Racing Fuel Systems

Helping a buddy with a holley 4412 on an asphalt car. Typical spring tear down before the season starts. He tried a used 4412 at the end of last year and wants me go thru it. Looks close to stock from what I can tell so far. Down leg boosters, 50cc, 28 squirter, brown cam, 6.5 pv. The metering block doesn't look modified, but I still need to check the metering block and bleed sizes. This is a gas 6cyl class @ 250cid engine. The car had a major stumble off idle on quick throttle moment last year. I haven't seen the engine run with this carb...

I haven't been around these carbs, mostly the 477x and similar QF carbs.
Should the throttle blades be drilled/have holes in them on the transfer slot side?

I've been on the site for a couple of years now. Usually I'm not quick enough to post when I can help....

Thanks for the help.
Scott

I've done a bunch of them, drill .080 for the throttle blades, use the smaller black oxide buttonhead allen screws for reassemble; drill the squirter to .031 (the tech guy won't care about the actual size as long as he see's the -28 on the squirter); swap the PV for about a 5.0 or 4.5 and you might have to restrict the PV holes in the metering block, I'm thinking .030ish? Leave the idle restrictor & bleed alone; you might have to add a spring to hold the antisiphon slug in place, the 6cyl have odd harmonics like a 4cyl that can dislodge them at high rpm and cause a stumble on the main straight as droplets of fuel get pulled out from the squirter. Adding a spring to the squiter is a complicated business, see if you have a problem first, I can walk you thru it if you do.
That will give you a solid carb to start with, no throttle stumble and reliable.

I think the help Tuner is giving on the other site will serve well

What is the name of the other site, I'd like to read tuners remedy.

Like mentioned, there is a similar thread on the other site.

Here's some more info:

This is a local 1/4 asphalt track. It's an 4412-5 with a factory choke horn and the older 2 hole emulsion block with the screw style (fuel level) fuel bowl.

I found out this carb came off of a v-8 and sat on a shelf for several years - outside of that we don't have any history on this carb. This makes sense as why the throttle blades are drilled... It basically was given to my buddy to try - low buck operation.

Based on Holley's List sheet - Stock 73 jets, 5.0 PV, 28 squirter.

Here's the metering block - (I only have #'d drill bits - using the shanks so the diameters are approx.)
2 hole emulsion block - @ .028" ea
drilled kill/anti-siphon bleeds in angle channel @ .026"
Recessed Main Well Cups & Flush Idle Well Cups (top of metering block)
Lower IFRs - @ .031"
Main Jets - #64s
PV - 6.5
IABs - @ .076"
MABs - @ .028"

Factory Brass float with .110 N&S
Fuel line has Inline reg set @ 3.5psi

Found no gasket on PV
The squirter had no gaskets & the weight underneath was stuck
The acc pump arm that rides the pump cam was partially froze up - not returning down on the cam

Cleaned out all passages
verified PV with vac pump
replaced missing gaskets
cleaned, reset accel pump arm
set the float to bottom edge of bowl screw threads.
Set mixture screws to 1-1/2 turn out to start with

I didn't change the existing jets - I wanted to see how the carb ran to get a base line.

With the Idle speed screw backed off throttle plates 1/8" (gap) - idle rpm @1200. The throttle blades are fully closed and t-slot is not even close to being exposed. Engine has a noticeable tip-in studder on a major/quick throttle opening (rev, cough, rev) from idle with no load - out of gear.

Still need to verify distributor timing - vacuum advance is connected to full manifold vacuum port currently.

Here's what my to do list is - let me know what you think:
Seal the holes in the throttle blades and get the idle position back to a more normal squared transition slot exposure with an idle rpm closer to 7-800 range.
Increase the squirter if needed.
Verify Dist timing (intial & mech) and see what the vac pot is adding to the timing.
Jet according to engine/plugs

We should have some time this weekend to see what the timing is after plugging the throttle blade holes and resetting the idle position. We might even get to the track weather permitting.

Thanks again for all the help.
Scott

Yup, seal the holes in the t/blades. If PCV is used, make sure it is working properly & not adding excessive air at idle.

Those throttle blades are # 215 I believe. Those were factory drilled for V8 cars. These are the same blades as a 750/780 holley # 172 , not drilled, about .440 " thick.
#173 blade are the same size , slightly thicker at about .640 " thick.

Do you know how much vacuum you have at idle now, and after you close up the holes in the throttle blades ?

Also , those well plugs are flush at the top, not solid brass?

Since the carb is 'used', check for worn throttle shaft [ air leak, fast idle ].

Well, the weather didn't hold out for us this past weekend - we're looking at next weekend...

I really haven't put hands on the engine yet besides getting the carb to idle, so I don't have much info on the manifold vacuum, timing, the distributor and or its setup/operation yet...

GTO Geoff - No PCV currently. If I need help in getting the idle position back to a more normal squared transition slot exposure after I seal the blade holes, I may need to add a PCV valve.... We'll see.

rgalajda - those are the blades #'s. With the hole finish and their positions being visually identical blade vs blade - I was wondering if that was the case. I'll take note of the manifold vac before & after sealing the holes. The flush plugs are above the Idle wells - I shouldn't have called them cups.... Yes, both the Main Well cups and Idle Well plugs look like brass.

I still have my to do list and will go from there.

Thanks again for the help.

Scott