Air/Fuel idle mixture - Holley
Thread Starter
Pro
Joined: Aug 2001
Posts: 683
Likes: 0
From: mississauga ON
Air/Fuel idle mixture - Holley
I have a Holley 4175 on my base motor 71. The carb has been
performing well, and the car running great. The other day I noticed
that the installation manual suggests using a Vacuum gauge to
set idle mixture. Out of curiousity I hook up the gauge, and play
with the idle mixture screws. I was able to get the vacuum from
about 15-16" at idle to about 20" at idle....and it actually
appeared to idle a little smoother. Today I start the car (cold
start) and the car was a little hard to start. It would usually
fire up 1st try, however today it took 3-4 tries. (it still fires up
on first try on a restart) Have I made the idle mixture too lean??
Any thoughts on this?
performing well, and the car running great. The other day I noticed
that the installation manual suggests using a Vacuum gauge to
set idle mixture. Out of curiousity I hook up the gauge, and play
with the idle mixture screws. I was able to get the vacuum from
about 15-16" at idle to about 20" at idle....and it actually
appeared to idle a little smoother. Today I start the car (cold
start) and the car was a little hard to start. It would usually
fire up 1st try, however today it took 3-4 tries. (it still fires up
on first try on a restart) Have I made the idle mixture too lean??
Any thoughts on this?
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
20" hg would be about right for a 350/270 in factory spec with timing around 10-14 degrees.
You may be too lean on the idle. The trick is to adjust that reading for highest vacuum on the rich side, rather than the lean side. Turn the screws out until you see the needle start to drop then slowly screw them back in to the point where you see the needle recover to the peak and go no more. You should see about a quarter to half a turn where there is no affect in the vacuum. You can also go a bit below peak vacuum to fatten up the idle. Having the "optimal" setting does no good if the car runs bad.
You may be too lean on the idle. The trick is to adjust that reading for highest vacuum on the rich side, rather than the lean side. Turn the screws out until you see the needle start to drop then slowly screw them back in to the point where you see the needle recover to the peak and go no more. You should see about a quarter to half a turn where there is no affect in the vacuum. You can also go a bit below peak vacuum to fatten up the idle. Having the "optimal" setting does no good if the car runs bad.
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
Originally Posted by comp
richen it a little and let us know 
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
i'm sure you increased the vacuum by leaning it out....and i'm sure the lean mixture makes cold starting harder. its all a balance.....how does the car smell? and do you mind the muscle car smell? or better yet put does the significant other mind the muscle car smell?
Thread Starter
Pro
Joined: Aug 2001
Posts: 683
Likes: 0
From: mississauga ON
I pulled the cover off the carb intending to richen
the mixture a little. However, I quickly noticed that
I had not reconnected the choke. I plugged it in
and the car fired right up (cold). I do like
the suggestion to richen it a little though, and
will give it try.
the mixture a little. However, I quickly noticed that
I had not reconnected the choke. I plugged it in
and the car fired right up (cold). I do like
the suggestion to richen it a little though, and
will give it try.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
