After I drive my 78 for 30 min or more, when I got to turn it off the engine almost acts like it’s dying a slow death. Just still runs, then shakes, and finally quits. It can last anywhere from 5 to 7 seconds or so, but just seems weird to me. It’s seems in my opinion like for a brief second after I turn it off it still is getting gas to fire and slowly dies as it runs out. I can start it up immediately after and it turns off immediately, just when I drive it a bit it does this. Any ideas on what’s going on? I don’t feel like there’s anything going on that’s wrong, it’s runs great overall, it’s just a little embarrassing sometimes if I pull into a car show or something with a lot of people. All eyes go to me in a wrong way haha.

 

Stock carb? My edlebrock 1406 caused my engine to do this from fuel pushing past the needle and seats with too high of fuel pressure. Fuel regulator fixed my problem. High mileage motor? Could be from carbon build up causing hot spots on the pistons. How is your idle? A high idle can cause this problem as well. Happened to me when I developed a vacuum leak.

 

Sounds like dieseling. Bad enough it might backfire through the carb. Suboptimal. If you haven't made any changes then obviously something did. A vacuum leak would be the first thing I'd look for. Before you park it take the air cleaner off and see what it's up to. Mine did the same thing and I saw fuel dribbling from the secondaries of the 4160 style carb, found the vac hose dangling on the wrong side of the check valve.

After that I'd check timing. Had a Jeep with an inline six and the hold down clamp for the distributor actually snapped on it's own. Entropy I guess. Wasn't immediately obvious and it caused some weird stuff.

 

Disagree. Dieseling is caused by incorrect ign. timing.

 

Understood, and I was editing as you were typing. But a vac leak really did cause dieseling for me. Saw it with my own eyeballs and fixed it.

 

Well Mr. Pigfarmer. That's a new one on me. And I've only been turning wrenches for 50 years. So abviously am still learning. But most of the time I think it's fair to say. that dieseling on is caused by improper ign. timing.
Hell, I remember my father struggling with this issue with his Pontiac back in the 60's.
And once I too had this issue on my Vette, badly worn advance arm bushings in the distributor turned out to be the culprit.

 

I would think that vacuum is involved in the idle circuit and that the mechanical advance arm limiter bushing is not, it'll come into play at a higher rpm unless springs are missing. Pinging under acceleration would be more of an issue with the limiter bushing.

Let's take a step back a minute. Not being snarky, this is just a general observation that applies. The hood release is usually closer than a PC and I am in favor of just having a look around, hands in pockets. True enough no matter what you're working on. In this case it's a hobby car and it's not in a shop with the clock ticking. The man said the car was fine until it wasn't. What changed if nobody had their fingers in it ? A few minutes of just looking around, observing it at idle with the air cleaner off is free. Plenty of time to go digging later. These old engines can be very forgiving and sometimes it isn't one big problem it's more of a death from a thousand cuts.

If he wants to get into the timing my best suggestion is that he contact Lars for his most recent paper on the topic. It's not an Artemis moon rocket fueling out on the pad, it's an old Chevy.

 

It’s interesting to use the term “dieseling” and not understand why it’s called that. A Diesel engine needs no spark to work. Compression is what causes the fuel to ignite. After an ICE is turned off, I can’t imagine how excessive ignition timing has anything to do with the run-on, when there is no current being supplied to the ignition circuit.
Dieseling is a byproduct of heat in the chamber, usually because of excessive carbon build up . The carbon holds enough heat to light off any unburnt fuel that may still be in the intake/chamber. Once the latent ignition event happens, the engine sucks in more fuel as a byproduct of normal operation, and any other hot cylinder fires that mixture off again, and again, and again,..,until the hot assed carbon cools enough to stop firing the incoming fuel.

 

I’m sure by now the OP is totally clear on all this …….

 

Well,
I might add, and I also wrenched for over 50 years that if I change octane to the crap they sell my engine diesels EVERY time. Timing, YES. need to retard timing to run crap gas and give up mega HP.
Only way my engine works great is with the proper octane. Any change to pump gas is a disaster.

Dom

 

You all are correct.
Anything that can cause high cylinder spot temperatures that persist after ignition shut off can cause the source of ignition for diesel or run-on. Ignition timing grossly off, vacuum leak which leads to a lean condition, carbon deposits good source of ignition, even a really hot spark plug ground strap if it’s hot enough.

