Hey there, C3 enthusiasts!
I have a 1976 L82 stingray manual 4-speed.

TL;DR: I (inexperienced DIY) replaced oil pan & brakes and now the engine races when it cranks.

Bear with me on the context.
I noticed my brake calipers were leaking, so I decided to replace the rotors, calipers, and pads.
I also noticed that I had a small crack in my oil pan, so I procured a replacement for that, as well.
I put my car up on the lift and did the brake work, then I worked on the oil pan.
I drained the oil and removed the oil pan, cleaned off the gasket, and installed a new pan with fel-pro gasket.
I finished the oil pan work, and will finish the brake work when I get that sorted out (passenger rear caliper & supply line will not deliver brake fluid).

I cranked the car to see if everything was good, and it took a minute to crank, but it roared to life. And kept roaring. As if the pedal was to the floor.
I have cranked it 2 more times since then, and I don't know why the engine is racing. The throttle (Accelerator pedal linkage?) does not APPEAR to be calling for this. Engine was cranked with air filter on.
I had my wife work the accelerator pedal, and the throttle moved as I might expect it to.



Busted rubber cap on a carburetor nipple



To be clear, the engine does not start normally, and then work up to a race, or gradually increase RPMs... it just seems to be max rpm as soon as it cranks. Turning the key to "off" resolves this by killing the engine, and it's the only thing I've tried.

I'm not an expert on any of this, but I'm trying to learn.
While I was working on the oil pan, I noticed 2 disconnected/broken vacuum lines- 1 at the interior floorboard of the passenger seat, tethered to the wall,


and 1 that appears to connect to the coolant line. I reconnected the coolant line, because I could find both ends of the break, but they broke again after I started the engine.

Peeking over passenger side, although my camera flipped the image since my phone was upside down


I'm not familiar with carburetors, nor automotive vacuum systems. A few years ago I decided to become my own mechanic, so I've had some chances to work on my other cars, but I haven't done anything too daring or exciting on the corvette yet. I mention this to point out that there may be "obvious" things that I've missed due to my lack of experience.

 

You have a vacuum leak. Start by replacing that deteriorated cap. If that doesn’t solve your problem plug all connections to the carb and intake and test one port at a time to isolate it.

 

Any place where the engine can suck air, except through the carburetor, will cause the engine to idle abnormally high. Replace that carburetor condom and I bet that fixes most of your problems. While you’re at it, look over all vacuum lines and hoses to make sure that they are all in good shape. These corvettes have vacuum lines running all over the place so your issue could be a combination of leaks.

 

Also, your last photo shows a disconnected vacuum line. The Black plastic valve conneted heater hose is where it should land

 

Hello,
It sounds to me as though the accelerator might be stuck.
Have you tried to cycle the accelerator cable and linkage manually to make sure it not stuck and that it has full travel?
It's in your second photo.
Regards....

 

It looks like it's stuck too (or being pulled). I believe the lower arrow is the idle set screw and there's clearly a gap between the screw's end and the lever. The upper arrow appears to show the accelerator pump raised off of its seat.

 

The vac hose disconnected on the inside foot well is one of the ventilation door hoses that connect to the A/C-vent control on the center console. The diagram below shows the various hoses to the controls - '79 should be very similar (exact?) to '72.

 

Id take a look at your fast idle cam.

 

Thank you all for the replies & suggestions.

I replaced what I believe to be the disintegrating "carburetor condom" and there was no change. I haven't gotten to troubleshoot any further yet.
I took a video of full press & release of accelerator pedal, followed by the start.


Better pic of carburetor with replaced condom?
Apparently I cannot upload an mp4 video file

 

Have you skimmed the Edelbrock tech videos?
https://www.edelbrock.com/edelbrock-...nd-tech-videos

Here's a short one on excessively high idle:

 

Is the large vacuum line that goes to the brake booster not inserted correctly?

 

I know more about women than I do about carburetors so maybe I shouldn’t be commenting. I know less about edlebrock.

But - the secondary jets are open. Should ther be a flap over them? A QJet has a spring loaded flap over the secondary jets.

 

Here is how the engine starts (impetus for this thread):

I replaced the line that goes from the carburetor to the distributor (is this "vacuum advance" line?), which is the shiny black line in the picture.


I reconnected the vacuum line (apparently 1/16" ID) to the heater hose.
I cranked the engine, and it didn't race! but it is... loud. Almost sounds like something's hitting.

While it was doing this loud (not fast) idle, I got out and manipulated some of the vac line connections to see if I could identify what change had this result.
I disconnected the new vac line from the carb and it didn't change anything.
I disconnected the tiny vac line from the heater hose, and that didn't change anything.

I let it idle noisily like this for about 5 minutes to try this troubleshooting. It does NOT sound right.

 

It's hard to tell from this far away, but I'm wondering if the clatter is an exhaust leak at your header. Do you have gaskets between the head and headers?

 

Prior to replacing the brakes & oil pan, it did NOT sound like this. It sounded like a regular 1970's muscle car idling.
I all the way believe there's a gasket at the exhaust header, but I hadn't peaked to check.

When I replaced the oil pan, I didn't get time to do the job all at once, so the car sat with the oil pan removed and all the oil drained out for about 3 weeks. I'm wondering if some gaskets/seals got dry during that time, and has imparted a new issue.
I can try to check on the condition of the header gaskets later today (hopefully I'll have time).

 

to
^^^^^^^ yup

that’s the first thought that came to mind when I saw the electric choke.

 

okay... what's the"fast idle cam"?

 

What is this vacuum to the heater hose? I don't see anything like that in your photos and that sounds really odd, do you have a pic of it?

 

the heater hose is circled in blue, and the vac line going to it in red.

 

I really can't speak to that. I didn't do much of my own work on this vehicle until just lately, and I haven't had much work done on it all together. It has MOSTLY sat in a garage. It does seem that the large vac line going to the brake booster is crumbling at the end.