Lean Burn Evaluation

Engine, Transmission & Driveline

  1. jason99

    jason99 Member FCBO Gold Member

    Messages:
    148
    Likes Received:
    56
    Joined:
    May 20, 2018
    Location:
    Atlanta, GA
    I may be looking at a '77 with a lean burn 400 in it. What things should I look at with these systems to avoid potential issues?

    Thanks,
    Jason
     
  2. 70bigblockdodge

    70bigblockdodge Old Man with a Hat FCBO Gold Member

    Messages:
    9,953
    Likes Received:
    3,655
    Joined:
    Jan 19, 2013
    Location:
    Avonmore Pa.
    The carburetor is the lean burn part. The ignition system is okay just not performance oriented. On a stock engine add a couple of degrees of initial and it should be fine.
    Most people will have problems with carburetor.
     
    • Agree Agree x 1
  3. Imperialist67

    Imperialist67 Active Member FCBO Gold Member

    My Photo Garage
    Messages:
    394
    Likes Received:
    103
    Joined:
    Jul 16, 2013
    Location:
    Washington, DC
    When I got my '79 Diplomat it had that lean burn. Ran poorly with L-O-N-G hesitation until it warmed up, which took a surprisingly long time. Replaced it with an Edelbrock, and never looked back......
     
  4. 1978 NYB

    1978 NYB Old Man with a Hat FCBO Gold Member

    My Photo Garage
    Messages:
    36,176
    Likes Received:
    8,287
    Joined:
    Apr 16, 2011
    Location:
    Classified
    Converted every Lean Burn to standard Mopar ignition used from 73-75 and added an Eddy or Holley aftermarket carb....no problems.
     
  5. jason99

    jason99 Member FCBO Gold Member

    Messages:
    148
    Likes Received:
    56
    Joined:
    May 20, 2018
    Location:
    Atlanta, GA
    I got the general feeling that's what most people do, but wasn't sure of the difficulty or if that was frowned upon.
     
  6. Polara_500

    Polara_500 Senior Member FCBO Gold Member

    My Photo Garage
    Messages:
    1,895
    Likes Received:
    725
    Joined:
    Apr 17, 2011
    Location:
    Up North, you betcha!
    That's one of the few areas where staying original is more likely to be frowned upon as long as you don't swap it out for a tunnel ram with it's necessary hole in the hood, etc..
     
    • Agree Agree x 1
  7. moparnutcase

    moparnutcase Well-Known Member

    My Photo Garage
    Messages:
    893
    Likes Received:
    522
    Joined:
    Feb 25, 2011
    Location:
    Linwood,New Jersey
    My '77 Cordoba is still running the Lean Burn system, albeit with less than 13,000 original miles. Yes, had the carb rebuilt but mainly do to sediment from the tank. The ironic thing is when I pulled every thing apart and flipped the base of the air cleaner over it said...."remanufactured" on the computer. The computers were known to have bad solder joints. So, even with barely over 11,000 miles when purchased my Lean Burn is technically not the original from the assembly line. Being my car is so original, I will try to keep it as is for as long as possible.
     
    • Like Like x 1
  8. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

    My Photo Garage
    Messages:
    2,405
    Likes Received:
    563
    Joined:
    Mar 27, 2017
    Location:
    Cornelius Or
    In '77 there was an early and a late variant of the lean burn system. The '76 and early '77 had two pickup modules in the distributor and a centrifical advance. These systems had many problems, the solder joints failed, the electronics had issues transiting from the start module to the run module, the location of the computer on the air cleaner resulted in vibration damage to the electronics and also heat from the engine cooked the computer. Unless your prospective purchase is a very low mileage unit, it is very unlikely that it still has the original lean burn computer. I bought a new '77 Fury with the 400 lean burn which was the early type and the lean burn system took a dump 3 times in 18k over the first year.

