Cougar hit it. 100%. I had an 850 DP Holley on my 68 Merc's 390 for awhile. It was too big for the 390, but it did work well and was in tune, just that if you nailed it off the line from a dead stop, it would kill the motor when all 4 bbls opened up at once. Just too much flow there off-idle for the application. If you got it out on the freeway, it worked well, once the revs were up to handle the extra cfm's. If you put that 700 on and tune it correctly but it also kills off the line when the gas is nailed, it too would be too much for it, but it's about right on the money for what you have. The 650 would give you better fuel economy and street performance though, if you can get the secondaries functioning correctly. I still stand by what I said about your dyno guy though, if he can't get it to work and there's nothing bent, broken or otherwise keeping the secondaries from opening, you need another tuner.
what carb would you put on this set up?
- Thread starter jerry S
- Start date
-
Sponsors (?)
Jets, metering plates and springs don't prevent secondaries from opening. You can also rev to 5500rpm if the load isn't too great and the secondaries may still not open under normal conditions. The secondaries open based on vacuum, or lack of it, and spring pressure, adjustable to tune when the secondaries open.
The only thing I can really think of causing secondaries not opening is interference or binding in the linkage.
I too question the tuner on this one.
DP's can work woth automatics of tuned properly. 700 cfm is not too much if jetted properly.
The only thing I can really think of causing secondaries not opening is interference or binding in the linkage.
I too question the tuner on this one.
DP's can work woth automatics of tuned properly. 700 cfm is not too much if jetted properly.
Max Power said:
If I test for vacuum, how many inches should I have before ruling out a lack of vacuum as the culprit and looking for interference with the linkage?
I think there is something wrong with the carb. It was brand new. I got the car off the boat and it never ran right. It stalled out on the highway on my way home from the inspection no less than 10 times. I would have to pull off the highway, put the car in neutral and restart. I could never get it over 50 mph without the motor dying off. Then I installed the new MSD and when we started it up, we got two huge back fires. Between describing the reports as sounding like a shot from either a .22 lr or a .45 +P, I would describe it like the latter. I finally got it to the dyno tuner and the needles were stuck. I don't think I should l have these kinds of problems with a brand new holley. After reading the comments to my initial post, I guess am not ready to give up on a vacuum secondary carb just yet. I think I will send it back for a Holley Street Avenger, either a 670 or 770 CFM carb with electric choke.
Max Power said:
That's backwards, vacuum flowing through the primary venturies is what opens the secondaries. There's a diaphram that possibly has a hole in it. There's also an o-ring that seals this to the main body of the carb. I had a quick change kit on my old carb that allowed me to change the spring. One thing you could do is pull the actuator off the side of the carb. You'll have to unhook the linkage that opens the secondaries, but that's fine. You'll see a little hole were the vacuum goes into the diaphram area to open it. You can suck on this yourself and see if it pulls the linkage up, and holds it up. If you suck on it and get air out and the linkage doesn't move, BINGO! The main problem I had was the four screws on the top of the diaphram. When you'd insert, or remove them, it would catch the diaphram and actually pull pieces of the rubber off of it, causing it to leak.
If you pull that off, apply vacuum (suck on it), and it moves the arm and keeps it there, the diaphram is fine and the passage from the venturies to the actuator/diaphram is blocked.
IIRC, when you look at the primaries, there'll be a small pin sized hole on the side of the secondaries actuator. That's where it gets the vacuum from. You could hit that with an air compressor and see if air comes out were the actuator was.
If that's free flowing, then perhaps there was a leak between the diaphram/actuator, and the center section of the carb.
Let me know if that helps, or you need me to explain it better. I'm at work, so typing in a hurry.
**EDIT**
One more thing to mention. I'm also running a 4000 stall torque converter, so I'm getting up there in RPM's before I romp on the gas. Sometimes I use my transbrake, which completely defeats the extra gas from the DP. It's already floored when I let off the button
**EDIT 2**
Are you in the Netherlands?
85MustGT
Founding Member
Explain how a carb can cause a backfire? I would look for another problem as well. Power Valves can be blown out with a backfire.
I have one final question. Mine is a street and occasional strip car. By "occasional," I mean maybe one or two times a year. Does a vacuum seconary carb tend to suffer from any kind of lag off the line when you stomp on the pedal? Assume I have a 2800 stall on my tranny and want to stick with a 4 bbl vac sec carb. I am thinking about either the Holley Street Avenger in 670/770 (80670 or 80770) or dropping a few more bills for Holley Pro Street 750 vac sec carb (82750). One thing I also need to ask about is that the Pro Street carb does not have a choke whiel the Street Avenger has an electric choke. What are the implications of a no choke carb?
