General Category > Quadrajet Carb Talk and Tips

Idle Fuel

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Cliff Ruggles:
If you need more idle fuel to the idle system, reducing the size of the airbleeds will do that.  No way to tell if any further modifications would be needed, without testing it to find out.  If every thing is completely "stock", the stock idle system usually has enough idle fuel and doesn't need to be modified anyplace.....Cliff

1970LS5:
Cliff,
 I finally removed the carb and measured all the relevant components. I have not yet removed the idle tubes to  measure those but they were supplied by you as part of the kit.

Idle down channel: .052
Upper idle air bleed: .070
Lower idle air bleed : .0785
Idle mixture screw holes: .070
No idle bypass air.

"If every thing is completely "stock", the stock idle system usually has enough idle fuel and doesn't need to be modified anyplace."
I agree with this completely which is why this problem is so frustrating.

Thanks again for your help,
Gene

1970LS5:
I finally overcame my anxiety about removing the idle tubes. They measured .035". using the specs in the book as a guide. I increased, the idle tubes size .003" to 0.038". After adjusting the idle speed to factory spec, the rpm no longer increases when the air horn is partially covered. There is a noticeable unburnt hydrocarbon smell at the exhaust, but this may be normal for this year of car. There is no noticeable black smoke at idle. Now off to adjust the mixture and hopefully this project is complete.

Gene

Cliff Ruggles:
All of the big block Chevy carburetors I've seen from that period have a LOT of idle bypass air.  I'm not sure why your throttle body wouldn't have been drilled for it?  I don't have one of those carbs in the shop right now to check it, but remember the last 1970 big block carb that I did has around .120" idle bypass air, and pretty large idle tubes in it......Cliff

1970LS5:
Cliff,
 I looked for idle bypass air in the throttle body with the book open to that photo and it did not appear to be drilled for it. Just for my edification, is .035 a large idle tube?

Thanks,
Gene

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