Fuel mixture is lean after headers installed.
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,877
Likes: 13
From: Doha
St. Jude Donor '08
Fuel mixture is lean after headers installed.
I checked my A/F ratios tonight using LM-2 after I finished installing the hooker super comps, I've noticed that the fuel is so lean when Idle ,about 18-19, and 14.5-15 when cruising and it goes down to 13 when WOT, I know I have to do something with the idle mixture screws in the carb, but what do you think about the cruise and WOT numbers? do I have to re-jet the carb?
my engine is almost stock LS5 except 9:1 CR and mild cam.
my engine is almost stock LS5 except 9:1 CR and mild cam.
Safety Car
Joined: Jan 2008
Posts: 3,795
Likes: 435
From: Aarhus, Denmark
I had the same problem with my 11:1 L46 1969 engine (std 350/350 carb)after I installed headers + 2½" exhaust.
I've incresed my jets and needles.
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,877
Likes: 13
From: Doha
St. Jude Donor '08
I thought the LM-2 is quite accurate, thats why I paid more for it!
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,877
Likes: 13
From: Doha
St. Jude Donor '08
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
the lm2 is not lying, when I start my car from cold its runs very lean, ie between 16 to 18, then richens up once it gets warmer
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Where is your sensor located? Could it be positioned too far downstream which might allow the hotter headers to be burning off the excess before it reaches the sensor? I don't see how a change of headers has any actual effect on the mixture of the carb.
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,877
Likes: 13
From: Doha
St. Jude Donor '08
its located right after the collector, I welded it in the collector to pipe flange.
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
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Most of the time after headers are installed it will run leaner. This is due to the fact that the headers will scavenge the combustion chamber better than the exhaust manifolds of burnt gases. Remember the engine is nothing but an air pump. More air in and out makes HP and more efficient. More air will require more fuel hence the increase in jet size. Most people don't run a wide band O2 sensor for testing the AF ratio so they just run it as is after a header install. The wide band O2 sensor can be mounted down stream a little because it is heated so the proximity of it to the cylinder head is not a crucial as it is with a non heated narrow band O2 sensor.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
Hamad...just play with the idle mix screws first. I would be willing to bet you have a vacuum leak somewhere.....that's IF it's really 18-1 afr...which I doubt... Did you do a heater Calibration and free air calibration? Does the lm2 show 20.9% oxygen when the meter is on and engine off?(and not run for atleast a few hours...)
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,877
Likes: 13
From: Doha
St. Jude Donor '08
Hamad...just play with the idle mix screws first. I would be willing to bet you have a vacuum leak somewhere.....that's IF it's really 18-1 afr...which I doubt... Did you do a heater Calibration and free air calibration? Does the lm2 show 20.9% oxygen when the meter is on and engine off?(and not run for atleast a few hours...)
the carb was rebuilt by Lars so I'm sure everything inside is correct, except the vacuum leak which was my mistake (the carb was not tight to the intake flange).
my jets are 77s and rods are 48s, I'll try to jump 2 numbers to 79 and see what will happen.
I need to tweak it well before I go to the dyno shop, the guys there know nothing about carbs!, they know how to increase fuel using laptops and kept their hands clean and they don't smell gas when they go home for their wives!
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
Hamad.... If you screw the idle mix screws all the way in, does the engine die? If not, the you still have a vacuum leak. Ofcourse with these cars the leak could be anywhere. I would disconnect the vacuum hose that feeds the vac reservoir/headlights/hvac so that eliminates those possibilities... Just for diagnostic purposes....
I wouldn't start messing with the jets/rods yet until you make sure you have no vac leaks affecting the idle AFR.
At wot what was the afr? You said 13 but.....13 what? 13.0? 13.9? Your jetting sounds pretty close out of the box but i would prefer your wot afr around 12.8 or so.......but 13.0 would probably be fine since you live on the outskirts of hell....(during the summer anyway)
I wouldn't start messing with the jets/rods yet until you make sure you have no vac leaks affecting the idle AFR.
At wot what was the afr? You said 13 but.....13 what? 13.0? 13.9? Your jetting sounds pretty close out of the box but i would prefer your wot afr around 12.8 or so.......but 13.0 would probably be fine since you live on the outskirts of hell....(during the summer anyway)
Tech Contributor
Joined: Aug 1999
Posts: 14,382
Likes: 6,401
From: At my Bar drinking and wrenching in Lafayette Colorado
If A/F was correct prior to the header installation, the installation of headers will typically require a 2-jet increase in order to maintain fuel ratio. The idle mixture screws will always need to be richened when switching to headers from cast manifolds, and often the IFR's will need to be opened up to flow adequate fuel at idle and into the transition circuit.
