OK, so I've read and read and read some more about multi-carb setups and I'm still having problems. My situation is as follows: I'm running a Weiand Drag Star 6X2 intake with Holley 94's (8BA's) on a Buick 401 nailhead that has been bored .040. Manifold vacuum is steady around 15 inches at idle. I don't know if it has an aftermarket cam but would assume not. I bought the intake and carbs in a complete set that had been run at some point with a solid linkage (non-progressive). The outside 4 carbs had their PV's leaded shut (the hole in the base) and the center 2 were running factory 7.5's. They were all running 51 jets. I rebuilt all the carbs and bench set them. I also switched to a progressive linkage using the center 2 carbs for primaries. It fired up and ran, but was really rich on idle. It did clean out once it was revved up, but when you burped the throttle it would belch black smoke. I took everything back apart and double checked everything. I adjusted the linkage so everything was timed correctly. I also bought and installed PV blockoffs in all 4 outside carbs just to make sure, and then installed 4.5's in the center 2 carbs. I checked all jets and they were correct, so I drilled out the jets in the outer 4 carbs to 53's and left the center 2 carbs at 51. I got in touch with Daytona Parts in Florida and ordered a mess of gaskets, as well as "better" leather accelerator pumps and "better" needle and seats that are supposed to be compatible with new gas (these seem strange to me because they don't have a rubber "nipple" or cone on the needle, but rather a little rubber "puck" that's flat). I installed these and reset the float level to 1 11/32 up and 1 7/16 down. I also set the outer carb idle screws at .75 turn out and the centers at 1 turn out. When I fired up again I thought the problem was solved. I hooked up the timing light and double checked timing. It appeared that my problem was solved. Idle was good and clean and you could burp the throttle with little to no black smoke. It still has a flat spot right off idle if you romp on it, but I'm not sure if that is a carb issue. I took the car on its first real trip, 150 miles south to Joseph Oregon which is about 2000 feet higher than here. It ran rich when I got there, as I expected, so I played with the idle mixture screws a little and it seemed to get better so I ran it that way over the weekend. When I got home I drove it across town and it keeps getting worse and worse..... The black smoke has returned. When I drive the car, I notice that when I come to a stop it wants to flood out and die. I thought this was a transmission issue (I'm running a switch pitch 400) but now believe that it is actually because carbs are flooding the engine due to the braking action. I came to this conclusion because I live on a hill, and when I come to the stop sign at the bottom (car facing down hill) the car REALLY wants to die, even if I rev the throttle, and smokes like a locomotive. When I saw this I went to another hill and drove up it....the car runs fine. Then when I point the car down hill it loads up bad. Next thing I noticed is that when I shut the car off I can watch fuel leak out from apparently the throttle shaft on the front passenger carb. It never did this before. While I'm looking at the fuel leak I can hear fuel dripping inside other carbs (not exactly sure if it was all or just some) which NEVER happened before. I've driven the car for about 30 miles since I got back and in that time it has gotten a LOT worse. This last drive almost asphyxiated me and its not just exhaust, but more the smell of raw fuel. MY THEORY: Those new "better" needle and seats aren't working and its allowing fuel to flood the float bowl and therefore pour into the engine. Would this get worse if the car was facing down hill? I guess I need to mention that my carbs are mounted "backwards" on the intake with the bowls to the back of the car. Anyone have any other ideas as to where to look? One major question I have is about PV's. Like I said I've done a lot of reading and even talked to a local drag racer from the 70's and I've found 2 totally different theories on PV function. One side says that you need to add up the rating for ALL of the PV's and that number needs to be less than the manifold vacuum at idle or it will just dump fuel in all the time. The other side says that you can have a thousand carbs hooked to an intake and as long as every single power valve is rated for less than the vacuum at idle you are fine. Thus the confusion on my part. Logically, I am siding with the "thousand carbs" theory because the PV's have a spring that won't allow it to open unless it is exposed to a lesser vacuum than its rated for. Therefore, if I'm running 1 or 10 PV's that are 7.5's and my vacuum at idle is 15 none of them should open because the vacuum at each PV is over 7.5 inches?.?.? I know I've written a short novel here, but I'm hoping someone can shed some insite on my specific situation. I know every engine is different and I will have to tweak my setup to work for me, but something is definitely not right currently and I'm not sure where to head from here. I've tried to give all the information for my situation, but if I left something out let me know and I'll edit the post. If I had the coin I'd just buy 6 of the new 97's and be happy but I'm not in that situation and just want to figure these out. I have read over and over that 94's are "easy to tune" but haven't had that experience to this point. Please.....bless me with you knowledge and advice. Thanks.
