Timing jumping around

"I timed my engine per the boa

"I timed my engine per the boat to 10 degrees before TDC. Did the base timing on teh 5.7L Merc with Thunderbolt V ignition. It recommended ground a wire to put it in base timing mode. When the jumper was disconnect and put timing light on, notice that the timinig mark is jumping around, did not stay constantly where it was supposed to. Thoughts?"

"Go under your distributor cap

"Go under your distributor cap, grab the rotor and check for wobble/play with your distributor shaft."

"Of course as Eddie says check

"Of course as Eddie says check the rotor. Alot of aftermarket rotors fit the shaft like crap and "float" around. Although all of those distributors have excess axial play, that is the distributor shaft will creep up and down in the housing. Usually the change in timming you are seeing is only at low or idle rpm's. As soon as the engine is reved up the helix in the distributor gear drives the shaft upward and stays there until the rpm drops. The only other explanation is the timing chain set is a little loose and will let the timing jump around. I would not have a problem with a 2 degree variability."