Afterfiring, tuning, Carb/Timing Gurus?
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,365
Likes: 0
From: Farmers Branch TX
Cruise-In VI Veteran
St. Jude Donor '09
Afterfiring, tuning, Carb/Timing Gurus?
I have just recently gotten my 78 back out on the road after sitting just due to lack of time to drive it and am noticing a few problems. When cruising at part throttle, I am getting afterfiring in the exhaust. I know this is a rich condition problem, but am unsure of where to start adjusting. I have a regular o2 sensor (not wideband) and guage in the car, can I use this to get anywhere near the proper settings, or do I have to have a wideband? Also, could this be an indicator of a cam going flat? My cam/engine have about 4000 miles on them so far. I am not getting any loud noises from the valvetrain, but the car does accelerate choppy sometimes like it isnt firing on all 8. I also have a somewhat surging idle. I have the summit house brand K1103 cam/lifters and 882 casting heads with 2.02/1.6 valves and flat top pistons. Otherwise the engine is basically stock. What time of vaccum numbers should I have at idle with this cam?
Any help will be greatly appreciated. Pictures added!
Chris
Any help will be greatly appreciated. Pictures added!
Chris
Last edited by Today's problem is..; Sep 10, 2007 at 10:53 PM.
Melting Slicks
Joined: Dec 2006
Posts: 2,669
Likes: 3
From: Kingston Ontario
Not sure what vacuum you should have but it sounds as if you have a problem with firing on one or more cylinders. Check all ignition wires, condition of ignition system (cap for cracking, etc.) and I would pull all spark plugs and read their condition.
Sometimes when we troubleshoot we suspect major components instead of the basics. How long have you been considering upgrading the valvetrain components?
Sometimes when we troubleshoot we suspect major components instead of the basics. How long have you been considering upgrading the valvetrain components?
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
I have just recently gotten my 78 back out on the road after sitting just due to lack of time to drive it and am noticing a few problems. When cruising at part throttle, I am getting afterfiring in the exhaust. I know this is a rich condition problem, but am unsure of where to start adjusting. I have a regular o2 sensor (not wideband) and guage in the car, can I use this to get anywhere near the proper settings, or do I have to have a wideband? Also, could this be an indicator of a cam going flat? My cam/engine have about 4000 miles on them so far. I am not getting any loud noises from the valvetrain, but the car does accelerate choppy sometimes like it isnt firing on all 8. I also have a somewhat surging idle. I have the summit house brand K1103 cam/lifters and 882 casting heads with 2.02/1.6 valves and flat top pistons. Otherwise the engine is basically stock. What time of vaccum numbers should I have at idle with this cam?
Any help will be greatly appreciated.
Chris
Any help will be greatly appreciated.
Chris
Stumbling acceleration is a lean condition.
Popping in the exhaust is a lean condition.
If this engine was rebuilt to have better performance it will have better volumetric efficiency. This will tend to lean out the mixture. The '78's were lean to begin with for emission purposes. If it's the stock carburetor it will be even leaner. Start tuning.
BigBlockk
Later.....
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,365
Likes: 0
From: Farmers Branch TX
Cruise-In VI Veteran
St. Jude Donor '09
Surging is a lean condition.
Stumbling acceleration is a lean condition.
Popping in the exhaust is a lean condition.
If this engine was rebuilt to have better performance it will have better volumetric efficiency. This will tend to lean out the mixture. The '78's were lean to begin with for emission purposes. If it's the stock carburetor it will be even leaner. Start tuning.
BigBlockk
Later.....
Stumbling acceleration is a lean condition.
Popping in the exhaust is a lean condition.
If this engine was rebuilt to have better performance it will have better volumetric efficiency. This will tend to lean out the mixture. The '78's were lean to begin with for emission purposes. If it's the stock carburetor it will be even leaner. Start tuning.
BigBlockk
Later.....
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Surging is a lean condition.
Stumbling acceleration is a lean condition.
Popping in the exhaust is a lean condition.
If this engine was rebuilt to have better performance it will have better volumetric efficiency. This will tend to lean out the mixture. The '78's were lean to begin with for emission purposes. If it's the stock carburetor it will be even leaner. Start tuning.
BigBlockk
Later.....
Stumbling acceleration is a lean condition.
Popping in the exhaust is a lean condition.
If this engine was rebuilt to have better performance it will have better volumetric efficiency. This will tend to lean out the mixture. The '78's were lean to begin with for emission purposes. If it's the stock carburetor it will be even leaner. Start tuning.
BigBlockk
Later.....
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,365
Likes: 0
From: Farmers Branch TX
Cruise-In VI Veteran
St. Jude Donor '09
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
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
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,365
Likes: 0
From: Farmers Branch TX
Cruise-In VI Veteran
St. Jude Donor '09
Ok, I pulled the plugs out of it tonight and found that I really need to buy a good digital camera so I can post pictures. Basically what I found was that cylinders 1,3,5,7 look ok, maybe just a tad rich. Cylinders 2&4 look textbook perfect, very slight tan color with the burn spot right in the bend of the ground strap. Cylinders 6&8 however are not so happy. Cylinder 6 has some hardened crud built up around the white part of the electrode and some on the ground strap as well. Cylinder #8 is completly covered in what looks to be hard/wet dark black carbon. I guess its oil because it doesnt smell like fuel. the entire electrode/ground strap are coated in it, no gap in between the strap and electrode at all. Here are the pictures I could take with my phone, they arent very clear but maybe will show some of what I am trying to explain that I see on them. The first two pictures are the #8, next two are #6, and just for reference to what the other plugs look like the last picture is of #2. Also, all my spark plug threads are coated in oil, even the plugs that look fine, what causes this?
Click to make bigger
Click to make bigger
Melting Slicks
Joined: Dec 2006
Posts: 2,669
Likes: 3
From: Kingston Ontario
Oil deposits can be caused by worn valve guide seals. (Cylinder 6/8 check modulator valve for diaphragm rupture/auto transmission.)
If electrodes appear to be destroyed/melting could be pre-ignition typically caused by timing and/or lean fuel mixture.
Install new plugs and see if your misfire is eliminated(this will check cap,rotor and wiring), then troubleshoot further.
If electrodes appear to be destroyed/melting could be pre-ignition typically caused by timing and/or lean fuel mixture.
Install new plugs and see if your misfire is eliminated(this will check cap,rotor and wiring), then troubleshoot further.
Thread Starter
Melting Slicks
Joined: Mar 2004
Posts: 3,365
Likes: 0
From: Farmers Branch TX
Cruise-In VI Veteran
St. Jude Donor '09
Oil deposits can be caused by worn valve guide seals. (Cylinder 6/8 check modulator valve for diaphragm rupture/auto transmission.)
If electrodes appear to be destroyed/melting could be pre-ignition typically caused by timing and/or lean fuel mixture.
Install new plugs and see if your misfire is eliminated(this will check cap,rotor and wiring), then troubleshoot further.
If electrodes appear to be destroyed/melting could be pre-ignition typically caused by timing and/or lean fuel mixture.
Install new plugs and see if your misfire is eliminated(this will check cap,rotor and wiring), then troubleshoot further.