WOW! I love it when people give this much detail! I'm about to get my Hemi going with a 6x2 and 6-Holley 94's, so I'm watching this carefully. You mentioned everything except one detail... what are you running for your fuel system (pump, regulator, etc.)? What is the pressure you have your carbs regulated to?
I've got a 6x2 dragstar on a y block with 97's, electric fuel pump with just under 3lbs pressure. I'm using the eelco progressive linkage but I've heard that 6x2 setups dont like progressive linkage so I set mine up so they all work together. The way I've been told to set them up is like this: Idle circuits plugged and throttle plates closed completly at idle on the end carbs, centers are primary carbs. You have to look at it like a 4 bbl (6bbl)carburetor, just seperated. Plug the power valves on the end carbs. I've fired this up with the center carbs only so far and got the motor running.
if your "better" needles arent seating properly, then yes, while pointed downhill your bowls are higher than the seats, and are gonna dump fuel(as per your description) While I dont have exp. with your manifold/carb set up, I was under the impression that vacuum operated in a similar fashion to an electrical circuit - parallel, and series(in this case, progressive linkage?) - in parallel, resistance is the same for all, but in series, resistance climbs as it passes each component(do I have that right? its been a long time since skool). At least that was how it was explained to me as to why I needed a Vacuum booster installed on my old Chevelle when I went to a seriously more potent cam shaft. I didnt stay at a Holiday Inn Express last night, so I could be entirely way off track with this.
Good point, scootermcrad. I'm running a continuous flow electric fuel pump with the Holley 12-804 fuel regulator. My fuel pressure is set to 2.5 lbs roughly and is monitored at the fuel log with a Moon pressure gauge. Here are a few quick pictures......they're supposed to be worth a thousand words, right?
There are (were) two styles of the flat fuel valves: 1) an aluminum plunger with a recess and the flat neopreme wafer inserted in the recess. 2) an aluminum plunger rounded on both ends and a separate hex aluminum piece with the flat wafer inserted. OPINION (others will differ!) We love type 1. Any of the type 2 that we have acquired in the past when we bought carburetor inventories WERE MELTED FOR SCRAP METAL! The leather pumps are definately an inprovement over the neopreme! Jon.
Couple comments on PVs: I can see no way that ONE PV is going to know how many more are on the manifold. Each does it's own thing - regardless of the number of carbs. On modern 4-barrel Holleys, the PV value needs to be 1/2 the manifold vacuum - and given the numbers they're available in, you go to the next lowest if need be. Example: for an engine that idles at 13 on your vacuum gauge, you need a 6.5 power valve. If it reads 12, then you need a 5.5 (as they don't come at 6.0). I'd imagine that the old 94's work the same way. Anybody who actually knows . . . care to comment?
Correct I worked at Kirks Carb Shop here in the motor city after high school for 5 yrs. The spring in the power valve is designed to let the valve open at the number of inches of vacuum stamped on it. Wether you have 1 or 100 of them. I think the school of thought is plug the secondary carbs P.V.'s because of the amount of fuel they release into the system at W.O.T. Most, but not all, Holley 4 barrels dont have a P.V. or mixture screws on the secondary side. The end carbs are in theory, like the secondaries of a 4 bbl carb (except we have 8 extra venturis instead of 2) and shouldn't need a P.V. The flow of fuel from the nozzles of the end carbs at W.O.T. should be sufficient.
