|12-10-2018 09:32 AM|
Its not so much as that weight is a problem with using a double pumper on heavier vehicles but more so is the biggest thing with them more so then weight is how quickly the rpms pick up.
Basically if you have a 4000 pound plus vehicle and a engine only making hardly much power and not much in torque and the engine is a slug taking off then a double pumper would not be the ideal carb to work as if you smash the throttle to the floor from a dead stop or a very slow speed your going to have trouble with a bog cause its taking the engine to much time to catch up and get up the rpms up and the instant lean spike from the throttle opening and the gush of wind it would take a heck of a lot of fuel to cover that gap and I have seen many times people put double pumpers on vehicles that were not setup properly and they ended up using 50cc pump shots to get rid of a big bog which is a band aid type of a fix and not a proper tune. About the only time a 50cc pump shot has ever been used is in all out racing builds and in all my research and through all my tuning books I have never seen any setup on the street with 500 plus horse power or less have a holley in the 900 or below cfm range ever have to use one.
Holley though does not help as they are trying to cover themselves as to what you can use a double pumper on as you can use them with automatic transmissions and you can have less then a 3000 stall converter and they can do the exact same mileage as a vacuum secondary and are fully streetable but holley says otherwise but that is being from what they feel helpful to keep a person from getting something wrong that won't work.
The main thing with a double pumper more then anything else is if you know how to tune with them the proper way and not doing band aid fixes and as long as the vechicle can pick up the rpms very quickly then all the other stuff does not matter for the most part.
Its all about having things matched up as even the vacuum secondary carbs if using a very light spring on a build that is a slug taking off has a high chance of a bog and using a spring that is so stiff it would never open up the secondary side all the way up. Yeah they have mods that can be done to make them even work better.
Bottom line if your vehicle has the right gearing and enough power to make that thing go then a double pumper of the proper size can work just fine regardless of weight and a few other things that are spoken against them.
|12-09-2018 06:56 PM|
From my limited carb rebuilding knowledge, I think the stigma against double pumpers for heavy cars with highway gears is bull. I believe that yes, if you want a "plug and play" carb, well then yes it may be true. A little bit of tuning goes a long way. I put a Holley 800 double pumper mechanical secondary spreadbore on my 454 with an automatic, 3.42 gears (at the time) and the car with me weighs 3920lbs. At first it had a hesitation opening up those giant rear barrels, but I played with it and got it better. Then bought a cam kit that I believe only had 1 cam that fit the rear throttle and it just wasn't what I wanted so I turned the cam to where I wanted it, drilled a hole, and put the screw in. Gave it a nice long squirt throughout the entire secondary throttle range. No hesitation at all at any rpm in any gear.
On the other hand, a vacuum secondary that opens like a slug from the factory can be *fixed* also. It won't be quite as crisp as a mechanical, but you can get it pretty close.
|12-09-2018 10:46 AM|
Your jetting was quite lean with 69 in the front and 73 in the rear and now your heading in the right direction like stated above and the 770 street avenger is just fine for your big block and if you went with a 650 double pumper your mileage would only be different if you would keep opening up the secondaries over and over again as I own several size holley carbs and regardless of running a 750 or a 650 my mileage was the same but some slight differences on other things but engines differ on how they respond and work as other factors will change things as well but in my opinion the 650ish range cfm carbs are better on small block builds then big blocks even in mild stock form as a big blocks have a heck of a lot of air coming in with way bigger bores and bigger valves and cubic inch size and your 770 street avenger would be better overall compared to a 650 double pumper as double pumpers are not as forgiving as a vacuum secondary carb as double pumpers are picky with vehicle weight and also rear gearing and how quickly the engine can pick up the rpms and respond with. If the engine takes too much time to get the rpms up the double pumper will only give bogs or hesitations and will be a nightmare if not matched right on the secondary side of things.
That is why vacuum secondary carbs are more often recommended over a double pumper in most instances. There are a lot of myths though on double pumpers as well but that is another topic there. Stick with your 770 and get it dialed in good and you will have everything your engine needs for carb wise with what you already have.
|12-08-2018 05:36 PM|
If you ask me, the old chestnut carb formula of RPM x CID divided by 3456 then factor some efficiency figure was designed by somebody's Grandma, cause it is overly conservative for everything I've ever done.
I guess they just assume every street engine is in somebody's daily driver and it has to be timid and meek in order to be reliable.
|12-08-2018 05:25 PM|
I think I'd stick to the 770. If you keep the vacuum secondaries so they don't open TOO fast, you will have all your low end response and torque.
With a 350 you can run those carbs, especially if it's a vacuum secondary on the bigger one. Just need to set it right, which could be that the secondaries never fully open.
