Ignition tune up

[cushjbc]

Mespe's recent issue got me motivated to tune up my ignition. I bought a Bosch MMD 540H automotive multi-meter that can measure RPM, dwell, injector pulse width etc. I found the ignition spec in 07.5.1-515. If I am doing things right, my dwell is way late / long.

I measured dwell in two ways:

1. Red to terminal 15 on coil, Black to ground
2. Inductive pickup on terminal 4 (hot coil wire), Black to ground

Both resulted in dwell around 45 degrees. Spec is 32, ideal is 30. An article I read on the internet says that too much dwell makes a weak spark because the coil charge fades.

I have never opened or pulled the distributor, never looked at points or pulse generator. I furthermore do not understand the inverse relationship between point gap and dwell angle referenced in the engine manual. I do not have spares for anything on the distributor.

Do you think a dwell that large can be corrected or should I shop for new contact breaker points?

[RadioTek]

Something isn't quite right. 45 degree dwell on a V8 would be the points not opening at all, a no start/no run condition. You might be on the wrong terminal of the coil. You can adjust the gap of the points to vary the dwell angle.

Make sure that you have a good ground for the ground lead of your meter. You might have to scrape some corrosion off a bolt head or something to get to clean, bare metal.

Make sure you have the meter set for 8 cylinders.

If you look under the distributor cap at the points, you should note there is a small notch in the points assembly. Next to it should be 2 small bumps, you should be able to loosen the screw just a small amount and use the bumps and notch to vary the gap of the contacts. The bumps and notch should be near the screw.

This is a serious oversimplification, but it sort of gives you the idea.


Think of the action of the coil like a toilet being flushed. The tank is filled up, then the handle is pulled down, causing the water to flow into the bowl and clean out whatever is in there. The tank would be the coil, the rotor pointing to the plug wire terminal would be the handle being pulled, and the points would be how long/how much water is being put into the tank. (Electricity would be the water) The "flush" would be the pulse to the plug. Electriticity moves a lot faster than water, so things can happen a lot more often.

The points give a pulse of juice to the coil, and letting it 'fill up' sufficiently will let enough out when it fires the plug. The Dwell angle is the amount of time, expressed as a number of degrees of rotation of the engine that the points are closed, 'filling up' the coil.

If you want a really detailed explanation, you can Google around until your eyes glaze over and your brains run out your ears; have fun.

The "quick and dirty" way of setting the dwell after replacing the points or having gotten them way out of adjustment is to turn the engine manually until the rider for the points is on the tip of one of the lobes of the distributor shaft. (you have to be careful to get it as close to centered as you can) Then loosen the screw and adjust the gap of the points to .014". About the thickness of a business card.

This should let you start the engine when the cap is put back on. You should then be able to measure the dwell with a meter, to "fine tune" it to the specified value. It's a bit of a 'trial and error' process, involving starting, measuring, shutting down, remove the cap, adjust, replace the cap, then repeat. Then you adjust the timing, and re-check the dwell and adjust if necessary.

There's a reason Pertronix and Crane/S&S kits are popular.

Hope this didn't confuse anyone too much
Scott

[cushjbc]

Scott,

Thanks for the descriptions. I did some more research and found that MB put a signal pickup shoe on terminal block 7 which lies on the inside fender just below the series resistors. If you follow the low signal green wire from the back side of the distributor, it ends at this terminal block. When I measured dwell and idle RPM from there, I had more reasonable dwell readings to work with.

As you said, it is frustrating to get the distributor shaft to a good adjustment point. In the end, I was able to clean up the distributor and points and adjust everything to spec. I noticed an improvement in throttle response and a smoother idle.

By the way, that Bosch MM is a good one for D-Jet.

[meltedpanda]

remove and put in a set of petronix magnetic points, does away with the dwell issues and elimintes any distributor shaft issues. Lots of information on how to do it, search my user name for details

[cushjbc]

I spent some time looking at Hall effect (Pertronix) points. I like the upgrade concept especially since it is low profile. Thanks to all who shared info on this topic over the years.

I have an idea about the power concern where leaving the ignition on could burn up the pickup if the distributor shaft just happened to be in the wrong position. The fuel circuit in our cars has a safety feature designed in the ECU (power for one second when ignition is in position I or II, power during start, then power if the engine is above 100RPM). If the magnetic pickup was powered by terminal 87 of the fuel relay (terminal 2 of the 12 pole connector of the engine harness that connects to the main harness) it would benefit from this safety feature, eliminating the need for a kill switch or manual disconnect.

I will replace my contact points with the Pertronix 1885 kit when they wear out or I get tired of chasing dwell angles around. For now, the original design is set and producing good spark so "don't fix what's not broken" prevails. I do see that it may happen sooner rather than later.

[RadioTek]

I had to go with the Crane/S&S upgrade for my car. The '75 has some peculiarities with the distributor that a stock Pertronix unit won't handle. I have wobble in my distributor shaft, which was eating the rider for the points so fast that I couldn't keep a dwell adjustment for more than a few weeks.

Scott