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Old 07-15-2010, 09:25 PM
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Initial timing related to dieseling?

How likely is it that my change in initial timing is causing the engine to run-on?

I was getting some detination so I recurved and now am running 15-16 degrees initial. The engine runs fine other than the dieseling. It doesn't do it every time. It drives away cold great and idles at 850 in gear and 650 in park. I checked the primary transition slots on the holley and they looked around .060 give or take. I had no dieseling problem in the winter when I was running a high initial timing, which was around 20-22 degrees but have also changed lot of other things too.

Specs
383 stroker
xe284 cam advanced additional 3 degrees
9.8-10:1 static compression
.04 quench
93 octane (runon is much worse with 89 now but used to be fine)

Thanks!

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Old 07-15-2010, 11:35 PM
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Run on

Too much initial timing + bottom line octane (87) will cause your problem.
Leave your timing backed down where you have it now and run 93 octane.
You might even back it down a couple of * and try adding some octane booster. Either way, it will take a couple of tank full's of gas before you will see any improvement. You also might have some carbon build up on the valves which can be knocked off with a couple of eyedropper's full of water
when the engine is cold on first start up at about 3,000 rpm.
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Old 07-15-2010, 11:42 PM
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Quote:
Originally Posted by kyle86
xe284 cam advanced additional 3 degrees
Zero the cam timing if you're talking about detonation.

Last edited by techinspector1; 07-18-2010 at 08:15 PM.
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Old 07-16-2010, 05:34 AM
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Hey Tach-1, 40 yrs. ago I had a 3/4 cam in a 360 ADV. 3* with 10:1 and never had a problem. Would that be because the gas was 100-101 octane?

I often wonder how engines can run on today's 93 with ratio's like he has and get away with it.
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Old 07-16-2010, 06:46 AM
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Retarded ignition timing can cause this because when the initial is retarded you must open the throttle more to maintain a certain idle sped. Opening the throttle blades can cause run on. Advancing your inital will allow you to close the throttle more at idle and reduce the chances of run on. Also, a cam being advanced can cause extremely high cylinder pressures because the intake valves will close sooner. High cylinder pressures can also cause run on.
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Old 07-16-2010, 07:27 AM
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The ignition spark quits when the key is turned off, so ignition timing should have zero to do w/it- there's no spark! The timing has a secondary affect because of how far the blades are open at idle if not enough initial timing is used as stated correctly by Greg T above, or if ported vacuum advance is used.

That said, I have seen on my own engines in the past, that advancing the timing seemed to cause the engine to diesel.

The one sure thing is the engine needs fuel to diesel. If the throttle blades are set to close further at shutdown (like the anti-diesel solenoids do), or are otherwise closed beyond some point, there will not be enough fuel flow to allow the dieseling to happen.

I would look for a way to close the transition slot more than it now is, and/or lean the idle mixture screws using the "lean drop" method.

Last edited by cobalt327; 07-16-2010 at 07:32 AM.
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Old 07-16-2010, 08:04 AM
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too much fuel is in the chamber at shut down.

this is cause by the front throttle blades opened to much at idle.

retarded ignition timing requires more throttle to get the same idle speed.

use manifold vacuum on the vacuum advance to regain timing at idle.

run 15 initial then add another 10-15 with the vacuum advance on a manifold source (30 total at idle). 25-30 degrees of timing at idle will increase idle speed which will allow you to turn down the throttle adjustment.

you could also try cracking open the secondary blades (flip the carb over to see the blade adjustment screw). this will add more air and allow the front blades to be closed more at idle. drilling the front blades also works to let in extra air (one 0.090" hole in each front blade).

I would try the the manifold vacuum advance first, but not sure the 284 cam will make enough vacuum for that.

The real fix is to get an adjustable distributor and limit the total advance to stop the detonation and run enough initial timing to stop the dieseling at shut down.

also, why advance the cam 3 more degrees? comp already puts an extra 4 in the cam. I would run it straight up as this could lower cylinder pressure and make it less prone to detonate in the first place which will allow you to go back to 20 degrees timing at idle.
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Old 07-16-2010, 09:02 AM
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kyle,
(combined with your recent previous thread info about light throttle acceleration cruise ping,,,probable too lean jetting condition source/cause explained by old bogie)
the dieseling problem is a second solid clue that your carb jetting is maybe/likely a hair too lean!!!
.............................
you only increased the base from (before) 14 to now 15 but that does mean the motor HP/TQ actual output at idle rpms increased (because the intake stroke Hg increased),,,that means the whole rotating assembly has more rotating inertia stored when you do turn off the key to be sensitive to run on...
"BUT"
the typical/normal "suggested" (start point amount) base for your cam duration is 16....
(your cam needs idle Hg help from added timing to help keep the chambers free of carbon build up when idling)
so it's not likely the minor base timing change itself is the root cause...

try simple first:
for a clue for is it carbon or too lean,,,just go do some hwy/interstate miles (50+?),,,the combination of lots of timing (your 15b+18c+9v) and rpms and lean fuel feed (20Hg at cruise) and light load does wonders for self cleaning the chambers....

if it diesels at the end of the hwy run when you turn it off,,that's another small clue it is a jetting problem...

