Holley 4150 850 cfm

[Meyer]

Brand new carb

Float height is correct
I have electric fuel pump with 6,5 psi in preassure on the needles
I have a lean spot i have tryed jets from 74-78 pri sec 82-86 and power valves from 5-8,5hg
Air mix screws is 1/4 turn out from buttom out on all 4
When cruising its 13.7 afr gentle accelerating it leans out for a few sec to 16+ and goes back to 13,7
Idle hg 10 aprox
There are no hesitation from a dead stop likr if the acc pump wastent adjustet correctly
Timing is 33 total
Msd distributor 6al box
Idle 1000 rpm wont go lower
4 speed
3:55 rear

Any ideas....

Should i switch to a quickfuel 750 vac sec!

[Chryco Psycho]

 If the idle will not drop down it could be there is a vacuum leak under the carb or through the brake booster for example , or the secondaries could be opened too much , there is a set screw from the bottom on the right side of the carb to adjust this .
The accel pump cams have different profiles so maybe a different pump cam will add a bit more fuel as the throttle is opened , the cams also have 2 positions so changing the position may be enough .
What is the vacuum at idle , the power valve should be 1.5 inches below that at least , the closer it is to the actual vacuum the quicker it will open adding extra fuel .
I wrote a Holley tuning guide in the reference section which should help also .

[ek3]

 where is the rpm when it starts dropping off ?   1/4 turn out on all 4 idle screws  should be to lean in most typical carbs.  do you mean 3/4 turns out ?sounds like lean through transition . all 4 base plate slots are adjusted square ?  what cubic inch engine ?

[Meyer]

426 hemi


If adjust them out 1 1/2" turn its to rich . 1/4 out is the sweat spot

It doesent have the transferslot same location as the primaries it sits higher

[ek3]

pleanty of room between 1/4 out and  1 1/2  that's   6 times where you are...  just 1/4 more would be double the adjustment.    you said its to rich at 1.5 turns  but its lean at .25 turns  ?   if you close up the throttle plates and it still idles hi , its pulling air from somewhere .   i don't think you have control over the idol mixture if its running at 1/4 turn out..

[Meyer]

Transferslot is where it needs to be, and the idle mixture is set with a wacuum gauge... highest wacuum when set its 14.2 afr at idle ... all 4 idle screws are 1/4" out ...

[ek3]

not sure on what you mean.. 14.2  for  afr or hg?  also you had in the first post , 10 hg with 1/4 out .  so are you now  "10hg on vac  with 14.2 afr " @ idol ? or are you 14 on hg ?  if you come out to 1/2 what is it. and at 3/4  turns ? and did it drop the idol down from any 1k ?

[jimynick]

I believe the man means that at the highest vacumn reading, the engine shows 14.2 air fuel ratio and he says the screw is out 1/4 " inch and that may be a misprint. 14.2 AFR is pretty much a perfect, stoichiometric reading in my wee, tiny mind. 

[ek3]

ok.  so we had 10 hg vac. with a  1/4 turn out on all 4 screws. now with no screw adjustment , we still have 10 hg vac.  with a 14.2 afr.. did  you close up the throttle blades ??     I have never run a holley carb with only a 1/4 of a turn out from bottom on all 4 screws ..      sounds like pv or idol mixture is dropping out in transition..   for the record its going lean in transition...

[Meyer]

You are right @jimynick

My highest wacuum is aprox 10 hg at idle


@ek3

I tryed from 5 to 8,5 pv’s the 8,5 was way to rich , but the lean spot was not in the lean area long before it dropped back to 12 at cruise wich is way to rich.

With jetting from 74-78 pri and from 86-82 sec.

Cant solve the lean spot when i accelerate gentle it leans out on the afr - - - - and jumps back to 13 or so ...

Would a wac sec carb be the right choice!


Im confused about the transferslot on the mec sec it is up higher in the venture compared to the primary and the rear blades are alittle more open than the primaries

[ek3]

The blade should come down over the slot and nearly cover the slot all but a small square usually. if it does not , it will want to pull fuel from the transfer  passage. it will idol higher too. I think cryco  posted that. I would db.check the accel.  pump shot  duration.... a small tweak can fatten a small hole. if its only lean during transition its right there.    only the front pump..

[Meyer]

Transfer slot is not in the same location on the sec compared to the primary slot !

[ek3]

you said it want idol below 1,000 rpm. did you close both the  front and rear throttle blades ? 

[Meyer]

Its a brand new carb i havent adjusted the rear but they are more open than the primaries

[Chryco Psycho]

here is an adjustment screw from the bottom to close the rear throttle plates .