Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

TA olds 403 carb needs tuning help

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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:

--- Quote from: Shark Racer on February 08, 2013, 10:54:20 AM ---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?

--- End quote ---

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?

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