Then again, this could all be crap. I'm just going from what I've learned in the past 5 or so years on my own car. I don't have experience with a lot of cars/different size motors.
But if you want to sell the 770 cheap send me a PM!
|12-08-2018 04:37 PM|
|12-08-2018 04:02 PM|
Get the 770 Avenger all tuned in and it will be faster than the 650 Double Pump, and get better mileage to boot.
On 408", nothing smaller than a 750 Double, and personally I wouldn't go smaller than a 800 double if you feel you just have to have a Double Pumper.
I run a 950 Vacuum Secondary on my 406 SBC, it is faster than both the vintage GM original 780 vacuum all reworked and polished to 820-ish CFM, and the 800 and 850 Double Pumpers I've tried on it.
In cases similar size engines/performance like yours, the yellow or purple secondary spring is where I usually end up too.
Sounds like you've made some good progress
|12-08-2018 02:37 PM|
|jjjaffo||Okay, here's the skinny: I tried the wire in the air bleeds trick and no good. Noticed when I would lean on the gas slowly and it went to the primary circuit it would misfire until the vacuum got low enough to kick the power valve. Took carb apart and found 69's in the primaries. put 73's in and upped the secondaries from 75 to 78. It was like night and day! The throttle is more crisp now, but I might try upping the jets a tad more. Of course fixing one problem always creates another. It's a 30 degree day here in Wisconsin and traction is non existent in first gear with my automatic. That little 408 big block chevy has power I've never seen before. I put the yellow spring in the secondaries and transition to full throttle is smooth and clean. Might try a hair more pump cam just to clean things up a bit opening the throttle. Been kicking around the idea of a 650 double pumper. Any thoughts?|
|12-01-2018 08:29 PM|
Holley uses the same metering blocks on the 600 through 750 cfm carburetors on there most sold street versions of the 4160 and 4150 models of the vacuum secondary and double pumpers.
The only difference is the idle feed restrictor sizes and the power valve channel restrictor size. On a holley brand 600 to 650/670 vacuum secondary carb the primary idle feed restrictor size is about .028 and on the secondary metering block about .031. The vacuum secondary carbs of the 600 and 650/670 cfm range will always be leaner calibrated then a holley 750 cfm vacuum secondary metering block or blocks.
A holley 750 cfm vacuum secondary would have about a .033 primary idle feed restrictor and a .039 secondary idle feed restrictor size. The power valve restrictor size I would not know right off hand but they on the vacuum secondary carbs normally have about a .046 or a little bigger. The 770 should be similar in specs to that but I have not had one on hand in years to be able to measure for you.
This is on a 2 corner idle carb. The four corner idle carbs will have the same idle feed restrictors in the rear as the front since you have idle mixture screws in the front and rear and also the rear idle air bleeds will be the same.
A holley double pumper from 600 up to 750 will be calibrated richer as well. On the 600 and 650 2 corner idle versions they have a .033 primary idle feed restrictor size and a .039 secondary idle feed restrictor size. The 750 double pumper is a 4 corner idle so it would have about .033 primary and .033 secondary idle feed restrictors. A 4 corner idle carb will usually always be square front and back with the same size restrictors and air bleeds.
The holley street avengers all come with a secondary metering block so you don't have to worry on those if you want to use different metering blocks that would have different size restrictors but its easy to alter and modify yours with just a little bit of time and some tools and drill bits and some brass allen screws which are cheap and a 6/32 tap.
Basically how you can modify your metering blocks to change out your idle feed restrictor size is to drill out the pressed in idle feed restrictors and then after you yank them out you would then tap the open holes with a 6/32 tap and then you can use 6/32 x 3/16 long brass allen screws from mcmaster.com which you can get a box of 50 qty for about 10 bucks give or take and you use a small allen hex to screw them in and out to change them and then you would buy a 61-80 drill bit gauge set off of ebay and use a small pin vise to drill the proper size hole in them for what you need.
I have done this many times and its pretty easy if you take your time and are careful. I was successful the first time I did it and I never tapped a hole in my life before that.
Here are a few examples of holley idle clibrations on a 2 corner idle carb and this is only a guide line as engine size along with cam size will dictate a carb tune and calibration and is just a rough showing on idle circuit tune.
Holley 600/670 street avenger has a pretty lean circuit out of box on most small blocks in a 350 cubic inch range running a mild cam with 215 or less duration @ 50 with minimum overlap and making about 17 inches of vacuum or more in park.
Idle air bleeds are .078 primary and .049 secondary with .028 ish high speed air bleeds front and rear. They have a .028 primary idle feed restrictor and a .031 secondary idle feed restrictor size.