And so it begins...... This is the most conflicting subject I've found with the Holley 94's. SinisterCustom says to ad them up and have the total less than the total vacuum at idle, whereas Bored&Stroked says that you consider each PV seperately. I'm on the edge of my seat waiting to see which side wins. Maybe Bruce Lancaster can shed some light on this topic...... As far as the fuel valve style......the ones I got from Daytona seem to be the #1 type that carbking described, with a flat rubber "disc" on the end.
Mopar 6 pack end carbs have no mixture screws and dont even have a metering block to put a power valve in. If memory serves me correctly, GM tri-powers are the same way. These were systems that were designed to be street driven. Totaling the power valves up would assume that all of them some how communicate with each other and sync up somehow. It's just no so.
If your running, say two Holleys on a big inch motor pulling 10 inchs of vacuum, EACH carb PV needs to be 4.5 or less......to add up less than engine vacuum. One Holley PV supplies plenty of fuel for a big motor........ So if you have a bunch of carbs....all with, oh say, 6.5 PV's, they'll all start to open at 6.5...that's alot of fucking fuel to add to the engine and plugs foul, black smoke, etc, etc.....by dividing the # of pv ratings by the carb count, you get it back to where it would be with one carb. It's true each carb sees the same vacuum....but that don't mean shit when all those butterflies open up and that vacuum is GONE instantly.......that's when the extra fuel enrichment comes in....and 6 pv's all dumping in fuel is not good. Why do ya think 97's were prefered "back in the day" over 94's on flattys???? Ease of setup for the most part....the Holleys were harder to tune for guys not real familiar with vacuum pv's.... I think B&S and I are on the same page....just worded differently.
Your talking 2 different things here. The number on the power valve determines when it opens in relation to engine vacuum. The number is not like that of a jet, where it would refer to the size of the hole in it. The power valve draws fuel from a circuit in the body that goes to the float bowl. Unless you change the size of that circuit you will still get the same amount of fuel just later in the RPM band.
Sounds kinda like you have more pressure than the can handle, or a needle and seat leak issue, and you never want the secondaries to be bigger than the primary main jets. The ideal size is 2-3 sizes smaller. And I sure wouldn't have the leather pumps either. Use the new "funny" fuel resistant blue flourelastomer cups, they will even handle alcky, or methonal. Also, the 4.5 PV's are good but be sure you haven't popped one. If you think about it, there is a reason the motor ran well for a while and then without changing anything it starts complaining.??????? Also, if and when you run the front and rear 4 as secondaires you have to not only kill the idle circuits but also the vacuum circuit in the carbies as well. I dare say just "screwin' them in all the way' won't get it. If you are going to make your front and rear drones, go ALL the way and you will notice a difference. Oh, as far a needles are concerned, you CANNOT get as better funtioning one that the original style ALL METAL pin that Ford and Holley used forever. Fuerl don't screw up metal.
Holy confusion Batman! This is the same conflicting theories that get me all in a whirl too. It's the reason I've been puting off asking about these PVs as well. GAH! Let's throw two more examples into the mix, just to help clarify the theories here... Let's say we know the vacuum at idle is 12. With that being known, let's examin the above comments with this scenario: 1) 1 single, solitary, Holley 94 and Powervalve of some rating = X (should be easy, right) 2) 6 Holley 94's (not progressively linked) with all six power valves (if used) of the same rating and = X Good constructive conversation here about a very important topic with the carbs!
