So I've had a problem child 347 stroker in my '82 F150, and most of the problems are fixed except for a bit of a hard start and dieseling on shut down. What could make the engine surge after turning the ignition off? I've never had a vehicle do this, so I'm not really sure where to even start

 

The throttle is open too far, allowing fuel/air mixture that allow it to diesel. You probly don't have enough timing to let it idle with the throttle cracked open just s bit at idle

 

I'll double check the timing, but my mechanic just set it to 16 degrees with a 20 degree advance, and the idle is about 700 rpm. I'm currently running 93 octane too

 

I may have misread what you were insinuating though, are you saying that the throttle position is physically cracked open, or the rpm is set too high?

 

16 initial is way too much. 10-12 should be all that it needs at that point. With anything but race gas, it's probably pinging it's *** off. Back it off to 10-12 and then retune the carb and see if it'll idle slower. You running an automatic or manual trans ?

 

There's no pinging at all with the 93, but when I switch to a lower octane there is. So even with a fully built motor 16 is too much? I'm running a zf5 for a trans, the c6 just wasn't cutting it above 45

 

typically, dieseling is caused by to much timing........if the idle is a little high, the advance might be kicking in causing this.....

 

Yea, I'd think 16 is too much. It could still be pinging just right on the edge where you won't hear it. You haven't mentioned cam specs, but with a manual trans it could idle slower. My 331 idles at 500 rpms with a Z303 cam (.587 lift, 228* @ .050 with a 112* LSA.) And that's with three 2 bbl carbs available adding fuel air, although the end carbs are dialed all the way shut, even so they'll still add a tiny amount of fuel air at idle. What's your compression ratio ?

 

+1 with too much initial timing (that's also the cause of your hard starts), 12° is a good starting point. Also need to know more about the engine combo, as asked compression ratio, what cylinder heads and cam specs, carb.

 

Double post

 

So for engine specs, I've got trick flow 170 heads with 61cc chambers, probe/scat rotating assembly with -13cc pistons, Camshaft is comp came Hydraulic Flat Tappet 250/260, Lift .460/.474, edlebrock 600 cfm carb and one of their performer intakes, stock ignition. From what the kit says when I bought the rotating assembly I should be at 9.52:1, but I'm not totally sure I believe it. I wish the engine shop had done the calculations for me, since I don't know deck heights and stuff to do the calculations on my own. I'll dial back the timing today and see what happens though.

 

That should take care of it, and will bring total down to 32° which is in the ballpark of where you want it to be. Let us know how it goes.

 

Update: I"ve backed the timing down to 11*, and it still runs on. I've brought the idle down to where it sounds like its gonna stall when i let off the gas and it still does it. Not as much as it was, but still 2-3 clunks. It still cranks for a second when heat soaked before it starts, and cracking the throttle open usually speeds up the starting process quite a bit

 

Could pull your spark plugs and peek at your pistons. Built up oil crud or carbon (from running too rich) will make hot spots that ignite your fuel and cause dieseling.

If you've done significant engine customizing (e.g. aluminum heads with special chambers, custom cam, etc) you may need or use a timing curve that you'd never think of on a factory motor so 36 max mechanical advance or 16 initial mechanical may not be too much. To be sure, I'm speaking with no vacuum here. If you're using full vacuum, 12 mechanical advance (with vacuum disconnected and plugged) is often enough.

Also, if you've customized your engine then it may like to idle at 850-900ish. Idling at 700 is more for factory builds with smallish cams.

 

I've got a new set of plugs I was gonna put in today actually, so as long as I can find a good flashlight I'll take a look in ther. Could running too rich at idle be a potential cause of this run on? I'm gonna hook up a vacuum gauge to it today so I can straiten the idle a/f ratio out.

unfortunately this timing stuff is pretty new to me, so how I go about modifying the timing curve is something that I need to do an it of research on to fully understand (unless it's simply referring to where your initial/full advance numbers are at, and not necessarily the line it follows going from A to B) I can tell you I'm running ported vacuum at the moment though, once I converted to a 5 speed it didn't like full vacuum at all.

The motor definitely sounds 100x better and healthier when it idles a little higher, I've got an rpm/dwell meter that'll be here tomorrow so I can be a little more precise with it