I have a HP500 and having trouble timing my motor. It seems that when I put the timing light on and give it a total of 32 degrees timing and shut down and restart the motor the timing either jumps up to around 40 degrees total. It also sometimes goes back to 28 degrees. I notice when I put the timing light on and hold the mark the mark is jumping back and forth (the light) It feels like to me like there is something wrong with the ignition system because when I accelerate the motor the advance timing is not advancing. Anyone ever run across a problem like this?

First try a different timing light. I've seen the lights with the advance knob in the handle cause problems .

also the pick up lead on the #1 plug could be reading another plug??

Then check the rotor. It has to be tight on the shaft, no looseness at all. Its supposed to be locktited on. Usually, if the light is bouncing around at idle, its a loose rotor.

Is it a stock ignition or msd or something like that? Change modules in any case.

Thank you for your help. I have checked all that you have said and still the same problem. This is a stock ignition. The next thing is to try and change the module.
I'll keep you posted and again open for suggestions...

before you go and change the module you may want to check two things,
1) if thunderbolt ign check the hall effect pick up in the dist, merc has a redesigned sealed unit the older ones will rust and short

2) check the dist gear at the bottom of the dist and the bronze shaft bushing ( wiggle the shaft should be no play)

What GOODT said, I had a broken roll pin in a distributor gear that gave me fits for a couple of weeks.

What Year Is The Motor? and did you ground the wire (purple)

If it's a '99 like your profile says, then it's a Thunderbolt V and , like Baja said, you need to ground the timing shunt to enter base timing mode.

Dave

And It Should Be Set At 14.AND A TOTAL OF 34