Learn, Tune, En­joy!

A Col­lec­tion of Sim­ple Hol­ley Carb Tricks to Make You and Your En­gine Happy

Street Rodder - - Conents - By Jeff Smith Pho­tog­ra­phy by the Au­thor

A col­lec­tion of sim­ple Hol­ley carb tricks

to keep you and your en­gine happy

We wrote the first of many elec­tronic fuel in­jec­tion sto­ries way back in 1988 in which we pro­claimed that the car­bu­re­tor, if not dead, was on the clear path to an­tiq­uity.

That was 30 years ago. To­day, the iconic Hol­ley four-bar­rel is both alive and thriv­ing with con­tin­u­ing in­no­va­tion. It’s now a long way from the en­dan­gered species list.

Pop­u­lar­ity also breeds car­bu­re­tor mis­ap­pli­ca­tion, frus­tra­tion, and the oc­ca­sional bouts of mis­in­for­ma­tion. The tra­di­tional Hol­ley four-bar­rel is in­cred­i­bly ro­bust and the in­evitable prob­lems of­ten have more to do with poor tun­ing than any­thing else. We thought we’d ad­dress a few com­mon drive­abil­ity is­sues that never seem to go away.

Over the decades, tribal knowl­edge has de­ter­mined that many car­bu­re­tor-re­lated is­sues come down to vac­uum leaks, ig­ni­tion, or fuel de­liv­ery prob­lems un­re­lated to the car­bu­re­tor. Yet the car­bu­re­tor is most of­ten blamed. So the first tip when look­ing for the head wa­ters of a prob­lem is to make sure the ig­ni­tion sys­tem is in a good state of tune, the en­gine is phys­i­cally in good shape, there’s de­cent fuel pres­sure, and there are no vac­uum leaks.

We also in­ter­viewed Hol­ley car­bu­re­tor tuner/re­storer Sean Mur­phy of Sean Mur­phy In­duc­tion (SMI) to doc­u­ment a few of his tun­ing and assem­bly se­crets. Di­vided into two parts, each of these tips will be quick takes, which un­for­tu­nately will leave some in­for­ma­tion on the ta­ble. But di­gest each of these and they will gen­er­ate the en­ergy to get your started on the path to pure Hol­ley com­pre­hen­sion. The knowl­edge base for Hol­ley tun­ing is not likely to be­come a lost art any­time soon, and if you be­come pro­fi­cient, you just might be­come the neigh­bor­hood ex­pert.

1. Set Idle Cor­rectly

This might seem way too sim­ple, but for a car­bu­reted street en­gine this is the most im­por­tant tun­ing step you can make be­cause street en­gines spend a ma­jor­ity of their time at or just above idle. The first step is to ob­tain a low-speed tach that will show mi­nor changes in rpm along with a vac­uum gauge. For this pro­ce­dure we’ll as­sume a Hol­ley with only two idle mix­ture screws. Many per­for­mance carbs now of­fer four screws but the pro­ce­dure would still be the same.

Be­fore the en­gine starts, check the po­si­tion of the idle mix­ture screws (lo­cated on ei­ther side of the me­ter­ing block) by count­ing the turns on each one in un­til it lightly seats. Let’s say that the left side is three-quar­ter turn out and the right is one-and-a-half turns out. The best route here is to split the dif­fer­ence and make them both one turn out.

With the en­gine idling at nor­mal­ized tem­per­a­ture, try lean­ing the mix­ture with a mi­nor clock­wise turn of both idle mix­ture screws. Make these changes very slight and note the ef­fect on the en­gine. If the idle speed and vac­uum in­crease, con­tinue with sim­i­lar changes in small in­cre­ments un­til the idle speed or vac­uum de­te­ri­o­rates. If the ini­tial change re­sults in a loss of rpm or vac­uum go back to the base­line and then turn the idle mix­ture screws in the op­po­site or coun­ter­clock­wise di­rec­tion.

With each small change, wait for a mo­ment to al­low the en­gine to sta­bi­lize. This is where the vac­uum gauge and tach can re­ally help. Make changes that sta­bi­lize the vac­uum gauge nee­dle. If idle speed in­creases, lower it by clos­ing the idle speed ad­just­ment. An air/fuel ra­tio me­ter can also help, but don’t fall into the trap of shoot­ing for a given num­ber.

Set the idle mix­ture for the best idle qual­ity along with the lean­est air/fuel ra­tio. If the en­gine will idle at 14:1 air/fuel ra­tio that’s re­ally good. Most per­for­mance en­gines will need to be richer—usu­ally around 13.5:1. There is a tip-over point be­tween rich and lean where com­bus­tion is ideal. This is near a lean mix­ture as long as the idle is stable.

