I've got a 73 302...(has rebuilt 69 heads) just replaced distrib. (stock Motorcraft-professionally rebuilt) and when dialing in the point gap, the dwell numbers wander around at throttle up.

My Pertronix Ignitor was defective, so I've re-installed the points until the replacement arrives. Vac Advance tested fine. Could this be the timing gear is worn out on the cam shaft?

I planned on replacing the camshaft/and timing chain/gear with stock ones...what was the throw on a stock 302. I don't want to get into a discussion on hot cams and all that really, I just basically want to get it running smooth again...This truck is a farm truck D.D, all I want is an original 302 Cam, or something not too extreme that's going to open a can of worms on me.

Thanks in advance for any help,
Jason

 

Adjusting point gap will change timing and dwell. Increasing gap retards timing and decreases dwell, reducing gap advances timing and increases dwell. That's the nature of points ignition.

Edit: I got this half wrong. See posts #6 and #8.

 

Is the dwell wandering when you change RPM? If so, I'd question the distributor rebuilder about whether or not he did a good job, as a varying dwell is a sign of worn out distributor bearings allowing the point cam to wander and vary the dwell time.

Changing timing chain and gears will have NO effect on the ignition dwell. They are two separate items. The only thing that could happen with a weak timing chain is the timing chain could jump a gear tooth, causing the cam timing to be retarded, and then the initial ignition timing, measured at the balancer with a timing light, would be retarded (instead of 8* BTDC, it might retard to 0*).

I hope this helps you understand what may be going on.

 

Hey AZ Bill,
Thanks for the post...I generally time via vacuum guage and double check with a gun. The rebuilt distrib. I installed
was nice and tight and smooth. it's possible the points are a dud, although new. I'm going to have the distrib. tested to confirm...ignition troubleshooting is not my strongest area, so I'm learning here!
Best,
Jason

 

No problem, glad to help out with whatever information I have accumulated over the many years of working on various makes and models of vehicles.

As for points, I have always had great luck with Borg-Warner or Echlin brand point sets, and will pay top dollar for those over other brands. Nothing like replacing points in the middle of nowhere with little to no tools, and no dwell meter and timing light. Been there, done that, learned a lesson the hard way. RELIGIOUSLY changed points annually after that episode with brand name points.

 

My worst experience with points was where the mounting plate slipped and the gap got smaller, to the point of not opening enough to trigger the coil, and stalling the engine. Of course it was in the middle of one of the rainiest night of the season, and I didn't even have a screwdriver with me to re-set them.

Are you sure about this? Unless I have it completely backwards, the way to increase the gap is to rotate the point assembly toward the cam block on the distributor shaft. This means the points will open sooner, and close later, as it's pushed open by the cam lobes for a longer period of time. This is the trigger that fires the secondary of the coil, so it's the ignition dwell time. Since the points open sooner with a bigger gap, it also means the timing is advanced. It's been a long time since I set a set of points, but I'm pretty sure this is the case.

 

Hey guys,
Thanks for all the input. I got the part sent to me for the Pertronix Ignitor I have...it was a bad set of magnets in
the little disk that goes onto the dist. shaft. I re-installed the unit and was still having trouble dialing in the timing...
turned out to be a work vacuum hose down at the end where it goes into the trans. The P.O had used too short
a hose, so the hose had been kinda cocked and became splayed out at the end, not to mention brittle.
Changed it, timed in nicely, and running like a champ. Haven't re-checked the dwell, but the engine runs so smooth
now, not going to bother.

Jason

 

Thanks xlt4wd90 for pointing out my error. After studying a spare distributor, I realize that I got part of my response in post #2 wrong. xlt4wd90 is right, as the point gap increases the timing does advance, for the reason he stated. However, I believe dwell refers to the time that the points are closed (actually the ratio of closed to open time), so dwell does indeed decrease when gap increases.

Keep in mind that the coil fires when the points open.

 

Thanks Beanscoot. I didn't realize that was the definition of dwell. I always read it as an angle off the instruments that I used to use to measure it, so i assumed it was the angle during which the contacts were open, and thus the ignition time.