Blower carb power valve help please

I have a bbc with an 8-71 blower and 2) 750 holley blower carbs. that are boost referenced (this is a street/strip car)

I am having idle problems (too rich)

I have only 2 hg vac. @ idle and about 4-5 hg just off idle

I am running 28 degree timing locked out. other than the over rich idle it runs great.

My question is; (to fix this) should I run 1.0 power valves boost referenced?
I believe this will cure the idle issue, but will it possibly run lean somewhere in the power band? I can not seem to get my brain around this.
I called holley and bds, neither were any help.
Thanks. in advance.
Geo.

 

http://www.holley.com/data/TechService/Technical/power_valves.pdf


changing your power valve wont cause it to lean out under throttle. you could create a hesitation caused by a delay in fuel delivery caused by a power valve not opening soon enough when you hit the throttle. but once vacum recovers your fuel delivery is supplied through the jets. based on the directions in the link above and the info you provided, a 1.0 is a good place to start.

 

What cam is in it? vacuum is way to low you need to correct this condition, are you sure you are reading manifold vacuum?

 

I is a comp cam #k11694-8
652 lift 300 int dur, 308 ex. dur.
it is a mech roller made to use with a bbc with a laege blower.

yes, I am measuring manifold vacuum . taking it from the manifold under the blower , same place my boost ref. lines are hooked.

 

Never heard of boost referencing carbs that sit on top of a blower before, I'm familiar with boost referencing on blow-thru carbs for turbos and pro-charger type superchargers, could you please explain the need. I don't recall ever seeing boost referenced carbs on a GMC blower before.

The purpose of a power valve is for it to add the needed extra fuel when you are under wide open throttle. It is not intended to be an enrichment circuit during normal throttle cruising.

A power valve won't help an idle issue. The idle mixture screws will fix the rich condition at an idle and won't affect fuel mixture off idle. The jets control that along with the power valve and squirters.

My engine only makes vacuum at an idle, any time I get on it boost overcomes vacuum so at that point a vacuum gauge is useless. I'm running a 6-71 with two 650's and never had any lean conditions even at 20 psi of boost.

On my twin turbo blow thru set-up I used to run we boost referenced the fuel regulator but all it did was ad 1 lb of fuel for every lb of boost. At idle it did nothing. This done one of two things. 1st it will gave the car the added fuel needed under boost or 2nd it gave it more than the needle and seat could hold and flood the intake still adding fuel just a little more unregulated. Either way it worked and never went lean according to the EGT meter in the exhaust manifolds. we ran 17 psi which in turn made the fuel pressure go from 7 at idle to 24 psi under boost I can't imagine making a carb use more than 9 psi of fuel without blowing thru it, it seems it would come out the airhorn and all over the place with out the added air pressure pushing it down the carb.

 

AH! There hasn't been a good boost referencing discussion since "Panic" was banned!

You should really get a oxygen sensor and map your Fuel Air Ratio to see what is actually going on then adjust the carbs from there. You idle vacuum seems really low.

 

This is off of Holley's site...

QUESTION What is a Manifold Referenced Power Valve?
ANSWER Nothing will kill a blower or Nitrous engine quicker that a lean condition. You want plenty of fuel available for the engine to use .There is a thing you need to know about the power valves on a roots style blower engine. The power valve is installed to keep the engine from loading up and running rich at an idle. On a normally aspirated engine the engine vacuum at idle will hold the power valve closed. When you step on the gas the throttle plates open and the engine vacuum drops as you accelerate. When the vacuum drops below the rating of the power valve , it snaps open and richens up the main system. On a blower with the carb mounted above the rotors there is constant vacuum all the time even under wide open throttle. The power valve will never open and you will have a lean condition. To remedy this there is a modification you can have done that is called manifold referencing the power valve. You plug the vacuum feed hole in the baseplate for the power valve. Then you drill a hole in the side of the main body into the hollowed out vacuum chamber for the power valve. You then insert a vacuum nipple in this hole. You will run a vacuum line to the lower intake manifold from the new vacuum nipple. Now you will have vacuum on the power valve at an idle, and when you hit the gas as the boost builds, it will force the power valve to open and richen up the main system. This can be done by most carb modifiers or even yourself. We offer quite a few different size blower carbs with this already done. Consult you local Holley dealer or our Techline for the correct application.

Now I guess I have another question that maybe others can answer, if you install a vacuum/boost gauge on a blown vehicle it will read 16 inches or so at idle then under full throttle go to zero then to boost. If its in boost how can the engine have vacuum?

