drilling air bleeds on a holley 750 vacum secondary carb

[brianbailey440]

i have been told that drilling my air bleeds out on my 750 holley will make a huge difference in throttle response.i have a friend that has done this many times, and swears by this. im just curious if any of you guys have done this with good results.he told me to drill all 4 rears to .032, and the front centers also. the front outers to .078.does this sound right to you guys?

 

[Crackedback]

It may have worked in his case. May not in yours. It's better to take the bleeds out ad tap them for brass set screws and then it's fully adjustable. Once you drill them out, it's tough to go backwards if it's wrong. Proform/ Bo Laws have a 750 VS main body that works pretty well.

If you have an A/F meter you can see if the system is rich at idle and cruise. Drilling out the bleeds leans the circuit out.

There's a lot more to it than that. The IFR's may be too large or the emulsion package may be off. Lots of trial and error in tuning by the seat of your pants. A/F meter make life a bunch easier and they are relatively cheap ~150-200 for a unit.

 

[67 coronet]

i have purchased a couple of the proform main bodys and they worked well.as crackedback said,drilling is a lot of guess work.make them threaded or get the mainbodys with them already threaded.much safer.

 

[brianbailey440]

i have had the carb apart several times.it is an 80508-1 and the pvcr,s were waaaaaay to big, .063 holes.it also had a secondary metering plate.i took the primary and secondary metering blocks off of a 4777-3 holley 650 double pumper, and used those.the pvcr,s were .0365 on the 650.no power valve in the rear.jetted it to 81 front and 85 rear.white secondary spring.car runs way better with this setup.but the idle is a little too rich,and i can smell unburnt fuel after a full throttle run.idle mixture screws are 1 1/4 turns out, and that is as far as i can turn them in for a good steady idle.my timing is perfect, 18 initial 38 total.all in by 2500 rpm. xe268h cam in a .040 over 383 with 452 heads.9-1 compression.my friend says that drilling the bleeds will wake it up,and i want to try it, but if it doesnt work im looking for a new main body. i do have an 850 double pumper that another friend let me try, but i havent rebuilt it yet.is it possible to use the 850 main body with the other metering blocks off the 750, and make it vacum secondary?just something i have been tossing around in my head.the 850 main body is allready drilled to .0321, and .078.what do you think?

 

[brianbailey440]

can you turn a double pumper into a vacum secondary?

 

[forphorty]

can you turn a double pumper into a vacum secondary?

Never heard of it being done. Why would you want to? Once I got my first double pumper, I never went back to a vacuum secondary. Why not try that 850 dp on your car and see if you like it?

 

[brianbailey440]

only because a vacuum secondary is more forgiving on the street,i might just try the double pumper anyway.

 

[Crackedback]

If by forgiving, you mean slower, absolutely!

I've got 750 DP's on 340-360 sb's with auto trans/2500-3000 conveters, 2.94 gears getting 20MPG of the freeway. It's all in the tune up!

 

[brianbailey440]

im sure the gears help with the mileage. im gonna give it a shot with the 850 double pumper, what do i have to loose?

 

[67 coronet]

just so you guys know,dp and vac secondary main bodys will not interchange.noplace for dashpot among other minor stuff.

 

[Phil McGrath]

i have had the carb apart several times.it is an 80508-1 and the pvcr,s were waaaaaay to big, .063 holes.it also had a secondary metering plate.i took the primary and secondary metering blocks off of a 4777-3 holley 650 double pumper, and used those.the pvcr,s were .0365 on the 650.no power valve in the rear.jetted it to 81 front and 85 rear.white secondary spring.car runs way better with this setup.but the idle is a little too rich,and i can smell unburnt fuel after a full throttle run.idle mixture screws are 1 1/4 turns out, and that is as far as i can turn them in for a good steady idle.my timing is perfect, 18 initial 38 total.all in by 2500 rpm. xe268h cam in a .040 over 383 with 452 heads.9-1 compression.my friend says that drilling the bleeds will wake it up,and i want to try it, but if it doesnt work im looking for a new main body. i do have an 850 double pumper that another friend let me try, but i havent rebuilt it yet.is it possible to use the 850 main body with the other metering blocks off the 750, and make it vacum secondary?just something i have been tossing around in my head.the 850 main body is allready drilled to .0321, and .078.what do you think?

