I have a 77 Corvette with a small block 406 in it and automatic transmission. I have a quadrajet that was rebuilt by an old timer here in the Akron area at Main Carb. I am having a few issues with it. First I'm having a difficult time making the carb idle. The car will not idle at first when I turn it on. The carb guy set my chock plates for me, and I have adjusted them since; however, the car will not idle when I first turn it on in the day (winter, summer, fall, or spring). I have to hold the accelerator pedal and little the engine run at 1500-2000rpms for a few minutes before it will idle. Then it idles at about 700-800rpms but its tough. I also feel the car does not have the power I'd expect from a 406.

The timing is set stock, I tried spraying around with brake cleaner but couldn't find any vac leaks. Any tips from the good QJet guys?

Almost thinking about just buying a JET PERFORMANCE QJET replacement, but I would like to save some money (its scarce around here) if possible.

 

my engine builder(he's been racing professionally since 1955 and is a hot rod hall of fame member) loves qjets. he says they are one of the best thought out carbs ever made. but, he also says that in high h/p engines that it's difficult to get them rich enough at idle. he says that the holleys are "infinitely adjustable", that's why he uses holleys on his big engines. check your idle fuel-air ratio.

 

Send an email to this address: v8fastcars@msn.com and ask if he can send you a copy of his Qjet and timing papers, they will give you what you need. Also you said timing was set to stock. Stock for what? A 350? In truth stock for a 350 SBC is an emissions setting. A simple adjustment on initial and full in timing will assist in overall driveability. AND you should always have the timing where it should be before you start doing any further adjustments to the carb. The paper will give you a starting point on the carb and will outline the adjustments from there. Good Luck.

 

It's not just the big HP engines that have this problem with QJets. A lot of QJets have excessively lean idle circuits for bone stock configurations. It's a battle I've fought with my '76 L82 for years. Runs great when I put my foot in it, but let her drop to idle and it's a non-stop battle to keep it running. I've read up on a variety of solutions to this ... everything from tweak this screw to drill out this passage larger, and most of them I don't trust myself to perform.

 

First, set timing to a performance spec. 36 degrees centrifugal all in by 2800 rpm. Add about 16 degrees of vacuum advance for cruise. I don't drive at idle so I don't set advance at idle.

For the carb:
www.cliffshighperformance.com

 

There's a separate adjustment for the high idle speed- completely independent from the choke plate angle.

 

As Mike Ward said the fast/cold idle speed is adjustable, there is a adjustment screw on the choke side of the carb below the choke housing. I would suggest you tune the carbs a/f mixture for your engine and the gas you are using, you can use the info you can get from Lars or there are tuning links at www.olescarb.com

I hope this helps Henry @ olescarb

 

Good news, the 1977 unit is an excellent carburetor. Bad news is that it was emission calibrated and pretty lean just about everyplace.

If you are using it outside of the original application, it's going to need some help in several areas.

Rebuilding it will not help, unless the rebuild includes recalibrating it, which doesn't sound like what has happened.

I just happen to use a 1977 Q-jet on my own engine. Here's a link to the final race at the Pontiac Tri-Power Nat's in 2006. This run was made with my old engine at 514hp, with a 1977 Q-jet sitting on a stock iron intake manifold.

http://www.youtube.com/watch?v=t9eWgLd0q-U

My new engine is closer to 600hp, uses a stock intake, and the same 1977 Quadrajet. The Quadrajet has never been touched with a sanding roll or grinder, it is just recalibrated exactly for the application.......Cliff

http://www.cliffshighperformance.com/

 

Exactly why I took my Qjet off in 1985 and replaced it with a Holley 4175 650 CFM spreadbore replacement and it ran perfectly for 25 years (simple to custom tune) until last month when I decided to rebuild it which was really easy. Idles perfectly now again, good power, and mileage has leapt back up to where it use to be. The 4175 is and has been an ideal replacement for the stock qjet-simple design, easy to tune, easy to rebuild, reliable-25 years!.

