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[GMCnet] Re: Exhaust bang bang bang [message #366789 is a reply to message #366788] Wed, 15 September 2021 09:13 Go to previous messageGo to previous message
James Hupy is currently offline  James Hupy   United States
Messages: 6806
Registered: May 2010
Karma:
Senior Member
Matt, it takes many years of setting "lean drop" to do it without a tach
and vacuum gage. (TacVac).
It requires a fully warmed- up engine, and 8 cylinders that are within
10% compression pressure of each other, really tuned to factory ignition
specs, good wires, plugs, rotor, cap, etc.
In other words, a textbook perfect engine. If anything has been
monkeyed with, like pcv, or vacuum advance, or air cleaner, etc. Then all
bets are off.
Then, the best a person can hope for is a low CO reading at the
tailpipe, and a somewhat steady state idle at 650 rpm + or - 100 rpm either
way.
I try to set idle speed so that no dieseling or "running on" happens
on a hot engine when the ignition is switched off with the transmission in
neutral, and the A/C system off as well.
Real world stuff applies here, most of these GMC'S are high mileage
with leaky headers, etc., etc. Makes idle tuning a challenge for sure.
Then you take in humidity, altitude, barometric pressure, etc.
Challenge for sure.
Jim Hupy
Salem, Oregon

On Wed, Sep 15, 2021, 6:49 AM Matt Colie wrote:

> boybach wrote on Tue, 14 September 2021 20:18
>> JohnL455 wrote on Tue, 14 September 2021 13:27
>>> Mine will “burble” during long engine braking, but I have my exact GM
> copy exhaust system installed so it is barely noticeable.
>>> I suspect your idle mixture may be adding extra fuel causing this to
> be more pronounced. If you have 3” stainless this may exaggerate it.
>>> Correct mixture is balanced best idle just at the lean drop edge.
> “Emissions” setting would be 25 RPM lean drop per side to get 50 RPM lean
>>> drop.
>>
>> I have no way to check or test emissions, should I adjust the
> carburetor? How do I do that?
>>
>> BTW, I have no idea what this statement means :d "Correct mixture is
> balanced best idle just at the lean drop edge."
>>
>>
>> Larry
>
> While John's description is good, it does not make two points.
>
> There are two idle screws in the carburetor base that must be balanced. I
> find the "around the corner" special screw driver and a mirror to be
> essential.
>
> The lean drop edge is when the mixture is lean and if it goes any leaner
> at all, there is a precipitous drop in the idle speed. Without a tachometer
> and possibly a vacuum gauge, I don't know how one finds this. I did know
> one man that could, but he passed years back.
>
> Matt
> --
> Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL,
> GMCES
> Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum
> Brakes with Applied Control Arms
> SE Michigan - Near DTW - Twixt A2 and Detroit
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