73 timing issue "keeps jumping around"
 

I have had my distributor rebuilt, I have tryed a used replacement unit, but my timing keeps jumping around. I put the gun on it and it's all over the place. What kind of checks can I do to find out what is causing this? I dont want to spring for a new distributor untill I know I have to. Could it be the chain tensioners are bad? Maybe I should put the money into upgrading to oil fed.

This is a 2.4 cis motor.

The distributor is driven directly from a gear on the intermediate shaft. The condition of the timing chains or tensioners should not affect the drive to the distributor.
How about the vacuum attached to the distributor? Pull the vacuum hose and see how it affects the timing.
When you say the "timing keeps jumping around", could you clarify that?
Of course worn out bushings and dirty dryed out distributors are famous for this, not freshly rebuilt.
You could also check the free play by simply grasping the rotor and turning it back and forth to determine if there is excessive play in the distributor gear to intermediate shaft gear.
Do the rotor and cap look like new? Excessive arcing?
Any possibilty of a loose wire, broken wire?

How much free play is excessive?

My timing is 5 ATDC, I set it with the gun, race the motor a bit and recheck it and it tends to move a bit (the mark) it does not hold at 5 ATDC. also when holding the light on the mark after resetting the mark its not steady its kind of vibrating around (shaking) if that make any sense. I reset it and repeat. the vacum checks out.

The wires are new and tight.

I thought the rebuild on the distributor might be questionable I took it to a place that doesnt speaclize on porsche.

If Bosch grease [or any other heavy grease] was used where it shouldn't have been, meaning anywhere but the cam lobes and points rubbing block ... it can cause erratic timing. Does the timing spring back rapidly when the rotor is used to advance the cam against the two springs of the centrifugal advance mechanism? If not, the cam may be sticking or stiff because of Bosch grease on the central drive shaft, or on the weights and pivots. Thin OIL is all that should be used on the weights and camshaft of the centrifugal advance!

http://forums.pelicanparts.com/showthread.php?s=&threadid=5432&highlight=distribu tor+lube

I live in Los Angeles where can I get my distributor rebuilt? I am starting to think that maybe this guy did not do the job right. How much should it cost?

Thanks

I'm assuming this distributor has points (not converted to magnetic or optical pickup)?

Could be the dist. cam (the piece that has the six "bumps" that open the points) is badly worn. It could be the points are older and the spring tension in them is weak. This would cause point bounce /erratic spark at high RPMs.

It could be worn bushings or weak springs in the advance weights, hanging up and not advancing smoothly?

But you say you had the distributor rebuilt? Any/all of the above should have been repaired by a competent shop.

Sorry to ask the obvious, but you DO have the dizzy tightened down, right?

And to reiterate Mo, they points are new?

If your advance springs are old and weak - or if you have really light springs for a quick advance curve, the distributor could just have issues with pulling the advance weights all the way back in at idle. One way to check this is to put a really tight spring in temporarily, and see if your timing still jumps around. If it is steady, then you may want to tighten up your old springs a little, or lube the weights. If this is happening, you should also see your idle speed hang up a little.