TA olds 403 carb needs tuning help

[EvelBist]

17059253 carb number. Rebuilt engine and carb. Has a Comp Cams Xtreme 208 int./212 exh, and mild port and bowl work. Stock compression and headers + true duals.

I've drilled the idle mix holes to .090 and added bypass air to both sides. Float is set at 1/4". I'm at 6500' elevation. Otherwise the carb is stock with stock jets, needles and secondary.

Starts and runs fine.

However, it has to idle at 1100 so it stays alive when in Drive at 750. Slightly rough idle, annoying but acceptable.

I'd like to get it down to stock idle in Park - 900 and 600 in Drive.

Do I need different Idle Down tubes or jetting? What could I do to improve the idle quality and slow it down?

[Shark Racer]

It sounds like you're running lean at idle. With that cam you should be able to get it to stay alive at 600 RPM. I'm running a 216/216-110 and it will idle smoothly at 700 rpm in neutral once it's warmed up. (manual)

How far backed out are you on the idle mixture screws? If it's much greater than 3 turns, then going up a size on the tube and down-channel may help. What size are the down channel restrictions and jets now?

[EvelBist]

How far backed out are you on the idle mixture screws? If it's much greater than 3 turns, then going up a size on the tube and down-channel may help. What size are the down channel restrictions and jets now?

I really wish I knew the sizes. But I am certain its stock. From poking around on the net, I think they are #75??? jets? I dont know the tube dimensions.

Anyhow, it runs better if I turn out the screws a lot farther than 3, but it doesnt seem to get any better after about 3 turns out ... maybe i need more fuel at idle?

[Cliff Ruggles]

Still using the hot-air choke?  It has a controlled vacuum leak to the housing, so less bypass air is typically needed.

Symptoms indicate not enough idle fuel to the mixture screws.  I'd open up the idle tubes a few thousanths, and idle down channel restrictions as well.....Cliff

[EvelBist]

Thanks Cliff. It still has the hot air choke. I drilled the bypass holes because of the elevation here, is the carb still usable? I'd really like to get that carb working right. It has the stock jets and tubes, so can you give me the numbers of the parts I need so I can order them? Do I need to drill some passages larger?

[Ethan1]

I really wish I knew the sizes. But I am certain its stock. From poking around on the net, I think they are #75 jets? I dont know the tube dimensions.

Anyhow, it runs better if I turn out the screws a lot farther than 3, but it doesnt seem to get any better after about 3 turns out ... maybe i need more fuel at idle?

 Stock jets for your carb were #73.

[Cliff Ruggles]

Idle bypass air is used to keep the throttle angle low with big cams/low vacuum at idle.  It adds additional air (controlled vacuum leak) and the idle system needs to have enough fuel to the mixture screws to compensate.  I would recommend larger idle tubes and opening up the down channel restrictions for what you are doing.  This will help the situation.  The bypass air holes may be fine as is.  If you can't get the idle speed low enough, they will need to be blocked, which is very easy to do.....Cliff

[EvelBist]

Thanks for the jet size Ethan 

How can I tell what size idle tubes I currently have? I can open up the down channel restrictions using the procedure in the book - right? How much bigger? Is there a recipe I might follow in the book? I am only looking to make the car run smoothly for cruising, not a tire shredder 

[EvelBist]

Does anyone know what those stock idle tube sizes are or where they can be looked up?

[Ethan1]

 Usually you have to measure the size of the orifice, but, most stock Qjet's came with .031-.033 sized idle tubes. Unless they have been modified already.

[EvelBist]

Ethan you have my vote for most helpful!

[Ethan1]

Ethan you have my vote for most helpful!

 Thanks, EvelBist. I try to help out when I can. Most Qjets I have had, did have pretty much in that range of idle tube orifice sizes.