I have an 87 pickup with a 22r in it (carb). I just installed an aftermarket tach. I plugged the signal wire into the open connector next to the coil. The tach reads fine at idle or if I rev in neutral but as soon as I put the truck in gear and put a load on the engine the tach starts jumping around. It is so bad I can barley tell what RPM the engine is running at. I would say it jumps +-500 RPM but only under heavy load, in neutral it is completely smooth and if I only give it a little gas while in gear the tach is smooth also. Anyone have any idea why this might be happening?