Hey guys,
I just got my rx7 running and driving after an engine swap, and it seems that after I go past 25 mph, the needle on the speedometer jumps to 50 mph and starts going haywire. The needle sporadically jumps between 40 and 70 mph, and it's impossible to tell the speed i'm going. Could this be caused by a loose connection at the back of the intrument cluster, or is it a problem inside the speedometer itself? I know the cable from the transmission works fine; I tested it by removing it and twisting the end with a drill, and when I did it, the speedometer still skipped around. Does anybody know what the problem might be?
Thanks

 

try some lube on the cable. A dry cable will give those symptoms

 

More than likely the speedo has gone bad. You can try taking it to a speedo shop to get it fixed. I had the same issue with my 80 SA a while ago and just swapped in a 79 speedo. My old speedo when I took it out was actually dragging on the bell that spins around the magnet to create the field that moves the needle, if that makes any sense to you. I tried tweaking it and did make it better but then I tweaked it one more time and it was really screwed up. Lesson learned, next time I'll send it out to be fixed.