LOL,,,make a positive out of a negative?????
typically,,, increasing the jet size just slightly on a cammed motor increases actual power output so you actually use less throttle when driving normal (motor responds alot crisper/quicker to a normal driving throttle change)...

"because" inherent in all high performace cams,,,you traded more high rpms HP for less actual street rpms range TQ output than the oem stock cam...

feed it slightly more "btu potential" (gas) to help the cam at street rpms,,, can help build more/higher cylinder pressure,,, so the motor can/will wind up quicker at street rpms driving normal...
from all you have posted,,, fair to good chance,,,,slightly larger jets will feel like a 20HP+ gain driving normal...
.......................................
a short add to that previous vac adv thread:

to see what that supposed to be 9* added max at 13Hg or higher vac unit is actually adding at your 20Hg cruise...
simply:
use the linkage idle rpms screw to turn up the rpms to cruise rpms and read the timing total with/without the vac plugged in....

Last edited by red65mustang; 07-16-2010 at 09:17 AM.
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Old 07-16-2010, 04:45 PM
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The 284H-10 cam needs a minimum of 26deg at idle. Can use up to "locked out timing" 36deg full time locked timing.

The engine is running on cause the combustion chamber is getting hot
(sharp, glowing edge, too hot spark plug hat range, carbon buildup, excessive total spark advance), and the carb is open too much at idle. remove the carb , flip it over look at the throttles and balance the front and back throttle opening at idle. .020-.030 on both.
Make sure the PCV is hooked up and functional. The engine needs the extra air flow at idle that the PCV gives. If its missing, hook it back up.

This engine will not operate on low octane or even mid grade fuel.
Must use 92 or better. Make sure mechanical spark advance does not exceeed 36deg and vacuum advance does not exceed 10-12deg at any time.

It should never be higher than a combined mech+ vac 46-48deg.
A manifold vacuum leak can also aggravate engine run on from a lean cylinder or two.

low octane, crap gas is not going to work.
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Old 07-16-2010, 11:09 PM
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My engine does this too. I was told my converter is the problem, but until I get a higher stall converter, I just shut it down in gear.
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Old 07-16-2010, 11:29 PM
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Hope theres a vacuum advance on the dizzy here Unless its only full throttle, full rich condition all day long, everytime its driven.
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Old 07-17-2010, 06:18 AM
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kyle,
correction/clarification to my post..

vac adv unit models are usually stated/written in "distributor degrees"...
(you multiply by 2 to know how many max crank degrees will be added to the base and cent at light load/high Hg )

your unit "adds max 9* at 13Hg and above" means it will add 18 crank *'s at 13Hg and above

your 20Hg cruise timing total is 15b+18c+18v=51TT

LOL,,,
the unit notations/script on your vac adv link in the previous thread are clear as mud!!!

do the timing light at cruise rpms (using the idle rpms screw) test I posted to know if it is a 9 or 18...

if it is a 18* added fixed unit that doesn't drop out completely till less than 5Hg is present,,,you don't want it on your combo...

a picture is worth a 1000 words...
your specific unit behaves same as this attached adjustable unit graph does "if" this one is set to 5 turns....
(degrees added vertical line is dist degrees)
...............................

add just abit to 454's and F-bird's posts????

for a carb to be able to fully atomize the gas at only idle rpms=slow moving pistons (atomize it fine enough to STAY in suspension with the air all the way into the chamber),,,the carb wants/needs about a 13Hg vacuum signal or higher...

your 284 cam at a 1000rpm's idle with only 18 base can only make about/maybe 11Hg....
(more than 18* base starting it when hot gets to be a issue)

having more timing at idle with your cam raises the idle Hg significantly!!!!
to see it,,just slightly loosen the dist hold down,,,start the motor then carefully twist the dist so there is more timing at idle,,,both Hg on the gage and rpms will climb!!!=the motor is making more power with the same amount of gas because it is a more better atomized A/F mix!!!
(so you can reduce the blade gap)

my $.02:
which way to get to 24-28 idle timing depends some on how you most often use the car????

more street and cruise/less strip=buy a vac unit that only adds 8-10 crank degrees (or a adjustable unit) and plug it into "full time" manifold vacuum as 454 explained...
(base at 16-18 it should start fine when hot if the starter system is in good operating condition)

more strip/less street=rework the cent to only add say 10*s so the base can be 24-26* which is what F-bird is saying...
(to get it to start hot,,,add a ign bypass toggle or momentary switch,,,you turn off the spark until it is spinning then turn on the ign so it gets spark)
I wouldn't use vac adv with that set up but if you do then buy a adjustable unit and plug it into ported,,,vac adv can/maybe be a beeetch to tune out mid throttle ping on a steep hill starting out with that much base...
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Last edited by red65mustang; 07-18-2010 at 05:23 AM.
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Old 07-17-2010, 10:08 PM
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Wow, lots of great info in these posts!!! Thanks!!