Id check all of that. To decarbon the cylinders run it till it’s good and hot then slowly pour water into the carb while the engine is running, keep it running with the throttle while doing it and don’t just dump the water in and hydro lock the engine. About a pint to a quart will do it.

 

To avoid the semantical discussions about sparkless ignition, I suggest we call your situation "run on". As in, you turn off the motor, but it continues to "run on".

For combustion to take place you need 3 things; 1) "Fuel" (mixture of gasoline and air, for this situation); 2) Compression; & 3) and ignition source.

1) you have a carburetor, and it works off of vacuum. As long as the motor is turning over it is creating vacuum, and there is gas in the carb so gas will get pulled out of the carb. With the motor turning, a mechanical fuel pump will continue to supply gas to the carb.

2) Compression is there as long as the motor is in decent shape.

3) THIS is your main problem area. You have SOMETHING in the combustion chamber which is remaining hot enough to ignite the fuel. Probably a carbon build up, or maybe a sharp edge on a valve relief. This is uncontrolled ignition, so it could happen way earlier or later than when the plug would normally spark, so the motor probably sounds like crap when doing this. And you may only have one cylinder that is doing this (but probably more than one).

- Automatic transmission? If so, turn off the key FIRST, wait for the motor to stop, and then move the shifter to "park". That will put enough load on the engine to - usually - prevent run-on.

- Remove the carbon. All sorts of ways to do this, search the internet.

- Prevent the carbon from building up. In my experiences, carbon is either from running an overly rich fuel mixture, or from oil getting into the combustion chamber. Use of a wideband can help determine the fuel mixture, and to tune it where it needs to be. Oil is either getting past the valve guides, or is blowing up past the rings.

 

Again, ignition timing has zero to do with this situation after the key is off. I agree that the other ingredient to dieseling is low octane fuel. Lower octane results in less tolerance of cylinder heat and, WHEN THE ENGINE IS RUNNING, is the cause of ping/knock/ pre-ignition and ultimately, detonation. The low octane in a hot cylinder after the engine is off with a significant vacuum leak to allow air into the cylinder is what causes dieseling, not ignition timing.

 

Run-on after ignition shut-off is caused by excessive idle speed at shut-down. Idle speed at shutdown is related to excessive throttle blade opening at shutdown, and does not have to be rpm-related. GM solved the run-on problem by installing an anti-dieseling solenoid on the throttle lever to close the throttle opening down once the key was turned off. You need to run the lowest rpm practical at hot idle. Timing can factor into this, since badly-set timing (retarded) will require that the idle speed screw be cranked up in order to maintain a reasonable idle speed. This excessive throttle blade opening induces the problem. You can allow the engine to idle with less throttle opening by correctly setting the timing with proper and adequate advance. Running the vacuum advance off manifold vacuum will also allow the idle speed screw to be backed out to assist in solving the problem.

Your '78 should have an idle solenoid, which if hooked up and working, will solve the problem. Is the solenoid installed and operational? Is it correctly adjusted? If it's there and set up right, your engine will not run on.

Your carb should look like this:



Lars

 

Huh. Common on lawn tractors. Think they call them ‘anti backfire’ solenoids or similar. Shuts off fuel flow when the ignition is cut. Nasty backfire when they’re not working. Never made the connection

 

...and the reason that your vacuum leak caused the problem was that the vacuum leak effectively simulated an excessive throttle opening (air path to the engine), so it behaved exactly as an engine would with the throttles opened up a tad too much. By eliminating the vacuum leak, you reduced the air available to the engine at shutdown.

Lars

 

Thanks everyone for so many replies, it seems like I’m looking for a vacuum leak, bad idle solenoid, or carbon deposits.

A little more about the car, it’s a 355 crate that the previous owner put in and I don’t know much about it. He said it had low miles but no way to know for sure, but I’d say it’s relatively low if anything. It has an edelbrock AVS2 carb on it and I run 89 gas in it. Idle is great and idle rpm is 500-700rpm or so. Overall runs great and haven’t had any issues in the 2 years of ownership. This problem also didn’t just start it’s been doing it for a bit, just kept forgetting to post about it. And only work I’ve ever done to the carb is take the air intake off to clean it and put a new gasket on it. No real work on the engine either that would apply to this situation.