 

Shizzle,
you know see where my confusion comes from....

I get the fact that there is going to be manifold vacuum under the blower. I believe this will be something (mine is 2hg) when the throttle plates open the vac. Rises slightly. Then the vacuum profile goes from -hg to a +hg (boost)

i get that. I also get that if you have power valves installed with the boost ref. Line under boost there will be a very high hg (suction at the top of the blower (bottom of the carbs)that without the ref. Line the power valve could be sucked shut and cause a 6-8 jet size lean condition that will eat pistons etc.

My question is with my low manifold what power valve do i use?

 

I think the problem is the low idle vacuum. The difference between 1 and 2 is nil? The power valves are open with that low of a vacuum at idle hence your rich idle. Temporarily, you could just plug the power valve and see if the idle improves. Again, a broadban oxygen sensor would be a lot of help.

I'm wondering if the idle vacuum would improve if you had a little less than 28 initial timing.

I've set up my distributor to have a curve with about 12 degrees advance at the crank which is half what a NA curve has. That way you can set the initial at 20-22 initial and get close to 32-4 total, then use a boost retard to pull out advance with boost on the top end.

 

Well, I started out that way.
I found that i needed to get the static timing 20 plus to get a decent running condition. and with that it was difficult to set because it would move around so much (mech. advance working backing forth i believe)
so I was concerned with too much total advance. (I have a 6btm box, but what if it didn't work???/)
I did not check the vac.with the timing set back some so i dont know if it made the vacuum profile any better.

I guess i see that a 1.0 and a 2.5 power valves are not that much different, but with as low an idle vac. that I have should'nt it work?
part 2 of the question, if it does close at idle as I hope, once its closed will it stay closed under boost?

 

George, the purpose of a power valve is for fuel economy - anybody puttin around with a blown big block isn't worried about mgp! Block off the power valve and richen the main jet, that will solve the power valve issue. Power valve are not precise devices, i doubt if there is a 1 or 2" power valve, if there is they'll as likely be open as closed at that value and would be inconsistant as well. Yes, i do have a vacuum power valve testing gizmo and i can tell you that the value quoted (like 6.5) is when it is fully open, they begin to open at a much lower value like about 4.0 or so.
I think the vacuum is a tad low at idle, look to see if the throttle blodes have been drilled. It sounds like it. Lots of carb builders do this, look into the carb and see if the throttle blades have about an .080thou hole drilled on the outsides. If so, with 4 blades @ .080 ea times 2 carbs that makes for a ferocious vacuum leak.
What rpm does it idles at? It should be 800ish or a little higher with an automatic tranny.
To keep that motor cool, it will have to be fat at idle - i reaize up in Maine you spend more time toboganning than roddin but it can get warm on an august afternoon or two - i keep it rich enough so that plugs won't foul but the odor is noticiable and i can go traffic light to traffic light in over 100deg temps and 135deg asphalt with 8-71bbc and the engine will get to 180 and recover quickly, on the highway the electrich fan shuts off and it cruises at 160deg. Over-rich can be your best friend if you do it right.
Give me a pm if those blades are drilled and you want to put the right ones back in, i have them in stock and can send them to you.
Is that the car in your avatar? Lookin good, early willys? More pics pleeze, oj

 

It is a well guarded secret that Anglia's make the best toboggans....

I get the power valve being for fuel economy, I just need to get it shut at idle and idle around cruise so the tourists at the ice cream stand will stop throwing rocks at me.

if i up jet it will it idle as lean as i want?

 

Where are you porting for the vacuum reading? Below the blower?

Rule of thumb I use is lose the PV's bump the main jets 2 points all around. Verify fuel pressures and levels.

Maybe good for us to know specifics on your set up like peak pressures, CR and fuel.

 

The idle ckt is independant of the other circuits. So yes, you can change main jets without affecting the idle ckt.
The idle ckt is always working, the total fuel that the engine sees is what comes thru the boosters as well as the idle ckt. The most ellusive tuning of that circuit is when you are -say- coming into town from a 45mph zone and drop to a 25 and you are 'coasting' along. At that point you are entirely on the idle ckt and it needs to provide enough fuel for the engine to run smoothly.?
"if i jet it up will it idle as lean as i want?" Yes, as long as you control the amout of air the engine sees. If all the idle air is coming past the throttle blades then you can adjust the idle mixture screws to get it as lean/fat as you need - but! if air is coming elsewhere (like a vacuum leak, intentional or not) then you'll loose control of the idle circuit - you can still lean out the mixture screws but you won't have control of the circuit.

