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C3 Specific Tech Talk 1968 - 1982


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Old 03-31-13, 08:50 AM   #1
eroc731
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Doctor in the house...?

Happy Easter to all who celebrate!

Ok so my initial euphoria at solving my car troubles last Sunday after my first rollback ride, was dashed yesterday when my 74 would not run right once again...history previous owner had put on edlebrock manifold and new holley carb about 2.5 years ago...work was done by mechanic, car ran well.

I am stumped, car was running great when I bought it, (last week) but on the third day, i was out and she started backfiring and sputtering and now she has a sluggish idle and seems to intermittently miss with small quick bursts of decent idle...I have put in one tank of gas, filled up at Costco, premium, 93 octane...

what I have found/corrected thus far:
new carb was not completely tight so I tightened and double nutted the studs,
i found several of the bolts on the new intake manifold were also loose so I tightened them...
removed and cleaned, wire brushed and sanded the spark plugs, thinking maybe they were fouled.... car ran a tad better but still choppy idle...

I'll admit my auto skills are rusty and I have a limited tool stash...I am thinking this is a fuel delivery issue...where do I look next?...fuel pump?, fuel filter?

as always advice and input welcome
Evan

Last edited by eroc731; 03-31-13 at 08:51 AM. Reason: added info...
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Old 03-31-13, 10:38 AM   #2
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Evan,

What is the age/condition of the spark plug wires? Are you satisfied they are not arcing?
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Old 03-31-13, 10:59 AM   #3
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Plug Wires

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Originally Posted by gradeiv View Post
Evan,

What is the age/condition of the spark plug wires? Are you satisfied they are not arcing?
I am not sure of the age of the wires...I know plugs were replaced 3 years ago...

I do know that the wires are not attached as they should be in terms of how they are hung from valve covers...they are dangling and I actually found one resting on exhaust manifold...

I confirmed this AM that the new carb is a Holley Street Avenger...670 CFM, four barrel, vacuum secondary
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Old 03-31-13, 12:10 PM   #4
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I would start with the fuel system. If the carb and intake were loose, I would pull both off and reinstall them with new gaskets. I would also replace the fuel filter. If the car was stored low on fuel it can rust the interior of the tank and you can have ongoing issues with a clogged filter. With a loose intake, it could have damaged the intake gasket. It also could be a carb issue that needs attention. I am not a carb guy so I would take to a carb expert to go over it. Did you try some dry gas?

I would also check the ignition. Those old vettes tend to burn points. so I would Replace the plugs, wires, points, condenser and cap. The Distributor should be checked overall for condition, end play, Etc.

By the way, how many miles on this car and how often was it driven?

Sometime, owners with a problem like this that they can't seem to solve sell them. There was an article in Vette magazine a few months ago about a solid axel car that had similar issues. you may want to read that.

Bob
Just some thoughts.
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Last edited by Bob Weiss; 03-31-13 at 12:12 PM.
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Old 03-31-13, 12:49 PM   #5
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Check close for vaccum leaks (loose hose somewhere) this happened to me leaving Cavalcade one year, this can make car run very rough. If you're not worried about how original it is, I would dump the points and change to electronic with Pertronix...I did and car runs great (and for years too...with no adjustments)
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Old 03-31-13, 02:28 PM   #6
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EROC731, If you need any tools let me know. I have vacuum gage.compression gage,dwell/tack, and any hand tools you might need.
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Old 03-31-13, 03:12 PM   #7
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Best solution is to trash the drivetrain and install a ramjet 572, six speed, heavy duty rear diff and suspension.
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Old 03-31-13, 06:04 PM   #8
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UPDATE
Thanks to all for the advice, ROSEY especially for tool offer, much appreciated...carboman...I may be a tad frustrated but not THERE yet!! LOL!...

So my high school summer automotive classes are slowly coming back to me and the very last thing I did today seemed to do the trick....new cap and rotor...I did not mess with points but will definitely check into Dr P's recommendation...

