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Holley jetting for different cfm size carbs

 
WScott
V-6 Camaros rule!
Posts: 40
Joined: 03/12
Posted: 10/08/13 06:29 AM

i recently bought a used holley 650 dp carb list # 4777-2, i disassembled it to
get ready to rebuild it and see that it has .071 jetting in both the front and rear
blocks,i did some research on that list # and the factory jetting varied from
67 primary , 73 sec. up to 71 primary 76 sec. with all venturis 1 1/4 and 1 5/16
front and rear.I can appreciate that this carb has most likely been tailored
to fit the former owners app. my ? is that i am running a 750 vac sec carb on my
engine and it has 72 prim and 76 sec jets, when i jetted down to 71 jets the engine
had a very slight surge at cruise so i re installed the 72 jets and the problem went away. if the 650 has 71 jets could i experience the same surge as i did with my 750
or will the smaller venturis in the 650 compisate for this, i know trial and error
and test and tune will tell the story but i am trying to get in the ball park before
i reassemble the carb, it is on a 406 sb with a stick, thanks, Wayne  

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wayne712222
My first time was in a Chevy
Posts: 245
Joined: 10/13
Posted: 10/08/13 08:01 PM

i don't have an exact answer for you...

but surging at the low end of the cruise range is usually a LEAN IFR

fine tuning the IFR is where you can make the carb one you will love..  and where you will get increased MPG.. at cruise engine speeds..


idle circuit is controlled by the idle mixture screw

the idle transition circuit is controlled by the idle feed restriction..

the main circuit is controlled by the main jet..

idle circuit
......--------idle transition circuit---
..........................------primary main circuit-----------

Holley 4150 At Idle Zpsdf8a4c9d

Holley Off Idle Zps85779e58

Metering Block Clear Zpsb13473b5

invest in an oxygen sensor bung installation in your exhaust.. and either a 1 wire oxygen sensor.. or a 3 wire oxygen sensor from a 93/95 gm 5.7 pick up application.. so you can have it hot enough to operate if its installed in the collectors..

then you can get a narrow band air fuel ratio gauge..  mount it on a temporary install so you can tune your motor..

please.. don't go crazy. drill only ONE step at a time..  or pick up an aftermarket primary metering block that has changeable idle feed restrictions..  

Post Reply
skyeking
I mow my lawn and find Chevys
Posts: 2738
Joined: 08/09
Posted: 10/09/13 11:33 PM

Laugh  Hi Wayne and the gang,
  Because of severe space restrictions My beast can only
  use a 10" air cleaner Shocked and I use 67s all round
  in my barry grant street demon..It gets past the pollution
  tests and gives good economy.
  At 1st I had a 14" air cleaner with 70s all round and the
  ZZ4 gave plenty of it's best.  Smile .
    I would not have another 750 anything on a 350 unless it
   was smoking HOT!! Grin
  There are a multitude of options but I would say that 68s up
  front with 71s down the back with a very firm spring on them.
   Keep up with research that will ensure that the motor will last
  while giving you the satisfaction that you seek...Good luck Frown  
skyeking

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