Author Topic: The 427 Thunderbolt and vacuum secondary carbs?  (Read 6156 times)

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gt350hr

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #15 on: July 10, 2018, 02:48:38 PM »
  What's wrong with the age old screw and nut in the linkage? Unless it's an auto trans I don't see any benefit to vacuum control.

BattlestarGalactic

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #16 on: July 11, 2018, 09:11:43 AM »
Forcing the secondaries open, with no pump shot just makes a big lean spot til the air moves enough to pull fuel through the boosters.  Even with light springs, it takes a specific time to open and gives the air time to pull fuel.

Larry

Falcon67

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #17 on: July 11, 2018, 12:05:37 PM »
LOL, when people did those kits to "defeat" the vacuum secondary, the bog would be justified with "I can feel them kick in now!"  Well, um - that's cool. 

Some good points here - one of these days I need to get a pair of 1850s set up for t-ram use.  I usually start - for a single- with the purple spring.  The few times I've used a 600 for bracket racing, the purple spring was more consistent.  The air vane setup in the Performer carbs works.  They are not adjustable like the newer AVS models and on the setup I ran you could feel the motor start to run out of air, then pick up as they started to open.  On that particular 302 combo, it was around 5700 RPM.  I'd like to try the AVS units but I have so many carbs ( mostly Holley) stacked up here if a couple more show up my wife may have me committed.  :)

gt350hr

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #18 on: July 11, 2018, 02:31:49 PM »
Forcing the secondaries open, with no pump shot just makes a big lean spot til the air moves enough to pull fuel through the boosters.  Even with light springs, it takes a specific time to open and gives the air time to pull fuel.

     I thought you "three pedal guys" were "on the mat". LOL  You are certainly one of the best at it. I am well versed in the dual quad stuff ( 50 years worth) , and yes "on the street" making the secondaries "mechanical" made a huge bog. At the track with my 4 speed Shelby, the fuel was already flowing from the secondaries at my starting line rpm and I didn't notice it. Now I run center squirters ( dual plane Ford intake) with my automatic Shelby.
     Randy

BattlestarGalactic

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #19 on: July 11, 2018, 02:45:56 PM »
I've never left at WOT.  Never had a 2 step.  Back when I first start racing, the nostalgia classes would not allow such a thing. I learned to do it the hard way........and 23 yrs later I'm still doing it.  Old habits....you know?

I hold the throttle at 5K, 5500, 6K, whatever the track will hold.   Then when I dump it, I mash the throttle.

I have a pair of 660's on the blower car, but still not happy with them.  Not sure if they are just too beat up?  Run okay, but still not right. I tried them on the wagon and got a huge stumble(like on blower motor).  Never really figured it out.  I have a second pair a friend gave me to rebuild and try.  Never have.  I have a pair of 735's I converted to single feed and going to try, some day.
Larry

gt350hr

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #20 on: July 11, 2018, 05:29:13 PM »
   I know about old habits ! 44 years of racing the same car!!!  No one can argue with your win record that's for sure.  I was still a teenager when I was racing the 4 speed stuff. I was sure "stupid" back then. On the floor slide my foot off the pedal , try to rip the shifter out to the floor, break axles because of wheel hop, driveshafts etc. If I had been a little less aggressive I would probably still be a three pedal guy. The automatic did teach me about how important torque is with respect to ET and 60's. I used to be so screwed up on starting line antics I raced for MPH with the 4 speed cars. Of course the tires were hard rubber compared to today's tires. I appreciate what you do , it's not as easy as the "grandstanders" think.
  Randy

mike7570

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #21 on: July 11, 2018, 08:10:10 PM »
I've never left at WOT.  Never had a 2 step.  Back when I first start racing, the nostalgia classes would not allow such a thing. I learned to do it the hard way........and 23 yrs later I'm still doing it.  Old habits....you know?

I hold the throttle at 5K, 5500, 6K, whatever the track will hold.   Then when I dump it, I mash the throttle.

I have a pair of 660's on the blower car, but still not happy with them.  Not sure if they are just too beat up?  Run okay, but still not right. I tried them on the wagon and got a huge stumble(like on blower motor).  Never really figured it out.  I have a second pair a friend gave me to rebuild and try.  Never have.  I have a pair of 735's I converted to single feed and going to try, some day.

