Holley power valve size

[gordo1968charger]

Hi there
When determining power valve size holley recommend half of your vacuum at idle.other people say 2 less than your vacuum at idle.
Carb is a 770 Street avenger vacuum secondary.
I ve got 11 inches of vacuum at idle.
I ve got a 5.5 power valve in now.
WOT I have a lean bog,AFR gauge reads 17 for a second or so the drops to 13.
Any advice would be greatly appreciated. Thankyou

[XH29N0G]

I may be off base, so see whether others say something else.  My understanding is that the power valve feeds the boosters just like the main jets feed the boosters.  The power valve is therefore a way to make it so the main jets are effectively a little bigger at low vacuum than at high vacuum.   I wonder if the lean spot is related to when the boosters come on as the secondaries open.  I do not know if this transition could be covered by the power valve opening and sending more gas through the boosters of the primaries as the secondary boosters begin to flow or not (something for someone more who knows more than me).  


If it were me, I would (1) check the floats (I think more or less gas in the bowl can influence how much draw is needed to get the boosters to start flowing); (2) check vacuum during the tests to verify that the power valve should be opening at the time the lean spot is seen and that it is closed on cruise (it isn't expensive to try a power valve that opens at higher vacuum); (3) try a test with the secondaries wired shut (to see if the bog is related to the secondaries and also to see what the A/F is off of the primaries); (4) check the spring in the secondaries (it could be too light - opening the secondaries too quickly - but it should be obvious that for best acceleration the fuel delivery should be optimized before using the spring to address the issue); (5) verify that the A/F stayed low 13 to mid/high 12's for the full WOT acceleration.

[John_Kunkel]

If it doesn't bog at heavy throttle (short of secondary opening) I'd look at the spring in the secondary diaphragm, go to a heavier spring to delay secondary opening.

[gordo1968charger]

Thanks

[gordo1968charger]

Has anyone ever shortened a vacuum secondary spring to get the desired results?

[XH29N0G]

Has anyone ever shortened a vacuum secondary spring to get the desired results?

I believe cutting springs or compressing them also affects how they compress.  

Just last week I tried to improve two springs designed to make the handle of my new floor jack pop up - the handle hit the side of my car and I got angry .  After cutting them, I found myself searching online for replacements and mired in spring rates, materials, and sizes to see how to get a replacement.  Now it works again and is 'improved' .

Experimental modification can improve things, but if you are like me, sometimes it is better to just buy the parts that were designed by someone else (e.g., https://www.holley.com/products/fuel_systems/carburetor_components/vacuum_secondary_parts/parts/20-13).  

[gordo1968charger]

I ve got a full spring kit but the white one is too light and the yellow is too heavy

[XH29N0G]

How do you know it is too heavy?  Not trying to be confrontational, just curious.

Maybe you have to experiment by trimming. The worst that will happen is you have to buy another spring.  

Alternatively, you can explore the fuel delivery to see if that affects and transition.   I forgot to mention the accelerator pump cam and squirter above.  That can sometimes help too.

[gordo1968charger]

I do nt really know what I m doing to be honest.thats why I ve posted on here.
I ve got the gist of idle circuit, more or less,
Holley say go for a power valve half of your vacuum reading but my car seemed to run better with the standard 6.5 power valve.

[XH29N0G]

I do nt really know what I m doing to be honest.thats why I ve posted on here.
I ve got the gist of idle circuit, more or less,
Holley say go for a power valve half of your vacuum reading but my car seemed to run better with the standard 6.5 power valve.

I may know a little more. If others pipe in, chances are they will most likely know more than me. If you think I suggest something that doesn't sound right, I could be wrong.

Let me ask a few questions and then explain what I think I know.  

I just did a quick scan of your past posts and it looks to me like you have recently put in a Firecore RTR distributor so your ignition should be good, have a 383 with a 4.10 rear and automatic.  You also said it has a big cam (but you don't know what it is).  And that your idle vacuum is about 11.  Let me know if this information is incorrect.  I am assuming it is a hydraulic cam so your upper operating RPM is still 5500 give or take.

1) Earlier in this thread you said you had a lean spot when you stomp on it (and more importantly a bog/hesitation) and then that the A/F returns to 13.  Is this high or low 13's?
(I ask because if it is high 13's you might change things a little by jetting so you are in the high 12s.)  

2) Following up on A/F.  Are you able to log the A/F so you can see a trace through the RPM range? at cruise? and at idle? (again, just looking for places to shift the curve a little lower)

3)  I think the adjustments you can do on the carburetor you have relate to setting idle speed and mixture (on all 4 corners), accelerator pump cam, accelerator pump squirter, vacuum canister springs, jets, and power valve, but not the air bleeds nor the idle feed restrictor.  Can you confirm, or let me know what can be modified or adjusted?

