I recently put in a 6A Box in my 88' Formula 350 and when I hit about 4000 RPM's my tach goes down to 0 RPM's untill It shifts. And then It reads it fine. Thanks

 

Mine did the exact same thing. There is something in the directions about removing some filter on the factory tach wire, but i didnt bother because it gave me a reason to buy an aftermarket tach which looks alot better.

 

Your Formula probably does not have the tach filter. The 87-89 generally did not. Check though, this will be with in 1-1 1/2 feet of the coil on the white wire of the factory harness. It is a small 1x1 black box with a white wire going in and out and a black ground wire. Basically unplug the whites and hook them together eliminating the filter. If this is not found you can get the tach adapter part number 8910 to fix the problem.

Thanks,

Joe
MSD tech