    The late '77 lean burns had significant improvements over the early style but they were still a POS. The late '77 units eliminated the troublesome dual pick up modules in favor a single unit and the centrifical advance was eliminated. The computer circuitry was improved and there were less problems with the solder joints. The units still had major problems with heat and vibration degrading the electronic components of the lean burn computer. Any vehicle that still has a functioning lean burn computer has probably had multiple replacements of the computer. The throttle position transducer or throttle position sensor was troublesome because the resistance wire inside the sensor frequently broke or got contaminated or the resistance unit would move out of adjustment which caused faulty readings to the computer. The vacuum transducer on the computer system was very sensitive to small vacuum leaks and was a PIA to diagnose.

    As a result of repeated failures on both the early and late systems most were converted to the earlier '73-'75 electronic ignition which was much more reliable. It is possible to convert the lean burn style carb to run without the computer, but it is generally considered to be more trouble than it is worth as the thermoquad carbs had other issues besides the lean burn components. That is why most people and shops went to aftermarket carbs.

    Dave
     
    Last edited: Nov 8, 2018
    • Thanks! Thanks! x 1
  9. 1978 NYB

    1978 NYB Old Man with a Hat FCBO Gold Member

    My Photo Garage
    Messages:
    36,176
    Likes Received:
    8,287
    Joined:
    Apr 16, 2011
    Location:
    Classified
    Mopar even converted Lean Burn systems at the Dealer because of all the problems.
     
  10. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

    My Photo Garage
    Messages:
    2,405
    Likes Received:
    563
    Joined:
    Mar 27, 2017
    Location:
    Cornelius Or
    The dealers in the '77 time frame would not have done the conversion as such a conversion was a violation of EPA clean air regulations and some potentially big fines. We had lots of customers very unhappy with the lean burn cars, especially the large fleet buyers. This led to a market based solution. Our techs did a booming business in home garage conversions. Service manager would tell the customer that the dealership could not alter the factory installed equipment, HOWEVER, certain individuals would make the necessary conversion "off premises". Techs would buy the parts, meet with the customer and install the necessary parts. Payment was cash only so there was no paper trail. Later on after the lean burn cars were several years old, the regulations must have changed because we did a lot of conversions at the dealership.

    Dave
     
  11. BillGrissom

    BillGrissom Active Member

    Messages:
    348
    Likes Received:
    40
    Joined:
    Apr 23, 2011
    Location:
    Sacramento, CA
    You must change the distributor, since I think yours has a Hall-effect pickup and no advance (centrifugal and vacuum). The later was built into the "spark control computer", which is what I recall was the label on my 1982 Aries and the same basic box as a "Lean Burn". The later was usually mounted on the side of the air cleaner in V-8 engines. Easiest would be to buy a "ready-to-run" distributor by Ningbo (China), for ~$65 on ebay (Steve White Performance and others). Look for the 383/400 model. Also use a better e-core distributor. If you want to stay Mopar, take one from a Magnum V-8 engine (~1990 to 2001). Another approach is a 1970's Mopar e-distributor with a GM 8-pin HEI module and coil (1985-95 V-8 trucks).

    The electronic feedback carburetor would be hard to maintain today. You could use a throttle-body EFI. There are many integrated ones now, w/ one <$1000. But, you must plumb an EFI fuel pump and return to the tank, though there are some under-hood integrated kits that don't require a return.
     
  12. Davea Lux

    Davea Lux Senior Member FCBO Gold Member

    My Photo Garage
    Messages:
    2,405
    Likes Received:
    563
    Joined:
    Mar 27, 2017
    Location:
    Cornelius Or
    Back in the day, the basic conversion used the '73-75 Mopar ECM and cable, Distributor appropriate for the '75 400 in this case and a T-Quad for same. If the customer wanted a really clean looking back grade the '75 air cleaner could be included to get rid of the ugly hole in the lean burn air cleaner left by tossing the lean burn computer. They did not get called spark control computers until the '78 model year. Pretty much the same system, but when the big blocks mostly went away, ( system first appeared on '76 400), Mopar went with a new name as the Lean Burn had established a well deserved bad rap.

