I know very little about dwell, so will start researching that now - thank you!
You are 100% right about wear on the distributor. It’s something I’m surprised isn’t discussed more - as it has a catastrophic effect on performance. I spent a week troubleshooting, finding lots of other problems (which were good to solve) – but the root cause of my issues were down to the distributor not reliably retarding. So no matter what I did with the car, it always appeared as if the mixture had changed or the timing was out as I could set it up to run, and then find after a short trip up the road it was all messed up again!
A refurbished distributor has transformed the car, so it’s now just down to final tweaks really. It’s just that I am a little uncomfortable at trying to solve for too many variables at once - idle, mixture, and timing. I’m really surprised at how hard it is to find a reliable idling number!
Regarding your comments about it being nice to see someone trying to make points and condenser work, thank you for the encouragement – that’s nice to read. I’m keen to prove to myself that I can make the car run as well as it did when it was produced 50 years ago. The beauty of the 500 for me is in its simplicity (and of course it’s iconic design), and I get a lot of satisfaction when I solve a problem! And I have lots of little problems to solve on this car
I really enjoyed reading your reply.
Dwell? It's more important than the points gap. Although, if set correctly on a distributor in good mechanical condition then the dwell should be acceptable. Also it requires a Dwell Meter. I may be telling you stuff you already know here - if so apologies - but others may be interested too? So, what's this "dwell" thing? And please Mike or others, do join in if you want to contradict/modify what I'm saying here. So, Dwell? When the points close battery current flows through the ignition coil, across the points and goes to earth. this establishes a strong magnetic field in the coil. when the points open the flow of current is instantaneously cut off which causes the magnetic field to collapse and it's this collapse of the field that generates the HT produced in the coil tower and is fed down the HT "King" Lead to the Dizzy. There are two main important things which have to happen - and a number of small ones we can disregard too - but the two main ones are that, and most importantly, the magnetic field in the coil must have enough time to fully establish and then, secondly, this field must have time to collapse entirely before starting to rebuild for the next event. The number of degrees through which the distributor cam turns with the points CLOSED is the DWELL ANGLE and is what the Dwell Meter measure. If it's outside the quoted angle then the points need to be adjusted one way or the other to either increase or decrease the dwell angle. Too small a dwell is indicative of too large a static gap on the points and vice versa. It's a bit time consuming as it means, if the angle is outside the recommended limits, taking the dizzy cap off, adjusting the gap a smidgeon and replacing the cap then running the engine to recheck and, when you're inexperienced this can take quite a number of goes before you get it right. When you get good at it you can actually adjust dwell with the coil king lead earthed to the block and the engine turning over on the starter while you "fiddle" with the points locking screw and gap adjustment, Depends on how accessible the points and locking screw are. Some of the later cars back in the day, had an external adjuster on the distributor specifically so you could adjust dwell with the engine running.
One of the interesting things about dwell is that it gets to be a really important factor when you either try to operate a high reving or multicylinder engine because dwell is actually nothing to do with the angle the dizzy cam turns through, it's all about the time the field in the coil has to build and then, more importantly, collapse. So, for instance, The old V8s would sometimes have a distributor with two sets of points, and two coils one for each alternately firing 4 cylinders So it was really like two 4 cylinder setups. Also "performance" coils were available for competition engines which could collapse their magnetic fields more quickly than a normal jobbie. A 6 cylinder competition engine, maybe an XK Jag or similar, was generally considered to be about as much as a single points controlled system could reliably work with. The other problem often encountered was the "condenser" - actually a capacitor - which was there to protect the point from sparking as they opened by acting as a "sink" to which the reverse current surge from the collapsing coil field could be diverted. This is very important as it's the instant at which the points open, so cutting the current to the coil and initiating the HT to the King lead, which dictates the exact commencement of the HT which fires the relevant plug. Sparking at the points, apart from leading to early points failure through burning, also blurs the precise moment at which the spark occurs thus "blurs" the exact timing and destroys performance. Our boss insisted on plugs, points and condenser always being done at the vehicle's annual service and I can remember hearing some of our more mechanically minded customers having quite in depth conversations with him about whether the condenser really needed replace at every "major" service. It probably didn't to be honest, but, it was good "insurance", cost very little on the total cost of the service and, probably most importantly, ensured reliable vehicles for the vast majority of our customers.
If you've understood all that then well done - I'm now feeling a bit light headed myself! One of the dead giveaways on those old Vivas or anything else for that matter, was connecting the dwell meter up (which for us was part of the Sun Tuner work bench we used on every big service) and observing the dwell angle shown. If it held a pretty steady reading - they always jump about very slightly - then the Dizzy was usually good. If the dwell was jumping around, up and down, (we had the advantage you could put it up on the screen as a graph) then it was usually an indication that the shaft bushing were worn allowing the shaft to wobble which, of course, made an absolute nonsense of the coil recovery times and ignition spark settings. The "nasty" thing about this for the DIYer was that you could set the points as exactly as you liked and the engine still wouldn't run well. The problem was, of course, that with worn distributor bushings the shaft would have been pushed to one side while you were statically setting your points but then, on starting it up the shaft would "wobble" thus making a nonsense of all the time you'd spent trying to get the points gap set absolutely "spot on".
Right Mike, koalar and all you others. What did I get wrong? Be gentle with me please.
Edit. Although of course, you need the engine running to check the dwell and dynamic (strobe) timing, so may have to "fiddle" with a carburettor to get the thing running, you must get the dwell (points gap) and timing set up correctly before finally "fiddling" with carburation. Also of course points gap, whether you do it with just feelers for gapping or by dwell comes before timing comes before setting fuelling. So it's POINTS, (maybe using a dwell meter) then TIMING then FUELING (which will likely also involve idle speed adjustment as older fixed jet carbs need to be idling for the mixture screw to be effective. Stuff like SU and Stromberg we can have another whole conversation about if you like lads?
