Res­ur­rect­ing a life­less fac­tory 780-cfm carburetor

Chevy High Performance - - Contents - ✜ TEXT & PHO­TOS: Jeff Smith

The most ubiq­ui­tous speed part on the planet has to be the Hol­ley four-bar­rel carburetor. While mil­lions have been pro­duced, there are a few that are spe­cial. In our case, we’ve been har­bor­ing a fac­tory-spec’d 780-cfm Hol­ley carb for our 1966 SS 396 Chev­elle for over 40 years—some­day in­tend­ing to bring it back to life. The job called for some se­ri­ous in­ten­sive care. The carb was miss­ing nearly all of its small parts, in­clud­ing the throt­tle shafts, vac­uum sec­ondary di­aphragm hous­ing, and the en­tire choke mech­a­nism. Plus, the main body had been dropped and bent.

To res­ur­rect this gem, we called our buddy Sean Mur­phy at Sean Mur­phy In­duc­tion (SMI) in Hunt­ing­ton Beach, Cal­i­for­nia, and he will­ingly took on the job of res­ur­rect­ing our be­yond-dead carburetor. The num­bers on the choke hous­ing was what made this carb worth sav­ing. The GM fac­tory list num­ber

3613 and ac­com­pa­ny­ing 3883229 choke hous­ing stamp­ing called this out as a 4150, dual-in­let, 780-cfm Hol­ley for a 375hp 396 Chev­elle in 1966. The 4150 num­ber refers to the over­all con­fig­u­ra­tion as a square­bore Hol­ley with me­ter­ing blocks on both ends.

This carburetor is also com­monly re­ferred to as the 3310, which is the orig­i­nal af­ter­mar­ket part num­ber for this style 780-cfm carburetor. Later ver­sion 0-33310’s were des­ig­nated 750-cfm car­bu­re­tors. Our Chev­elle is the 360hp ver­sion that used a smaller, sin­gle-in­let, 600-cfm ver­sion (a 4160 ver­sion) so we

planned to use this larger carb as a “Day Two” up­grade for our big-block resto pro­ject.

“As beat-up Hol­leys go, you’ve prob­a­bly seen worse, right?” we asked Mur­phy as he quickly dis­as­sem­bled our lump of miss­ing parts.

“Well, this is pretty bad!” Mur­phy said.

We were cha­grined but he dove right in. The first is­sue was that the choke hous­ing ap­pears to have been dropped and bent the hous­ing so the choke blade would not fit. Mur­phy worked on it with light ham­mer taps on a large punch and then with some

mild pol­ish­ing of the hous­ing on the in­side and some mi­nor tweaks to the choke blade. It all fit again.

But rather than per­form just a straight restora­tion, Mur­phy dis­cov­ered a kin­dred spirit be­cause we were more than will­ing to mod­ify this OE piece to make our Rat run bet­ter.

Among the big mods were idle cir­cuit changes. Mur­phy re­lo­cated the idle feed re­stric­tor from un­der­neath a plug in the top of the pri­mary me­ter­ing block to a re­place­able jet that could be eas­ily changed to fine-tune the idle cir­cuit. He also mod­i­fied the sig­nal side of the pri­mary idle tran­si­tion slot cir­cuit. This is lo­cated in the main body and is an­other way to im­prove the drive­abil­ity of these car­bu­re­tors at part throt­tle. The size of this re­stric­tor would de­pend upon the ap­pli­ca­tion, but for our big-block he re­duced the chan­nel size from 0.120-inch to 0.093inch. This re­duces the sig­nal to the idle feed on the chan­nel side, es­sen­tially re­duc­ing the fuel de­mand through the tran­si­tion slot.

There is also a non-ad­justable fac­tory idle feed sys­tem for the sec­ondary side on al­most all Hol­ley four-bar­rels. This is added to en­sure that the fuel in the sec­ondary bowl is con­stantly re­freshed. In­stead of the orig­i­nal 0.040-inch re­stric­tor, Mur­phy re­duced this to 0.036-inch, which is still lo­cated un­der the cup plug in the top of the me­ter­ing block.

He also mod­i­fied the idle bleeds to im­prove idle qual­ity. There are two air bleeds lo­cated on the top of each ven­turi on a typ­i­cal Hol­ley carburetor. The out­board po­si­tion is the idle air bleed while the in­board one is for the high-speed cir­cuit. This carb em­ployed a large 0.076 idle bleed but Mur­phy ex­changed the pri­mary bleeds for smaller, 0.068inch ver­sions. A smaller bleed al­lows less air, which richens the cir­cuit. Mur­phy does this to com­pen­sate for mod­ern, re­for­mu­lated gaso­line that now in­cludes 10 per­cent ethanol.

