creepingdeath

I got a new holley 750 double pumper dichromate with manual choke yesterday It will be going on a 355-383 vortec, but for now i want to put it on my 305. I know it's way too much carburetor but how bad will it be? will it run good enough to drive daily? I have a mild cam 3" dual exhuast and 416 heads. I will also be getting a performer rpm intake manifold when i install the carb.

jeffcj

iTrader: (3)

I'm sure with proper tuning/jetting it will run. Throttle response and gas mileage will suffer though.

online170

Theres no such thing as over carburetion. You could tune a 1250cfm carb to run on a 283. The engine will only suck in as much as it needs, you just have to jet it to run correctly, so the mixture is perfect.

Undercarburetion is possible though, where the cfm is not enough to sustain the engine, or is not enough at wide open throttle. That being said, theres a formula that you can use to determine which carb is "best" for you application.

i have a 750 double pumper on my 355, and it seems to run nice. It was on a 383 before, and is still jetted for that, so i need to lean it out a litte. I think you will like the 750, but it wont be good on gas.

creepingdeath

Thanks guys, I'm in alberta gas is cheap here and i get paid for it at work so i Don't car for that. So it should run good with the 750 for now and i won't have any major problems? I have a buddy who went from an old dirty q-jet to a holley 650 dp he siad his car has a completely diffrent attitude and will smoke the tires sometimes if he's not careful

online170

Yea the rpm intakes help the engine breathe for sure.

In any case, it CAN run well, but it probably wont. The carbs are "recommended" for a certain cubic inch range, and are over jetted from the factory to run rich. Rich wont damage your engine, but lean can blow it up.

I can almost gurantee the carb will be too much, if you slap it on the 305, you must rejet it to run correctly.

It will basically dump fuel into the engine, and u will likely stall at stop lights. Jetting will fix this.

xpndbl3

iTrader: (5)

less of a booster signal from a smaller engine usually means BIGGER jets not smaller....

don't know what you're trying to tell him online170. Furthermore stalling at a stoplight is an idle mixture issue not jetting. Perhaps can I recommend a holley tuning book for you to check out to better understand these concepts? A few of them are really helpful.

online170

Youre right i got that backwards.

Just going by experience on my carb, which was tuned for a higher displacement engine.

Point is, if its a 750 cfm from factory, or was used ona 383, it wont work on the 305 as is.

xpndbl3

iTrader: (5)

actually i put a 750 double pumper on my 305 when it was running 13.0 at 103 and I was within 4 jets total front and back of a perfect tune. I think the primary needed 2 jets lower and the rear needed two jets higher from what I recall. either way a dyno tune or a wideband will tell you what's really going on.

online170

Well obviously i dont know much about tuning a holley. Mine had #71 all around, and the primary bowl was set really high, and the secondaries a little low.

I had gas pour into my primaries, which stopped after i lowered that level. This holley was from a 383 drag car.

xpndbl3

iTrader: (5)

so it had the power valves blocked off and was jetted square? hope you changed that around to drive on the street

online170

I adjusted the 4 corner idle, and readjusted the floats so the fuel level was just below the sights. The jets are still there, but i have to resolve a few issues before those can be dialed in correctly.

It was only driven about a week after the holley install. The float adjustment helped, and so did the idle mixture, but its still running rich. Wont stall at stop lights, or anything like that, but it hesitates from part throttle to WOT, and is definately not at peak power. Itll be fixed before spring this time around.

Gotta fix the fuel pump, and timing before i get to the carb tuning though.

Sonix

iTrader: (1)

Main thing you'll notice with the double pumper is it'll dump in as much fuel as you tell it to. ie. if you're in top gear, 2000RPM and you punch it, it'll probably drown the engine because it can't use that much mixture that fast.
You'll probably need to shrink the pumpshot and tweak the jetting. But I know all of "diddly" about Holleys', so if in doubt, RTFM eh?

