I have a 1984 Z28 Camaro that has a stock carbureted 305 engine. The engine runs great and only has 58k miles on it, however it has issues starting and stopping. On cold start up it sometimes dies out and I'm not sure why. It can take a turn or two of the key sometimes. I always pump the accelerator until it starts and everything but thats not my main issue-

My main issue as of today is that the engine is now clanking when I turn off the car. It almost exactly sounds like the movie affect of a car that breaks down while driving and has to pull over. Strangely it is almost as if the engine is trying to continue to run, as the car shakes a little too, and some components are pressing against each other. I live in PA and it is December - so it is 25ish degrees outside but I'm not sure if cold weather just causes something like that.

While the car is running and driving there are no strange noises, ticking, or shaking. She just purrs!

I'm guessing (and I'm not a mechanic by any means) that the valves are touching the pistons when I shut off the engine. Can anyone weigh in on this? I don't want to destroy my engine... 😭

 

It’s called dieseling

 

You need a tune-up. Your idle speed could be too high, and your ignition timing could be too far advanced.
Could also have carbon deposits on the valves or sparkplugs...

 

Old school work around to help this is to leave the transmission in drive when you shut off the engine. The loading on the engine will shut it down quicker. This problem might be connected to your cold start problem too.

 

I thought that only ever happened in diesel powered engines?

 

This might be it. I acquired the car this year, it sat for its previous 20 years not being driven - in a car port in Deleware owned by the original owner. The guy who sold it to me put a lot of work into it but hardly touched the engine.

Is it possible for me to correct the timing myself/ clean valves? Spark plugs he bought for the car but I want to replace em anyways because they were lower end - probably won't last too long.

 

Can't hurt to try this for the short term, thanks!

 

What's the idle speed at operating temperature?...

 

Edit: read the question wrong sorry

Idle speed at start up is about 1k, when driving seems like it's about 800 when stopped, maybe a little bit less.

 

What is it in Neutral when fully warmed up?

 

I'm not sure if I'm being honest, I hardly throw it into neutral and let it sit there. I don't remember the RPMs being in a weird place the last time I put it in neutral.

 

Neutral/ Park, makes no difference...

 

Ah in that case it's same as starting up, usually when put in park the RPMs will shift up to about 1k again.

 

You need to do what T.L. said and perform a tuneup and check and adjust your timing. Also, GM has a substance called "Engine Top Cleaner". Not sure if they still sell it. I have a can in my garage. Its' purpose was to clean out carbon deposits from the top side of the motor. It is poured into the carb and the throttle is blipped to keep the motor running. Then you let the motor die and pour the rest into the carb. Your supposed to let it sit for some time then start the engine. Blip the throttle again to clear out the carbon and compound.

Fred

 

Well 1k isn't crazy, but it's a bit high for for a stock engine. ASSuming your tach is accurate, turn down your idle speed to 750 to 800 RPM and see if the problem persists.
Checking for vacuum leaks would be a good idea as well...

 

Would this be it?

 

Got it, will do! I believe the tach is correct as far as I can tell. Will have to take it to the shop to check for vacuum leaks because I'm not sure I have the tool for that unless there is an easy way to check.

 

This "Shop" place of which you speak is a sure way to an empty wallet.

For real, if your gonna own a vintage car, and your not dirty filthy stinkin rich, you'd best learn how to maintain / repair it yourself.

 

What is the operating coolant temperature?

 

Yes, that is it.

Fred

 

My '86 LG4 did this at times. It's called run-on or dieseling and it affects carbureted gasoline engines. It doesn't happen on fuel injected engines because when you shut the key off, it shuts down both the ignition system (spark plugs) and the fuel injectors. No spark, no fuel, no run.

In a carbureted engine, when you shut the key off, it shuts down the ignition (so no spark), but fuel still flows through the carb jets and into the intake while the engine is still turning. If there is enough heat in the combustion chambers, the air/fuel mixture will ignite and it becomes a compression ignition engine (albeit a very poorly running, inefficient, knocking one). This can happen from a lot of carbon buildup in the combustion chambers (increases compression ratio and hot spots in the carbon buildup act like glow plugs). Some other reasons it can happen are from running too hot and a common one is from shutting the motor off with the idle too high. On my old LG4, if the idle was around 1,000RPM or higher when I shut it off, it would usually diesel. This only really happened when my electric choke stopped working correctly and the cold start high idle stayed on all the time. Until I fixed the electric choke, I had to let the clutch out a little bit to drag the RPM down to about 750 and then shut off the key.

I also recall that in my case, the primary throttle shaft was pretty worn in the Q-Jet carb and would cause the idle to stick up around 850RPM.

I would do a full tune up of ignition stuff (plugs/wires/cap/rotor), set the base ignition timing and set the idle speed and the issue will likely go away.

 

Usually run on happens when the throttle blades are open to far at idle. This allows to much air and fuel to enter the engine at shut-down. 1000 rpm is to high an idle for a mild engine. I would turn down the idle. Years ago I had a friend with a 68 corvette that ran on .I convinced him to IN CREASE the timing advance, that in turn caused the idle to increase. Then by lowering the idle speed back down the throttle blades were closed at idle and the run on was gone.

 

I'm definitely not rich, trying more and more everyday to understand engines and such without breaking anything 😅

 

Fully warmed up and running is about 190 degrees

 

Extremely informative and helpful! Thank you! 😁

 

Makes sense - from what everyone is saying I'll need to figure out how to adjust the stuff on the carb/ full tune up and clean the top end of the engine from possible build up. Thank you!

 

It happens in gas engines when you shut it off and engine runs or tries to, without electrical ign source. Like a diesel.

timing, vacuum leaks, high idle, high float, carbon.
If you still have a quadrajet on it, I'd bet it may need removed and some screws tightened on the base to main body.