alek72us

i been looking around to see if any one else has my same problem but can't find anything.

i have a brand new roller 305 and i but a new quadrajet 750 on it. and it doesn't want to run. i can turn the fuel/air mixture screws all the way in and it will still run, idles rough. so i thought i had a vacuum leak i changed all hoses put new plugs on all port holes not being used. i can spray the sh&* out of it with carb cleaner and can not find the vacuum leak. i noticed if i unplug the pcv valve it idles a lot better. but still cant tune the carb. does anyone know what else i can do. i have timed it and retimed it and redone the valves. i just can not figure this thing out.

five7kid

iTrader: (14)

Exactly what q-jet did you put on it?

What distributor are you using?

alek72us

This carb
http://www.summitracing.com/parts/JET-35001/
and this distributer
http://www.summitracing.com/parts/SUM-850001R/

five7kid

iTrader: (14)

Where did you set the timing? Did you have the vacuum advance disconnected when you set it?

Randy82WS7

may be leaking bowl well plugs if it runs with screws all way in

its a really commmon occurance on the Q jets

unplugging the pcv valve gives more fresh air oxygen into the fuel mixture

i bet its running rich


how spakr plugs look?

dark, sooty?

wet?

R45TSX would be the way to go, for spark plugs, in all the old sbc engines 72 up


extended tip plugs, run really good as long as no other issues

alek72us

I have the timing set to 8 and have changed it around a little bit trying to get something to change yes i disconnectd the vacuum advance.

i haven't checked the spark plugs i will tonight. i also think something is internally wrong with the carb also. i have another quad carb on a truck that i know runs good i think i'm going to throw it on this car and see if thay narrows down my problem.

SDTransAM

iTrader: (5)

a 750 is WAY more carb than you should ever need on a 305. You should really have a 550-600 cfm on there, and those will be plenty big.

alek72us

You really think so?

ur the only one that has said anything about that so far. even when i was building this engine everyone told me to get a 750.(its not a built 305 just .030 over stock dish, stock cam and stock heads)

five7kid

iTrader: (14)

There isn't a q-jet smaller than 750.

IROCtheRoad

Have you used your service manual and adjusted everything you possibly can?

alek72us

Yeah i have done everything. i have been messing with it all weekend. the only thing i can think it is is the carb itself. its just brand new so i was seeing if there was anything i am missing. i'm going to put a carb that i know is in running order on it and see what happens i will let u guys know.

Camaro86IrocZ

iTrader: (1)

Most commercial Reman Carbs even aren't worth the time it took me to type this... To many generic settings for the idle tubes, jets, rods, springs, ect... The Qjet was designed and built application specific, but when the remans get at em, they mix em all up...

Best bet is to find an unmolested original core and redo it yourself - http://www.cliffshighperformance.com/buy_book_2.html

Also Doug Roes book together are gold - http://www.amazon.com/Rochester-Carb.../dp/0895863014

Cosmik Debris

The 750 is standard on a 305.

Ozz1967

iTrader: (4)

Sounds like he needs to reset the idle mixture screws and the rich/lean stops and go from there. I'm not realy familiar with the non-ccqjets so I cna't add much more to the discussion on tuning, however I'd bet his settings are off. At least that's where I'd start.

ZF6camaroZZ4

The non electronic control carbs have a spring that controls the primary needles. You have to select the correct one to go with your manifiold vacuum/cam. Just sounds like his is running rich at idle.
I would get with jet and what they have to say, might have needed the stage 2.

Ozz1967

iTrader: (4)

Ok. I bought a stage 1 from Jet and used that for 3 years before I put the 355 in my car...I looked back over the work order from when I had my carb rebuilt and it's pretty much what they do to a Stage 2 carb. Same work done, just set up for a larger motor.

'84thunder

holy crap...a 750 carb on a 305? really? did u really need that big of a carb? whats the rest of ur set up?

i have a 305 with edelbrock nascar edition intake mani, trickflow super 23 175cc cylinder heads with trickflow roller rockers, lunati cam n lifter kit and im doin just fine with my holley 650.

im just curious

Camaro305SB

Q jets are considered 750 but I don't think they open up to 750cfm right outta the box. Someone may chime in to correct me, but I believe there are modifications to be done before they actually flow that much.

'84thunder

are we talkin aftermarket qjet or stock qjet from showroom floor? cuz i took my stock qjet off to upgrade to my new one, i looked up qjet's online and at dealers...they all said stock qjets were 500 or some low number like that. thats just wut i have been told by ASE cert mechanics and vendors(summit, jegs) im not a carb pro but...

Street Lethal

iTrader: (16)

Confused, does it not want to run, or can you not stop it from running...?

Ozz1967

iTrader: (4)

Every Qjet ever made was either a 750cfm or 850cfm from the factory. The way they are built, they only use what they need, so it's really not possible to "over carb" with a Qjet, unlike a Holley or Edelbrock. Now, from some of the mods that are done that we make possible to open up the secondaries more, lends me to believe that they would not hit their max potential which was probably by design. I saw the math one time, so someone correct me if I'm wrong, but I believe you won't need a 750CFM carb until you hit 427+cu or your running something really wild with high RPM like a race motor.

Camaro305SB

Ozz i think your right, or at least close to it. Its very common for the inexperienced to over carb an engine. Just like on later models people installing larger injectors on a stock car.

five7kid

iTrader: (14)

That pretty well sums it up.

For a 100% volumetric efficient engine (very rare) with a single plane intake manifold, the formula is CFM=CIDxRPM/3456. A small engine running high RPMs can flow as much air as a large engine running low RPMs. So, it isn't so much the size of the engine, but the size and max RPMs of the engine that determines carb size need.

Oh, dual plane intake engines need a little larger carb than the formula above, because the single plane effectively takes flow from all four barrels, while dual plane intake splits the carb in two (although the additional flow capacity is only like 15-20%, not twice).

'84thunder

i agree with that last part...you really dont need 750carb on a plain jane 305/350 unless you have some serious mods.

i guess the techs at the dealer and vendors were wrong in telling me the size of my stock Qjet i took off. makes me kinda mad that they cant be reliable.

Apeiron

And keep in mind that carburetors are flow-rated at an arbitrary pressure differential.

Camaro305SB

Well to be fair I think its more important that the technician knows how to properly service the carburetor rather than knowing every single little detail about it (not knowing exact CFM is not going to affect your ability to rebuild and adjust it properly). Hell, at my dealer maybe only half of the techs have an actual knowhow of quadrajets - it is seriously dead technology from a dealer standpoint. We don't even have a choke angle gauge at our shop.

'84thunder

hahahaha, bet ya some wouldnt even know wut it looks like or wut to use it on

five7kid

iTrader: (14)

Well, 2bbls and 4bbls are rated at different pressure differentials, but the 4bbl differential is pretty standardized. But, there are still ambiguities regarding dry or wet flow.

Apeiron

By arbitrary, I mean that the 1.5 inHg that a 4 bbl is flow-rated at isn't a vacuum level that most would like to see at WOT.