why would my timing mark be 4 inch low when running good on my 76 390 motor. when i put piston #1 to top dead center the mark is right on.

 

Slipped harmonic balancer maybe?

 

That would be my guess too. The rubber ring gets old and allows the damper ring to move. On an internally balanced engine it's no big deal, all it does is throw the timing marks off. On an external balance engine you will REALLY know when the balancer has slipped, the engine shakes like it's running on three cylinders.

 

when i bring #1 to tdc the mark is right on.

 

Actually it won't make any difference internal vs external balance because all the external balanced weight is in the center hub NOT the dampener ring that is supported by the rubber. And that is on ones that actually do the inbalance on the dampener itself. 385 series external balanced engines use a neutral balanced dampener and use a hatchet weight spacer behind it.

Ok What are you actually asking? That for it to run smooth at idle it is 4 degrees off what you think it should be? If that is what your issue is, maybe check for other issues like your idle speed, vac leaks, bad PCV valve etc.

 

Monsterbaby, I stand corrected. I was thinking in particular of the Mopar 360, the balance weight is in the outer ring. I had one in a Dodge Ramcharger that slipped about an 1 1/2". I had to drive that thing from Cincinnatti, O to Orlando like that. That was one miserable trip.

 

Are you using a dial back timing light?

Josh

 

the truck runs great weather it is at idle or going down the road, i would just like to know why when i put a timing light on it the mark is 3-4 inches below the tab. and no i dont think i have a dial back light.

 

#1 cylinder is the front cylinder on the PASSENGER side. Sounds like you are connecting to the #5 which is the front cylinder on the Driver's side.

4 8
3 7
2 6
1 5



Josh

 

no i am connecting to #1

 

Are you checking the timing with the vacuum advance disconnected and plugged?

When you connect the vacuum advance are you connecting to a PORTED vacuum source and not FULL manifold vacuum?

If the timing marks line up at TDC, and you're showing the ignition timing as that far off then the distributor is adjusted WAAAAAY out or you are on the wrong cylinder.

Josh

 

yes i unplugged from carb and capped off. if i move the dist to put on 6 deg below tdc the truck wont run good at all.