modifications for a good idle

[timothy47129]

Let me start by listing my engine set up.
chevy 350 40 over, 10:1 compression ,big valve heads,elgin cam  Lift: .429/.429, Adv. Duration: 295/295, Duration @ .050": 220/220, 110 LC,dual plane intake,and headers. 12" of vaccum at idle also this is in a 1980 full size chevy truck

I rebuilt the factory q-jet to replace my junk holley and now i cant get it to even run at idle untill around 1200rpm.From what I have read it is because the throttle plates are open so far just to get it to kinda idle its just dripping the fuel in and causing a real rough idle.What kind of modifications do I need to do to get this thing to run? There arent any carb shops around that can do it.So im stuck trying on my own.I dont really have the money to send it off to someone.Im sure most of my problem is using an emissions carb on a non emissions application

thanks,
Tim

[1985C10]

I'll leave it to Cliff to advise you on the exact modifications, but anyone trying modify a Q-jet on his own should have a copy of Cliff's book.  It is very specific on what a street performance Q-jet needs to run its best, making it the best tuning tool you can buy.

And while I doubt very much it is the source of your idle issue, that cam sounds like it may be a restricted lift cam, meaning it is designed to appease rules for certain classes of dirt oval cars.  A good cam with 220 duration should have about .455" - .465" lift with 1.5:1 rockers.  A higher intensity cam alone should help vacuum and power.

[1985C10]

that cam sounds like it may be a restricted lift cam, meaning it is designed to appease rules for certain classes of dirt oval cars.

Scratch that, I was thinking .425" lift.  But it is a very low intensity cam by today's standards (or even yesterday's).

[omaha]

I agree with 1985C10. If those are the numbers for that cam, that would be a strange cam. Even if you got the numbers mixed up and the lift is .492 (instead of the ".429"). It's too much of a difference from the advertised duration figure to the duration at .050. I would think you could get that lift with an advertised duration in the ~286 zone. That duration is just gonna kill the vacuum and the idle. Is it a drag truck or something like that. Ok.....these are just my opinions. You did not say what you are using the truck for....Is it stricktly drag or more of a street strip vehicle.
     Anyway, back to the question. Wth that cam you are gonna need more fuel at idle. These mods are in the book. I will tell you that it involves enlarging various parts of the idle, fuel metering orifices. If it is the original carb, the idle is setup very lean. These mods are not extremely hard to accomplish if you have the patience and the knowledge and some mechanical ability. Get the book. Enjoy!

[timothy47129]

Im building a street strip truck.And I am going to order the book asap.From what i have gathered from research i have the throttle blades open too far to get it to idle so its not running on the idle circuit.so i need more air so the throttle blades will be at the proper position.Is there a way to do that without drilling holes in the throttle blades.And it is a 1980 quadrajet



thanks

[Cliff Ruggles]

Tim, do NOT drill the throttle plates!

That carb uses a much better idle bypass air system.  Our book describes how it works, and provides specific modifations to the idle system for using big cams and having low vacuum at idle, etc.

What cylinder heads are you using on the engine, casting number, manufacturer, etc?

Did you use one of our HP kits for the rebuild?  They contain the high flow Viton needle/seat assembly, secondary cam/spring (manditory to replace IMHO), idle tubes (drilled out to increase idle fuel for the big cam), secondary cam/spring, new small float, complete HP accl pump assembly, power piston springs (for the big cam/low vacuum), and everything else needed for a complete/correct high performance carb build........Cliff