generallee440

i have a 91 gt with a 306 trick flow top end, 24 pound injectors with matching mass air flow sensor, long tube headers, just did a tune up, new plugs, wires, rotor cap, new pick up in distributor and module and coil. it has a miss in it about 3000 rpm. when it start to miss the car it shake and the tach will jump up and down. i think it is in the ignition some were but dont know what to cheak. i have set the timing at 12. even put in a spare distributor and still does the same thing. it could be fuel but would the tach jump up and down if it was fuel. the car was tuned about a year ago and has ran fine untill now. any help will be appreciated

mgmuscari

Are the RPM's jumping or just the tach? Maybe some bad wiring between the TFI and the computer? I'm imagining a situation where there could be some capacitance added on the tach signal from the TFI, which would make it difficult for the computer (and tach) to sync to the engine speed: higher RPM's = higher frequency signal from the TFI module, and added capacitance on the line could cause that signal to be "filtered" - if there's some kind of line capacitance it would essentially act as a low-pass filter and drown out the high-rpm signal.

This is just my (technical) guess based on my BS in electrical engineering; people more familiar with the Ford ignition system and TFI module may be able to help you pinpoint it to a part or maybe the harness better.

Does it miss past 3000 rpm?

Best thing to do if you have a Ford computer is probably to start with a KOEO test, and then a KOER test.

generallee440

it will miss at 3000 and up . i dont relly know how high is does it because i let out of it when it starts to miss. the rpm are jumping around with the tach. i will get a snaner and hook up to it and see what it says

lxman1

I would first check the fuel pressure at high rpms under load since you have basically replaced the entire ignition system. You might also check the alternator output to make sure it is at about 13.5v or more.

Venomantidote95

If you just slowly roll on the pedal does it still do it? or just at abrupt romping! Sounds like fuel do you have a stock or aftermarket fpr?
also was your car tuned for 12* base, did you decide to change it?