 

Yes, below the blower.

509 bbc,
edelbrock 24degree heads.
comp cam mech roller
652 lift 300 and 308 dur
8-71, 2) 750 4150 holleys
8.0 comp.
timming locked @28 degrees

running shitty 91 octane gas with 10% ethenol (thank you al gore)

 

More cam info please

 

<TABLE class=overvw><TBODY><TR><TD class=overvw-labels>Brand</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>COMP Cams</TD></TR><TR><TD class=overvw-labels>Manufacturer's Part Number</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>K11-694-8</TD></TR><TR><TD class=overvw-labels>Part Type</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Camshaft and Lifter Kits</TD></TR><TR><TD class=overvw-labels>Product Line</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>COMP Cams Blower and Turbo Cam and Lifter Kits</TD></TR><TR><TD class=overvw-labels>Summit Racing Part Number</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>CCA-K11-694-8</TD></TR><TR><TD class=overvw-labels>UPC</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>036584034087</TD></TR><TR><TD class=overvw-labels></TD><TD class=overvw-midmargin></TD><TD class=overvw-data> </TD></TR><TR><TD class=overvw-labels>Cam Style</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Mechanical roller tappet</TD></TR><TR><TD class=overvw-labels>Basic Operating RPM Range</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>3,500-7,000</TD></TR><TR><TD class=overvw-labels>Intake Duration at 050 inch Lift</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>255</TD></TR><TR><TD class=overvw-labels>Exhaust Duration at 050 inch Lift</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>262</TD></TR><TR><TD class=overvw-labels>Duration at 050 inch Lift</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>255 int./262 exh.</TD></TR><TR><TD class=overvw-labels>Advertised Intake Duration</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>300</TD></TR><TR><TD class=overvw-labels>Advertised Exhaust Duration</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>308</TD></TR><TR><TD class=overvw-labels>Advertised Duration</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>300 int./308 exh.</TD></TR><TR><TD class=overvw-labels>Intake Valve Lift with Factory Rocker Arm Ratio</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>0.652 in.</TD></TR><TR><TD class=overvw-labels>Exhaust Valve Lift with Factory Rocker Arm Ratio</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>0.652 in.</TD></TR><TR><TD class=overvw-labels>Valve Lift with Factory Rocker Arm Ratio</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>0.652 int./0.652 exh.</TD></TR><TR><TD class=overvw-labels>Lobe Separation (degrees)</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>114</TD></TR><TR><TD class=overvw-labels>Intake Valve Lash</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>0.020 in.</TD></TR><TR><TD class=overvw-labels>Exhaust Valve Lash</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>0.020 in.</TD></TR><TR><TD class=overvw-labels>Grind Number</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>CB 300BR-14</TD></TR><TR><TD class=overvw-labels>Computer-Controlled Compatible</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>No</TD></TR><TR><TD class=overvw-labels>Lifters Included</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Yes</TD></TR><TR><TD class=overvw-labels>Lifter Style</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Mechanical roller</TD></TR><TR><TD class=overvw-labels>Valve Springs Included</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Yes</TD></TR><TR><TD class=overvw-labels>Outside Diameter of Outer Spring (in)</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>1.540 in.</TD></TR><TR><TD class=overvw-labels>Retainers Included</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Yes</TD></TR><TR><TD class=overvw-labels>Locks Included</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Yes</TD></TR><TR><TD class=overvw-labels>Valve Stem Seals Included</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Yes</TD></TR><TR><TD class=overvw-labels>Timing Chain and Gears Included</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Yes</TD></TR><TR><TD class=overvw-labels>Timing Chain Style</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Double non-roller</TD></TR><TR><TD class=overvw-labels>Assembly Lubricant Included</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>No</TD></TR><TR><TD class=overvw-labels>Pushrods Included</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>No</TD></TR><TR><TD class=overvw-labels>Rocker Arms Included</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>No</TD></TR><TR><TD class=overvw-labels>Gaskets Included</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>No</TD></TR><TR><TD class=overvw-labels>Valve Springs Required</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>No</TD></TR><TR><TD class=overvw-labels>Quantity</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Sold as a kit.</TD></TR><TR><TD class=overvw-labels>Notes</TD><TD class=overvw-midmargin></TD><TD class=overvw-data>Requires thrust button and wear plate.</TD></TR></TBODY></TABLE>

 

I am sure someone will correct me but with 114 lobe sep,I would think vacuum would be higher,but Squirrel will know

 

A quick chat with these folks may help as well -

http://www.aedperformance.com/

I've got boost referenced AED 750 double pumps on top of my 671. I have almost the identical solid roller cam as you in my 383. It's got a tad more duration 256 I and 264 E at 50 at 110 lobe center.