    Dave
     
    Last edited: Nov 8, 2018
  13. CBODY67

    CBODY67 Senior Member

    Messages:
    2,286
    Likes Received:
    480
    Joined:
    Mar 27, 2011
    To me, if I was looking at a Lean Burn car, I'd look FIRST to see if anybody had mucked with it! Many would get a reman distributor and a reman carb, both of unknown/verified specs, and the result generally was not good. Many fine Chryslers were ruined by those poor conversions, by observation.

    The first '76 LB C-bodies I saw at the dealership sounded fantastic. Instant throttle response off idle, more energetic, quieter, AND no catalytic converter was needed@! Hence the "Non-Catalyst" sticker on the lh door shell. THAT impressed me as GM and Ford were struggling with AIR pumps and massive amounts of delay valves and such in their underhood equipment to get th engines past emissions.

    The "lean" part was that the carb would meter a 16.0 A/F ratio at cruise, rather than 14.7. At that leaner ratio, CO and HC were basically zero, as NOx was a little higher, but NOx was not a federal emissions spec at that time. In order to get the learner mixture fired-off, the computer had MORE spark timing in its map. Chrysler, as usual, was building a more efficient motor, it appeared to me, which I liked. But it might well have been that it was "too far, too fast" for good reliability AND understanding of how it all worked by the people who were working on them (outside of the dealer network). Back then, by observation, the repair shops were usually a little quick to condemn something they didn't then know anything about. Two years later, they DID know and they WERE working on them in their shops. by observation.

    The whole system worked on voltage and resistance. The Chrysler "magic box" tester was not required, as it seemed that they failed more than they passed, to me. We got a Volare used car in the shop with a suspected Lean Burn problem. I found a Chilton Aspen/Volare book with a diatnostic of the system in it. I gave it to the tech and he got his meter and went to work, finding the issue and fixing it. GROUNDS are important, too!

    There were a few Chrysler TSBs to re-locate the computer into the passenger compartment. Probably more needed in B/RB police cars than on a 318/360, I suspect. But I digress.

    To me, the best "change" would be a Mopar Perf electronic ignition kit (or similar, which gets you a new distributor with a good timing curve) and some kind of new carb (Holley 4175 spreadbore for a Chrysler B/RB application) or a Edelbrock AFB/AVS (with a matching intake manifold). This way, you KNOW what the timing specs are on the distributor AND the metering specs of the carb being used. You just don't have that information on any reman parts, by observation, where the parts are more "will fit and work" rather than "fit and work as well as they can".

    At some point in time, if it hasn't already been reached, I suspect the orig Lean Burn computers will not be remanned any more, unless somebody wants to "un-pot" the circuit board and re-touch the solder joints and such so everything works well again. Many of the sensors are probably already gone, I suspect. So . . . doing the electronic ignition upgrade and carb change might be planned on. One of those deals, if you know where you're eventually going to end up, why not get there sooner and be done with it?

    Enjoy!
    CBODY67
     
  14. 70bigblockdodge

    70bigblockdodge Old Man with a Hat FCBO Gold Member

    Messages:
    9,953
    Likes Received:
    3,655
    Joined:
    Jan 19, 2013
    Location:
    Avonmore Pa.
    If it ain't broke.....
     
    • Thanks! Thanks! x 1
    • Agree Agree x 1
  15. jason99

    jason99 Member FCBO Gold Member

    Messages:
    148
    Likes Received:
    56
    Joined:
    May 20, 2018
    Location:
    Atlanta, GA
    Thank you all for this wealth of information.
     
  16. 70bigblockdodge

    70bigblockdodge Old Man with a Hat FCBO Gold Member

    Messages:
    9,953
    Likes Received:
    3,655
    Joined:
    Jan 19, 2013
    Location:
    Avonmore Pa.
    Your welcome and thank you for not making your old car complicated.
     
    • Agree Agree x 1
  17. jason99

    jason99 Member FCBO Gold Member

    Messages:
    148
    Likes Received:
    56
    Joined:
    May 20, 2018
    Location:
    Atlanta, GA
    We will see what follows me home this weekend.
     
    • Like Like x 2