Carb Tuning .. thoughts from wayne

[wayne petty]

Hi Rodders.

over the past few years i have been messing around more and more with Idle Transition feed restrictions..

  in a Holley 4150, 4160. 4165 .. they could be in several locations depending on the age of the carb and who's metering block it is..

at idle the fuel is pulled from thru the Low Speed Circuit... idle feed restriction, thru the round idle feed passage below the primary throttle blade.  this idle fuel is limited by the tapered tip of the idle mixture screw...   the idle feed restriction and idle air bleeds measure in slightly at this point but lets speed this up..

  if your idle speed setting is proper... less than 10% of the idle transition slot is visible below the bottom of the throttle blade...  so any manifold vacuum pulling on that 10% below the blade pulls air and very little fuel from the idle transition slot..

as soon as you start to open the throttle you expose a larger percentage of idle transition slot.. you start feeding fuel into the air stream passing the ends of the throttle blade.. lowering the pressure on more and more of the idle transition slot to pull fuel progressively out...

from just off idle to around 1800 RPM... your engine is only getting fuel from the idle transition slot..  just above 1800  the air flow thru the venturi lowers the air pressure and starts to trickle the fuel out of the primary booster... the primary circuit increases until around 2400 RPM... as the idle transition slot is uncovered more the idle transition slot stops flowing..  no pressure drop across it..

if you have an air fuel ratio gauge...  a timing light... a mirror.. and a tachometer... look down the primary booster with the timing light flashing so you can see freeze frame of fuel  starting to trickle from the booster..  back off slightly till its not trickling..  look at the Tachometer...  look at your air fuel ratio... is it lean.. or rich...

at this RPM your engine is running on fuel pulled thru the idle feed restriction.. if its too small... the engine will be lean.. if its too big the engine will be RICH.. 

holley ships their new carbs with 0.031 IFR  sizes as that is what their flow bench is set up to measure...

a 0.031" idle feed restriction gives the proper low speed circuit air fuel ratio for a 327 to 331 cubic inch engine..

a 347 or 355 needs a 0.032" idle feed restriction..

i have found a 40 over 429SCJ with a holley needs a 0.035" idle feed restriction... 0.036 makes it a little tough to drive as you may not be able to feather the gas pedal enough to pull away from stop signs without spinning the tires with the lightest touch of the gas.

every engine/carb combo is different..  so you have to check yours..

ever seen a summit 8600VS carb..   actually a holley 2110...  those have idle feed restriction jet kits available for them... those are at the ends of the primary booster mounting flanges.. there is a neat pdf file ..

but wait.. i have an Edelbrock...

edelbrock AFB/AVS carbs have the idle feed restriction (economizer) hidden behind the air bleeds in the sides of the primary boosters...  so look down your primary boosters just below the speed the primary boosters start flowing.. are you rich or lean...

here is a sand away image of an carter AFB primary booster.. the end of the dental pick is at the idle feed restriction or Economizer 


changing main jets or main metering rods won't help tune out the flat spots.. as the primary main circuit is not even flowing..

why do you think that dyno runs start at 3000 RPM... because Both Holley and Edelbrock removed tuning the low speed/idle transition circuit from their carb manuals... why.. both answered to give professional carb tuners something to work with...

except so many professional carb tuners tune for wide open throttle..

guess what... lets tune the primary jets...

bring the rpms up till just before the secondaries are starting to open or flow... so the max amount of fuel is being pulled thru the primary main jets.. are you rich or lean at that point..  3500 to 4500 RPMs... again... before the secondary starts to flow..

enjoy the incredible throttle response off idle. the fuel economy in cruise at 1500 to 1800...

i hope this helps a few...

i got complaints from the guy with the 429SCJ... i described them above.. wayne.. i can't pull away from a stop sign or signal without spinning the tires.  i told him i can dial out some of the power.. he said.. no.. i love it...

once you get an edelbrock idle feed restriction tuned correctly...   you may want to have the engine idling in gear.. somebody on the brake.. loosen the screw on the primary rod piston cover.. just one..  swing the cover away.. is the manifold vacuum at idle in gear pulling the piston all the way down..  with the driver on the brake.. blip the throttle just slightly..  the spring pressure should start to overcome the manifold vacuum at around 1000 rpm against the converter. and start to lift... you should see fuel from the primary booster.  the coming up amount is tiny.. you may need to do this by slowly accelerating looking at your air fuel ratio sensor to make sure the engine does not go lean between the idle transition and the beginning of the primary fuel circuit delivery..

when doing idle feed restriction tuning... i have a dial caliper good for 0.001" or a micrometer also good for that.. so i can use the smooth end of my 61 to 80 drill bit set to see what size the idle feed restriction is.. so i know i can take it 0.001 larger at a time...  or you can get a Lincoln KH534 electric welding tip drill that has a bunch of sizes ...

[jaybee]

Thanks, Wayne. That's really good information and great to see you.

[chimp koose]

good stuff there! nice to see you back