I'm rebuilding my 4150, which is a 750 CFM double pumper, list # 9379. It's on a 350, moderate build (yes, I know it's over-carbed). The secondary metering block has a dummy power valve - it's a plug. But the rebuild kit for this carb came with two power valves and no plug.

Should I use the plug or the valve? Do 750 double pumpers generally have a power valve plug in the secondary metering block?

thanks!

Remove the plug and install the power valve. Standard double pumpers come with two power valves. Since the secondary power valve was plugged I suspect the jets on that side are fat so you will probably want to jet it down a bit.

According to the holley carb chart the List #9379 didn't come with a sec power valve... The jetting is 68 pri, 81 sec to make up for no PV. The pri PV is a 6.5 from the factory...

Dennis I didn't look up the carb number. Is this by chance a blower carb? If so that would explain it. If this is not going on a blown engine and it is intended for the street I think you would want to run a power valve. I should have looked up the number before I responded. All the double pumpers I have and have owned excluding the blower version came with power valve on the secondary side as well (inclding the HP series). Some drag only cars like to remove the secondary power valve and fatten up the secondary jets but, according to most of the research I have done this is not "suggested". You might want to give Holley a call and see what they recommend.

Hmm, well I think I'll try it just leaving the secondary plugged and see how it runs with the primary changes. It's already been running rich, and the only other jets I have are 70's from the primary side (but are now 66's) so unless I use those on the secondary, I can't lean it out.

Unless of course you guys have strong opinions to the contrary.

I'll let you know what happens.

thanks

My 4777-2 650 DP doesn't have a secondary PV either. I've had several Holley DP's over the years, and don't remember any of them having one, but I've never had a carb bigger than a 750. According to this site that I found, the classic DP's (4776 - 4781) didn't start getting secondary PV's until 800 CFM.

http://www.prestage.com/Tech+Info/MiscSpecs/Holley+Carburators/default.aspx

Luke, If you are near sea level I'd set it to factory specs - it should be darn close. Adjust as nec from there.

BPOS, that is news to me. I guess you learn something new everyday. I have 6 Holley DP's here right now (all 750 or bigger) and they all have powervalves on the secondary side except the blower carbs. The 650 I built for a friend also had a secondary power valve.

got it back together and fired her up with the dummy power valve in the secondary. Changed the timing from 16 to 20. Checked the vaccume - at 10 in so I leaned out the idle/mixture screws and get this - the RPM INCREASED. I think that means I'm still running too large of jets (66) in the primary, so I plan to swap in some 64's and see.

Overall it runs a lot better, but it still backfires out the carb a bit.

The primaries really have very little to do with the idle circuit. Check out our tech reference area www.camaros.net/techref (http://www.camaros.net/techref) go to "Series II" for some carb basics... Some good links to info on Holleys web site too.

Royce, I think you have an unusual batch of carbs layin around ;)

I'm used to seeing no secondary PVs except on the really old carbs -- like the O-3310 vac secondary carbs that came on 1st gen z28's, etc or on carbs that were converted from a 4160 style metering plate design using a 4150 style front metering block (not a good idea). Some of the high Dollar HP series carbs may still use this design -- don't ask me why though.

I think it is better to plug the secondary PVs and jet up to compensate. When you accelerate hard, all the gas sloshes to the back of the bowl uncovering the secondary PV. The sec jets are much lower in the bowl and do not have this problem like the power valve.

LOL, Eric I guess you are right. I have played with Holleys quite a bit and I swear the only ones I have seen that didn't have a secondary PV were the ones that were set up for race only applications and blowers. My Holley book also says not to remove/plug the PV. Now I am going to have to go out and double check the carbs I have laying around. I know for sure my 750 HP series has a secondary PV cause I remeber tuning/changing it. I am pretty sure my 830 Anuular does as well (I have two of those, but have only had one apart) I bought the other one and it is still in the box. I just took pictures of my 750HP and willpost them (PV's on both sides), I didn't think I was crazy (well maybe a little crazy) http://img38.photobucket.com/albums/v117/camaroman7d/IM000228.jpg http://img38.photobucket.com/albums/v117/camaroman7d/IM000229.jpg


On page 119 of HP books "Holley Carburetors and Manifolds" It says, Power Valve tuning "You may immediately think " I know just what to do-- take it out!" NO, regardless of all the material written to the contrary, there is seldom any real reason to take out the power valve and replace it with a plug, even though Holley sells plugs for that purpose.