Depends on the weather where you're at. Just where is that anyway? If you're in a warm climate, you won't miss the choke at all. I live in south Louisiana and that's the first thing that I remove from a Holley carb. Completely. I never use them and the whole assembly just impedes the air flow. On cold mornings, I just give it a couple of "pump shots" to prime it then start the engine, then baby it a few minutes til the idle takes over.
D.Hearne said:
I am in Northern Europe, about 320 miles due north of Paris. We are in the middle of an unbearable heat wave for the moment. It is 82 right now and I feel like I am about to die. Worse off are my dogs. Fortunately, we return to better weather tomorrow and the weekend (low 70's/high 60s).
My car is only a May - Sept. weekend only driven vehicle and is garaged the rest of the time. It never gets out in the snow or rain.
So it seems like the choke is unecessary. WHat do you think about the two carbs I am considering? In your opinion, will I encounter any lag at a street light or from a 2800 rpm launch at the strip if I use either of the vacuum secondary carbs I listed?
Not if you tune it correctly. You can experiment with heavier or lighter sec. springs, to tailor the secondary opening, and the accellerator pump if it does bog. But with a stall converter, I doubt it'll bog. As for which carb I like better, I can't say, I've never had either. I've always stuck with the std 3310 750 cfm, & the 1850 600's, I've also run a 650 DP and 850Dp, in addition to the triple 250 cfm 2 bbls on my 331. The 670 would give you the best throttle response, but your motor would likely try pulling the secondaries open at idle, the 750 is about perfect in size, the 770 getting a tad large, but the great thing about a vac. sec. carb or you can tailor it's size with the secondary spring. If it needs more(or sooner), go lighter, if it needs less (or more cfm later) cfm, go heavier
I'm sorry, I misunderstood. I thought you said that it was lack of vacuum that opened them.Max Power said:
Heat wave?? I wish it were only 82 here. It's 82 when I get out of bed at 5:30. I'm sure Hearne's the same, but with more humidity.jerry S said:
I do remember it being cold when I was in northern France several years ago. I had to wear a light jacket because I couldn't take the 60* weather
I'm not sure which carb is which, but I didn't notice any lag when I used my trusty 650 vacuum secondary carb. If you're worried about any lag, they sell a quick change kit for the spring, and it comes with an assortment of them. I might still have my springs if you're interested. I'll have to look. I might even still have the carb with the quick change kit on it. Basically, you replace the top part of the vacuum actuator. There are 4 screws and you can't open it up while it's attached to the carb. The replacement top piece has a hollow center attached with 2 diagnal screws and an extra door piece that's attached by the 2 remaining screws. It allows you to take that door piece off with 2 screws and swap springs out. A heavier spring means that the secondaries open slower and later. I lighter spring allows them to open earlier and faster. You could play with the different springs and look at your 60' & 330' times in relation to your 1/4 time & MPH at the track to see how it affects performance.
D.Hearne said:
regarding your comment about the 670 pulling the secondaries open at idle, I have a pretty decent cam in there at
ADV DUR: INT 297, EXH 308
LIFT: INT 538, EXH 534
DURATION AT .50 DEGREES: INT 236, EXH 242 @ 108 centerline
I know I did not have enough vacuum to run power brakes so I had to add an electric vacuum to the brake booster. this does not affect crankcase vacuum however. Would the size of the cam and the fact that I don't have too much vacuum act to keep the secondaries shut at idle?
jerry S said:
Vacuum in the intake does not govern whether the secondaries are open. Vacuum flowing through the carb opens them. At idle, there's very little air flowing through the carb, so the spring holds the secondaries closed. When you go to WOT, the air flow increases with RPM. When the air is being sucked in, it creats a vacuum in the primary venturies. That vacuum is what opens the secondaries.
I believe D.Hearne's comment regarding them being open at idle was in reference to the fact that as soon as you give it gas with the 670, it's going to pull a lot of air through there, creating a higher vacuum in the primary venturies at lower RPM's versus that of the 750.
