Engine Dieseling - Options?

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billsdartgt

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I think I have figure out the source of my woes but over the years have realized that once you peak at frustration its often easy to overlook some of what you know is right so I thought I would share some of my source of frustration to see if I have overlooked anything.

my engine (to start)

Static and Dynamic Compression numbers were from a few internet site calculations and some comments here on the board

72 360 block - bored .030 over
pistons - confirmed TRW 11.5-1 (probably shaved some)
CC volume of cylinder calced out @ - 5
Head volume calced out @ 69cc's
Static compression came in @ 10.5ish
dynamic compression came in @ 8.3/8.4
Edelbrock airgap intake
Holley 750 DP with mech secondary's. (carb is stock now with 70-MJ and 80SJ and 6.5 PV)
Plugs Autolite AP66
Comp Cam - 247/254 @ 508 int/exh lift 108 lobe separation - cam degreed in at 106 ICL
Doug's long tubes
engine has pretty stable 9" vacuum
idle 1150/1200 (lose about 150/175ish in gear)

Currently have distributer set with the FBO plate locked at 12 degrees of mech timing, moved from 12 initial up to as much as 22 initial - still searching for the sweet spot - with full charged battery no issue on starting at 20 degrees initial - may be good at 22, but most happy at 18

I started the engine back around labor day - broke the cam in, changed the oil, pulled and cleaned plugs - engine sounds awesome - feels strong under the foot on the accelerator - had a few issues with brakes, alternator charging, etc. but the main issue is the engine dieseling when shutting down, still a few other issues to deal with once I get the engine where I feel good about hitting the street with it.

What ive done:

I tore the carb apart to confirm clean and what the jets were, primary was actually 73s, they were changed to the 70s that were listed as stock on Holley website, didn't try to confirm, measure, or change anything else

added fuel regulator to be able to make sure what fuel pressure was and no spikes

confirmed T slot exposure with carb off and set appropriately - no adjustment made after installed

changed to colder autolite 3923 plugs

added 6-8 ouces of octane boost (royal purple) just over 1/2 tank according to gauge - after adding the heavy dieseling slowed down a bit, moved the initial back to 20, car seem to like 18 as the best - easy start and idles good there - its a little rich and didn't try any adjustments

checked cylinder pressures engine slightly warm, all plugs out and carb open all the way through the plates - first few were 210 - 220 range - Not too bad I said to myself, realized battery was down on charge - after some quick charging - rechecked a few cylinders, looks like Im gonna be in the 225 to 235 range on all 8, stopped to let the battery fully changed and to get over frustration before setting the car on fire

My conclusion is that my dynamic numbers are off and I just need some higher octane fuel to run the engine, I have a preliminary thought on how to achieve but want to see if perhaps I could also tweak something, adjust something, or almost anything that I don't have to go and spend an assload of money on, I reran all my numbers on the same websites I got my original numbers from and it looks like if I could get 72 - 73 cc's on the head that could drop the dynamic down closer to 8 - does it look like Im heading in the right direction?

(this is by far my most long winded post i ever did - and Tim I would have sent this straight to you but your inbox is full LOL.)

I could have missed something I did or tried, hope the post isn't to hard of a read
 
fuel level was a little high, adjusted down just under mid way of sight glass, carb has the vent diverter shields stock - carb gasket is about an 1/8-3/16 thick fiber gasket, 4 hole type

engine temp holds steady at 180/185ish on autometer gauge, stock gauge temp below 1/2 way mark, of course hottest day when running was probably high 60s at best
 
billsdartgt said:
I think I have figure out the source of my woes but over the years have realized that once you peak at frustration its often easy to overlook some of what you know is right so I thought I would share some of my source of frustration to see if I have overlooked anything.

my engine (to start)

Static and Dynamic Compression numbers were from a few internet site calculations and some comments here on the board

72 360 block - bored .030 over
pistons - confirmed TRW 11.5-1 (probably shaved some)
CC volume of cylinder calced out @ - 5
Head volume calced out @ 69cc's
Static compression came in @ 10.5ish
dynamic compression came in @ 8.3/8.4
Edelbrock airgap intake
Holley 750 DP with mech secondary's. (carb is stock now with 70-MJ and 80SJ and 6.5 PV)
Plugs Autolite AP66
Comp Cam - 247/254 @ 508 int/exh lift 108 lobe separation - cam degreed in at 106 ICL
Doug's long tubes
engine has pretty stable 9" vacuum
idle 1150/1200 (lose about 150/175ish in gear)

