Thread: Strange Tach bounce and rough running!! HELP!!!

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Hello fellow carguys
I have a strange problem that occured this sunday. I have never driven my 1986 Mustang to the track and back(300km), so I did it. It is a trailer queen, so I just cruised up, no racing. I drove it nice, with occasional evil passes, but very obedient. Everything was fine and ducky untill I was heading home. I started it up, went to drive out the gates of the track, car starts idling high. I have an edelbrock 750 on it and the throttle has been sticking occasionally but not this time. So i turn the idle screw down, shut the hood and get it. I noticed the tach was jumping up and down, not really showing me the propper RPM. It was probably idling at 900(my ear) and the needle was jumping anywhere from 2000 to 4500rpm. The idle was as steady as it gets for my cam. Started driving, noticed a loss of power. I got in 5th gear, where before it used to have instant giddy when i slam the gass from 2000rpm, is now a lag(no bogging). It felt like I was driving a different car, and lost about 100 horse. I have had this thing from 100km/hr to 280km/hr within a minute, so I know what the initial feel is when you honk on it.
My issues are:
-severe/not accurate or real RPM fluctuations
-rough/low idle - gets better as the rpm goes up, but no power
-smells very gassy, like incomplete combustion. Extremely rich, unburnt, no black smoke
-kind of missfires, won't run smooth unless you go above 2000rpm

Its an 86 rustang LX trunk
It has a 302 bored to a 306, pistons and rods I am unsure(i didn't build it)
E303 cam
E7 heads done to 202/160, roller 1.6 scorpion rockers, blue valve springs(not sure what spec), blue valves(??)
weiand dual plane intake with a 2" spacer on top
edelbrock 750 carb(engine will not run with anything else since ive had it)
MSD 6al box with plug in rev limiter
Accel yellow coil
plain distributor with no timing advance(3 wire)
External holley electric fuel pump with inline regulator(6 psi to the carb)
Autometer monster tach with shift light(which never turned on when this problem happened.

The tach seems like its getting scrambled signal from something, I think. It seems like when you see static on a tv screen. It keeps fluctuating but not for 1 second does it actually show me an rpm. I wiggled wires, checked powers, grounds and everyting is the same as when I left home, intact and connected.
This setup has essentially been the same for the past 3 yrs minus the spacer, suspension components and other proper fuel system mods I have made to have a bullett proof system.
This car runs constant 12.60s in the quarter without nitrous or power adders, with 3.73s in the back, so I know when shes not all there. Its driving me, quite nuts. Its going away but I want one more cruise!!!

Please guys, any info you can throw my way will be greatly appreciated!! My iontuition is pointing towards ignition, since it smells like an extremely rich mixture.

Thanks in advance!!!
Val

Could be the coil, could be the MSD box would be my best two guesses. An ignition problem can also leave a lot of unburned fuel, aka a very rich condition......

So, I have looked up the diagram for both: tach and msd box. The tach wire is supposed to go straight to the msd box. That is where the tach is getting the scrambled signal. I couldn't get to rip the wiring loom apart today, but now its down to which one. I am going to measure the resistance in the pickup coil(what should the resistance be?). I will try to measure the ignition coil(what resistance trst should be done?). Perhaps i'll run a spare tach under the hood isolating my tach completely.
I have to spend a good couple of hours checking this right, so I can replace the actual issue. Its all the time but with this msd 6al box i have no experience with.
Any advive helps thanks alot man!!!

MSD does make a tester for the box, some MSD retailers keep one on hand to check customer's boxes with. Might check around locally and see if anyone has one. There is some troubleshooting help on the MSD website, and in the past I've got a lot of good help from their techs!

I'm with Dave, I'd try a different coil. I wouldn't trust anything made by Accell.