FCBO Gold Member
- Mar 19, 2013
- Reaction score
- Coral Gables, Florida
I know that most of you out there will give not a whit about any version of Chrysler’s Lean Burn Spark advance system.
In fact, most people regularly ditched the entire system fairly early on and converted their cars back to Chrysler’s earlier and indestructible electronic ignition system. And, not to be a hypocrite, I may end up doing the same..... eventually.
But, here goes... at least for posterity....
I bought my NYB with a failing spark advance computer in December of 2017. As I started sorting the car and the ignition, it became obvious that I would be tangling with the lean burn in one way or another.
I was forced, by my very stubborn habit of keeping my cars as original as practicable, to actually learn a bit about the system..... as much out of curiosity as anything.
Unlike the first generation Lean Burn System, the second version is actually much simpler. This later Lean Burn system eliminated both the second reluctor pickup (for the separate starting program in the computer) and the centrifugal advance in the much simplified distributor. Because it no longer ran a separate redundant ignition system just for starting the engine, the computer is also simpler. It has a single plug connecting it to:
Throttle position transducer
Coolant temp sensor
Throttle idle switch
(Vacuum is measured on the module from a conventional vacuum pickup connected to a rheostat)
The carb is a specific version of the venerable Carter Thermoquad with a few unique changes made to have it produce a very lean mixture (16:1) and give the proper input to the spark advance computer.
- a vacuum operated enrichment circuit for the idle when cold that is governed by a thermal vacuum switch set to close at approx 150 degrees of engine temperature.
- Leaner main jets and needles
- Idle set screw placed on the right side to contact an idle switch and operate the throttle position and translation transducer
- Idle screws have a finer taper at the end for more precise tuning
With no centrifugal or vacuum advance built into the distributor. Base timing is key to the operation of the system. The FSM and the decal on the yoke both call for basic timing to be set at 12degees BTDC. I’ve found that this car is a bit more responsive with about 4 additional degrees of advance. Any more yields pinging. I also run non ethanol premium 93 pump to eliminate this problem.
No vacuum leaks allowed!
With such a lean mix, any vacuum leak will yield exaggerated misfires and rough and unstable idle beyond what we’re used to on more convention earlier systems.
Carb must be very carefully balanced
Those fine idle mixture screws need to be tweeked just so to get as perfect a balance as possible to achieve a smooth idle..... regardless of how rich they are set. Also, richer is not at all better. Richer setting of the idle will force closing of the primary throttle to reduce rpm to spec. Transition from idle to power will therefore be more abrupt and could cause stall or backfire beyond poor responsiveness. I have set mine almost dead even (bit less for the left side) at 1-1/2 turns right now and am still tweeting it DOWN.
You need all 12 volts to make it run correctly...
This system does not like voltage drops much below 11v at any time. Battery, alternator and voltage regulator must we working correctly.
After all the above are done...How does it perform?
Car starts very eagerly with the first turn of the key, warm cold or hot.
Transition idle control and stability from cold to full operating temp is the biggest challenge. Standard choke and choke pull off need to be working correctly as well as carb balance.
Throttle response varies with temp of engine and how it’s driven. The ignition computer advances timing by 10 degrees at initial cold start for about 90 seconds and for about 60 seconds after warm restart to stabilize the temp and mix. Performance at these times is not different from the earlier non lean burn system. Like any cold engine with a carb, it’s a bit less powerful until it builds a little bit of temperature.
The timing advance is completely controlled by the computer. It is responsive to both throttle (transducer and idle switch) as well as load ( vacuum sensor) and, unlike a conventional vacuum/mechanical system, it can more quickly adjusts the advance and retarding of ignition timing.
However, emissions control was also its reason for being and, as such, it has some mid throttle quirks that are not what we’re used to.
The electronic spark advance will not snap the advance of timing back up like a vacuum advance will. Instead it builds it up to its max advance over time (5 degrees at a time) after the throttle is off the idle stop. Once built up, the advance will remain in memory for up to 5 seconds after closing the throttle idle position switch. After that, it will start reducing the advance in memory by the same steps until you get back to basic running timing.
If, for example, you coast to allow the engine braking to slow you down. You will activate the idle switch which will return the advance back to basic timing if you coast long enough. Depending how long you leave the throttle on idle will affect your initial advance upon re-opening the throttle. This can be felt as a momentary lack of power/response at part throttle... it’s not a stumble like a misfire. Just a reduced power level as the computer rebuilds the advance.
This can be a bit irritating but it’s not “wrong”. These ain’t no road runners were talking about. So, although slightly noticeable, the momentary delay in full available power at part throttle is not a deal killer. Should you need it, you can increase throttle to make up the difference or you can give it a little stab which helps bump up the advance based on the transducer signal.
In normal operation, it is really no big deal and, to most people... probably not even noticeable.
Like previous Thermoquad equipped 440’s a push past the primaries will yield both full power and that glorious sound whenever you want or need it. Hit it at the right rpm and speed and you get a nice kickdown and nudge in the rear. (You want a kick in the rear.... get a hemi)
Adjusting the transmission kickdown to get the trans to hold in gear a bit longer also helps both performance of the overall package and allows for a little more revs you may want to have at the ready should you need to mash it good.
I set mine so it shifts to 2nd at 10-12moh and into 3rd at 25 mpg..... taking it old lady easy. A bit more aggressive driving takes the above to 28/29 for second and as much as 60 for third. When I got it, this car was shifting to second in as early as 5mph and third at 15mph. It was crap.
Finally, it took me several months to sort out the car and well over two months to get the lean burn to work as it should. I can see where people could get frustrated very easily as this system is pretty finely tuned. It will not react well to what are minor issues on the previous system of just electronic egnition. I can also see where it can become the focus of symptoms of some other system weakness..... such as weak or faulty wires, plugs and coil as well as an out of tune carb.
Like everything else, KISS is the buzz word for it and don’t think it is the first culprit.