Like many I don't have cam numbers memorized so xe268h doesn't tell me much? But even the Vac Sec carbs have Hi and Low speed bleeds.... If it were I, I would use a drill bit as a pin gage and I wouldn't get crazy but I might go one step larger on just the low side and not touch the Hi side bleed at all. All carbs are sensitive too bleed adjustments after going up one size on the low side if it worked out GREAT you came out ahead, if not then you didn't really hurt anything. However if your looking too play the adjustable bleed game you can buy the tap and do it at home for a lot cheaper than a new main body costs.

Metering blocks are metering blocks for the most part, if you use a metering block from a dbl. pumper on rear of a Vac Sec the pump shots passage is blocked by the rear bowl and carbs main body but it still meters.

 

[brianbailey440]

i have a good understanding how the fuel cuircuts work,just never touched the bleeds.but, i was told by a pro builder to drill them. .032 on all 4 out back, and .032 in the front center, and .078 on the front outer.he knows his #$%^ and has helped me get my car running mint.i think i should give it a shot. i could drill and tap them for bleeds after if it doesnt work!!!

 

[Bigal75]

Increasing your main air bleeds will possibly lean out the circuit at WOT.

 

[brianbailey440]

yes,both low and high speed.it is rich on both right now.cruise is perfect, but too rich at low and high.

 

[Bigal75]

Have you tried adjusting your jetting? PVCRs? I wouldn't start drilling anything without trying these things first.

 

[brianbailey440]

i have tried just about every configuration with jetting from 72 front -76 rear to 84 front 88 rear.2.5 power valve to 6.5 power valve.i had two different metering blocks, the stock metering block had .063 holes in the pvcr,s, and the other is .036.the smaller pvcr,s work best with the 81 front and 85 rear,but the ifr,s are a little bigger on that metering block.i have been studying carbs for about two months now , and found all kinds of interesting theory on the web, but nothing has worked better for me than what my friend at bjr racing has recomended.he told me to drill the bleeds,so im gonna give it a shot.here are a couple links.http://www.bjrracing.com/about.htm. http://members.tccoa.com/392bird/tuning.htm. http://www.bob2000.com/carb.htm.i have a pretty good understanding on how carbs work,and have tried things that work, and things that dont.i think that drilling the bleeds as bobby at bjr racing has told me to do will help,he knows whats up.i just wanted to know if any of you have tried it with good success before i drilled mine.

 

[Bigal75]

Are you using a WBO2 sensor? Working the bleeds will definitely change the fuel curve and probably help in your case. Keep us posted.

 

[brianbailey440]

i have to get it back on the dyno for that,i dont have a wide band, so right now im tuning by reading the plugs.i will deffinately let you guys know how it turns out after i drill them.

 

[383man]

I do the same as Phil said. I have the carb drill bit sets and rather then just go to the size was said I would use your drill set and find the size they are right now. And then just open them up a size or so at a time and see how it likes it. That way you dont have to worry about going to much. Just open them a very small amount until you get where it works good. Good luck , Ron

 

[brianbailey440]

i called holley, and they told me my carb had .025 inner bleeds on the secondary side, and .028 on the outer.he also said the primary are .028 inner and .078 outer.if the front outers are at .078, then ill probably have to put a wire in the ifr,s , or change the metering block to a smaller ifr.the problem is that the other metering block that i have has way too big pvcr,s.im gonna try drilling to what i was told, and if its too lean, then ill have to jet up, or drill for screw in bleeds.ill probably end up drilling for removable bleeds, but im gonna give it a shot anyway.cant hurt to try it, ill keep an eye on the plugs,and tune from there.right now its deffinately rich.my friend says that he does this to every 750 holley vac secondary, and it makes a huge difference.im dying to try it.i just got my .032 bits in the mail, and im still waiting for my pin drill vise to come in.hopefully today ill have it, and i will drill and test as soon as it does.ill keep you guys posted.