Sure, your right, with one carb....but multiply that by 6.....now you have 6 carbs each dumping fuel.....and when ya crack the throttle on 6 carbs, vacuum is GONE....... Haha...sorry Scooter....I sometimes hava hard time typing what I mean... An engine at idle with 12 inches of vac will keep any amount of pv's closed with ratings under 12.....the problem comes when the throttle is cracked....the more carbs, the faster vacuum is lost. Each carb dumps the same amount of fuel so X many carbs dumps X amount of fuel. #1) a 6.5 PV should be fine... #2) 1.5's (if they even make 'em??) or plug 'em and jet accordingly.....Littleman has alot of good info on his setup. Even better.........use 97's.....
Okay, I feel I understand your theory the most, however your answer to #1 now doesn't make sense. Why wouldn't you have said 11.5 (if they made one)? I have such a love hate relationship with all this crap. Proof I've never been good at dialing in carbs.
I have an opinion on the powervalve theory, too. But I'm not gonna bother sharing it. I don't think that the powervalves are the problem at all. If you're running a progresive set-up then you need to close the idle circuits completely in the secondaries. you also need to make sure that the butterflies are closing completely. You didn't mention changing the butterflies when you changed it over from non progrsive to progresive. If not then that will be a lot of your problem. The secondaries should close tight otherwise they are essentially just leaking vacume.
Beleive me Scooter, I had the same love/hate thing with this too years ago, trying to get my big cammed 440 to pass emissions...that's when I started messing with this crap... You don't want alot of fuel dumping immediattely...the PV's are for dumping fuel under quick throttle responses, not minor changes in throttle pos......if the pv rating is too close to idle vac, any minor drop in vac will supply more fuel that really isn't needed..... Many big cammed cars have such a rough idle that vacuum is all over the place under 1000rpm...so using a PV at 1/2 or so solves the overly rich idle issues.
This thread rules. so Sinister I don't want to put words in your mouth I just want to try to reword your answer in a way I can understand it. If I'm wrong here please correct me. Basically what your saying is that it's no so much that you need to have 1/6 the pv number for 6 times the carbs. it's that you should use a smaller power valve to delay the time it opens because of the quick dump in vaccuum due to more carbs. AND that dividing by number of carbs is a good rule thumb to get you in the ball park. Correct?
Yeah....that's pretty much it. Tough for me to explain....I'm a terrible writer.haha Idle vacuum will hold 'em closed, no problem there, it's when all those carbs (PV's) open at once is where the problem begins.... And trying to get old carbs to completely seal as secondaries and not have the butterflies want to stick in the bores is another issue altogether...haha.
With due respect to all commenting on the power valves; as long as progressive linkage is being used, then discussion of a power valve in the "dump" carbs is moot. The purpose of a power valve (or economiser valve, depending on which company's product is being used) is to enrich the mixture under W.O.T. conditions. As the "dumper" end carbs are being used ONLY to provide additional air and fuel under W.O.T. (using progressive linkage), then take a look at the dumpers on factory Rochester tripowers. Plug the power valve circuit on the dumpers and increase the jet size to be equivilent to the original jet + the power valve. If the carbs are being used with solid linkage, then the power valve on all MAY (or may not) be a good idea. Jon.
Questions for the originator. Have you backfired this engine at any point? Someone mentioned to me that backfiring has a tendancy to blow out those PV seals. Just adding input and content to the discussion. Not saying that's the reason. Feels like an episode of House, or something...
Exactly. Which is why you need to eliminate 4 power valves out of the system. Changing their number still lets them all open. Dumping the same amount of fuel. They are not like a jet. They have the same size hole in them regardless of the number on them. The number dictates When they open not how much they flow.