On your Q-Jet, if you're going to be running 79 or 80 jet sizes, fuel flow at WOT will be high enough that the standard needle seat will restrict fuel flow and cause a lean-out at WOT. Make sure you obtain and install one of the larger, high-flow seats for that application.
\\
Lars
On your Q-Jet, if you're going to be running 79 or 80 jet sizes, fuel flow at WOT will be high enough that the standard needle seat will restrict fuel flow and cause a lean-out at WOT. Make sure you obtain and install one of the larger, high-flow seats for that application.
\\
Lars
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,877
Likes: 13
From: Doha
St. Jude Donor '08
Hamad.... If you screw the idle mix screws all the way in, does the engine die? If not, the you still have a vacuum leak. Ofcourse with these cars the leak could be anywhere. I would disconnect the vacuum hose that feeds the vac reservoir/headlights/hvac so that eliminates those possibilities... Just for diagnostic purposes....
I wouldn't start messing with the jets/rods yet until you make sure you have no vac leaks affecting the idle AFR.
At wot what was the afr? You said 13 but.....13 what? 13.0? 13.9? Your jetting sounds pretty close out of the box but i would prefer your wot afr around 12.8 or so.......but 13.0 would probably be fine since you live on the outskirts of hell....(during the summer anyway)
I wouldn't start messing with the jets/rods yet until you make sure you have no vac leaks affecting the idle AFR.
At wot what was the afr? You said 13 but.....13 what? 13.0? 13.9? Your jetting sounds pretty close out of the box but i would prefer your wot afr around 12.8 or so.......but 13.0 would probably be fine since you live on the outskirts of hell....(during the summer anyway)
Idle : 16.5
Cruise : 14.6
WOT : 13.5
these numbers are subject to 0.1 +/-
I didn't try to close the needles all the way in, I will try tomorrow, so the engine should tall when I do this? .. and if it continues to run that means I have air leak, right?
Lars : Thanks for the tips, I just ordered a 79 jets and 48 rods (prima
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,877
Likes: 13
From: Doha
St. Jude Donor '08
If A/F was correct prior to the header installation, the installation of headers will typically require a 2-jet increase in order to maintain fuel ratio. The idle mixture screws will always need to be richened when switching to headers from cast manifolds, and often the IFR's will need to be opened up to flow adequate fuel at idle and into the transition circuit.
On your Q-Jet, if you're going to be running 79 or 80 jet sizes, fuel flow at WOT will be high enough that the standard needle seat will restrict fuel flow and cause a lean-out at WOT. Make sure you obtain and install one of the larger, high-flow seats for that application.
\\
Lars
On your Q-Jet, if you're going to be running 79 or 80 jet sizes, fuel flow at WOT will be high enough that the standard needle seat will restrict fuel flow and cause a lean-out at WOT. Make sure you obtain and install one of the larger, high-flow seats for that application.
\\
Lars
Thread Starter
Melting Slicks
Joined: Jan 2003
Posts: 2,877
Likes: 13
From: Doha
St. Jude Donor '08
OK, the PCV connection was leaking and that brought the A/F up to 16.5, I plugged the carb side and it went down to 13.5 right away.
Also today I tried to screw the mixture needles all the way in and I did manage to stall the engine and eventually shut it off, so I assume the carb is leak free.
But I still don't get any change of A/F ratio when I play with the mixture needles/screws! .. why is that???
Also today I tried to screw the mixture needles all the way in and I did manage to stall the engine and eventually shut it off, so I assume the carb is leak free.
But I still don't get any change of A/F ratio when I play with the mixture needles/screws! .. why is that???
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
Not sure why idle mix adjustments are not affecting afr but right now I would concentrate on finding your mega vacuum leak.....going from 16-1 down to 13.5 just from plugging a vac port is huge...
You may never completely get the vac system air tight but you should get it closer then that...ultimately you want to adjust your idle mix with a vac gauge connected. Adjust the screws until you get the highest rpm/vac readings....then lower your idle to the desired rpm via the idle speed screw. Expect your idle afr to be around 14.5 or so after the engine is at full temp. I would like your cruise afr in the mid 13 range....may have to change/raise the needles to get it....
Just take it one step at a time. Find the major vac leak first....prolly the headlight cannisters or valves...pcv valve or brake booster...
You may never completely get the vac system air tight but you should get it closer then that...ultimately you want to adjust your idle mix with a vac gauge connected. Adjust the screws until you get the highest rpm/vac readings....then lower your idle to the desired rpm via the idle speed screw. Expect your idle afr to be around 14.5 or so after the engine is at full temp. I would like your cruise afr in the mid 13 range....may have to change/raise the needles to get it....
Just take it one step at a time. Find the major vac leak first....prolly the headlight cannisters or valves...pcv valve or brake booster...