Cars with au­to­matic trans­mis­sions may need a slightly richer idle mix­ture set­ting to al­low the en­gine to idle smoothly in gear. For tight con­vert­ers, we’ve had to add a half-ra­tio richer mix­ture to keep the en­gine run­ning in gear. Much of this is de­ter­mined by the tight­ness of the con­verter.

If your en­gine has an an­noy­ing stum­ble just off idle that you can’t seem to fix, check the ac­cel­er­a­tor pump link­age. Lightly open the throt­tle link­age and watch the ac­cel­er­a­tor pump squirter in the pri­mary ven­turi. The mo­ment the throt­tle link­age moves, fuel should exit the noz­zle. If it does not, look first for slack or clear­ance be­tween the ver­ti­cal ac­cel­er­a­tor pump link­age arm and the lever that moves the ac­tual ac­cel­er­a­tor pump di­aphragm.

There is some con­fu­sion around this as older tun­ing sto­ries men­tion re­quir­ing clear­ance here, but only with the link­age at wide-open throt­tle (WOT). At idle, there should be zero clear­ance or even a slight preload be­tween the link­age and the ac­tual di­aphragm. If clear­ance ex­ists, merely ad­just the pump link­age with a pair of 3/8-inch wrenches. Once this is prop­erly ad­justed, it’s also pos­si­ble that the ac­cel­er­a­tor pump noz­zle size can be re­duced. Too much fuel from an overzeal­ous tun­ing ap­proach can be just as detri­men­tal to good throt­tle re­sponse and too lit­tle noz­zle size, al­though the drive­abil­ity symp­toms will be dif­fer­ent.

3. Con­vert That 4160 to a 4150

So you bought a newer 0-3310 vac­uum sec­ondary 750 Hol­ley for a great price, but later you re­al­ize it is the 4160 style with the sec­ondary me­ter­ing plate. These are still great car­bu­re­tors but you’d rather have the more race-in­spired look of the 4150-style Hol­ley with the sec­ondary me­ter­ing blocks. No sweat—there’s a con­ver­sion kit for that.

Hol­ley makes sev­eral kits—mainly for ei­ther the 0-3310 750-cfm or the 0-1850 600-cfm carbs. The con­ver­sion kit adds the rear me­ter­ing block along with the re­quired dif­fer­ent me­ter­ing block gas­ket. The kit also comes with the cor­rect jets to re­tain the stock jet­ting. For ex­am­ple, the 0-3310-5 4160 750 car­bu­re­tor comes stock with a sec­ondary me­ter­ing plate (PN 134-21) that is equiv­a­lent to a 75 jet that is in­cluded in the kit.

This 4160 to 4150 me­ter­ing block con­ver­sion can be ac­com­plished on a sin­gle-in­let 600-cfm car­bu­re­tor as well. The best thing to do here is to also con­vert to dual in­let bowls at the same time. This can get a lit­tle ex­pen­sive so it might be worth­while to find an older dual in­let Hol­ley that will do­nate the fuel bowls.

4. Trav­el­ing Down the Slot

Here’s where a lit­tle knowl­edge can make you a master carb tuner. A con­tin­u­ing prob­lem for street en­gines run­ning big camshafts and lots of over­lap is low idle vac­uum.

With min­i­mal vac­uum, these en­gines de­mand in­creased throt­tle open­ing in or­der to idle at an ac­cept­able speed. Un­for­tu­nately, when the curb idle ad­just­ment is opened past the stock set­ting, the throt­tle blades un­cover the trans­fer slots. These slots are in­tended to pull ad­di­tional fuel from the car­bu­re­tor’s idle cir­cuit that is con­trolled by the idle feed re­stric­tor. The trans­fer slot adds fuel to com­pen­sate for in­creased throt­tle open­ing be­fore the main me­ter­ing sys­tem kicks in. This pre­vents a stum­ble un­der light ac­cel­er­a­tion.

When the throt­tle blades are opened far enough at idle to un­cover the trans­fer slots, this adds idle fuel that is not con­trolled by the idle mix­ture screws. This in­creases fuel flow, mak­ing the idle air/fuel ra­tio ex­tremely rich. Yet when ad­di­tional throt­tle is added for light ac­cel­er­a­tion, the en­gine stum­bles.

The tra­di­tional so­lu­tion (which works very well) is to close the curb idle back to where it just touches the trans­fer slot and then drill a small ini­tial hole; usu­ally 3/32 inch (0.093inch) in each pri­mary throt­tle blade. If the idle speed is still too low, larger holes or two more in the sec­ondary throt­tle blades can be drilled. It’s also pos­si­ble to slightly in­crease the sec­ondary throt­tle stop to slightly in­crease air­flow but again only enough to avoid open­ing into the sec­ondary trans­fer slot. The ideal place­ment of the closed pri­mary and sec­ond throt­tles should be with the blades un­cov­er­ing barely 0.030to 0.040-inch of the trans­fer slot.