The power valve in the secondary is especially important because it allows using smaller main jets. Braking forces don't cause the engine to run excessively rich and there is no tendancy for the engine to load up or run rich at part-throttle. Holley's engineering staff has found many advantages by leaving the power valves in place.

The valves have an important purpose or they would not be installed in the first place. If they could be left out, Holley could reduce their manufacturing costs. The power valve is the "switch" between the mixture ratio for cruising and that required for full power"

I know in this case he did not remove the valve because it didn't come with one (Dennis pointed that out). So I don't know weather I would install one or not (I think I would).

In the same book on page 126 it has more on removing the power valve, I will spare you unless you are interested in what they have to say.

Once again I am not arguing or trying to prove anyone wrong, just want all the info on the table and we can all learn from it.

[ 03-22-2004, 11:30 AM: Message edited by: camaroman7d ]

Well the info on 126 is relevant so I will post it.

"Removing Power Valves-- We see an alarming amount of this. The only reason for removing the power valve is where the induction system is so restrictive that manifold vacuum at WOT gets above the rating of the power valve. In this case the valve will close and lean the mixture. the correct solution is to use a higher rated valve. If you remove the power valve you must jet up 6---8 jet sizes to compensate. Then the part-throttle mixture will be 6---8 jet sizes too rich."

Well I'm not sure I understand everything you guys are saying :D , but here's my plan from what I gathered:

Current Status: 66 jets in primary, 81 in secondary with the power valve plug. Engine runs better and leaner than it did, but still backfires out the carb.

Step 1: Rejet the primary to 64's. See how it runs. If it still backfires or seems rich at WOT.

Step 2: Put the original 70 jets from the primaries into the secondaries (reducing it from 81's) and install the power valve (I think it's a 6.5) in the secondary. I'm running about 13 in HG vaccume.

Comments anyone?

You say the carb pops...what action makes it pop? Many backfires are caused by lean conditions so going more lean will not help.

Is it when you slam open the throttle? Just cruising along?

Also, when in doubt put it back EXACTLY like it came from the factory. Unless you are at 5000 feet or something it will be very close.

You have something else causing the backfire out the carb, jet changes are not the answer... Start by putting the carb back to factory spec's, jets, pv rating pump cam etc. then adjust the floats, the accl pump and the pri and sec throttle bores and rich lean mixture screws to holley spec's. Use the info in techref and the holley links to get your carb back to factory spec's. Next bolt the carb back up and and get the engine running and up to temp with minimum adjustments... Next get your timing set somewhere in the 8-14 degs initial with the vac advance disconnected and the source plugged. also make sure you have a good PCV valve... Now using a vacuum guage set your idle mixture for most vacuum and them maybe 1/8 turn in toward the lean side. Set your idle and check your timing again... if you end up with one idle mixture screw at 1 turn and the other at 3/4 turn split the difference, set both at 7/8 turn... When the car idles and doesn't smell too rich go drive it and pay attention to bogging, stumbling and backfiring and when any of it occures. now start tuning the accl pump and cam and squirters. don't mess with jetting until your engine runs properly, no bogging, stumbling or backfiring... Use the techref info and holley links to learn about the different circuits within the carb... Fix the problem within the proper circuit and you'll have a much better tuned carb.

When you're happy with driveability, then tune for power and proper WOT fuel mixture...

I understand what you are saying Royce and it IS an interesting topic. One that I've personally done a lot of research on.

First, I think we need to be carefull to differentiate between removing/plugging the front power valve and removing/plugging the rear power valve. I'm not sure what the context of the exerpts you quoted were, but it sounded to me like they were either talking power valves in general or speaking about removing the front power valve. IMO Front power valves should always be left in place except when you have a VERY radical race car with virtually no vacuum at idle.

But for drag racing IMO removing the rear power valve and jetting up to compensate in back is a good thing. Again, the logic is that the PV is smack dab in the center of the metering block. The float level is typically set so that the top of the PV is barely covered when the car is stopped. When drag racing, rear power valves are easily uncovered and cause a brief lean condition.