Another fun HAMB thread! Sinister and others have it right. The big issue is that you probably don't want 6 power valves all dumping fuel and bogging the crap out of it. If you're running a progressive linkage, then you probably only need PVs on the center two carbs - put block-off PVs on the outer four. Also, you may need to go to lower numbers on the two centers -- instead of say 6.5 - maybe 5.5 (it all depends). If you're going to run all 6 of them in sync, then you may need to lower the PV numbers - to have them come in a bit later. OR . . . you may find that it is a good idea to stagger the values -- maybe 6.5 in the middle and 3.5 on the outer four. We can't bench tune here . . . running 6 carbs (of any style) takes quite a bit of tuning, checking plugs, screwing with sync devices, etc.. I've never seen a "bolt em on and go" situation with 6 carbs (or even 3!) . . . which is one of the reasons that big-ass 4-barrels really took off - a hell of a lot easier to tune and KEEP in tune. If you're not a carb guy, then hookup with somebody who really is. Also, buy a vacuum gauge, buy a Unisyn, learn to read plugs and be prepared to spend some time with jets, linkages, PVs, idle circuits, etc.. If you don't consider this "fun" - don't run a multi-carbsetup (at least past 2 of them) Best of luck! Dale
I do not believe I have backfired the engine, and I'm sure I would have noticed it if I had. I will be tearing down the carbs tonight though, so I will check out the seals and see if I can notice any damage. By the way, how do you tell if you've blown a PV? It now makes perfect sense why some people were saying to add up the values. All this time it had seemed like that school of thought was trying to say that you needed to add up the numerical values of all PV's on an intake to determine when they would open. Now I understand what they were saying.....the PV's, no matter how many you hook to an intake, will not open until the vacuum in that intake drops below the numerical value of each specific PV indiviually. HOWEVER, if you have 6 PV's that are rated at 7.5 inches you might as well hook a garden hose to your gas tank when the intake vacuum drops below 7.5 inches because they will ALL open at that point and DUMP fuel. That makes perfect sense...... My next question would be do they make different flow rates for PV's? I understand that the different numbers (7.5, 4.5, etc.) designate when the PV will open, but do they make PV's with smaller or larger openings to better regulate the amount of fuel that is dumped when the vacuum drops? I'm not sure this would matter as much for me because I'm using a log style manifold, but it seems like it would still be twice as much fuel as the setup was designed for. I'd say just eliminate all but one PV, but the split log intake wouldn't allow that. I'm guessing that my problem now is the darn needle and seats. I've never used the "puck" style, but it didn't have any problem like this before I changed them out....just ran heavy at idle. Can someone verify for certain what the float levels are for 8BA carbs? I set mine at 1 11/32" up and 1 7/16" down. Also, what is the deal with the whole idle circuit debate? It seems like it is as divided as the PV issue. Some guys say to block all the circuits for the outside carbs, but others say not to worry about blocking off the fuel....just turn the idle mixture screws in further on the outside carbs. I can see both sides, but does anyone have any definite facts about this topic? I've taken the carbs apart a few times now and am getting pretty good at it, so if I have to tear them down again to block something off it won't be a problem. I didn't say this but it might make a difference.....I did not replace any shafts or butterflies when I rebuilt the carbs. They all seemed pretty good so I didn't bother. I've also sprayed carb cleaner all over every carb while the engine is running to see if I have any vacuum leaks and it doesn't effect the engine, but I'm not sure if that is because I'm dealing with so many carbs at once. NOW FOR ANOTHER POINT: I do have a brand new unisyn tool and opened it the other day when I was messing with this setup. I read the instructions and disconnected the primary carbs from the linkage. I set the unisyn on the left bank primary carb and adjusted it so the ball was right in the center. I then moved it to the right bank carb and the ball was only about a quarter up in the tube. I adjusted the throttle stop so the ball would rise to the center of the tube to match the other carb, but that raised the idle RPM of the engine. I reconnected the linkage, and tried to back the idle down but as soon as I start to back off the idle screw on the right bank carb it drops the ball on the unisyn while the unisyn stays the same on the left carb. I admit, I've never used one of these, but it seems like I'm doing something totally wrong. I assume I've got a vacuum problem????? Thanks again for everyone pitching in their 2 cents. I know there are plenty of threads about multi-carb setups, but even after reading them I still felt confused. I finally think I understand the PV issue so I can check that one off my list of "DUH".
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