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72 D-Jet tuning

1991 Views 9 Replies 5 Participants Last post by  MBGraham
Hello everybody. Thank you for the great information that has been posted. I have spent a lot of time reading through all of the posts, especially the encyclopedia germanica sticky. I want to start off by asking what browser is best for this website. I usually spend 5-10 minutes trying to load the page I want :( I am working on tuning my 72 350sl right now and I am getting a lot of misfires. Timing is set 5 degrees BTDC (about 27 total advance @ 3k rpm), but it jumps with misfire. Dwell is at about 33 and all injectors seem to fire just fine (I listened to them). She seems to misfire a lot before reaching operating temp, but not as much afterwards. Plugs are a little white, I set them at 0.032" Ecu is set to all the way rich, she misfires more if set leaner. I did check the air temp sensor and water temp sensor, but map.... there isn't a test for it? Right now I plan on cleaning the injectors and then replacing the fuel filter. Fuel pressure is also good, 31 psi. Can I soak the injectors in carb cleaner, or will that destroy the electronic or rubber parts? I haven't seen much information on the best way to clean them yet. I am also going to replace the soft top and paint the hood. I don't like the way the soft top bows up right now (on the back), is there a thread that deals with this? What is the best manufacturer for the soft top?
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Thank you all for the useful information. Winter is coming and I will have more time to start addressing the problems that I have had. I believe that my misfires are ignition related. My tach started dropping recently, coordinating with a loss of power that is noticeable at highway speed or any decent amount of acceleration. I will replace the points with a Pertronix set up this winter and hopefully find out why I'm loosing spark as well. The tach drops every 2 seconds or so and has a major cut out about once every 5 minutes that causes a decent loss of power very briefly.
Read the section in EGV107 about setting the Throttle Position Switch (TPS). Make sure it is set properly so that it will signal to the ECU that you are at idle. You should be able to set knob in middle or range and fine tune idle mixture by going a click at a time followed by short burst of throttle.

Resistance testing of MPS is covered in the shop manual. Also check with Mityvac or similar that it will hold say 20" of vac for 15-30min. To test if it is working properly and make adjustments, you need an exhaust analyzer or a wide band AFR meter (I could never get my car right until I installed an AFR meter, but hardly anyone here has one!)

Probably not your problem, but you can immerse the injectors in carb cleaner. There are small screens on the inlets. You can remove those for cleaning by screwing in a suitable sheetmetal screw and pulling to extract. I set up a kind of crude system to flow test the injectors using my pressure brake bleeder. I think it is in the Comprehensive guide in the stickies. Later, I did 4 at a time using spare fuel rails and compared the amounts collected. At first I used Seafoam to clean, then carb cleaner. For flow testing I used clear mineral spirits/Varsol (not gasoline!)

Don't know why you are getting misfires. Usually timing or ignition problem. Follow what Cush said and get things in good shape firts (Sounds like you are well on your way). Pertronix eliminates points as a potential problem - covered in Stickies. Make sure rotor and cap are aligned. Check for damage to contacts in cap.
MBGraham. Thank you very much for your response as well as your extensive sharing on this forum. I decided to replace the points with the Petronix kit and the coil as well, following your posted material. The car is running almost perfectly now :) I will still clean injectors and check spray patterns and do some minor tuning, but I am very happy with the new ignition :) What would be a good spark plug gap for the higher coil output of the flamethrower? I will add a air/fuel sensor soon as well and adjust the car for a little better mpg if possible. I'm guessing that fuel pressure is the only way to do this? Now it is time to start addressing the cosmetic issues. I have a new soft top and I'm going to strip the hood and hard top for paint soon. That is going to take some time and money. Lol But... I get to drive plenty in the mean time.
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