There's no reason you can't tune it fairly well with your current engine. Just be aware you shouldn't go WOT until you're at a certain RPM. With a vacuum secondary (like the q-jet) only the primaries would be open if you punched it, and the secondaries would open up as needed. It's nearly impossible to "drown" the engine with that. Dummy proof if you will.
Don't be a dummy and you'll be fine. Let us know how it goes.

five7kid

iTrader: (14)

It is possible to overcarbureate an engine. xpndbl3 already gave one reason, boost venturi signal. A venturi works via flow velocity. The larger the carb, the lower the velocity. It can get to the point that you don't get enough of a signal for it to even operate. 750 on a 305 isn't close to that, of course, but you are compromising when you do it.

See http://www.holley.com/data/TechServi...Carburetor.pdf . Two parts there - carb size vs. engine size, and carb size + engine size vs. WOT RPM. First, size - you aren't going to use the full 750 CFM until well over 8000 RPMs on a 305 with 100% VE. That means a lot of mis-calibration at lower RPMs. Then, you need to keep RPMs above about 1700 when you go WOT or you're going to bog - no amount of tuning will help. I found (back in the days of yore) that I couldn't go WOT with a 310 (4.060" bore x 3.00" stroke) under 2500 RPMs (that was partially a cam issue), so take that chart with a grain of salt.

On a Vortec 383, the 750 DP would be the cat's meow. On a 305, it's going to be a huge compromise.

online170

i see.....

My basis for this argument was, that the engine will basically suck in as much as it needs. So although its NOT the best performing, you could still go the biggest available sized carb, on a relatively small V8.

Is that wrong to assume?

Apeiron

The engine will draw in as much air as it needs, but the carburetor's ability to meter the correct amount of fuel will be impaired.

creepingdeath

Wow thanks guys I'll let you know how it goes. I got the carb but i need all the brackets, choke cable, fuel line, intake mani, and all that stuff so it wont be on until weekend after next. BTW I killed a JDM JZ1 twin t56 ball bearing turbo toyota soarer the other day (toyota supra engine) with a posi rear With a near stock 305 and a crappy cruddy q-j and slipping clutch, my freind driving the car was super embarrased i loved it.

Sonix

iTrader: (1)

That's hilarious. Your custom "cutoff" exhaust must have just added insult to injury there. mwahaha.
Bah, too much hate for the Q-jet man. tsk tsk. If you're throwing away your old Q-jet i'll carry it to the dump for you

creepingdeath

My 25 year old ******* rebuilt camaro killing a high tech japanese car haha. I smoked him off the line with one tire spinning and i had him by about 3 cars after 200 feet or so then He almost caught me after about a mile then he governed out at 180km I really didnt think i was going to win. You can have both the q-jets jay my junk is your treasure . I like how the holley sits lower than the q-jet i just gained an 1" of hood clearance.

Sonix

iTrader: (1)

Right on, you can afford a nice tall air cleaner, or a nitrous plate

Hey if you're serious i'll come by and pick them up on my way back to Calgary this time. I'm in Edmonton again. I might be driving back on thursday or friday evening. I'll give you a call when i'm on my way through.

Were you planning on porting your 305 heads, or are you using a Vortec 350? I forget, I remember you changed your mind when you were getting the 350 in the truck?

Dan W

I'm not as familiar with Holley carbs as edelbrocks but they should be similar enough that my advice should hold true...

The Velocity through the venturi as already mentioned is one issue. The Venturi are air foils like an air plane wing. They create a pressure differential to move fuel. This differential is created due to the velocity of the air passing through the air foil. The smaller the engine, the longer it may be before the venturi start working.

Similary, a big engine has a larger fuel demand at idle or in the transition area before the venturi are really working. In these areas a carb designed for a larger engine may give wrong ammount of fuel for the smaller motor. The idle adjustment may not be enough to fix it as this also affects the transition fuel. Some times, air bleeds need to be modified to make both the idle and transition fuel tune in.