 

This is why you see the poor suction numbers. In reallity we need to know that the carb base is seeing (above the blower). That is the value that determins how a PV reacts. Should be a bigger vacuum reading like 22inhg with teh blower sucking on the carbs. That is why the PV's are eliminated on carbs used for draw through blower apps.

 

I agree

 

Manifold vacuum at the carb base is irrelevent when using a boost referenced power valve.
I agree I think I should be seeing more vac. at idle.

Other than being over rich at idle it runs great.

 

Sorry I missed this:

At idle see if the boosters are dribbling. If so make sure the two carbs are synced good and that the fuel levels are good.

Or Move in more initial and see if you can shut down the throttle plate opening.

 

BTW, Thanks for all the help and thinking material

Ok,
I rechecked the timing and reset dead on at 28 degrees.
I closed the blades some and have 5hg at idle.

Now that it is at least somewhat normal what can/should I use for a power valve?
I bought 1.0 power valves, will these work ok and not cause any issues?

 

The PV is not going to be the cause of your rich idle unless there is a problem with the carbs. So if you are happy with the way it is running while under power I'd leave the PV's alone and focus on the rich idle condition. We don't run PV's on our blown engines.

If you have a high throttle plate angle to achieve a satisfactory idle speed you are probably exposing progression holes which are not metered by the mixture screws and it will be hard to calibrate. I have been taught to run more timing then 28* locked out on a blower motor. It enables better suction and low plate angles.

Have the idle circuit fuel areas been messed with? Drilled ETC.

 

George - when cruising, at what RPM do you cross over from manifold vac to boost?

 

Have not yet.
I found at last outing we were cruising at 5-10 hg

is there a chance it is too lean at cruise to damage anything?
I dont think so, because it will load up rather quickly.

All this info. Is/has been great.
Thanks, to all

 

I was trying to get a feel for your power valve needs. They usually open at cruise when you put some gentle pedal into it to the extent you loose manifold vac- like a gentle up grade. And they will only open on vac. So if you had told me that you were running enough blower drive as to be already making boost, the PV would not be needed. Your cruise numbers above seem to have a big spread. Too much for a steady state operation I think. But if a typical cruise turns out to be pulling, say 7 hg, and when you add some throttle is drops to 4 - that's the neighborhood you'd want your PV in. Then there is the multi card factor. So you might find you need to go lower to avoid getting too much PV input too early. I think you have some other issues going on as well, but if you started with 2 hg PV in both carbs should tell you where you need to move toward.

 

The idle ckt is seperate from the power valve ckt. Changning the power valve should have no effect on the idle ckt.
The idle ckt richness is adjusted by those screws on the metering block, you have holley #80576 blower carbs? Then you'll have 8 adjusting screws, snug them down against thier seat and back them out about 5/8 - no more than 3/4 (one flat and a piece of a screwdriver). Lite off the engine and give it a few minutes to work its' way thru the new settings. it should sit there and idle fine.
What rpm is it idleing at, it should be 800ish a bit higher if it is ab automatic. If you are 'idling' at 11 or 1200ish then you aren't working off just the idle ckts as the volume and depression will be great enough that you will pull thru the main wells. Look down into teh top to see if there are 'droplets' of fuel coming thru teh boosters - there shouldn't be.
Cam, boost etc will have no effect on idle rpm, what has an effect is how well it breathes.
A possibility is that you have one of teh carbs that have defective zinc (that shiney stuff) coating and it is coming loose inside the float area, it'll look like a pond that has dried up all craked and flaking - now that'll mess with your idle ckt.

 

Old post, but I have the same problem as George(my name is also George)

Blown bbc, 671, two 750 dps, runs great, but loads up idle.
76 primary's , 6.5 pv's, 89 secondary's , running a Ronco mag
Can I just delete power valves and bump the primary's to 78's or leave the power valves alone and drop primary's to 74's?

The carbs are setup with billet metering plates and were built as blower carbs.