I took a quick look after a drive tonight and I did look around and found a slight leak under the electric clutch landing on a vacuum or some kind of line. I didn’t see it dripping but the tube was definitely wet. I’m not sure what line this is, but it has a filter looking thing on it and runs from the distributor to the front of the carb. I took the air intake off, and everything looks and seems to work fine. Also not seeing any lose vacuum hoses.

I’ve also been having another problem where if I turn it off hot and wait 5-15min or so the car sometimes doesn’t want to start. I’ve put in a new mini torque starter and new battery because before the starter wouldn’t even try, but now everything works fine and yet in this scenario I have to hold the key in start and rapidly pump the gas to get it to start. The starter sounds like it’s working great and is turning the flywheel, I just don’t get an ignite for the engine to actually crank until 5-10 seconds of gas pedal pumping and it takes multiple tries of this. This is only in this scenario though, normally my car is a crank and 1-2 pumps and start kind of car, even after sitting for days. Only happens when it’s hot. Maybe vapor lock since it’s only when the cars hot? But idk if or how my two problems are related or where to look next.







Electric clutch with leak

Filter thing attached to distributor with line that has the leak on it. Runs from distributor to front of carb

 

Your POS AVS aftermarket carb has no idle solenoid on it. That's part of your problem. The line you're looking at is an EVAP system purge line. There are no "filters" in any of your photos above.

 

Ok, so is this now an add an idle solenoid on and should be fixed problem or should I still be look for something else? What about the leak I saw, any ideas on if that matters? Also are you sure that’s the EVAP line? The metal filter/vacuum thing I thought it was is connected to the distributor.

Also, I didn’t notice, but apparently it smells like a lot of gas when the car is running. So now I’m not sure what’s going on. Some friends are saying maybe it’s vapor lock is my reason it won’t start unless I rapidly pump the gas which I can see since the lines run close to the manifold but I’m lost overall and not really sure what’s going on with it now.

 

If you're going to create and operate a custom car that has been modified with a bunch of aftermarket mis-matched parts, you're going to have to educate yourself significantly on the modifications that are done, why they have been done, and what systems they affect. You can't just bolt on random parts and hook up things in a completely random manner. Here is what you have along with my evaluation:

• Your first through third photos above show that you have no anti-dieseling solenoid. You have photos of the top of the carb, the choke, and the choke linkages. This has nothing to do with a solenoid or run-on after shutdown.
• Your fourth photo is a photo of the distributor and it's vacuum advance control unit. It has nothing to do with any filter.
• Fifth photo is a photo of your EVAP vent line with a splice in it. It is not connected to the distributor. Your distributor vacuum line is connected to the carb. If that EVAP vent line is connected to a manifold port on your carb, you effectively have a huge vacuum leak.
• Your car smells like gas because the EVAP system has been hacked.

The fact that you are unaware what these components are, or what their function is, tells me the following, and I recommend the following actions for each item:

• You have not checked or properly set up your timing. It is, therefore, likely grossly retarded, causing you to operate with an excessive throttle opening in order to maintain engine idle speed. You need to properly set up your timing and timing advance curve.
• You have not verified that your distributor vacuum advance is running off of manifold vacuum. It is highly likely that you have it hooked up to ported vacuum, further retarding the timing at idle and aggravating the run-on problem due to high combustion temperature and further excessive throttle opening. You need to verify operation of your vacuum advance, verify its spec, and hook it up correctly to a manifold vacuum source on the carb.
• That awful AVS carb has no provision for installation of an anti-dieseling solenoid. Put it on a shelf in your garage as a display item and buy a good carb with solenoid provision.
• Your vacuum/EVAP system lines are not hooked up correctly, and are likely inducing a huge vacuum leak and venting gas fumes into your garage after shutdown. Get a shop manual and an assembly manual for your car and hook up your lines correctly. If you intend to eliminate the EVAP system you need to educate yourself on its operation before you start randomly bypassing system components, disconnecting things, or hooking up hoses to the wrong ports on the carb.

Once you have done these things, you will not have a run-on or gas-smell problem after shutdown.


Lars