Author Topic: Problem with sloppy nozzle drip  (Read 3074 times)

Offline Knightvd

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Problem with sloppy nozzle drip
« on: August 09, 2016, 12:34:22 PM »
I have a 1970 Corvette with stock 300 horse 350.  The engine is all stock with 10.25:1 compression ratio and a Quadrajet.  The carburetor is not original, however.  It has not been altered and I assume was purchased as a stock replacement.  This is one of those carbs with the large main air bleeds (.125). I don't have the idle air bleed sizes at the moment  The idle tubes are .040 and the channel restrictions are .060. The main jets are .076 and the primary rods are .044 at the last step with .026 tips.
The needle and seat, float and seals are new. Float is set at exactly 1/4".The carb has bypass air.  Initial timing is set at 10deg advanced and ported vacuum advance.
The problem is with nozzle drip, just off idle as the transition slots are fully exposed to engine vacuum. This occurs from about 1200 RPM as a drip and increases to a splurge to 1500 RPMs then turns into a proper emulsified stream above 1500.
The engine idles fine at 875 RPM and the idle screws are effective at adjusting the air/fuel feed.
What would you suggest to remedy this problem?
I thought of decreasing the idle tubes to .037 but without also reducing the channel restrictions will this lean the idle mixture too much?
Is there a best way to resize the channel restrictions ?
I know in your book you suggest reducing the air bleeds and using smaller jets on these early Chevy carbs but I would rather not have to go through all that.


Offline 77cruiser

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Re: Problem with sloppy nozzle drip
« Reply #1 on: August 09, 2016, 01:39:46 PM »
I don't think it's an issue as long as it doesn't do it at idle.
Jim

Offline Knightvd

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Re: Problem with sloppy nozzle drip
« Reply #2 on: August 09, 2016, 02:12:20 PM »
Sorry, I forgot to mention that when dripping in this RPM range it misses and runs rich.

Offline 77cruiser

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Re: Problem with sloppy nozzle drip
« Reply #3 on: August 09, 2016, 02:54:18 PM »
I've been tapping the DCR to 8-32 & drilling brass set screws to what I need.
Jim

Offline Cliff Ruggles

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Re: Problem with sloppy nozzle drip
« Reply #4 on: August 10, 2016, 03:34:32 AM »
What is the carburetor part number?

What size are the upper and lower idle airbleeds?

Offline Knightvd

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Re: Problem with sloppy nozzle drip
« Reply #5 on: August 10, 2016, 05:17:09 AM »
Carb part number 7041211

Upper idle air bleed .093, Lower idle air bleed .070

Thanks

Offline 77cruiser

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Re: Problem with sloppy nozzle drip
« Reply #6 on: August 10, 2016, 05:47:41 AM »
Have you tried lower the float?
Jim

Offline Knightvd

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Re: Problem with sloppy nozzle drip
« Reply #7 on: August 10, 2016, 08:42:54 AM »
Thanks for the suggestion.  I was thinking about it but want to see what Cliff says.

Offline Knightvd

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Re: Problem with sloppy nozzle drip
« Reply #8 on: August 10, 2016, 02:15:36 PM »
Correction on the idle channel restriction; it is .055 not .060.

Offline Cliff Ruggles

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Re: Problem with sloppy nozzle drip
« Reply #9 on: August 12, 2016, 03:00:03 AM »
No big flags going up with the basic set-up.

I don't like the Chevy carbs with the HUGE upper idle airbleeds.  They seem to make the engine skip and run rough at idle and just off idle.  I prefer to re-size the upper bleeds and use much smaller idle tubes and DCR's.  I've even relocated the upper IAB's to the airhorn with good success on those units.   Some of the earlier Chevy carbs were actually set up that way from the factory.  The huge upper idle bleeds were common in the early 1970 Chevy carbs, probably an attempt at cleaning up the idle for emissions, but they never work well and in later years they quit using that set-up......Cliff

Offline Knightvd

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Re: Problem with sloppy nozzle drip
« Reply #10 on: August 12, 2016, 04:09:34 AM »
I'll go with recipe 1 and take a look at blocking those IABs and drilling them in the air horn.
Thanks!