Idle Mixture Screws Do Not Adjust Anything on 1974 454.

[68post]

If you bought a "brand new" quadrajet then it is the problem (only chinese are making them, and not correctly), if it's a commercially rebuilt unit then it's the problem.
The only quadrajets to trust off of ebay are untouched cores, and you must be careful with them being rebuildable, but they are the best to start with. Qjets are very application specific and parts are not necessarily universally interchangeable.

 

[fast 99]

Can try dribbling fuel in each side of primary venturis when running. If engine slows down rich, speeds up or smooths out lean. That can also tell you if it's isolated to one side or both.

 

[Ricko1966]

If you are only pulling 15 in of vacuum at idle,I'd check for vacuum leaks harder,I do not see where you say if you sprayed carb cleaner around the perimeter of the intake manifold. Also you can put a vacuum gauge on the dipstick tube pull and plug the pcv valve[I'm assuming it's hooked up and working] duct tape over the valve cover vents and VC pcv opening, read your gauge,this will tell you if you are pulling vacuum through the bottom side of the manifold.

 

[JBswth]

Know you said vacuum leaks were checked but try disconnecting anything that uses vacuum except advance assume you checked that, brake booster all of it. Check fuel pressure and volume. Was intake and carb base checked for leaks?

Although don't think timing is related to idle issues 12 degrees sounds a little high, what is the total?

Was that carb new or used? If used, it probably needs new gaskets. Also, you went from a 305 to a 454. What rear end do you have and what transmission do you have, (if automatic)? You can't use a Turbo 350 in back of a 454, nor can you use a 10 bolt rear end. Both will soon fail. You need a Turbo 400 trans and a 12 bolt or 14 bolt rear.

J. B.

 

[DoubleDingo]

Hello Everyone first post here! I bought a rough, but not rotten 1984 C20 Squarebody! I bought it thinking that it was a 350 small block, but it turned out to be a 305 ): I wanted a bit more get up and go than what it had so I bought a 1974 454 Big Block for it. Once installed in the truck I bought a brand new Ebay Quadrajet measuring 750CFM. (Bare with me! I am still learning about Quadrajets and carbs in genral.) The internet thought 750 CFM was fine for this engine so on it went. Immediately after starting it would not idle without some choke and even with choke it would run stumbly. As soon as you open the choke it would die. After some looking and researching I adjusted the timing and idle mixture screws. Timing went fine and I set it to 12 Degrees with Vacuum advance disconnected. One thing I noticed is that it ran better the more timing I gave it past 12. I tried to adjust the mixture screws, but nothing was happening when I would turn them. Vacuum, engine speed and quality would stay the same. I consistently have 15-17 on the vacuum gauge no matter what I do with the screws. After some playing and some substantial run time I was able to get it to idle without choke. It spits and sputters like its really lean. I assume lean because it gets substantially better and idles much higher when the choke is partially closed. I have not found any vacuum leaks, but I would assume that the leak would have to be very drastic to cause this type of problem. In higher RPMS it runs good! Sidenote that big block is a force to be reckoned with! Nearly broke the tires loose in a hard shift to 2nd...and its stock! I am all out of ideas the carb is brand new, but who knows what that means anymore. The timing is dead set at 12 degrees. The mixture screws do absolutely nothing unless almost taken out. It only runs not awful with choke and if I get it to run without it, it sounds awful. Where should I check first? Reading some of these forum post I can see a lot of you are brilliant when it comes the notoriously misunderstood Quadrajets so your help would be greatly appreciated! I have a lot more work to do on this truck, but I will post about those plans another time. Thanks!

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You bought the wrong carb if you bought a new q-jet off of ebay. Take some pics of that thing without the air cleaner on top of it. If it's not a rebuilt, it's junk chinesium and you were swindled out of a chunk of cash.

 

[DoubleDingo]

I don't know if this forum is against the forum I am inserting a link to here, but if the OP did in fact buy a crappy chinesium quadrajet, they at the very least need to know what Lars went through to get a 4MV to operate somewhat normal to a factory Rochester...

https://www.corvetteforum.com/forums/c3-tech-performance/4523417-new-chinese-q-jet-inspected-evaluated-and-tested-the-complete-results.html

 

[fast 99]

That's news to me didn't know that was copied. I don't like much of the new or reproduction parts out there. If you can rebuild/repair OEM stuff your better off.

Just today bought an emergency brake cable from a national parts chain. Spent the next 1/2 hour trying to get it to fit. Seems none of this crap is any good today. SAD