 on: October 22, 2020, 04:49:11 PM »
Hi Steve,

My initial thought was/is accel pump except it wasn't doing this before. All I changed was the diaphragm during the rebuild. There were three diaphragms to choose from so I picked the one which looked closest to the one removed. The others not installed had much more stroke to them. That and new  power valves as mentioned. The stumble doesn't happen off idle but once the throttle blades are cracked and the throttle is pushed it stumbles.
Thanks for helping!
Hank   

Henry Cloutier

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Re: Power Valves
« Reply #20 on: October 22, 2020, 05:16:48 PM »

I rechecked what came in the kit and figured out what they sent. Two shallow and two deep diaphragms obviously for single or double pumpers. Silly me... I wonder if rebuilding this carb uncovered issues previously masked by other things?... I honestly don't know what accel pump cam is on it now. It was not removed during the cleaning.

Thank you for helping!

Hank

rszmjt

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Re: Power Valves
« Reply #21 on: October 22, 2020, 10:05:01 PM »
The kits come with 30 cc diaphragms and 50 cc. R4053 carbs use the 30 cc. The green diaphragm is a GFLT type used for alcohol, I use them a lot because today’s ethanol based fuel seems to petrify the stock rubber diaphragm.

As suggested the dist may need a recurve, the stock GM advance curve is brutal, you might want to try bumping initial timing to 10-12, also make sure vacuum advance is OK.

Mike

rszmjt

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Re: Power Valves
« Reply #22 on: October 22, 2020, 10:16:34 PM »
Stock jetting is 68/76 BTW, typically it’s 1 jet size leaner per 1,000 ft altitude

Henry Cloutier

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Re: Power Valves
« Reply #23 on: October 22, 2020, 10:31:39 PM »
Thanks for the jetting, Someone in the Z's previous life has re-jetted, why they went up on the primary side and down on the secondary side makes no sense to me. I was used to the other way around back in the day. Wichita altitude is 1300 ft. prior to rebuilding the carb the z was running fine, no stumble or accel problems I just couldn't live with the fuel leaks any longer. I've already had one fire, definitely don't need another.

Hank

Henry Cloutier

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Re: Power Valves
« Reply #24 on: October 22, 2020, 11:58:27 PM »

Hey Steve,

Who is MacNeish? I have no doubt the distributor needs some attention.

Thanks,

Hank

z28z11

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Re: Power Valves
« Reply #25 on: October 23, 2020, 01:34:52 AM »

Hey Steve,

Who is MacNeish? I have no doubt the distributor needs some attention.

Thanks,

Hank

Hank,

Jerry MacNeish, authors the "Definitive Facts" books for 67-69 Z28's and 1969 SS 396 First Gens. Web site for his company "Camaro Hi-Performance" is z28camaro.com . Surprised you've not seen his books (as a Z owner) -

Steve
1968 Z28 M21/U17 BRG/W 1967 Chevy ll Nova SS 
1969 Z28 X77/M20/VE3 LeMans/W
1969 L78 X66/N66 Cortez/BVT
1969 Z11 L48/M35/C60/C06  1949 3100 5wd 235/6

Henry Cloutier

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Re: Power Valves
« Reply #26 on: October 23, 2020, 11:37:25 AM »
 Hi Steve,
Thanks for the info. I haven't run across his stuff and now that I know about him I'll for sure look him and company up. I've only owned the Z since mid July so I'm a rookie at it no doubt. i have wanted one ever since I was a kid. Closest I got back in my teen years was a 1968 Camaro with a 327, 4 speed Saginaw  and 10 bolt rear. Fortunately the 10 bolt was at least a posi rear. I drove the 68 well into my 20's until I finally sold it to a friend. By then it was far from stock. The engine had a Crane 510 solid lifter cam, angle plug fuelie heads,11.0/1 pistons, Offenhauser dual plane manifold equipped with a Holly 3 barrel carb. Headers which had to be shorted about 4 inches a tube since they were off a small block 1957 two door coupe (I had never welded before, man were the beads ugly) and a 4.11 rear gear/Muncie trans.
I knew when I purchased the Z she wasn't perfect and would need some work which I'm happy and excited to do. I'm retired now from 40 years in aviation maintenance and for sure have the time. Unfortunately The are no speed shops like there was back in the day to go visit and build those kinds of relationships. Fortunately this forum exists and I can and am learning from you all when ever possible. I realize I have a ways to go yet and am so appreciative of all the knowledge and advice the forum and everyone on it gives me!

