Re: Engine tach rpm drifts at 4000-5000 rpm
I'm gonna guess here. Tach's that I remember had a rotating part and a stationary part (internally) . The rotating part had a magnet on it and a piece of steel. The faster it spun the more times the magnet influenced the steel and pulled harder on it. Hence the needle would deflect more and more. But if your needle is dragging because of dirt, which is nonuniform around the circle, the needle would hang and get free and so on. The faster it turned the more noticable it is.<br /><br />I'm going to guess that your tach is dirty and these little jewels (that the needle pivots on) need a bit of maybe WD 40; something real light preferably with some cleaner in it. Or maybe pour on some alcohol and hit it with 20ish pounds of compressed air to blow out any crud (keep the air pressure low so as to not damage the little fine parts). <br /><br />Jewels (if used) don't need lubrication (like in a watch....I remember Bulova used to have 21 jewels in some), but yours may not be jeweled. If metal on metal would need some oil I'd think.<br /><br />I have a service manual but is says nothing about the care and feeding of instruments.<br /><br />Mark