That is a leaner setting starting off with. If you are needing a slightly richer idle calibration leave the air bleeds alone as they normally don't need changed to often as changing out idle feed restrictor sizes has a way bigger effect on idle and transition circuit just by going a .02 up or down in size.
By leaving everything as it is and only changing the primary side to .031 it would increase the idle calibration a few steps richer but not overkill richer.
The .031 primary side and .031 secondary side resctrictor size would still be enough to make a change without being overly rich depending on the came size.
If that was not rich enough then you could then go up again .02 on just the primary side to .033 and leave the secondary side alone at .031. If that is close but still not quite depending on how your carb responds and adjusts etc then you would change up the secondary side to .033.
The thing is with changing out idle feed restrictors is a trial and error thing just like changing out jets but it takes a little experience and knowing what to look for while cruising below 40 mph.
I go by my idle mixture screw adjustment range along with how much range I can adjust my primary idle setting screw setting adjustment in relation to transfer slot exposure as well as my secondary opening setting.
Its one of those things that engine size and cam specs and how much overlap it has and a very critical thing as timing and engine vacuum can play into a carb tune that can vary.
I have a 377 dart shp small chevy build that I just put together a holley 650 vacuum secondary carb that has all the identical metering blocks to a holley 670 street avenger and I have a 228/232 @50 547/547 hydraulic roller cam and have 14 inches of vacuum in park and I am running my carb with a richer idle circuit of .033 primary idle feed restrictors and .033 secondary idle feed restrictors and .074 primary idle air bleeds and .049 secondary idle air bleeds and it runs perfect with my engine and I run 16 degrees of initial timing and I also run my vacuum advance of 12 to 14 degrees on full manifold giving me around 28 to 30 degrees of timing at idle.
My timing is all in around 3000 or a little over rpm. Your holley 770 street avenger should be about right or if anything maybe a little rich on the secondary side on the ifr size but vehicle weight along with rear gearing and transmission type being used all can play into things as well.
The holley street avengers most notorious for being really lean and off idle hesitation is the 670 cfm one more so then the 770. The best thing you could do first is actually measure the idle feed restrictor sizes in the metering blocks and post back. You don't want to go changing them things until you have exhausted everything else on off idle throttle response and tuning the shooter size and pump cam assortments.
Tuning the pump shot is also a trial and error thing as well and sometimes you have to change out one and then the other and see how it reacts and then sometimes go back on one while not changing the other.
How I do my pump shot tuning is I make sure everything else is good and I kind of know if I am close on my idle circuit calibration by feel and how it runs as I have a bit of experience with many different builds over the years but I start off with the stock being stock out of box setting.
I will go up in shooter size first and see how it does and if there is less hesitation then I know I am in the right direction and then I go to a pump cam change. I don't know what color your 770 came with stock as I have not read this whole post in a while for example on every quick fuel carb I have bought brand new they always come with a pink pump cam and it has always given me a bog off idle or even while opening up my throttle slowly on every build I have had.
I first went up in shooter size from a .031 to a .035 and it would not change much so I knew I need a different pump cam profile as the pink would not give me enough initial shot so I would put the shooter size back down to where it was out of box at .031 and then would go for a pump cam with a quicker profile for a bigger shot and in my case using the middle of the ground chart I used an orange pump cam and then just with that change all the off idle hesitation and bog was gone and I could snap my throttle in all driving conditions and not have any issues.
Another thing too is to make sure your pump shot is instant and that its adjusted to not have any slop in the accelerator arm but another thing to look at as well is the accelerator arm itself is to make sure its making proper contact with the pump cam as I have cam across to where they were bent sideways and would not travel on the pump cam right and I had to bend them so they made proper contact with the pump cam being used.
|12-01-2018 04:51 PM|
|11-27-2018 12:50 PM|
|eric32||Ok I just posted a big write up on the holley 670 street avenger sticky about secondary tuning options and some good information on how to get the best function on the secondary side of things. Once you get your primary side stuff sorted out you can check it out and hopefully you can get something from my write up and it will help you get the most out of your secondary side of things.|
|11-27-2018 09:12 AM|
|eric32||Below is a pump cam chart and if you look at it the more cc's it does then the lesser the initial shot but has more throttle duration travel and the lesser cc's it has a bigger initial shot but less throttle duration travel. I will also get with the secondary side of things later on as I got something that went on yesterday.|
|11-27-2018 06:15 AM|
|TommyK||Try a blue cam.|
|11-27-2018 04:23 AM|
The red cam is one of the smaller cams if I remember correctly.. Will likely work it's best on the second hole in the cam, second hole in the linkage plate.
|11-26-2018 08:36 PM|
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