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Old 09-19-2012, 08:58 PM
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Fuel haze and rich idle problem

Hey everybody,

I recently installed a QFT 650 cfm carb on my 355 chevy and im having some idle issues. The carb is a chokeless race carb with 4 corner idle.
The engine:

355 small block chevy, aluminum angle plug heads
Victor jr intake
Comp 280R solid roller cam 242/248@.050
11:1 compression,
12deg base timing with no vac advance.

the question i have, when the engine is warmed up idling it lets off a grey fuel haze and smells really rich. the AF meter that i have hooked up shows about 11.5:1 A/F. Neutral idle is 1100 rpm and when dropped in gear against a 3200 stall converter it drops down to about 800 rpm.

When driving the A/f stays rich, and wot wit 10:1 A/f.

when sitting at lights in gear or neutral their is always a haze of grey around the truck. So my question is, what tune is off, or how do you tune the idle of a 4 corner carb?

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Old 09-19-2012, 09:24 PM
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whats you vacuum lookin like? power valve may need to be a 4.5 instead of the 6. also if you are rich while driving switch to smaller jets. or larger high speed air bleeds. jets first. get the afr right then tinker with the timing. you can get about 40hp in timing, seen it many times. i have a 355 with a thumpr cam and 78 primary jets and 80 in the rear makes a slight lean condition on the dyno at wot. no lean pop while driving normal tho. just a thought
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Old 09-19-2012, 09:32 PM
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try fully closing the rear idle screws too. if the throttle plates are barely cracked open back there you shouldnt need them at idle. my quick fuel is like that. 750hp conversion, billet metering blocks. rear idle screws are shut in on mine. idles fine. i had the haze and burning eyes, power valve for me but my idle vacuum was 9Hg. so the valve was open at idle. changed the pv and voila! hope its that simple for you. keep it simple.
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Old 09-19-2012, 09:36 PM
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Ok that makes sense. the power valve is stock, and in drive it makes about 9 or so, neutral idle is around 11 inches not sure of the jets but i think they are 68 and 74 from factory
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Old 09-19-2012, 09:36 PM
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12 degrees timing at idle might not be enough? what is the idle speed?I run about 16-17 at idle.You are using a fairly small carb?
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Old 09-19-2012, 09:38 PM
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Idle speed in neutral is around 1100 and drops to 850 in gear. i was told the carb would be fine for driving around the street and such
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Old 09-19-2012, 09:49 PM
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power valve,add timing at idle, with total at 34 ish.then change jets,then air bleeds.650 is ok
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Old 09-20-2012, 09:01 PM
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Well guys i took the carb apart to see what i have for jets and such and this is what i found.

primary main jet .068
secondairy jet .076
idle air bleeds 73/73W
hi speed bleed 31/31
power valve 4.5

Should i go to a lower vac power valve? any suggestions from here?
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Old 09-20-2012, 09:09 PM
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At your elevation I'd run 20-22 degrees initial with 12 degrees in the dizzy all in by 3200. THen play with the carb.
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Old 09-20-2012, 09:16 PM
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Quote:
Originally Posted by RWENUTS View Post
At your elevation I'd run 20-22 degrees initial with 12 degrees in the dizzy all in by 3200. THen play with the carb.
With the vac advance hooked up?
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Old 09-20-2012, 09:26 PM
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why are you running a vacuum advance?
bump initial to 17 or 18 with out vacuum advance.make sure your total timing is less than 36,with out vacuum advance. check float levels and replace that PV just because you have the carb apart. use a 4.5 or 3.5.take 3 jet sizes out of primaries if its still super rich.
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Old 09-21-2012, 02:08 AM
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make the mech advacne curve 10deg.

This will allow 26deg inital at idle and 36deg at max mech advance.
The timing must not drop off in gear.

Vac advance is a separate thing. Use ported advance. Should add 10-12deg max at hi vac cruise.

The increased 26deg base inital advance at idle will clean up the "rich idle" and allow you to readjust the pri and sec throttle position.
at idle.

12 deg at idle is not near enough for a cam like that. You can make the curve limit 10deg
or simply lock out the emch advance and set the locked fixed timing to 34-36deg.

Set the idle with a manifold vacuum gauge.
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