[project89]

alrigh theres the deal,last week my tach started to jump up and down like 500 rpms,even though u could clearly hear the motor stay at the same rpm.ive had this happen before and it was due to arcing plug wires,so that the first thing i check.nope there all good,so i pull the plugs to see if any are fouled,nope all a nice tan/brown color just like they should be.so i pull the dizzy cap(oh boy)well the rotor had cracked and was running off center causing the rotor to rub the top of the cap,so i clean out all the debris,and drop in a new cap and rotor,put it back together and its still jumping around.i checked the wires to the dizzy/coil/stock tach/and my aftermarket tach.there all good.the tachs run off seperate leads,and they both bounce the same amount.the mnotor dosent miss at all.the only thing it does is when i take off or am driving and the rpms go above 3 grand the tach starts wildly bouncing to almost 7,000 triping my revlimiter making the car backfire threw the ex.

im out of ideas on things to check.u guys have any idea on what else could screw up the tach signal/make the tach bouonce around like that,that i could have missed