Also, obviously, the accelerator pump shot from a carb designed for a 350 would be larger than needed for a 305.

sofakingdom

iTrader: (1)

That's really the deal right there, in a nutshell.

The bigger the carb, the smaller the signal, for any given amount of air moving through it. A small motor moves a small amount of air. Meaning, a "too big" carb, will have trouble being "sensitive" enough to the small signal produced by the small motor's demands.

It'll make power and all that, just fine; more likely, it'll cause trouble with part-throttle driveability, if anything. Might work plenty well enough though. 750 isn't THAT outrageously big on a 305.

creepingdeath

I've been thinking of putting on a small plate system since i need a new hood Im either going to hunt down a 82-83 fg hood or 3" cowl. I will only be able to give you my old carb (the one you thought was strange in my shop) until the weekend after next when the holley goes on. If the holley acts up too much I'll try jumping up two jet sizes and see what that does. Also the carb needs slight tuning becuase holley sets there carbs at sea level and recommend a tune if your above 2,000 feet and I'm at near 3,000. I guess i will have to wait and see how it is on the engine, I dont think it will be too horrible since i do have a fairly strong 305.

The the truck was a no-go the guy couldn't even start it and I dont want to take everything out of the truck. The vortec will be on the stand in the shop on the 15th

Sonix

iTrader: (1)

I'm toying with the idea of the powershot setup for spreadbore. We'll see. $550 I need to come up with first.

Ok, i'll grab your "junk" carbs another time then.
Whoa hoa, Vortec 350 with the holley 750 eh? Winning combo right there. It just desperately needs headers and a roller CAM!!

RandyZ

Im doing a cam swap right now ,my car is in the driveway with everything off just going to pull the lifters out and im all set to go.I'm using my stock Q-jet and im not going to have any problem your crazy for changing you carb my father in law ran 12s with his . every thing is staying stock be sides my 470 lift cam in my 305 with hooker headers and true daul exhust .

creepingdeath

I might build my bottom end with a powerhouse 383 kit to go in the vortec and i will certainly be using a roller cam (maybe going solid). Everyone at the performance parts shops have been telling me that the holley 750 dp/vortec combo will be amazing so i guess i picked a good carb haha. Is that the NOS kit or edelbrock? what do you think youll be running this summer for hp?

I just wanted something new that i can trust.. I know q-jets are great but I'm no q-jet expert and dont want to put the work into them or fork out the cash for a re-furb. Also i think the DP will be better suited to my setup with the manual trans . Plus I only drink coca cola and I prefer blondes so naturally i gotta have a holley right?

Sonix

iTrader: (1)

hahhaa, blondes and coke eh? Ahh, don't want to upset all the sterotypes. Glass packs only go with Holleys anyway.
He's a lost cause Randy :LOL:. I offered to help him tune it a few times but he's set on the Holley. I got my Q-jet running pretty perfect with my cammed up 350, 224/230 and .52x" lift.

I was looking at the NOS kit. The Edelbrock one is about $100 more for the same thing so..... I generally only like Edelbrock for intakes.
I'm hoping i'll be able to hit close to 400HP at the crank. I'm going to tune it with my wideband, but then get it on the dyno later. I'm shooting for 310HP or thereabouts at the rear wheels. Then a 150HP shot, then go on up to the 250HP when i'm ready

85SILVERBULLET1

over carb??? hmmmmmm I am running a bigs 950 hp on a mild 383. Hehehehhe... all though it is an 11.1 roller motor with a big roller cam and aluminum heads that gets twisted around 7500rpm. seriously, that 750 might be a we bit large for a 305.

creepingdeath

I get the vortec this weekend and the holley goes on friday night, i know the 750 is huge for the 305 but it will probably run better then my stinky ol' q-jet, low rpm power will suck but i'm not racing or anythin so i should be fine hopefully. If i was getting a carb for the 305 i would've got a 600 or 650 but i was thinking ahead I'll post vortec pics saturday in a diffrent thread, and holley swap pics in this thread maybe friday.