 

"but, he also says that in high h/p engines that it's difficult to get them rich enough at idle. he says that the holleys are "infinitely adjustable", that's why he uses holleys on his big engines. check your idle fuel-air ratio."

This simply comes from lack of knowledge about how to set up the q-jets idle system for larger than stock camshafts. The Q-jet actually has an EXCELLENT idle fuel system, and after 1967 they are equipped with a built in idle bypass air system to keep the throttle angle low enough to keep the sensitive main system off line at idle speed. No drilling of the throttle plates is required, and they can be set up to idle perfectly on engines with huge cams and very little vacuum at idle speed.

One of my customers has a 67 GTO that runs 9.90's at 135mph. He has a Quick Fuel custom carb at his disposal and used to use it before we sent him a custom full race Q-jet for his engine. He ended up using and prefers the q-jet because it idles better and is easier to stage the car when he comes up on the throttle (foot braking).

He tells us that there is not measurable difference in ET or MPH between the Holley and the Q-jet at the track. He actually uses an adjustable air door on his q-jet to keep the car from going quicker than 9.90's, so there is no need for a throttle stop either......Cliff

 

As stated above, get your timing squared away first. 36 degrees (with the vacuum advance disconnected) at 3000 RPM will put you real close to the specs needed for performance. Let the 'initial' timing fall where it falls, which be around 14-18~ degrees.

A couple of questions:

1) What vacuum level (hg) do you have at idle?
2) I'm a bit confused. Are you saying she idles ok after warm-up?
3) Are you running manifold or ported vacuum to the vacuum advance? (Go with manifold for much improved idle charactoristics and other benefits).

I run a Q-Jet on a 450 HP 406 and wouldn't swap it for anything.

 

I have a great Q jet on my 68 vette....327-350hp...I once replaced the carb with a holly replacement carb...it was like removing 2-3 pistons...awful One week and I put the quad back on. The new QJ on my vette was put on in 84, new from Chevy and has never been touched. When I first start the car, I press the gas to the floor, that sets the choke, as soon as the car starts, the choke pull off opens the butterfly about 1/4 of an inch...and heat eventually heats the spring on the manifold and moves the butterfly to wide open...mine idles very nice...the same thing happpens on my 49 ford with 305 chevy in it with a 72 carb...day one on the 49 ford, the rod on the back of the QJ on the pass side that keeps the back barrells closed until it heats up, was hanging up...causing some bad running and missing idleing...of course, it was hard to hear as the 49... only had headers and no exhaust...first day on the road after rebuild...but we found and fixed it...then it was fine.

Take the cleaner off your car and see if it does what I said it needs to do to operate and idle correctly when you first crank it.

My 49 ford carb was built 3 times by me and my mechanic before I was happy...not because it needed building...I was trying for 24 MPG at 70-80 mph with a turbo 350 trans...not even an overdrive...third combo of jets and needles and we got it all...smoooth no surging...lots of power and great reponse and 24 mpg.....that was 2-3 years ago...no problems ever with it.

The quad on my vette makes it a monster...when it cuts in...no better sound or push.

Jimmy in NC

 

You have a PM

 

Update: I tried adjusting the carb some more, adjusting timing, messing with the choke, nothing helped.

I'm kind of on a time constraint with the car, as it needs to be shipped to Denmark in early Feb. I went to Summit Racing and bought a Summit Racing Quadrajet 210200. The descriptions says for up to 400cubic inches. It had a slightly different vac. config setup then my carb, but everything went on fine after I studied the carb for a few minutes. Put the key in, cranked it for about 10 seconds (fuel bowl was dry), and it turned on and idled without me having to do anything to it. I let it run until it warmed up, set the idle to where I liked, and then adjusted my timing back to stock (Has to be at stock to pass Denmark emissions testing, owner can adjust after it passes). I drove the car a little up and down the road, felt good. I'll get some fine tuning done soon after I get my horn fixed.

 

I hope they never start checking the timing here.

 

A 406 powered vette in Denmark? Won't you run out of road before you get up to speed?
sorry just joking. Its is a nice place.