**EDIT**
That's a pretty hefty cam, too, it's bigger than mine :damnit:
cougar694u said:
Which is a good thing, right? This is why you are getting the better throttle response with the 650 than with the 750, correct?
jerry S said:
I would think that's correct. It has to do with the air velocity. I always think of it like blowing through a 1/2" tube vs a 5" tube. The air coming out the other end of the 1/2" tube will be moving faster. But, the 750 will allow it to flow better at higher RPM's. The 650 can only flow 650 and the motor will starve. Your cam shouldn't have any problems with that 750 vacuum secondary. Play with your springs, start in the middle, then go two steps one way to see how it reacts, then two steps the opposite way of the middle and see how it does. On the dyno, you'll be able to see everything you need to know. You'll end up with the same peak HP, but you'll be able to see how your torque curve shifts.
ive got a few suggestions and an observation.
first off ive been thinking about the accelerator pump if that's set up so tight that the accel. pump digphram bottoms out before the butterflies are completely open wouldnt that hold up the secondaries from opening all the way?
second what about a rip in the secondary pod diagphram that is sealed when closed and becomes more exposed when opening?
for my observation i've had 3 different carbs on the same motor(302, e-cam, ported early 289 heads) an el-cheapo deluxe holley 600, road demon 625 and an edelcrock 750.
first carb the holley, ran beautifully no dead spots, i figured one day to see what a different carb would do (also i needed one for a tamer engine, so i used the holley) and bought the demon, stepped on the gas for the first time and i had more low end torque -wtf?, whatever maybe the holley didnt atomize the fuel as well(played with different springs and jets in the holley). but the demon had a dead spot when the secondaries were partially open, differnt springs just changed the rpm which it farted. no external adjustments changed it. so i figured the transition circuit is at fault here(dont want to mess with it, dont exaclty know how), so i traded it for a used 750 edelbrock and another el-cheapo holley.
then came the 750 edel same grunt as the demon with a little more top end, and better gas milage to boot, one big problem with it though, at light throttle it sputters the PO said it did the same on his chevy and thats why he got rid of it. i think im not gonna mess around with it and just buy a 700 dp mechanical secondaries.
but no i dont think a 750 will be over kill
first off ive been thinking about the accelerator pump if that's set up so tight that the accel. pump digphram bottoms out before the butterflies are completely open wouldnt that hold up the secondaries from opening all the way?
second what about a rip in the secondary pod diagphram that is sealed when closed and becomes more exposed when opening?
for my observation i've had 3 different carbs on the same motor(302, e-cam, ported early 289 heads) an el-cheapo deluxe holley 600, road demon 625 and an edelcrock 750.
first carb the holley, ran beautifully no dead spots, i figured one day to see what a different carb would do (also i needed one for a tamer engine, so i used the holley) and bought the demon, stepped on the gas for the first time and i had more low end torque -wtf?, whatever maybe the holley didnt atomize the fuel as well(played with different springs and jets in the holley). but the demon had a dead spot when the secondaries were partially open, differnt springs just changed the rpm which it farted. no external adjustments changed it. so i figured the transition circuit is at fault here(dont want to mess with it, dont exaclty know how), so i traded it for a used 750 edelbrock and another el-cheapo holley.
then came the 750 edel same grunt as the demon with a little more top end, and better gas milage to boot, one big problem with it though, at light throttle it sputters the PO said it did the same on his chevy and thats why he got rid of it. i think im not gonna mess around with it and just buy a 700 dp mechanical secondaries.
but no i dont think a 750 will be over kill
67GTCOUPE
Founding Member
jerry S said:
Get a QuickFuel ProVAc 750...........can be had on ebay from time to time as well.........put one on my 351w...needed to go up a size on the squirter, and down a size on the jets....BUT HOLY CRAP! Way better throttle response than my EFI cars! UNREAL.
Actaully, if you run a too small carb like a 600 on a big enough engine or one that pulls high vacuum at idle, it can pull the secondaries open far enough to affect the Idle. I've observed this twice now recently with a 600 cfm Holley. the first time I had put it on the 390 in my 68 Merc, with the purple sec. spring, you couldn't get the idle down far enough to put the C6 in gear. And when you did romp on it, even in neutral in the driveway, it would pull them far enough open to see it first hand. The second time, was when I put the same carb on my "spare" 5.0. This is an engine I pieced together last year for my Ranger. It consisted of a 30 over roller 5.0 shortblock from a rolled 93 E150 van. I swapped in an F4TE "high torque" roller, topped it with ported E7's and 1.7 Cobra roller rockers, and a Ford A321 intake. This one also generated enough vacuum for me to have to pull the white spring I had put into it out and use the purple spring. It too was pulling the secondaries open far enough to affect the idle.cougar694u said:
Similar threads