Currently have distributer set with the FBO plate locked at 12 degrees of mech timing, moved from 12 initial up to as much as 22 initial - still searching for the sweet spot - with full charged battery no issue on starting at 20 degrees initial - may be good at 22, but most happy at 18

I started the engine back around labor day - broke the cam in, changed the oil, pulled and cleaned plugs - engine sounds awesome - feels strong under the foot on the accelerator - had a few issues with brakes, alternator charging, etc. but the main issue is the engine dieseling when shutting down, still a few other issues to deal with once I get the engine where I feel good about hitting the street with it.

What ive done:

I tore the carb apart to confirm clean and what the jets were, primary was actually 73s, they were changed to the 70s that were listed as stock on Holley website, didn't try to confirm, measure, or change anything else

added fuel regulator to be able to make sure what fuel pressure was and no spikes

confirmed T slot exposure with carb off and set appropriately - no adjustment made after installed

changed to colder autolite 3923 plugs

added 6-8 ouces of octane boost (royal purple) just over 1/2 tank according to gauge - after adding the heavy dieseling slowed down a bit, moved the initial back to 20, car seem to like 18 as the best - easy start and idles good there - its a little rich and didn't try any adjustments

checked cylinder pressures engine slightly warm, all plugs out and carb open all the way through the plates - first few were 210 - 220 range - Not too bad I said to myself, realized battery was down on charge - after some quick charging - rechecked a few cylinders, looks like Im gonna be in the 225 to 235 range on all 8, stopped to let the battery fully changed and to get over frustration before setting the car on fire

My conclusion is that my dynamic numbers are off and I just need some higher octane fuel to run the engine, I have a preliminary thought on how to achieve but want to see if perhaps I could also tweak something, adjust something, or almost anything that I don't have to go and spend an assload of money on, I reran all my numbers on the same websites I got my original numbers from and it looks like if I could get 72 - 73 cc's on the head that could drop the dynamic down closer to 8 - does it look like Im heading in the right direction?

(this is by far my most long winded post i ever did - and Tim I would have sent this straight to you but your inbox is full LOL.)

I could have missed something I did or tried, hope the post isn't to hard of a read
Click to expand...
I just spent the last two years going through this with my 340.
You are to rich at idle, back down the idle mixture screws. Most of us set them too rich when they should be set toward the lean side.
The high initial timing will cause a to fast of idle speed, you may have to back it off. To get mine to idle down I set it at 14* BTDC and added 18* of mechanical advance.
What is your hot idle speed?
Auto or shifter car?
 
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T slot is a decent "square" shape, mixture screws 1 turn out fronts, 1/2 turn out rears - idle is right at 1150/1175 -

one thing to note that the dieseling didn't stop, just "lightened" a bit, was gonna put a hose in the tank and blow some air in to try and mix the octane boost, figured what the hell, as I noted with the battery low I called it quits, previously when the battery was low it noticeably affected running, figured I needed everything to be good when checking things - I do know just by exhaust its a little rich - I will be dialing back idle screws when I get back at it, also thinking about rejetting my 670 and putting on there
 
You think 1150 for idle speed is "RIGHT"? You're badly mistaken, Bill. That's the whole problem "right there". Find why it will not idle down. Do you have a vacuum advance on the distributor? Is it hooked to manifold vacuum by mistake?
 
no vac advance hooked up yet although FBO strongly believes in manifold vacuum advance to 30 degrees - info somewhere on here indicated the idle Im at, really haven't tried to slow it down to much - Im all for rechecking, rechecking, and rechecking, I can easily close off mixture screws and shrink the T slot exposure more, even if a hammer is needed, I didn't hesitate to smack those Doug headers LOL
 
billsdartgt said:
no vac advance hooked up yet although FBO strongly believes in manifold vacuum advance to 30 degrees - info somewhere on here indicated the idle Im at, really haven't tried to slow it down to much - Im all for rechecking, rechecking, and rechecking, I can easily close off mixture screws and shrink the T slot exposure more, even if a hammer is needed, I didn't hesitate to smack those Doug headers LOL
Click to expand...

I wouldn't have a problem with going smaller with the T slots. But before I did, I would look HARD for vacuum leaks.
 
