Ignition Timing question

[JJRJR]

Hey guys,

Down here in South Florida, it gets pretty hot so my engine runs a bit hotter than I'd like. After running her a bit (hot) and I flip the kill switch, the engine runs on a second or two. I've done a little reading on the subject, and seems like it's either an octane problem and/or a timing problem.

Many of you know my build, but in summary: 383 .030 over, Eddy Aluminum Heads and CR is about 9.6:1. Pretty big Comp Hydraulic Cam, and a ProSystems 840 CFM carb, Eddy Aluminum Performer RPM intake, with 1" spacer. Dyno at 440HP/440TQ. I would think that pump 93 would be fine, but the motor consistently does this at shut down. I am pretty sure that they dyno operator set her at 38 advance, but I really don't know too much about this topic.

I'm going to order some octane boost to see if that helps, but what are you guys running your timing at? Any other suggestions? The car runs great, just has this issue at shut down. Like to get it solved before it does any damage. How are you doing on pump gas with similar builds?


Thanks guys,

John

 

[george68hemirr]

From what is the general consensus of all the sites I've seen, that 34-36 deg total timing seems to be the magic number for timing on Mopes once you reach your "all-in" rpm. That extra 22-24 deg above initial timing is called "centrifugal advance". That is the force that causes the weights inside the dizzy start to work against the springs as they want to fly outward as rpm increases, and increase the advance [initial+centrifugal=total]. Fuel burns at the same rate generally, thats just physics. So as engine speed increases, the ignition needs to start earlier. Depending on the springs, that rpm can vary. Now the all-in-by rpm depends on your build. Once again...clear as mud? :drive:

 

[JJRJR]

From what is the general consensus of all the sites I've seen, that 34-36 deg total timing seems to be the magic number for timing on Mopes once you reach your "all-in" rpm. That extra 22-24 deg above initial timing is called "centrifugal advance". That is the force that causes the weights inside the dizzy start to work against the springs as they want to fly outward as rpm increases, and increase the advance [initial+centrifugal=total]. Fuel burns at the same rate generally, thats just physics. So as engine speed increases, the ignition needs to start earlier. Depending on the springs, that rpm can vary. Now the all-in-by rpm depends on your build. Once again...clear as mud? :drive:

LOL. Yea George, it's very clear. I've read a dozen different articles on this. All it does is reinforce the reason I am not an engine tech. Thanks for the input sir.

John

 

[69hemibeep]

Backing the timing off some will allow it to run cooler and also reduce detonation and run on :yesnod:

 

[mac]

i would also check your fuel levels and where your idle set screw is set.

 

[droptop]

i would also check your fuel levels and where your idle set screw is set.

:yeathat: to fast idle rpm will cause run on.

 

[JJRJR]

i would also check your fuel levels and where your idle set screw is set.

:yeathat: to fast idle rpm will cause run on.

The guy that built the carb said idle needs to be between 1,100 and 1,200. I think we're at 1,000 to 1,100.

 

[mac]

if the float levels are set too high or if the idle is too high, you may be drawing fuel in from the main circuit instead of the idle circuit which would supply fuel to possibly make the engine diesel. :cents:

 

[moparchris]

Low compression motors that idle high are prone to run on. Easy way to fix it? Put it in gear and load the engine (let the clutch out until it starts to drag) just a little with your foot on the brake when you kill it. The hard way to fix it. Change the cam out to one that has a smooth idle at a lower RPM then you can close the throttle a little more and put more timing in it. Factory Mopar multiple carb cars have an idle solenoid that holds the throttle open at idle and closes when you shut the key off. This allows the throttle plates to shut off completely thus preventing run on.

 

[JJRJR]

Timing was at 38 advance, brought down to 28 advance, still got the run on. No clue what the deal is. Just had it towed AGAIN, second starter failed this morning. Going with a better starter this time. Seems something else is wrong. Don't know. Wits end!!!

 

[mac]

if 38 is your total timing i would put it back to that and really check your float levels and your idle screw mixture settings.