357 small block chevy carb problems with vacuum

Old 09-20-2010, 06:33 PM
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bp1x71
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Default 357 small block chevy carb problems with vacuum

I have a 357 small block chevy motor in my race car it has dart pro 1 alum heads srp dome pistons.... runs 6.6's to 6.8's depending on weather. but anyways i have a queston about the carburator. it has a proform 850 4 circuit idle screws the center section 850 holley. the rear vacuum port has to remain open for the car to run those numbers and to idle right. if i put the plug on the port and turn the idle screw way up the car will run 8.20s and backfire going down the track. i believe that has some to do with consistency. it also has 80 squared jets. i was wondering if someone else has had this problem if so please help thanks alot this is my first year racing and i would love to win a race before the season is over
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Old 09-20-2010, 10:05 PM
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cepx111
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Something is way off if your having to "add" that much air to make it run.

I assume your talking about the 3/8 manifold vacuum port that is sometimes used for the power brake booster or a PVC system.

Others will chime in I'm sure but you've definetly got a big problem somewhere.

Cp
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Old 09-21-2010, 04:40 AM
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itsabird
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yep, i agree, sound like something is blocked, air bleeds maybe need cleaned, or something going on with metering block gaskets.
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Old 09-21-2010, 05:53 AM
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qtrmile2
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When did it start doing that? Was it running OK before? If it just started I'd pull the rear bowl, metering block, gasket and inspect it for blockage.
If the carb is old I'd tank it and buy a rebuild kit for it.
More info would help.
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Old 09-22-2010, 07:00 PM
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bp1x71
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well everything in the carb is good and has a perform center section i was thinking about cracking the back butterflies has anyone else done this and did i work. everything in the carb is good i mean its just a 357 and runs 6.6s in good weather in a 2500lb car
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Old 09-23-2010, 12:18 PM
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prowagon55
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If you have an adjustment screw for the secondaries and you adjusted your idle using the primary screw you probably exposed the transfer slot too much allowing fuel to dump in at an idle. Pull the carb and check to make sure the primary butterfly is closed enough to make the transfer slot appear to be squared on all 4 sides. Then use only your secondary adjuster for your idle. Its an easy mistake to make. I did it my self when i started using a race style carb.
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Old 09-23-2010, 06:14 PM
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bp1x71
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ok so i need to close the front butterlies and just adjust the idle by the rear butterflies. but the front ones need completely closed? right now we are adjusting only by the front. do we need to leave them cracked the way they are or close them. im just thinking that it if we could plug the rear vaccum it would make the car more consistent when the same air and fuel mixture gets into each cylinder. thanks alot for your help!
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Old 09-23-2010, 08:03 PM
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prowagon55
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Heres a link to a picture of the carb transfer slot. Pull your carb and turn it upside down, now turn down the idle screw for the front butterfly to a point where the transfer slot looks like a square. Thats where you will leave it. do not close it all the way. Put it back on the car and try to establish a baseline idle off the rear butterfly adjusting screw only. If you can do that with minimal adjusting with the port plugged off, then you can readjust your idle air/fuel screws. Hope that helps you out. Skip

http://www.mustangandfords.com/techa.../photo_19.html
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Old 09-23-2010, 09:57 PM
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cepx111
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Originally Posted by bp1x71
. im just thinking that it if we could plug the rear vaccum it would make the car more consistent!
Well I sure hope so, I cant believe it's running that good, 660's? dude.....when u get it right, it should FLY!
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Old 09-24-2010, 12:55 AM
  #10  
Tod74
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Originally Posted by prowagon55
If you have an adjustment screw for the secondaries and you adjusted your idle using the primary screw you probably exposed the transfer slot too much allowing fuel to dump in at an idle. Pull the carb and check to make sure the primary butterfly is closed enough to make the transfer slot appear to be squared on all 4 sides. Then use only your secondary adjuster for your idle. Its an easy mistake to make. I did it my self when i started using a race style carb.
Everything you said is true but that wouldn't have anything to do with it slowing down A SECOND AND A HALF just by sealing off a huge vacuum leak.


this statement doesn't make sense to me.

it has a proform 850 4 circuit idle screws the center section 850 holley

So is the main body a PRO FORM or is it a HOLLEY? Is it a Holley main body with proform metering blocks? Did you just put the 4 coner idle metering blocks on a 850 Holley body not designed originally for 4 corner idle?

You can't do that.

Is it a Double pumper or vacuum secondary?
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