I have not looked for a vacuum leak at all - damn except when I left the PVC hose off - thats an easy one to check and one that could be a source of my angst

the high compression and large cam has had me on edge about what to expect, its new territory for me - generally one of the first things I check after a manifold and carb install is for leaks - duh - gonna suck with my juiced up gas if its the issue or part of the issue
 
billsdartgt said:
I have not looked for a vacuum leak at all - damn except when I left the PVC hose off - thats an easy one to check and one that could be a source of my angst

the high compression and large cam has had me on edge about what to expect, its new territory for me - generally one of the first things I check after a manifold and carb install is for leaks - duh - gonna suck with my juiced up gas if its the issue or part of the issue
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Get you a spray bottle of water and spray all around everywhere on the intake there's a gasket. If you have a leak, you'll find it quick. Don't use carburetor cleaner or anything flammable. You don't need to burn your car to the ground. Water works fine.
 
I'm at the same place with mine and I know exactly what it is. don't put octane boost in there actually put a gallon or two maybe even three of 110 Sunoco... Your problems will be over.. notice when you put that octane boost in their things got a little bit better. I'm not a big believer in potions well it may have helped a little it wasn't enough.. I don't like my idol down as if it had a factory cam in it. When your cam gets Beyond 240 duration at 50 it starts needing a little bit more idle speed not only to keep it idling smooth but to keep oil splashing up on the cam at idle.. I have a 10 gallon fuel cell in the trunk of my car so I usually mix 3gallons of 91 non ethanol with 1 gallon of 110 Sunoco... I've acted all the way off 2 about half that much Sunoco and still shuts off fine.. of course I could just pop the clutch when I shut it off but I'd rather shut down solid...
 
RustyRatRod said:
Get you a spray bottle of water and spray all around everywhere on the intake there's a gasket. If you have a leak, you'll find it quick. Don't use carburetor cleaner or anything flammable. You don't need to burn your car to the ground. Water works fine.
Click to expand...
Hush dummy, this is about performance...
 
j par said:
I'm at the same place with mine and I know exactly what it is. don't put octane boost in there actually put a gallon or two maybe even three of 110 Sunoco... Your problems will be over.. notice when you put that octane boost in their things got a little bit better. I'm not a big believer in potions well it may have helped a little it wasn't enough.. I don't like my idol down as if it had a factory cam in it. When your cam gets Beyond 240 duration at 50 it starts needing a little bit more idle speed not only to keep it idling smooth but to keep oil splashing up on the cam at idle.. I have a 10 gallon fuel cell in the trunk of my car so I usually mix 3gallons of 91 non ethanol with 1 gallon of 110 Sunoco... I've acted all the way off 2 about half that much Sunoco and still shuts off fine.. of course I could just pop the clutch when I shut it off but I'd rather shut down solid...
Click to expand...

The problem with octane boosters is they aren't octane boosters anymore. They used to slap work. Now they're just crap in a brightly colored bottle.
 
@RustyRatRod - I already set the car on fire once, wont do it again - will check and see what I find

@jpar - maybe it was wishful thinking but it sure seemed like the octane boost helped, but it was probably 40ish in the garage yesterday, car was running around 170ish then, would climb to 190 off with the heat soak - I looked a little and couldn't find anything around me that has any high octane - still more looking to do
 
billsdartgt said:
@RustyRatRod - I already set the car on fire once, wont do it again - will check and see what I find

@jpar - maybe it was wishful thinking but it sure seemed like the octane boost helped, but it was probably 40ish in the garage yesterday, car was running around 170ish then, would climb to 190 off with the heat soak - I looked a little and couldn't find anything around me that has any high octane - still more looking to do
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Baxter's Auto Parts has it. it's expensive and you got to buy it by the 5-gallon jug...
Probably like 12 bucks a gallon or something. You could also go to a small private airport and get 100ll... it's usually between 5 and 6 bucks a gallon. You might have to put a little more in of that since it's a little bit lower octane but the LED sure helps as well... I used to run it exclusively but it's just too much of a pain in the butt to get that much to run it on it exclusively...
 