Sonix

iTrader: (1)

Boy if I had a nickel for every time someone said that on this CAMARO forum, i'd be rich...
Kinda like the guys who think these cars are for driving to church every sunday
Good luck, let us know how she goes.

creepingdeath

I got her on today, took me about 2 hours. I just got a square bore to spread bore spacer instead of the new manifold. I had to hack off a vacuum port on the passenger side of the manifold off, and my $75 holley chrome hardline with the fuel psi gauge on it is too long.. It is about 1/4" away from my alternator (i wonder how i could fix that) i only have neoprene gas lines for now becuase of the alternator problem. I got a stainless braided AN fitting line but it wont work on it right now. my fuel psi is reading 8-9 psi is that good? As soon as the car started and i revved it a few times i knew i gained a massive amount of power, I swear i gained 50hp from the q-jet. The engine even sounds really diffrent sounds kinda "rippy". the only drawbacks i have right now is a very very slight stutter if i open the secondaries too soon and a weird jumpy 750-1000 rpm idle. My idle speed screw is all the way out and I can't find a vacuum leak, i think it might be my rotar and ignition timing that is making my idle a bit high, my rotor is really loud right now for some reason.

online170

Hey now, the q-jets arent that bad (the pre 1980 ones anyway). Yours was probably gumed up good.

In any case, do you plan to run an electric fuel pump at any point? Might be a good idea, the holley sucks your tank dry pretty quickly. IF SO, switch your fuel line around. ie: make the inlet face the firewall. You can then place your regulator on the firewall later on. But for now, just get some fuel line, and direct it away from your exhaust and to the fuel pump.

creepingdeath

Thanks I never thought of that. I know Q-jets are good but they're too much work and holleys are so much easier to work with and tune. I have a 1974 and 1984 q-jet and i couldnt get them to work so i just got the holley (jay i got them sitting in my shop if you still want em). I'm not experiencing horrible overcarb problems besides the quirky idle and stumble when the secondaries open (saw that coming) I can't smell unburnt fuel anymore so it seems like my A/F ratio is better. Also when i was playing with her today i noticed my gas mileage was a little bit better and i was hitting 6,000 rpm. I noticed also when I shift I get alot more power once i hit the throttle again. Tommorow I'm going to play with my ignition timing and see if that idle problem goes away if not i think I might need new intake manifold gaskets or something..

xpndbl3

iTrader: (5)

why would you EVER tell him to place the regulator on the firewall. That is a big NO NO.

creepingdeath

Why is it a big no no? Ive seen lots of cars with them.

xpndbl3

iTrader: (5)

why would you want a fuel source located on the firewall that is there to prevent a fire from coming into the passenger compartment?

Sonix

iTrader: (1)

Ok, i'll keep that in mind next time i'm on my way up to Edmonton/Red deer

You can HEAR your rotor?!? That's weird. I'd guess it's something else.

Is your PVC hooked up right to the Holley?
Have you adjusted your idle mixture with a vacuum gauge?
Does this carb have secondary idle as well, or just the front?

You're still going to need to tune this carb a fair bit to get it running a-ok. But the fact that you have more power on an upshift is pretty normal - that's a double pumper for you. With a vacuum secondary the secondaries snap shut if you lift your foot off the gas on a shift, so they need to re-open (takes a sec). Powershifting and there's no difference.

online170

If a regulator fails at the fitting, the fuel spray can pretty much spray anywhere. It will most likely spray on a plane parallel to the cross section of the fitting though. That doesnt leave many places for the regulator to go, thats out of harms way. Just because its MOUNTED on or OFF the firewall, isnt going to prevent the fire from happening should it fail.

My reasoning was just to keep the fuel line length to a minimum.