What causes run-on?

[alchemy]

Besides lots of beer and chili.

I've got a SBC with stock cast iron intake, Quadrajunk, and single point ignition. Has stock ramshorns and a TH400. All wrapped in my daily driver '72 Suburban.

I tune the ignition to stock specs and the points-wires-cap-plugs are fairly new. The cam is a slightly rumpy no-name, but that's the only non-Chevy part. I may have the idle turned up a bit to overcome the rumpy cam and keep it running at stoplights in this cold weather. I use 98 octane ethanol blend gas.

Any suggestions?


alchemy

[safariknut]

Crank the idle back down.My wife's 72 T'Bird does the same thing at high idle.Once it comes off the fast idle cam,it's fine.

[manyolcars]

idle too fast made mine do it.

[alchemy]

If I lower the idle it'll die at lights. So I just have to live with the run-on or change cams?

Could I change the degrees of advance any to get a better result?

alchemy

[DrJ]

Mid 70s (76-77Olds,I know, maybe others.) they put an electric solenoid on the carb that clicked on and off with the ignition and the idle was set to it in the on/out position so when you turned the ignition off the solenoid moved back in letting the butterflies close completely. Maybe you could retrofit this item if you don't already have one or fix/replace it if you do and it's not working.

[FLAT6]

Just wondering you mean by "run-on", I have never heard that term before.
Mike

[gregg]

Just turn it off while you're still in drive, then put it in park.

[alchemy]

What I mean by run-on is the engine still actively fires (or seems like firing) a few revolutions after turning off the key.

My carb never had any such solenoid Doc, but I'll look into it.

I have been turning the truck off in drive, but 50% of the time still have run-on.

Thanks for the responses so far.


alchemy

[Revhead]

I have heard that a build up of carbon on the tops of the pistons will cuase run-on or Dieseling. Not sure how true it is though.

[FLAT6]

That is what I thought you meant, my sisters car had this problem too. The only thing I could figure was that it happened when her electronic choke was still on, and it didn't happen when the choke was completely off. If you have an elec. choke, maybe check that the choke is opening fully and not staying shut a little bit. Just a thought.
Mike

[Anderson]

My Olds does that all the time. I've messed with the carb but it persists. Sometimes it deisels, sometimes it doesn't. I rebuilt the engine a year ago, so there can't be too much carbon buildup (though that could still be the problem in some cases). I just need a new carb.

[G V Gordon]

Run on or dieseling is usualy caused by carbon build up. Cardon has sharp points the hold heat and will act as a glow plug to ignite any raw fuel that enters the cylinder.
When I worked as a car salesman back in the seventies and we got a car in that was carboned up we used to have someone hold the throttle at about 1500-2000 rpm and pour a glass of water slowly down the carburator. The steam blows the carbon off and out the exhaust. Not sure how good it is on the engine but it works and we never killed one doing it. (Disclaimer) I of course take no responsibility for any damage that may result from this proceedure! http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif

[av8]

Back off the idle stop screw and raise the idle speed with the idle-mixture screws.

[burndup]

My truck did it quite a bit before the headgasket blew, the idle was up a tad high, cause the valves were shot... needed to smoothe the idle a bit there. There was a LOT of carbon in the chambers and on top of the pistons, as the valve guide seals were sot and I would use a quart in 60 miles.

Now that everything is back up to snuff, it never diesels, and I have it idling at 450-500 just because I CAN... still smoov. NO noticeable oil consumption, and other than scraping the piston tops with a popsicle stick, I didn't touch the bottom end.

So, you prolly have more going on... Whats you're oil consumption like? fouling plugs?

good luck,
J

[1oldtimer]

you can fine tune the points/timing and idle mix to get it to idle better and them lower the idle rpm to about 750 in drive (or 650 if you can). my elco has a high lift long duration cam and the idle is at 700.

[DrJ]

AC Delco makes a de-carboning fluid that works like a champ.
has a weird name like X3000 or something I forget but Killer knows if he's watching... use as directed...
Or you can put a mix up equal parts of water, tranny fluid and Carburetor boil-uot cleaner and put it in a ketchup bottle and squirt it down the carb slowly while attempting to keep it running by hand.
Do this where no one's going to freak out (or report you) at the billows of white smoke coming out the tail pipe.... http://www.jalopyjournal.com/ubbthreads/images/graemlins/blush.gif
Had a 6 month old Oldsmobile spit out what looked like a poker chip size piece of carbon once.

Oh, and change brands of gas... In the 70's Shell was the one that carboned them up then they reformulated so just change to anything different from what you're using if it turns out to be carbon.

[homebrew]

You can cure the carbon build up by holding the engine to 1500 rpm and slowly poring auto trans fluid thru the carb. about 8oz. will usely burn the carbon off and will not hurt the engine but if EFI will harm the oxygen sensor. good luck ---Homebrew---

[alchemy]

Thanks for all the advice guys. It sure is nice to have the HAMB respond to my request for help. (sniff)

Looks like I'll find some of the AC Delco carbon remover, if unobtainable I'll use the water-or-trannyfluid-or-mixture to remove the carbon. Then I'll (attempt to) adjust the idle mixture. Then I'll mess with timing a bit.


alchemy

[hudson_hawk]

oreilly has this stuff made by seafoam, its in red/white can it burns the carbon out. it will smoke the shit out of everything so do it by the highway

[LIMEY]

There is a couple of other factors that may cause it to run on, over advanced ign timing, eng temp to high or fuel mixture setting even the grade.
Do you experience any pre ign when driving under load, you may call it something different like pinking, detonation or knock. sometimes this will happen also with a motor that runs on, eventually it will cause damage.

[Smokin Joe]

[ QUOTE ]
When I worked as a car salesman back in the seventies and we got a car in that was carboned up we used to have someone hold the throttle at about 1500-2000 rpm and pour a glass of water slowly down the carburator. The steam blows the carbon off and out the exhaust.

[/ QUOTE ]

Used car salesmen... Bet ya put heavy weight oil in it after you burned out the carbon the rings were riding on so it wouldn't smoke too... I bought a couple of those 140,000
"Speedometer says 40,000." wonders before I learned. Amazing what they'll stick in a tranny or rear end to quiet them too... http://www.jalopyjournal.com/ubbthreads/images/graemlins/wink.gif