I changed all the computer stuff of my 82 z-28 - The only carb I could find at the time is a 750 vaccuum secondary holley. Is this carb to big for a 305?

Cars seems to run ok, not really fast or anything, but sort of a gas hog compared to the original CC carb. I do a plug check and the plugs are tan - and its not running overly rich or anything.

I also get 19 inches of vaccuum at idle, if thats of any help

 

It's larger than the engine will ever be able to use. Whether it's too big to work, probably not. It isn't surprising that your gas mileage has dropped, though.

What distributor are you using?

And, of course, the big question: Why did you remove the computer stuff?

 

To tell you the truth the car was running fine before I changed the distributer and carb-

However the previous owner had the wires and sesors all cobbled up- only a couple were plugged in anywhere, and half the stuff was melted on the exhaust. It looked really nasty.

As far as distributer- I installed a older Vacuum advance HEI

 

that 750 is way to big if you put something mor like a 600 on it you would probaly get a noticable increase in power and fuel economy

 

Its a vacuum secondary... it wont necessarily need 750cfm, so the secondaries wont open enough to flow 750cfm... only what the engine actually needs. Don't forget, the stock Qjet was 795cfm, and GM put that on a 305.

 

If you want better mileage from a Holley you should look into the emissions-legal version of their 600 CFM unit. People practically GIVE these junkers away at swap meets, but for a mild performance engine they work just great (if they're in good operation condition). They have much leaner idle circuits and overall leaner primary calibration than the performance-oriented versions of the same carb.

 

so what you are saying is that a vacum secondary carb takes care of all the problems of running to big of a carb. if so then how come on my 350 when i was running a 725 vacum secondary road demon and swapped on a 600 edelbrock i got a noticable increase in power off idle, part throttle, and full throttle

 

Your demon wasn't tuned.

 

No, it's a crutch. Bandaid. Reduces the problems, doesn't necessarily eliminate them.

One effect of having too large of a square flange type carb is a reduction in the air flow velocity at part throttle. This will cause boost velocity issues, bleed circuit issues, throttle response issues, and fuel economy issues. Being vacuum or demand-based secondary doesn't solve any of those things. You could disconnect/lock out the secondaries, and you'd still have those porblems.

All of this, of course, requires a return to first principles: Namely, what's your defination of "too big" (note that I didn't say "to big"...)? I suppose I should have included the paragraph above in my original response.

 

the demon was tuned

 

thats pretty much sums up my 305 -. I know that the Q-jets flowed about as much, but part throttle they seemed a lot more responsive.

BTW anyone have a cheap, Non cc Q-jet for sale that dont need to be rebuilt?

 

I think I still have one in the garage attic. It hasn't been run for a few years, it'll probably be okay if you just install it, but it isn't that hard to put a kit in them.

 

If you do any idea, of what you would like to get out of it?

 

these are about as common as **** in china...
I've got 3 in my garage... They're not worth a whole lot....


You could probably get a board member to rebuild one for you, and jet it somewhat close to what you need.... for a pretty fair price...

 

The typical 750 holley (VS and DP) is calibrated for general high performance applications. ( it's a little rich out of the box)

Not suprising your mileage dropped. You can fix it.

You need to fine tune in the primary side of the carb for better efficientcy.

The stock primary jetting is 72. drop it down to a 66, 67 or 68. Leaner (smaller) is better. Experimentation will find the best jet for you. if a small #66 or 67 jet results in drivability problems go up a bit in jet size till the driveability comes back. Best jet should be between #66 and #70.

If you are crafty you could further leanout the idle feed restriction in the primary metering block a little (.002" at a time to lean out the idle circuit.)

I use thin mig wire shaped in a V incerted in the idle feed restriction holes.

You'll know when you are too lean cause the car will stumble and surge while driving around town.

be sure the car is fully warmed up before evaluating a jet change effect.

As you lean down the calibration naturally the cold temp operation will become more critical. Hook up the choke and use it. Manual choke linkage kits are cheap and simple to install.

Try these mods and you'll get better mileage with a 750VS (3310) on a mild motor.

These universal performance carbs are jetted to work well in most customers cars. but you can and should fine tune it to get sharpe performance in you specific car.

Your car will likely favour the changes I recomended.

The carb is not too big. I hope you are not trying to make this carb work on a stock QJet spread bore manifold with a carb adapter.

It isn't going to work. get a manifold that will allow direct mounting of the carb without an adapter. Too much flow disruption.

The secondary jetting will be in the 76 to 82 range. ( #21 sec plate.)

A 750 may seem "way too big" to those who don't or won;t tune a carb in for best performance on their car. But now you know what to do.
A late model Qjet flows 795cfm when the throttles and air door are fully open. a early pre 1974 Qjet flows 725/750cfm. Your carb is not "too big". But it does need fine tuning.