I decided to run the cam advanced another 3 degrees in addition to the 4 comp grinds in because that's how I was running it before I blew the motor. We originally did not know comp grinds in 4, but the 3650 lb car ran a 107mph trap so we figured it was pretty darn close and it would be best put it back like it was. Ya know if it ain't broke... it wasn't dieseling back then either.

As for the timing I have tried the vac advance connected to manifold vacuum before. The cam dosn't get enough vacuum in gear so it pulls most of the timing out between park and drive which gives me a huge drop of rpms. 6" in gear, 11" in park currently.

Hot spots in the engine compartment? I have noticed that every set of plugs I use in these heads give me funny burn marks on the insulators. I'm currently experimenting with 2 non-projected tip plugs and they seem to not be affected like the rest of the projected ones. Also experimenting with different heat ranges and swapping plugs from cylinder to cylinder. Currently I'm running 6 NGK UR6 plugs, one autolite 144, and one autolite 145. The 145 is always clean white like new while the other plugs porcelin look darker. Not sure which one to use.

The engine will not take any more than 32 degrees total timing right now at WOT. If I bump it to 34 degrees or higher, I can hear it chattering on a WOT acceleration above 3k. I might still need more fuel like was posted earlier even though I finally gave in and bumped it up from 72s to 74s. I have a pair of 76 jets I can try out however it's not surging or anything so I don't want to rejet if I can avoid it.

As for a high initial, that's the way I had it with the engine before I blew it the past couple times. I was running around 22-26 I believe somewhere around there and had the total limited with welded up slots. I have an interupter switch on the ignition power so starting is not an issue. I changed it to 15 initial because I was getting some detination and thought maybe the curve was comming in too soon so I retarted it and filed out the slots. Now that I look back it was prob just lean. Sure are a lot of variables haha.

Edit: Oh yeah, I tried cracking the secondaries some by adjusting the set screw, but I just couldn't get it to run right like that. It would be perfect wark, but then with a cold motor, I would have to idle it up a lot higher to get it to run right, so I put them back to where I started. This is a 80508 vacuum secondary carb with no secondary idle circuit, maybe that has something to do with it.

Last edited by kyle86; 07-17-2010 at 11:28 PM.
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Old 07-18-2010, 06:20 AM
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kyle,
what the 11" in park and only 6" in drive (5Hg drop!!!) tells you is the motor is making so little actual TQ at idle (illustration say 75 ft/lbs?) that just the slight added drag/load from the slipping torque convertor (illustration say just 30ft/lbs? drag) is to much for the motor to handle!!!
suggestion:
till you sort this all out you would be wise to kick it into neutral at red lights and raise the idle rpms to 1000...
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Old 07-18-2010, 06:25 AM
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Quote:
Originally Posted by kyle86
Wow, lots of great info in these posts!!! Thanks!!

I decided to run the cam advanced another 3 degrees in addition to the 4 comp grinds in because that's how I was running it before I blew the motor. We originally did not know comp grinds in 4, but the 3650 lb car ran a 107mph trap so we figured it was pretty darn close and it would be best put it back like it was. Ya know if it ain't broke... it wasn't dieseling back then either.

As for the timing I have tried the vac advance connected to manifold vacuum before. The cam dosn't get enough vacuum in gear so it pulls most of the timing out between park and drive which gives me a huge drop of rpms. 6" in gear, 11" in park currently.

Hot spots in the engine compartment? I have noticed that every set of plugs I use in these heads give me funny burn marks on the insulators. I'm currently experimenting with 2 non-projected tip plugs and they seem to not be affected like the rest of the projected ones. Also experimenting with different heat ranges and swapping plugs from cylinder to cylinder. Currently I'm running 6 NGK UR6 plugs, one autolite 144, and one autolite 145. The 145 is always clean white like new while the other plugs porcelin look darker. Not sure which one to use.

The engine will not take any more than 32 degrees total timing right now at WOT. If I bump it to 34 degrees or higher, I can hear it chattering on a WOT acceleration above 3k. I might still need more fuel like was posted earlier even though I finally gave in and bumped it up from 72s to 74s. I have a pair of 76 jets I can try out however it's not surging or anything so I don't want to rejet if I can avoid it.

As for a high initial, that's the way I had it with the engine before I blew it the past couple times. I was running around 22-26 I believe somewhere around there and had the total limited with welded up slots. I have an interupter switch on the ignition power so starting is not an issue. I changed it to 15 initial because I was getting some detination and thought maybe the curve was comming in too soon so I retarted it and filed out the slots. Now that I look back it was prob just lean. Sure are a lot of variables haha.

Edit: Oh yeah, I tried cracking the secondaries some by adjusting the set screw, but I just couldn't get it to run right like that. It would be perfect wark, but then with a cold motor, I would have to idle it up a lot higher to get it to run right, so I put them back to where I started. This is a 80508 vacuum secondary carb with no secondary idle circuit, maybe that has something to do with it.
You're going to promote detonation because the cam is advanced and raising cylinder pressures. Retard the cam to straight up and use a looser converter. Problems solved.
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