So after a day of tinkering, (wife's working a double shift today - nurse) I shook off my mechanics cob webs and did the following...
New plugs -
New pcv valve
New fuel filter
Snugged up valve covers
confirmed gas delivery to carb
New cap
New rotor

Had her running again and kept running for a good 20 minutes plus with no rough idle, no bogging and a healthy gallop with some throttle applied...occasionally she would sound a bit like a stall but recovered on her own...

too bad it started raining otherwise I would have test drove her...soon as the sun is out again...we shall see....fingers crossed...

be well all
THANKS AGAIN!!!
E-
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Old 03-31-13, 06:42 PM   #9
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FYI, depending on mileage I'd suggest a new timing chain. It's often overlooked but in higher mileage small blocks it can cause inconsistent running as there can be quite a bit of slack as the factory nylon gears wear and the slop in the chain affects valve actuation and timing since the distriboputor is drives off the cam.
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Old 03-31-13, 07:41 PM   #10
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Sounds like it me be a good idea to set point dwell, ignition timing and adjust manifold vacuum. You may want to change out the points and condenser. I also have a timing light if needed.
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Old 03-31-13, 08:00 PM   #11
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Mileage

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FYI, depending on mileage I'd suggest a new timing chain. It's often overlooked but in higher mileage small blocks it can cause inconsistent running as there can be quite a bit of slack as the factory nylon gears wear and the slop in the chain affects valve actuation and timing since the distriboputor is drives off the cam.
Carboman, mileage is 59,355...
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Old 03-31-13, 09:24 PM   #12
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If you are near Warminster, You are welcome to stop by or I'd be happy to come by and give it a once over. My strengths are really with big blocks, but small blocks can't be that much different. Everything is just smaller...... right?

Seriously though, as Don pointed out, vacuum leaks on these C3's can cause many issues. I don't recall if 74's had vacuum actuated headlights or not.

You should check/set the dwell and get a timing light on it. It would also be a good idea to check the advance while you are there. A vacuum gauge would be helpful checking for vacuum issues and setting the idle.

Replacing the parts as you did is a good start and sounds like you may have fixed some of your issues along the way. Plus you had some fun getting your hands dirty I hope.

Do you know if the engine is still stock or has it been modified (other then the intake and carb) in any way?

Tom
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Old 03-31-13, 09:47 PM   #13
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74's do have vac headlights. You might want to replace the points with electronic module. I did this on my old 72 and it made a big difference and eliminated the need to adjust the points. Most Corvette parts suppliers sell an inexpensive kit that's easy to install and it looks stock since it's under the distributor cap. The mileage sounds low for a timing chain issue but it's still worth changing if you plan to do any work, clutch fan, etc on the front of the engine since the nylon gears are probably fairly worn by this point. If you want help with anything let me know, always happy to pitch in.
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Old 03-31-13, 10:53 PM   #14
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Carboman and thoyer...i am close by i am in churchville so around the corner...i checked out the elctronic ignition and it looks promising...i did enjoy getting my hands dirty and it brought back a lot of memories of working on my 72 chevelle back in high school...

I will freely admit that i am lost when it comes to dwell and Timing stuff...scares me a bit..
I am reading and trying to re-learn the basics...

I am planning on attending the next meeting and hopefully the cruise night on 4/20.... I have an appointment with sean at vette shop on 15th for inspection and some front end suspension issues so if nothing else he can attend to any lingering idle issues...

You guys are great and i appreciate the offer of assistance...i would love to hook up with someone who can guide me as i turn the wrench!!...

As far as i know thoyer the engine is stock except manifold and carb...i am the 5th owner so i cannot be 100% certain...i am sure someone with more in depth knowledge tha me could decipher what is stock vs replacements...i am "bagging and tagging" any thing i take off just to be safe...

My cell number is 215-880-7456
Evan
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Old 04-01-13, 12:45 PM   #15
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I'm sure Sean can get to the bottom of your issues when you go in for inspection.

I expect to be at the cruise night with my "newest addition" as long as the weather cooperates.

Tom
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Old 04-01-13, 08:19 PM   #16
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bummer....

Ok so apparently my supposed fix was once again short lived...came home after work to take a drive and once again idle off and although I took her out on the cul-de-sac...she wanted to stall when trying to drive...

really confused as to why she would run so great in garage yesterday to then be doing the same thing again....

this is now twice she has done this...ran great and next time...not so much...

oh well I will keep tinkering until Sean gets her...
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