Calling any ‘79 Points Distributor experts
 

Folks,

I have two bone bone stock ‘79 SAs with points distributors. I’m looking for someone to help diagnose why one of the distributors works absolutely fine in both cars while the other distributor (the shiny one pictured below) will start one of the two cars and idle fine, but stumbles terribly with any throttle inputs. Points in both dizzys are gapped to spec. The one that works well has old points and condensers, while the newer distributor that runs poorly has new Mazda OE points and condensers. The trailing pots on the bad (shiny) dizzy appear to be aftermarket. The only visible setting difference is the bolt position in the plate “slot” at the 7 o’clock position (see pics below).

Before folks pile on and tell me to convert to electronic ignition, keep in mind the my goal is to keep these both bone STOCK. Jeff20B recently rebuilt the carb and timing is properly set on both cars. He and I have discussed this issue and have come up empty. Any thoughts on remedies or things to try would be greatly appreciated! Thx 👍🏼

https://cimg6.ibsrv.net/gimg/www.rx7...33d78c57a.jpeg
Good dizzy
https://cimg5.ibsrv.net/gimg/www.rx7...3fbef46e4.jpeg
Malfunctioning one
https://cimg7.ibsrv.net/gimg/www.rx7...65fb5f153.jpeg
Note difference in slot position on Phillips head screw at 7 o’clock.

If these cars were brought to me for this issue,after a close inspection which i assume you've done by now,1st thing i would check is quality of spark to all plugs on both cars. Pull all plugs from the engine,put them into their respective plug wires and lay them on top of engine for a good ground,have someone crank engine while you look at plugs closely.Though points ignition is not as strong as later FB ignition,the spark generated should be blue in color on all plugs and all should fire consistently. Compare both cars to see if any difference,If car with new dist has weaker spark,that's a clue,start there.

2nd thing i would check is base ignition timing on both cars with a timing light,leading and trailing to be certain that both line up with pin on front cover. Next,pull vacuum hoses from both advance units and plug hoses. Slowly increase rpm while watching timing light(a dial back light is perfect for this)and see if timing advances smoothly and how far from the pin it advances by @3500 rpm. Leading and trailing should advance the same amount.Take note of distance advanced on each car,or how many degrees if you have a dial back light. No two cars/distributors exactly same but should be reasonably close.This is checking centrifugal timing

3rd thing to check,with cap/rotor off,apply vacuum to each vacuum advance can with vacuum pump and note how much each leading/trailing plate moves. They should begin to move with @ 8-10" of vacuum and should be fully advanced by @ 14-15". Leave pump connected to each can and watch to see if vacuum drops. A good advance can will hold that level of vacuum indefinitely,one with a leak in diaphragm will drop vacuum,faster the drop,bigger the leak. Compare test results between both cars.
Connect both vacuum hoses to cans and start engine and slowly rev to @3500 rpms and watch timing advance. It will advance more quickly and further than last test,take note of how far or amount of degrees with dial back light. Timing advance should be smooth,not jerky and should be about same on both cars.Base/centrifugal/vacuum advance should have a steady flash from timing light at all rpms,any dropouts/glitching or dancing around of timing marks is another clue to follow up on. A 1st check for this is to remove cap and attempt to wiggle leading and trailing points plates in both distributors for any play,should be none. Next remove rotor and try to wiggle dist shaft side to side looking for play,should be no appreciable play in either dist,compare.

At this point i would scope the ignition,if there's a fault in primary or secondary ignition electrically it would show possible fouled plugs,weak ignition coil,shorting pointset,shorting condenser.
Without access to a scope,if none of previous tests showed anything,about all you can do is look for common denominator and diagnose by process of elimination. Most likely cause at this point will be ignition components:points,condensers,distributor wiring and connectors.
Just because points/condensers are new does not mean they are"good". This is not as common with Mazda oe parts,but happens,Mazda doesn't actually manufacture them. OP doesn't mention if problem existed before new pointsets/condensers installed and replacing them was meant to repair problem. Distributor looks to be a reman judging by screws,hardware,etc.
Process of elimination...set both engines to TDC,be certain of this. Remove both distributors,swap pointsets/condensers,wiring with connectors from "old" distributor to"new". Set pointgaps and install distributor,set base ignition timing and run/drive car(s). If problem is gone and both cars run fine,swap"new" points,condensers,wiring with connectors one piece at a time back into "new" distributor til problem"comes back" to determine which part(s) to be defective.
Systematic process of elimination...tedious but will get you there. Happy hunting! Post results.

i had one where when i got the car it ran ok, but the tach jumped all over. it turned out that the base plate was just shot, a new one of those, points, cap, rotor, wires and plugs sorted it.

the next thing to pop up (same car) was that the power wire to the points failed. this one was a bugger (and the only time since 1994 i've needed a tow truck), as the insulation on the wire was fine, but it had broken internally

Guys, thank you both so much. GSLSEform, thanks for the most generous info. I’ll try some of your troubleshooting ideas. The rough running began just a few short hours after installing new points and condensors with my previous problem being weak to little fire on the trailing side. Initially it ran great after install. Coils are good, so I’m betting it’s either the trailing vacuum pots, or wiring to the trailing points & condenser. We’ll see!!

Follow testing suggestions in sequence so you’re not chasing your tail...
I don’t think you’ll find the trailing vac advance can to be source of your problem.
Don’t pigeon hole yourself into thinking problem is only to do with trailing ignition. The car will run fine with little to no spark from trailing ignition,you might never know as makes no difference in power or drivability. Trailing ignition only function is to help clean up emissions.

+1

Understood. I will.