The only prob­lem with drilling these holes in the throt­tle blades is that this be­comes a semi-per­ma­nent mod­i­fi­ca­tion. If the carb will be sub­se­quently used on a milder-cammed en­gine, it will prob­a­bly not be pos­si­ble to re­turn the curb idle speed low enough be­cause of the per­ma­nent holes. This can be re­paired but will re­quire new throt­tle blades.

When Hol­ley re­designed the Ul­tra XP line of car­bu­re­tors, they added an ad­justable air by­pass fea­ture lo­cated un­der­neath the air cleaner stud. With this car­bu­re­tor, idle speed ad­just­ments should be done with the by­pass ad­just­ment rather than at the throt­tle speed screw on the link­age.

5. Vac­uum Sec­ondary Tricks

If you’ve ever tried to change the vac­uum sec­ondary spring in a Hol­ley di­aphragm mod­ule, you know it re­quires dex­ter­ity and pa­tience. Sean Mur­phy showed us how to do it with­out tear­ing the frag­ile rub­ber. Place the rub­ber di­aphragm into the lower hous­ing and then set the base on your vise with the steel shaft ex­tend­ing through the jaws. Po­si­tion the rub­ber di­aphragm on the base in the cor­rect ori­en­ta­tion. Make sure the hole that seals the di­aphragm trans­fer slot is un­cov­ered. Lightly clamp the steel di­aphragm shaft in the vise.

Now shoot a lit­tle WD-40 or other lube on the threads of the four screws. Place the vac­uum sec­ondary spring in the boss on the un­der­side of the lid. For most street ap­pli­ca­tions, Mur­phy likes to use the pur­ple spring. With the di­aphragm held in place by the vise, lightly com­press the spring over the di­aphragm and align the holes be­tween the base and the cover. Start each screw by hand as the lid is held in place on the di­aphragm. Be care­ful here to not catch the rub­ber with the threads of the screws.

Mur­phy also prefers to elim­i­nate the stock steel ball that Hol­ley uses to trim the open­ing rate. In­stead he drives in a lead ball and drills it to 0.040 inch to act as a re­stric­tor so the sec­ondary opens at a slow enough rate to pre­vent a bog or hes­i­ta­tion.

■ Ac­cu­rate tun­ing of the idle mix­ture with a com­bi­na­tion of a vac­uum gauge and sen­si­tive tachome­ter can of­fer ex­cel­lent re­sults—and the best part is these tun­ing ef­forts cost noth­ing to achieve.

■ Ad­just the link­age on the ac­cel­er­a­tor pump so that the in­stant the throt­tle moves the ac­cel­er­a­tor noz­zle will squirt fuel. If it doesn’t, the link­age is im­prop­erly set. There should be zero clear­ance or a slight preload on the link­age arm to the ac­cel­er­a­tor pump lever.

We’ve seen the stan­dard ac­cel­er­a­tor pump di­aphragm be­come brit­tle with age. Hol­ley sells a green Vi­ton rub­ber di­aphragm in­tended for al­co­hol carbs but works great with the 10 per­cent ethanol in to­day’s pump gas.

■ The con­ver­sion kit (fore­ground) re­places the me­ter­ing plate like on the 4160-style carb on the left and re­places it with a me­ter­ing block like the 4150-style car­bu­re­tor on the right. In case you’re tak­ing notes, the sec­ondary me­ter­ing block is the main dif­fer­ence be­tween 4160 and 4150 car­bu­re­tors.

■ In a prop­erly ad­justed curb idle set­ting, the tip of the throt­tle blade should just un­cover the end of the trans­fer slot (as shown here).

■ This Hol­ley Ul­tra XP and some older HPs em­ploy a by­pass idle cir­cuit with an ad­juster screw lo­cated un­der­neath the air cleaner stud mount­ing boss. You can iden­tify these carbs by the four ver­ti­cal holes ad­ja­cent to the air cleaner stud that al­lows air (con­trolled by the ad­just­ment screw) to by­pass air past the throt­tle blades.

■ Here’s the trick to as­sem­bling that tricky vac­uum di­aphragm can­is­ter. Place the base on top of the vise and slide the di­aphragm through the lower half un­til it sits flush. Clamp the shaft in the vise and then care­fully lower the lid in place. Start the screws lightly to avoid wind­ing up the di­aphragm, which could tear the rub­ber.

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