What size squirter are you using?  my bog using 660's went away with .043 size orifice and 50cc accelerator pumps

Drew Pojedinec

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #22 on: July 11, 2018, 08:42:26 PM »
Most of them came with a .025

Thumperbird

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #23 on: July 12, 2018, 06:43:15 AM »
Mike:

Can you briefly describe your motor setup?
Most threads you read never suggest such a large squirter let alone a 50cc pump, especially on twins.  I just finished a moderate 445 stroker build with a high plenum volume intake and want to minimize the # of iterations it takes me to get them dialed in so soaking up all I can on carb configurations. 

Overall, it seems to be very thirsty; using a set of stock BJ/BK carbs (032 squirter) and wondering just how much more fuel a stroker is looking for than a 427/428 back in the day.   

I tend to see an ever so brief rich pulse on my A/F followed by lean right now.  My setup currently has some segments of RPM range that are excellent and others that I can tell are off, same goes for response from various throttle input rates at various rpms.

Thanks.

BattlestarGalactic

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #24 on: July 12, 2018, 07:55:17 AM »
Most of them came with a .025
I used my pin drill set and put the biggest hole I had.  Can't recall what it is off hand.  It didn't really make any improvement.  I know that big windmill needs LOTS of fuel.


Quote
with .043 size orifice and 50cc accelerator pumps
I thought they had 50cc pumps on them?


Quote
try to rip the shifter out to the floor,
I used to be known for my "aggressive" driving style.   Back when I still ran a toploader, I did rip the shifter plate off the transmission.  I then replaced the 5/16" bolts with 3/8" and never had that problem again.   I'd heard them say I was really jump'n around in the car shifting.  I told them I had ONE JOB, and I did it with all my effort!  I never missed a gear.  With a stock toploader, with brass blockers, you couldn't drive it easy and powershift it at 6K rpm.
Larry

mike7570

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #25 on: July 12, 2018, 12:25:44 PM »
Mike:

Can you briefly describe your motor setup?

Thanks.

Mine were changed with a pin drill also .043 was one of the sizes my cousin had in his tool chest. (We were just taking a stab at it)
The bog off the line went away once we got enough fuel in it. At the time I was running a tunnel port at 427ci with 2 660's.
The car originally had no trans brake and only a 3500 converter. I could bring the rpm up some but when I went full throttle I would get a lean hesitation (bog) from all the air introduced.
It also helped some to delay the accelerator pump shot to coincide with the throttle position so I didn't use up some of the travel and pump shot bringing it up to stall rpm.





mike7570

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #26 on: July 12, 2018, 12:34:16 PM »
Mike:

Can you briefly describe your motor setup?
 
Overall, it seems to be very thirsty; using a set of stock BJ/BK carbs (032 squirter) and wondering just how much more fuel a stroker is looking for than a 427/428 back in the day.   

Thanks.

Also 660's are not progressive linkage, It's all or nothing.  You should be able to use less accelerator pump on progressive linkage and vacuum secondary carbs.

HvyFt4spd

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #27 on: July 12, 2018, 03:10:37 PM »
  Stock is 1 to 1 but if you're a 660-870 fan you'll often find the secondary cam reshaped to delay the opening rate. You can do as much damage as good there real quick but it's something I still mess with to even out the bogg or surge you can get from them.

427LX

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #28 on: July 12, 2018, 08:14:26 PM »
Thats a neat little concoction there!  How did it work?

Worked fairly well on my then 351W. I had it designed so I could remove the secondary pump arm on my 750 DP and then swap on the air valve unit which was held in place by the air cleaner stud. I had to go up in size on the secondary High Speed Air Bleeds to lean the mixture a bit as the door opened. Door tension was adjustable.
Somewhere I have Time slips from a back to back run at the track.

TorinoBP88

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Re: The 427 Thunderbolt and vacuum secondary carbs?
« Reply #29 on: July 14, 2018, 06:18:02 PM »
Neat evenly sure, but comments above look good.

What i can say is i used to have a 3310 times by a local reputable shop, and the secondaries started opening at like 25% throttle.  It ran great, but got poor mileage, LoL.