Engine missing after couple of mods
Thread Starter
Intermediate
Joined: Mar 2020
Posts: 29
Likes: 6
Engine missing after couple of mods
Hello all....wondering if anyone can give me some Ideas for my engine missing after smog pump removal and header/side pipe installation. 1974 C3, 350, auto, a/c, power brakes, 186,000 on car, 60,000 on engine. I think everything is stock mechanically, except for my mods, smog pump removal and headers/side pipes.
First drive after removal of smog and header/side pipe installation....ran great for about 5 miles, then started missing really bad....didn't think I could make it home but would run if I babied it.
Second drive......ran good until it gets warm/hot then starts missing and sputtering. It will run ok if you baby it. Idles great, starts right up....stock quadrajet carb.
My thoughts are:
1. Timing
2. Points
3. jets
Help me out.....this is my first corvette and i will have to say, they are a different car.
First drive after removal of smog and header/side pipe installation....ran great for about 5 miles, then started missing really bad....didn't think I could make it home but would run if I babied it.
Second drive......ran good until it gets warm/hot then starts missing and sputtering. It will run ok if you baby it. Idles great, starts right up....stock quadrajet carb.
My thoughts are:
1. Timing
2. Points
3. jets
Help me out.....this is my first corvette and i will have to say, they are a different car.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
They are a different car but the engine was basically the same as anything else back then. No mystery to it.
But yes, Timing, points and maybe jets.......
Set your dwell first to 28 to 30 degrees.....you will need to find a dwell tach to do this. Borrow or buy a used one off E-bay......
Then set timing for 36 degrees with the vacuum advance unplugged.
It is highly unlikely that you need to change jets.......Q-jets have a pretty wide range that is self adjusting via vacuum on the primary and airflow on the secondaries.
Take the plugs out and post a pic.
Jebby
But yes, Timing, points and maybe jets.......
Set your dwell first to 28 to 30 degrees.....you will need to find a dwell tach to do this. Borrow or buy a used one off E-bay......
Then set timing for 36 degrees with the vacuum advance unplugged.
It is highly unlikely that you need to change jets.......Q-jets have a pretty wide range that is self adjusting via vacuum on the primary and airflow on the secondaries.
Take the plugs out and post a pic.
Jebby
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
I agree with this to a point....I think it leaned it out enough that it brought other issues to light.
You need to power tune it before you change the jets anyway to get a good baseline so I would go through all of the checks first. It's cheap and is good practice, then you know for sure where you are at.
Jebby
You need to power tune it before you change the jets anyway to get a good baseline so I would go through all of the checks first. It's cheap and is good practice, then you know for sure where you are at.
Jebby
Thread Starter
Intermediate
Joined: Mar 2020
Posts: 29
Likes: 6
Thanks guys....wealth of knowledge on this board....I'll look into the dwell tach and adjust the timing....hoping I don't need to change out the jets but not that big of a deal. Hopefully someone will chime in on the jet sizes.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
when you do something and something else goes wrong, you just did it. go over the plug wires that hadda come off and go back. point gap degrades slowly. nothing wrong with checking timing-dwell, etc, but it is your plug wires most likely.
Last edited by derekderek; May 10, 2020 at 08:02 AM.
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Le Mans Master
Joined: Aug 2017
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From: Cool Northern Michigan
Yup. Pay particular attention to numbers 5 & 7 wire routing. Because of SBC firing order, it is somewhat common for those two to "cross-fire".
Make sure those two are separated as far as possible in the looms.
Make sure those two are separated as far as possible in the looms.
Last edited by HeadsU.P.; May 10, 2020 at 09:12 AM.
Race Director
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2025 C3 of the Year Finalist - Modified
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Cruise-In VIII Veteran
Gotta wonder, all the stock heat shields would have been removed to install headers.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Would no longer fit.
As most people find out, you have to bite the bullet and get a set of plugwires that can handle the exposure to the extreme heat. Something with a good insulation and superior, angled silicone boots. A good set of pre-made with any quality, will run you $70-$90.
Last edited by HeadsU.P.; May 10, 2020 at 09:13 AM.
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 787
From: Lebanon Township New Jersey
All good suggestions above. One other thing - did any spark plugs get damaged where the porcelain might be cracked either by dropping the plug or wacking it with a wrench?
Melting Slicks
Joined: Apr 2001
Posts: 3,399
Likes: 279
From: Olmsted Falls, Ohio
Good rule of thumb here.
If your going to do "mods" you should know what the cause and effect will be. Headers = leaner mixture, reroute spark plug wires, etc.
And with 186K miles on the car, you may have opened a can of worms.
If your going to do "mods" you should know what the cause and effect will be. Headers = leaner mixture, reroute spark plug wires, etc.
And with 186K miles on the car, you may have opened a can of worms.
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
Jebby
Le Mans Master
Joined: Oct 2010
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From: melbourne florida
2023 C3 of the Year Finalist - Modified
I agree with BK AND JEBBY. I was going to say plug damage and you guys beat me to it. Wires are next
jebbys suggestion of tuning after you get this issue resolved is solid.
jebbys suggestion of tuning after you get this issue resolved is solid.
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 787
From: Lebanon Township New Jersey
Yep. I was talking from personal experience. That problem drove me nuts.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
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From: Hermosa
Runs good until it warms up? Like after the choke opens up maybe? Lean mixture would be my first go to due to this symptom and addition of headers/ side pipes.
ive never cracked a plug that I know of but wouldn’t that produce a miss all the time, not just after it warmed up?
ive never cracked a plug that I know of but wouldn’t that produce a miss all the time, not just after it warmed up?