The added ethanol tends to make the orig­i­nal cal­i­bra­tion slightly leaner.

He also showed us a tech­nique where he blows com­pressed air across the pri­mary ven­turi that lo­cates the sig­nal to open the vac­uum sec­on­daries. He uses a small air noz­zle blow­ing across a very small hole (or

some­times a small brass tube) in the mid­dle of the pas­sen­ger side pri­mary ven­turi. This cre­ates a siphon/ vac­uum ef­fect that pulls against spring pres­sure in the vac­uum pod and opens the sec­ondary.

An­other tip when as­sem­bling any Hol­ley carburetor is to make sure that there is no clear­ance or gap be­tween the ac­cel­er­a­tor pump link­age and the ac­cel­er­a­tor pump arm at idle. The ac­cel­er­a­tor pump arm should move and squirt fuel the mo­ment the throt­tle link­age moves. This en­sures that fuel is squirted to com­pen­sate for added throt­tle open­ing be­fore the main me­ter­ing sys­tem kicks in. If there is clear­ance be­tween the throt­tle link­age and the ac­cel­er­a­tor pump arm, this will re­sult in a slight hes­i­ta­tion at throt­tle open­ing, which is an­noy­ing.

If we could put all of Mur­phy’s knowl­edge down in printed form, it could eas­ily fill a 140-page tech book with all his Hol­ley carburetor se­crets. But for now, we’ll just cel­e­brate one more mus­cle car Hol­ley that has been re­sus­ci­tated. CHP

03 | The big­gest is­sue was where the carb had been dropped and cracked at the base of the choke hous­ing on the pas­sen­ger side. Mur­phy had to lightly ham­mer the hous­ing back into place and then lightly trim the choke blade so it would op­er­ate cor­rectly. This was dou­bly crit­i­cal be­cause the choke hous­ing con­tains all the fac­tory iden­ti­fi­ca­tion.

01 | There wasn’t much to our orig­i­nal carburetor be­yond a stripped throt­tle plate, the main body, me­ter­ing blocks, and the float bowls. Most of the other pieces were miss­ing, but luck­ily we came to the right place.

02 | Sean Mur­phy laid out all the new parts needed to bring this 3310 back to life. He keeps a col­lec­tion of elec­tro­plated small parts to give the carburetor a new look and bet­ter-than-new per­for­mance.

04 | The throt­tle bores were heav­ily scored, so Mur­phy re­moved the notches and made sure the sur­face was flat be­fore re­assem­bling the throt­tle shafts. 05 | Mur­phy says that all Hol­leys should be milled to en­sure the seal­ing sur­face is flat. Our carb only needed about a 0.005-inch trim, but he’s seen carbs that needed as much as 0.060-inch re­moved to make the gas­ket sur­face flat. He says over-tight­en­ing the bowl screws causes this de­flec­tion.

06 |Mur­phy blasted all the body parts and then sub­merged them in a quick dip in an acid wash that re­stores the out­side ap­pear­ance to much like the orig­i­nal.The orig­i­nal zinc dichro­mate process in­volves car­cino­genic chem­i­cals that the EPA re­ally frowns on now.

07 | Mur­phy prefers to add tiny re­stric­tors in the pri­mary idle cir­cuit chan­nel in the main body to the tran­si­tion slot. The orig­i­nal di­am­e­ter was 0.120inch and he re­duces this to 0.093 to re­duce the sig­nal from the tran­si­tion slot so less fuel is in­tro­duced in this “cruise” pri­mary throt­tle po­si­tion. This im­proves fuel mileage and drive­abil­ity.

08 | After the main body and me­ter­ing blocks have been cleaned and re­col­ored, Mur­phy adds threaded 6-31 jets sized with a 0.032-inch re­stric­tor into the idle cir­cuit after re­mov­ing the orig­i­nal re­stric­tors lo­cated un­der the brass plugs in the me­ter­ing block. These are eas­ier to ac­cess and can be quickly changed to fine­tune the idle cir­cuit. 09 | Later drop-leg boost­ers (below) cre­ate a small ra­dius at the junc­ture where the two drilled pas­sages meet. This ra­dius im­proves the flow through the booster along with a slightly larger in­side di­am­e­ter of 0.160inch ver­sus the orig­i­nal booster’s 0.140-inch. The larger pas­sage im­proves fuel aer­a­tion.

10 | Two of the orig­i­nal boost­ers were not square in the ven­turi, which can dras­ti­cally af­fect WOT me­ter­ing. Mur­phy uses this cus­tom-made tool to in­stall the boost­ers prop­erly in the ven­turi.

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