billsdartgt said:
T slot is a decent "square" shape, mixture screws 1 turn out fronts, 1/2 turn out rears - idle is right at 1150/1175 -

one thing to note that the dieseling didn't stop, just "lightened" a bit, was gonna put a hose in the tank and blow some air in to try and mix the octane boost, figured what the hell, as I noted with the battery low I called it quits, previously when the battery was low it noticeably affected running, figured I needed everything to be good when checking things - I do know just by exhaust its a little rich - I will be dialing back idle screws when I get back at it, also thinking about rejetting my 670 and putting on there
Click to expand...
Dude your idle is waaay too tall. Shoot for 800 rpm at hot idle, temporally plug the PVC that may be part of your problem. I assume you are running pump gas, if so forget about mixing fuels and adding snake oil to the fuel it will only make it harder to tune..
What do the plugs look like, always go back to the basics before you start making changes.
Again, automatic tranny or shifter car?
 
its automatic, plugs were ehh, car was pretty rich with the carb straight out of the box, a step I generally do with new carbs is take them apart to check for cleanliness and what guts it has - I skipped that step this time around due to wanting to get the car started initially, I pulled and cleaned them after cam break in but didn't look at them again until the new ones went in, the new cold plugs are clean as can be after two 30 minute run times, I can get a pict of the plugs later this week
 
Check some Shell stations around you. We have two locally that sell 110 out of the pump. It's not cheap though, almost $7 a gallon, or at least it was last time I checked.
 
billsdartgt said:
its automatic, plugs were ehh, car was pretty rich with the carb straight out of the box, a step I generally do with new carbs is take them apart to check for cleanliness and what guts it has - I skipped that step this time around due to wanting to get the car started initially, I pulled and cleaned them after cam break in but didn't look at them again until the new ones went in, the new cold plugs are clean as can be after two 30 minute run times, I can get a pict of the plugs later this week
Click to expand...
Automatic's can hurt your diesel issue due to the spinning inertia mass of the convertor. What stall speed is the TC ?
This is not rocket surgery basic tuning will get rid of the dieseling.
What you will discover is this is a fine line of balancing fuel mixture and initial timing to achieve a smooth idle at a lower idle speed while using pump fuel.
 
There is only one cause of Run-on, after shutdown; and that is, that the engine,
by it's speed of rotation,
is able to pull fuel and air in, in about the correct ratio, that either ;
the heat of compression is able to cause it to combust, or
the chamber has hot-spots in it that light the fuel off, like a glow-plug.
If you remove one of the three/four, your troubles are over.

The heat of compression is already built into the engine, and cannot easily be changed. Hot-spots well you mighta missed something.
Of course the easiest to do is to close the throttle.
But if your timing is already way too much, then it won't idle.

So here is my tune from when I was running a 292/292/108 in my 11.3Scr 360. The 292 is advertised as [email protected] IIRC. See note 1

Note 1
Back the IDLE-timing up, to 16*. This will force you to open the curb idle screw.
Put the mixture screws to 7/8 turn. This will force you to open the throttle some more
Make sure the PCV is correctly plumbed and working.
Defeat the Vacuum advance for this test.
If your DP carb does NOT have a 4-corner idle, make sure the secondaries are closed up tight but not sticking, both sides the same.
If your DP carb does have a 4-corner idle; IDK what to do with those on this size cam; I didn't need it.
Start the car and warm the engine up. If it doesn't want to stay running, just crank in the speed screw.
After it is warmed up, reduce the Idle speed to 800ish.
That was my base tune.

If it runs like crap and you are 100% satisfied that the lifters are lashed right, and you have NO vacuum leaks anywhere, including inside the crankcase, the brake booster, and the PCV circuit; AND
the fuel is fresh, the level is correct and stable, AND
the plugs are clean, not compromised, and the gaps are not Too Tight.
Then
Put a shop rag in the secondaries. The rpm should nor change much if at all. The secondaries must NOT be pulling DRY air. If it does, the back cylinders will run lean.....making the engine run like crap and the exhaust will burn your eyes. Fix it.
Next,
Begin covering the primaries.
If the rpm goes up, your primaries are lean
If the rpm goes down, your primaries are already rich.
Fix this and your problems will be gone. see note 4
Before you begin, look for a tip-in hesitation. see note 2
After you finalize your Idle-timing, you will have to revisit your max power-timing, your rate and shape of the advance curve, and finally, start tuning the Vcan.