Best regards,
Hank
       

PHAT69AMX

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Re: Power Valves
« Reply #27 on: October 27, 2020, 04:20:11 AM »
As to the original Question, imho no functional difference between 2-window and 4-window power valves.  Both are just to prevent the PV from being a fuel flow restriction.  Very early Power Valves with the round drilled holes in theory could be a fuel flow restriction, so the 2 & 4 window units were developed.
  The PV windows do NOT determine the amount of power valve enrichment.  That is determined by the PVCR's, Power Valve Channel Restrictions, which are drilled orifices in the metering block itself, one on each side of the cavity that the power valve screws into.
  Would imagine after the rebuild the Accelerator Pump Arm was re-adjusted?  If not that might cure that stumble.  The Accelerator Pump arm adjustment is directly affected by the idle screw adjustment.  Any time the Idle Speed Screw is changed, an adjustment to the Accelerator Pump may also be needed.  This is due to the Accel Pump Cam being mounted on the Bellcrank, change the idle speed screw, it moves the bellcrank, which moves the Cam, which can change pump arm adjustment.

  Imho, the Primary Idle Speed Screw Adjustment should be done on the bench and set to achieve proper primary transfer slot exposure below the butterfly blades, only a 'square' no more.  Then the accelerator pump and arm can also be adjusted on the bench.  No the tricky part.  Do NOT touch or adjust the Primary Idle Speed Screw again !  LOL  Leave it right there where it was set on the bench to give correct transfer slot exposure...

  The final idle speed once back on the car is done by way of the almost always overlooked Secondary Idle Speed Screw...  which is a pita... but it can remedy a number of tuning woes.
It is a very small flat blade screwdriver 'set screw' with a 'played locking action' is one way to put it.  It is recessed into the #10 iirc threaded hole in the baseplate.
This prevents the Primary Throttle Blade Butterflies from being too far opened at idle, messing up the Primary Accelerator Pump, over exposing the Primary Transfer Slots, and loosing all controlled of the idle mixture delivered by way of the Idle Mixture Screws...  The Idle Mixture Screws get their emulsion from the same circuit as the Transfer Slots.  The Transfer Slots are upstream of the Idle Mixture Screws.  Over expose the Transfer Slots and all the idle emulsion discharges through them and nothing go past the idle mixture screws and the mixture screws no longer ahve any effect.
Here is an image from a Holley document about it.


Henry Cloutier

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Re: Power Valves
« Reply #28 on: October 27, 2020, 12:02:14 PM »

Thank you so much for the adjustment info. I've done a lot of reading on Holley's in the last several days. Enough to know there seems to be a division in the adjustment and use of the secondary idle screw as a final adjustment. I have considered trying this and I did notice when the carb was on the bench the secondary's were in fact cracked open; since my only issues were fuel leaks and float adjustment sticking, I didn't change secondary butterfly adjustment figuring they must be alright as adjusted. Cleaned everything up with Gumout and re-assembled. I sure didn't expect the problem created by only replacing gaskets, a diaphragm and PV's. Although I fully understand the possibility the stumble was somehow masked by the way the carb was before I tend to believe the theory if it wasn't broke before I worked on it and now it's broke, I did something to upset the operational balance. Thank you for clearing up the flow question regarding the PV's as I'm finally beginning to piece all the info provided together! I have received and installed new 8.5 PV and re-installed the front bowl. I did notice the accel pump adjusted differently this time; before there was not enough length in the bolt/spring/capture nut to get anywhere close to the .015 desired at full travel. The clearance before was probably .060"-.080." Now it adjusts correctly so I am wondering if perhaps the geometry of the accel pump linkage was changed enough to create the stumble. There is freezing rain, ice and snow here for a couple days so we are parked for awhile. If the stumble is still present I'll probably remove the carb and go back thru it paying more attention to the secondary idle adjustment screw and how much it is actually open. I've also considered the accel pump system is no longer supplying the appropriate amount of fuel following the clean and reseal and was somehow masked by the way the carb was thus requiring a larger nozzle or more aggressive cam. Thanks to you and the group for putting up with my questions and silliness, I will fix this!       

Kelley W King

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Re: Power Valves
« Reply #29 on: October 27, 2020, 07:31:36 PM »
just my 2 cents, if it ran ok before I would put the PV you took out back and drive it before changing other things. It appears the only change you made other than gaskets. Maybe the "new" one is defective. Also while you are in there check the gaskets for holes that do not line up or are not there.
69 Z28 RS Scuncio Hi Performance
69 SS L78
67 SS Chevelle
64 Corvette
66 GTO Tiger Gold
77 Trans Am Special Edition