The PVCR in back is typically equivalent to 6 - 8 jet sizes. When you plug the rear PV and increase jetting in back to compensate, there is no adverse affect on a double pumper -- only now you have a source of fuel that stays submerged in the float bowl. If the jets do somehow get uncovered in back (ie: during really hard Pro-stock style acceleration) you can add jet extentions; you cannot do this to a power valve. There is no function a rear PV performs that I cannot tune around on a DP with the rear jets and rear accelerator pump.

The power valve only serves to richen the mixture during low vacuum conditions (part or WOT). Or you could say that it serves to lean the mixture during cruise or light throttle conditions. Either way you look at it its the same thing.

Since the secondary side pulls fuel from the main well only when the throttle is cracked open enough to create a signal across the secondary venturi, the presence of a secondary PV is almost irrelevant on a mechanical secondary carb. You have to be into the throttle for the secondary to start pulling fuel anyway, so why have a PV to lean the mix at light throttle?

The old O-3310 vac secondary carbs used a secondary PV because the engine would go lean for a split second when the secondaries initially opened. Since there was no secondary accelerator pump the PV would in theory help "cover" the flat spot. However, the primary accelerator pump, primary PV, and vac can spring can be tuned to smooth the transition too -- and I also believe that the size of the signal passage to the vac secondary can was later reduced to help smooth out the secondary opening rate (a better solution IMO).

For road race or oval track engines, there may be some use for secondary power valves since hard braking under load and lateral g-forces come into play. I'm not really up on tuning for road racing or oval track.

In this quote they said, "Holley's engineering staff has found many advantages by leaving the power valves in place." I'd be interested in knowing what they are . . . there have been quite a few threads over the years here that have dealt with the subject -- some seem to feel strongly one way or the other, but I have never seen any hard data that removing a rear PV slowed a car down or made it less driveable.

Not arguing, just enjoying the discussion graemlins/beers.gif

Luke --

Sounds to me like your primary jets are awefull small. Try going a little richer in front (like #70) and a 6.5 PV. Leave the back PV plugged with number 81's. You have waaaaay too much difference in jet size from front to back - 6 to 10 jet sizes is about max IMO. (I know Holley sayd 68/81 on this carb, but that is kind of rare that they spread that much front to back). The popping indicates lean to me.

The popping is probably due to low fuel pressure at high RPM. OR if it pops when you nail the gas out of the hole, it is an accelerator pump issue.

If you want go back to the bone-stock factory setting with rear power valve and see what happens. When in doubt, go back to the stock settings and work from there.

Eric that carb came with a rear plug bone stock... The use of a rear PV would not be stock...

Eric68, Need another nail? You nailed that one!

Originally posted by DjD:
Eric that carb came with a rear plug bone stock... The use of a rear PV would not be stock... Oops. You are right Dennis.

I wonder if a primary metering block was installed on the secondary side with this carb? I've heard of this being done when someone wants to have "4 corner idle circuits" (doesn't work BTW). You can tell because the secondary block would have idle mixture screws that would have no effect on the idle.

If that is the case, you need the right secondary metering block -- the wrong secondary block could be causing the engine to go lean up top . . .

sixd8rs - thanks graemlins/thumbsup.gif

Eric I know you aren't arguing we usually see eye to eye anyways. I understand your point and I totally agree if we are talking about a drag only car. The book was talking about the secondary power valve for most of the discussion, I don't know why anyone would ever remove the primary PV anyway.

I don't have a HARD opinion either way, as usual I believe in doing what works for you.

Your theory makes sense to me. I have read this book on several occasions and for some reason (as you could tell) they are firmly against ever removing the PV.

I wonder why Holley installed the secondary PV in the HP series 750 I have (it was bought new so I know it wasn't tinkered with)?

Good stuff.

I also agree the popping is caused by a lean condition. though you have not mentioned when the "pop" occurs, this would help narrow it down.

Last night I jetted down from 66 to 64 (left the secondary at 81 w/ power valve plug). Popping out the carb occurs at low RPM now and it doesn't have the power at WOT that it did. Just now read your posts. graemlins/clonk.gif

It sounds like you gents mostly agree that I should reset it all to factory specs with the 68 jets in the front - the rest is the same as factory as far as I know. Then test and tune as DJD describes from there.

I am hesitant because it seemed to run real rich with 70 jets in the front and so 68 isn't much less. Are 66's out of the question with 81's in the secondaries (and the PV plugged)?

FYI - it does have a secondary metering block w/ idle/mixture screws. Where should these be set or does it matter?