Note 2
The tip-in hesitation is unrelated to the accelerator pump. You just very slowly,gently tip the throttle in. If the rpm hesitates, that's gonna be PITA forever; you gotta get rid of it.
The tip in hesitation indicates that your throttle is too far closed and the transfers are too slow to get moving, crank up the idlespeed 1/2 turn and try again. Repeat as may be necessary, to get rid of the tip-in hesitation.
You can fudge the mixture screws but when finished, at the end of the tune, they should be at about 3/4 turn,+/- ,1/8 turn.If the rpm climbs past 850, retard the timing.
When you end up idling on the transfers and the mixture screws centered in their adjustment range, this is IDK, 70/80% of the battle.
When you get to this point; Do not change the curb-idle screw setting! Leave it where it is! if you change it, your tip-in hesitation will come back. If you need to change your idle speed, you will have to figure out a different way to do it. see note 3

Note 3
Your other choices for setting the idle speed are; timing and idle-air bypass; mostly timing; and changing the fuel level or the IABs, or the IFRs. If you get to thinking about IABs and IFRs I'm not your guy. I have never had to mess with those.

Note 4
My 292 cam wanted more air. I drilled holes in the throttle blades, one in each valve, on the front sides between the transfer slots and mixture screws, about a quarter inch back from the front edges. I started with 1/16 each and ended at 1/8" each, but 1/8 holes were too big. So I had to solder them up, move over, and start over.
Maintain your idle speed after each drilling. and retest with the shoprag. I doubt you will get completely rid of the rich running with drilling the holes (Idle-air bypass), So I suggest you stop at 3/32, and drive it for a while. Note; adding air in this way will increase your idle speed! so take it easy on the holes.
The whole purpose of doing this is so you can leave the throttles in the correct position up on the transfer-slots, to prevent the PITA idle tip-in hesitation, and to provide the correct AFR for the slow-speed circuit.
If you crank in the timing: your engine will pick up power, and you will be able to cruise down there on less throttle, and so the engine will be lean all the time in that zone. If you drive too slow, the engine will pull air around the throttle valves thru the T-slots, but the T-slots will go lean. Then your spark plugs will get hot. And then,when you stomp on it, it all goes to crap. IMO, there is no good reason to run a lot of timing at idle. With an automatic, there is no good reason even to run a lot of timing below stall. If you think you need more timing down there, put it into the rate of advance, or into the V-can system.Your engine will thank you for it. Anyway, that's my two cents.....
Think about it;
At idle...... the engine is at idle! it only needs enough power to idle, and to not stall when you put it into gear. My 292 cam idled at 600 in first gear/A833 at 5* advance, pulling itself thru the parking lot. This is my proof that your engine doesn't need mega idle-timing.
If you put it into gear and stomp on it, the Rs fly up to stall speed and the engine doesn't care what the timing was anywhere below stall.
The only time the engine cares about the timing between idle and stall, is at Part throttle..... and that is where your Vcan comes into play. So instead of trying to force an arbitrary timing number on your tune; let the engine tell you what idle-timing it wants. If she tells you 16/14 or 12, or whatever, is enough, listen to her. Your idle-timing is only not enough when the engine stalls going into gear,lol.
 
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Lean out your idle mixture and lower the idle RPM. With your compression readings, it may be that you will need to add an electric idle solenoid. That way when you turn off the key, the idle solenoid will relax and the throttle will slam completely shut.
 
I was able to do a little check tonight - got the engine warm and checked for vacuum leaks - nothing shows up any where - good and bad I guess but will eliminate that as an issue - next I backed everything off on the carb, mixture screws front to 3/4 and rears to a 1/4, idle screw backed off completely, reset timing to 20 - idle down to around 900, vacuum now moving between 7 and 9, not jumpy or fluttering though. heavy rpm drop when going into gear, exhaust smells a little rich still - dieseling was nearly gone, cranking still ok with no kickback. Looks like I will find a decent spot for initial from 16 to 20, so looks like Im heading in the right direction, I would be OK driving as is to go top off the gas tank and finish diluting the octane boost juice down. starting to hope again I can tune out and run on pump gas - Im gonna get back at it this weekend, first thing is the carb is coming off for a good thorough inspection if I can get a rebuild kit with all the gaskets, may even re-jet my 670 and put in on there - it has been a proven performer on my Mustang - thanks for all the info - will see what I can do with a little refresh of thinking
 
Sounds like it needs even more initial timing. Classic symptoms are the big RPM drop and smelly exhaust.
 
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Not likely your dieseling issue....but all four idle mixture screws should be set the same.
 
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