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My 1985 Mustang 5.0 throttle Body Injection engine is running lean with a engine missfire that gets worse when the engine is warm. I have also noticed that during my check and setting of the initial timing, I found that the timing holds well throughtout the RPM range. However it will advance thru 2000 RPM to around 29 degress before top dead center then retards backwards back to about 12 degrees BTD at about 4000 RPM and thru redline.
The sparkplugs read lean both from idling and also from a hard pull. I just had this car thru emmissions and had a little trouble. I had advanced the timing for good running and when I took it down to have it tested it failed the first test. After retarding the timing back to manufacturer spects, the car passed with great readings, per the site tester. But the engine became rought and irratic in running. It didn't die but I'm just not happy with the idle at manufacturer specifications.
The air gaps on the plugs are good and as I said earlier, the plugs are very clean. Maybe too clean. Not sure what is going on here. I will be getting new sparkplug wires soon. The rotor and distributor cap are almost new.
I also have another issue I would like some help with. I have this Mustang with the 5.0 TBI and auto trans. Then I also have a 1988 5.0 Multiport Fuel Injection from a Mercury Grand Marquie. I have the complete engine wiring harness and the computer. Can I remove the multi port upper and lower and install it on my Mustang engine. Also will I need the computer changed to get the engine to run. Lests just ask how complicated is this and can it be done with out any major complications.
Thanks in advance for any help.
 

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That TBI system is a PITA. You can't really improve it, and once it acts up it's a pain to cure.
Swapping to the actual EFI would be a good idea, but you'll have to swap the computer to do it.
I can't remember how much pressure the TBI fuel system has, that's what'll have to be sure of. You'll have to change the fuel rails, regulator, and wiring harness.
 

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Want to thank you for your info. It has helped greatly in determining my direction. I know the speed density system I have is not as upgradable as the newer mass air systems but I plan to acquire the materials for the upgrade to mass air in the future. Just wanting to get this car to run a little better right now. Budgets are what the are. Thanks again.
 
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