You are just guessing on the jetting, go back to the factory jets and make the car idle and run properly. Learn the carb's different circuits and learn to tune each. The carb is kind of like a car, the car is made up of different systems like the engine, tranny, steering, brakes and rear end. They all make up the car but you don't fix the brakes by taking the steering wheel off and putting a different one in it's place. Back to the carb, you can't fix a rich idle problem working on the power circuit (jetting), you have to tune and adjust the idle circuit...

If the engine is popping it is too lean. period. Unless there is something else wrong like a bad cam lobe, but your cranking compression would probably have been off for one cylinder.

How do you know it was rich with #70 jets? It sounds it was too lean with #66s and for some reason went leaner with #64s. :(

You need to verify WOT fuel pressure to be at least 5 psi THEN richen up the primary jets to at least #68s.

Then if the problem still isn't solved you need to replace the metering block in back with the correct one. You have no idea what air bleeds this non-stock metering block in back has -- that can be causing all kinds of trouble.

Finally, if you have done the things above and it still pops, then you had better look hard at your camshaft because thats the only other thing that I can think of that would cause an engine to pop like that. You can sometimes tell by watching your vacuum gauge . . . if it is steady you are probably OK, if the needle jumps around then you probably have an intake lobe gone.

Great discussion, guys.

Just to add one thing - I've never seen a Holley with a plug where the secondary PV should be. Those secondary metering blocks on the carbs I have seen aren't drilled and tapped for a PV, just a blank. If Holley ever did have a plug in place of a PV then I stand corrected.

Carry on.

BPOS,

Holley did! :D

Every part number 4779 750 DP that I have taken apart had a PV in the front and what looks like a PV in the back, only it's a solid piece (plug).

DJD - tried idle/mixture screws FIRST. Then went to the primary jets since it was running rich, and those were 70's, while stock is 68. I hear that you want to jump a couple sizes b/c one doesn't make much difference.

Eric68 - cranking compression is perfectly even at 195 PSI. How do I test fuel pressure?

How steady need the vaccume pressure be? The needle flickers a total of 1-2" HG.

I got the stock 68 jets today and I'm going to put them in and try to adjust the idle circut as DJD suggests. Any advice?

Im wondering if maybe you have also blown the power valve with the carb poping as often as it has. Usually it will have a hard time idling though if this happens. But I have seen them blown and idle but will run very rich. This wouldnt cause it to pop but would could cause it to run fat. Be sure you have the correct valve in when you get the 68 jets in the primarys.
A fuel guage will determine your fuel pressure. But using a mechanical pump Im not sure you can unless there is a special guage, Diaphram type maybe that will do it.
A really good book about these carbs that I would recomend to anyone who is rebuilding or tuneing a Holley is the Haynes Holley Carb Techbook. Found in the manual section at most auto parts stores.

I've seen Holleys with and without power valves in the secondarys.

Use a fuel pressure gauge to check fuel pressure.

I just use a cheapo gauge connected close to the carb inlet -- I use a plastic line to allow me to move the gauge to where I can see it while driving. I like to tape it to my windshield. Then just make a couple blasts and glance at the gauge when the engine is rev'd up.

Just be CAREFULL you don't hit anyone/thing and keep the gauge outside the passenger compartment in case that plastic line splits or leaks.

If the accl pump isn't adding enough extra fuel during transition off the idle circuit it will creat a lean condition as the air rushes throught throttle bores until the fuel from the jets catch up. I know I sound like a broken record but learn the many circuits or systems that make up your carb and work them one at a time...

Fuel delivery and storage (float bowls and floats) then idle system then transition (accl pump, pump cam and squirters) Refer to the techref area series_II for a lot of basics...

sixd8rs - I just now replaced the primary power valve since it's been backfiring out the carb, so that should be fine. It's a mechanical fuel pump so it's hard to test. I think they either work or they don't.

Dennis - the accelerator pumps appears to work instantaneously with throttle movement with a healthly squirt.
Though I haven't detailed here every step that I've taken, I really have read your tech article, a Holley tuning book, the Holley web site and followed each step as closely as I can, but still have the problem. I've tried to understand and tune each circut in order.

I did start another post under Trouble Shooting because my original question here about the power valve plug was answered. So if 'yall wouldn't mind taking a look under "Holley Carb - rebuilt and now runs worse!?" I'd appreciate it! graemlins/thumbsup.gif