4) Is the accelerator pump circuit working properly and properly adjusted?  Does gas come out of the squirters at the moment the throttle is touched?  Also, what color accelerator pump cam is being used?

My thinking has been that it is related to the transition to the main circuit (the transition to getting the gas flowing through the boosters in a way that they are controlled by the main jets) .  I am assuming the A/F of idle, cruise, and WOT are where they should be, then you have the idle circuit and main jets correct and that your float levels are correct.  I am guessing that the issue you are seeing is because the secondaries open fast enough so that the air flow either slows in both primaries and secondaries or is simply not fast enough in the secondaries to draw gas through the boosters.  You can either do something to cover this up (that is where the accelerator pump circuit comes in, or make it so the transition occurs at lower air flow rates (lower RPM) (probably not possible because it relates to carburetor size, air bleeds, IFR, and booster design which I believe are not adjustable for the street avenger).  The power valve also could come into play, and you could change this too, but I don't know exactly which way to go.  I am still running a 6.5 and have 11 " idle vacuum.  When I am at cruise, the vac is high teens (like yours); so my thinking is that this means the power valve opens when I depress the gas and drop vacuum, but not at cruise.

If you have not started by working with accelerator pump cams and squirters, I would start there.  

Pump Cam:  Unless something has been changed you will have a 30cc  pump (30 cc for 10 cycles) which means that the brown cam is too big.  I would look to the orange or blue cams in the first hole (you can work with the different holes, the amount of gas that is delivered probably should be checked to make sure what you think it is doing is correct.  There are charts on a site I just looked at http://garage.grumpysperformance.com/index.php?threads/holley-accelerator-pumps-cams.1790/ and other places.    A larger cam will allow more gas to be delivered.  

Squirter:  Also, unless you have changed something, I am guessing you have something like a #28 squirter nozzle. I would change that in steps of 2 or 3.  A larger squirter will allow gas to be delivered more quickly by the pump.
You could try both of these, and see if they address the issue.  

A final two thoughts:

Since you have 4.10 gears, the engine may be able to take more, rather than less gas with the accelerator pump, and you may want to try increasing the size of the pump to a 50 cc pump and seeing if the brown cam would work.  I would try this after the other changes.  But if you are going that route it also might be that a double pumper is what you want.  

Also, I do not know whether the carburetor size that is too much for the 383.  On my set up, I tried a carb that was way too large (1.56" venturies) and had the same issue you have.  I use a carb with 1.39 venturies that is feeding 450 c.i. with 4.30 gears. Unless I am reading wrong, the 770 SA has 1.375" front and 1.438" rear venturies.  The air flow issue could be in part related to the larger rear venturies.  That noted, I read of people running similar size 750's on 383s and liking them.  But I have not done so myself.
 

[gordo1968charger]

Yes it's hydraulic cam
Af reading was low 13s after lean bog subsided
No onlogging on Af meter it's just a autometer gauge
It has green cam on position 1
Accelerator pump is properly adjusted
Gas squires straight out
Jets are 71 primary and 80 secondary
Float level is correct
31 squirter
Was thinking the 770 may be too big myself but heard of people running an 850 on a 383

[XH29N0G]

Wow.  It looks to me like you have most of the things I suggested covered already.  See what others say. 

I think this is above what I can advise on. 

[gordo1968charger]

I ordered up a 8.5 power valve today,if that's no good I ll put the 6.5 back in.
If that does nt work, I ll get the holley trick kit with everything in it.
I m reading David vizards book too so hopefully I ll get there eventually.
Thanks for everyone's help

[c00nhunterjoe]

Have you blocked the secondaries and either verified or denied that the bog is indeed the secondaries yet?

[BSB67]

Options, assuming everything else is right:
1) Stiffer secondary spring
2) Increase the secondary float level higher
3) increase the IFR
4) decrease the secondary MAB

I have no idea why Holley states to put a PV rated at 1/2 the idle vacuum.  Dumb.  I guess it is a starting point.  That said, I don't know if that will help your problem.  That would be something to look at if you have a part throttle lean condition.

[gordo1968charger]

I have nt had a chance to look at it since last sunday,hoping to get a good look at it this weekend

[gordo1968charger]

Update,
Finally sorted out.
74 primary jet
83 secondary jet
7.5 power Valve
Heavy yellow spring,
Running between 12.6 and 14 AFR throughout the throttle range.
Idling in gear at 900 rpm and 13.8 AFR.
Thanks for everyone's help