I have 1500 miles on my new motor, almost all city driving.. my car started to run on after I shut it off, I changed the plugs it helped quite a bit.. but every now and then it will still run on.. what causes this? is there anything I can do to stop this? I have a msd adjustable timing, I tried retarding when I shut it off, but that really dont help..

 

It is usually caused by the idle speed beign too high....

 

I had the same problem on my fresh 427. Lars suggested switching to manifold vacuum for the advance can (which increases idle timing 15* or so, and also increases idle speed)and then lowering the idle speed back to normal.

It worked.

 

I had the same problem recently when I changed the distributor. I'd swapped the advance springs and went with way too light springs giving too much advance even at idle.

 

is 800rpm to high for idle? whats the idle rpm on your cars, I alwyas seem to keep it in that range..it's a 385 with 195cc AFR and a comp extreme cam H274

 

It's not so much the RPM at idle as it is the throttle blade position at idle. Running more advance at idle allows you to close the blades a little more yet retain the same idle speed.

Edit....The reason he suggested switching to manifold vacuum was because this allows the higher idle timing without affecting the total timing. You could do the same thing by turning the distributor and reducing the amount of mechanical advance to keep the total under 38*...

 

well, In my case it's not the vacume, I have a msd with out the vacume advance can, my mistake when I purchased it many years ago.
also you mention throttle blades, I have the summit throttle return spring set up, I also use to use another spring from the throttle to throttle bracket because the pedal was a bit soft and It felt like it stuck a tad from fully closing.. I removed the the spring I used, to give more tension on the blades to shut.. now that I think of it, this started about the same time I removed it.. I wonder if that had anything to do with it.. well I'll try it out over the weekend and see if that makes a difference, but do you think that 800rpm is a bit to high? should I go for 700 to 750?

 

I agree with the throttle blade position factor discussed above.

In my experience, one of the biggest contributors to run-on is when the blades don't close completely (and I mean *completely*). This allows a certain small quantity of air into the manifold which in turn allows mixture to enter the cylinders where hot spots can ignite the mixture and cause run-on. If the intake column of fuel mixture is stopped completely, the engine will stop almost immediately. (In fact, you can stop an engine even with the ignition on, by placing your hands and sealing off the carb in this manner.)

The manual states 750 rpm idle for manual cars, and 800 for automatics in neutral. Those figures are for stock small blocks.

 

What kind of engine? What's your timing, at idle and also 3000 rpm? What did your plaugs look like when you pulled them? How's the idle quality?

 

I cant remember what the timing is, it's been several years since it's been touched, I hardly drive it.. but the plugs were black.. but that could of been from the distributer cap having so many cracks in it. thats been replaced when I did the plugs, I was getting a lot of missfire.. it idles real good very steady at 800rpm, I'll be taking it out sunday for a hiway ride to a friends house. we'll see how it does with another return spring on.. I am almost positive thats it, I noticed today when I started it, that when I pulled back on the throttle by hand it did drop down a bit in rpm

 

Madvette74,
You need to think of the "combustion triangle". For fire to occur, you need three things:- oxygen, fuel and an ignition source. The only reason the engine runs on is that petrol (and air) is getting into the cylinders and is finding an ignition source. If the spark plugs are not getting current, then your combustion chambers have a "hot spot" that is igniting your fuel. Usually this is a large amount of carbon build up that glows red after the key is switched off.

But petrol is still getting from the carb to the cylinders. Cars of this vintage have a solenoid on the driver's side just in front of the throttle linkage that should allow the throttle blades to completely close when the ignition is turned off. Check that first. If that is working OK, then you have to get rid of the carbon build up. Usually a good high speed drive will do it, but the carby must be tuned properly first. Another method is to trickle alcohol (methylated spirits) or water into the carb while the engine is running at a fast idle (air cleaner off). Take precautions in case of backfire. This will dislodge the carbon build up and you should notice a lot of black smoke from the exhaust if this is happening. The smoke comes out in an anti-clockwise spiral down here, but it may go in the other direction where you are.

Good luck and,

Regards from Down Under

aussiejohn

 

Be sure and just "mist" the water into the carb..use a spray bottle...and keep revs up at 1500 +...don't let it die...you do not have to do it to long...

My idle is 900..but I have a stick...when the idle was at 1100,my engine ran on..

 

Drive it!! Don't fix a thing til you drive it. Put a thousand miles on her then let us know how she runs.

 

This is what I got from my friends site!

"Quote"
This is a common problem and it's caused by a couple of things. Some people call this "dieseling" or "run-on". What is happening is, when you shut the ignition off, something, somehow is causing a couple of the cylinders to keep igniting, which in turn keeps the engine running, although "sputtering" pretty badly. This can be caused by excessive carbon build-up in the combustion chambers or on the piston tops which when hot, acts like little glow plugs. It can also be caused by an excessively high idle with a little too much ignition advance. Dieseling is a common problem with high compression / high performance engines, especially in warm weather or when the engine is hot. In this case, it is usually caused by running "cheap" gas with inadequate octane. It can be remedied by either switching to a higher octane fuel or by laying the timing back a few degrees. In cars equipped with automatic transmissions you can simply shut the engine off while still in "Drive" and the load from the tranny will cause the engine to shut right off. Manual transmission cars can be shut off while in gear (with the clutch pedal down) letting the pedal back up slowly when the key is turned off to load down the engine.

I use this method when I need to.

 

I use a GM idle solenoid, used on some HP engines to completely (as someone else said) shut off the air to prevent dieseling. I doubt that GM still offers the beast, but Holley still makes them to kick up the idle when you turn on your AC.

Basically it kicks your idle setting up to the run position when the car in running and when you shut it off, it TOTALLY closes the throttle preventing the run on.

The system has worked for me since '77 on an iron slant plug 2.02 head, 11.0:1 CR L-46 (about 120,000 miles) and my current Brodix -8 Pro Aluminum 2.055 heads with 10.67:1 CR and a Comp Cams 294S cam (22,000 miles and climbing).

 

I do what 72 Ray's post suggested. . . I keep my car in drive until I've shut it off. I actually don't need to with the way it is setup at this point, but I like to keep the RPMs as low as possible before shutting it down just to be on the safe side.

-dath

 

Can someone explain whay having too much initial timing causes runon?

It's not like the plugs are actually firing when the ignition is off...

 

I believe that the reason initial timing being more heavily advanced is that the plug tip temperature is higher which provides an preignition initiation site.

 

I had the same problem with a Edelbrock carb..I fixed the problem with a fuel pressure regulator.
Good luck

 

I think in your case you had some oil dripping into your combustion chambers. When rubber seals loose their oil coating they dry out and crack and of course leak. The oil that gets into the combustion chambers will burn and leave a carbon residue which stays hot and like aussiejohn said it acts like a glow plug and keeps igniting the fuel...you may also be hearing some "pinging" due to the air/fuel mix igniting before it gets fully compressed. You can also get a carbon residue from high temps where the Carbon is stripped from any oxygen, hydrogen, and sulfer and deposited in the combustion chamber...usually on the piston. I've never had any success with the water trick...it blasts the top coating off, but at the risk of getting water into the oil through the piston rings. With large deposits it just polishes the deposits which helps for a short time, but will return.

The only sure solution is to pull the heads, clean the pistons off, fix the cause of the high temps or oil leak, and put it back together.