SB chevy timing retarding

[JimL]

I have no experience with MSD, but do have a dumb question.  How do they control the dwell time?

The reason I ask, is that years ago (about 1980) I had a "semi-transistorized" ignition (used points to fire an igniter) which was good up to 9000 rpm or so.  The problem I had, strangely, was at idle or very low rpm.  With the turn signals on, the idle speed would rev up and down, and my timing light showed the advance drifting back and forth.  Insufficient current was changing the dwell time for the coil and igniter.

It was a corroded wire harness feeding the igniter power, combined with low alternator output at low rpm.

so....fast forward to 2000 and a Pectel T2 fuel injection on a turbo banger.  The actual timing, at the plugs, would not stay stable under load, and it just wouldnt rev up during high cylinder pressures.  Pectel told me to go get Ford Ranger coils and wire them "Menard V6" style (small 6-volt battery in series to the coil 12V terminal).  That worked and solved the problem of not enough dwell to build enough voltage in the coil secondary to get the spark off on time.  In fact....it was pretty late, I suspect until cylinder pressure let down enough to make the jump!  

Might have been a fuel vaporization issue, also, because you can't spark on time if the spark plug is surrounded by mostly compressed air.  The timing light only flashes when the spark actually jumps, not when the ignition asks for a spark event.  Anyway, hot coils and 18V took care of whatever was going on.

I still dont get it, but their recommended fix worked just fine.

JimL

PS:  The Ranger coils held up to 18 volts ok, when the engine spent most of its time at high rpm.  They were the coils from about '96 or so, when it took a lot of spark to fire lean across a wide gap.  I dont think most coils will put up with that, but those never failed us. 

[Ron Gibson]

Do you run an alternator or is it a total loss system? If total loss, you need an alternator. Check for resistance between MSD box and battery, possible corrosion. If no resistance problem, try running it up with a battery charger hooked up. That will give more voltage and amperage.
On that thought, if you're running an alternator, are you sure it's putting out what it should. If a diode fails, alternators get weak on output but still work, then low voltage like was pointed out.

Ron

[sdroadster]

I'm going to try to get the car started today with the points distributor. The car has had 4 ignition boxs in it, and 2 of them were new. When I installed the last one I ran the harness over the core support near the radiator, and wired in an ignition switch directly off the battery. This eliminated all the ignition circuty inside the car. We put the car on a chassis dyno at JBA and it reved well untill 5200 rpm. The motor then sounded like the driver lifted, and shifted gears. The horsepower on the computer screen peaked, then made a u turn and dropped like a rock. I suspect the problem is electrical, but what are the chances of 4 boxs, 2 of which were new, and 2 new distributors. The car is wired nicely. It has plenty of ground straps, and the msd box is wired directly to the battery. Beats me.. I'll keep you informed. Thanks for all the response.

[sdroadster]

I just saw the last 2 posts. I am running an alternator, and I have had it checked, although I will have it checked again. The timing does retard then at about 4500 it starts to drift. The timing will drop off then come back and then drop. I have noticed with the tach pluged in the condition seem worse. Hopefully I will run this again today, and then reinstall the MSD distributor and add the battery charger. Thanks!  Oh, based on the symptoms, I installed a new tach too.  (money pit)

[Sumner]

Do you run an alternator or is it a total loss system? If total loss, you need an alternator....

I agree that it is best to run the alternator, but this year we ran out of time and didn't get an alternator on the car and ran on the battery only with an MSD Box and had no problems, even 4 runs on the same day back to back without having a chance to recharge the battery.

We will have an alternator on the car before it goes back as that is the best scenario,

Sum

[Ron Gibson]

Years ago,(amazing how time corrupts some memories) 1968 when I had a service station (yes service), I had a customer that the same problem as yours on his 396 Chevelle, Your last post reminded me. This was before MSD existed. The problem was chased down to a faulty tach. It was semi-shorted and drawing way too much power therefore lowering voltage to the coil.
Have you checked voltages while doing your tests? Good Luck.

Ron

[manta22]

JimL;

Regarding the question of how an MSD (or any modern electronic ignition box) controls the dwell time-- it doesn't. "Dwell" was the % time that the points were closed, allowing current to flow in the ignition coil primary. It takes some time for the current to rise when driving an inductive load such as a coil, so the voltage must be applied to the coil primary winding for enough time to allow the current to rise to a high level. The magnetic field increases as the current increases and when the points open, the current flow stops and the magnetic field collapses. This sudden change in magnetic field generates a high voltage in the coil secondary winding and that voltage fires a spark plug. This was the venerable "Kettering System" that was used for used for years.

The fundamental problem with the points system is that at high RPM the points must still be closed long enough to build up the magnetic field; to do that, the needed dwell time gets to be a larger percentage of the crank cycle; at some RPM there is simply not enough time to build up the required spark energy.

Ignition boxes such as the Crane (MSD may be similar) fire a short, high voltage pulse into the primary of a special coil (actually a pulse transformer) and the secondary voltage fires the plug. Since the pulse is very short it is not a big percentage of the crank cycle time even at very high RPM. These ignitions can deliver very high energy to the plugs.

A common problem is that noise pickup can cause the box to false-trigger and the spark timing goes spastic. It is important to have a clean 12VDC power source and a good ground. The most vulnerable place for noise is the timing pickup wire. Whether it is from a crank trigger or points trigger, it needs to be routed away from other wiring or ever shielded. Don't forget to use a heavy bonding strap or wire to ground the engine block to the chassis. Good grounds on everything are a must.

I hope this helps.

Regards, Neil  Tucson, AZ

[Ron Gibson]

Question??? Doesn't the tach work off of the trigger from the distributor or crank trigger, whether points or electronic wouldn't make much difference if the tach is robbing or distorting that signal.

Ron

[Seldom Seen Slim]

Further on what Neil said about the wire maybe picking up stray signals from other wires - and therefore run the wire away from others:  Shielding might conceivably cause some capacitance effects, especially at higher revs, and those might maybe perhaps farkle with the signal to the coil(s).  If you can, run the wire so that it crosses the other wires at a 90-degree angle - don't cable it neatly in with the other wires.  It might take some doing -- but if that's the problem you're having, this "crossing at a sharp angle" trick might help a bunch.

(Ham radio training on building electronic stuff to the rescue, right, Neil?)

[manta22]

Yes, Jon. We learned that stuff the hard way. 

Low capacitance shielded wire is a good idea if the length isn't fairly short.

Regards, Neil  Tucson, AZ

[Crackerman]

Also twisting in a couple of ferrite beads is a fairly cheap and effective method of filtering noise if you cant run  Shielded wire  for whatever reason.

I would still be tempted to look at the mechanical end of things if you have already tried different boxes and arrangements.
Mostly wear in distributor gear and shaft play up and down. The shaft play is easy to check. Pop the cap off and pull. It will rotate if it has any travel.

[sdroadster]

Thanks for the ideas. Please keep them coming.Today I got the motor started with an old Chevy points distributor that I found it in a drawer with related "stuff". The timing jumps around a bit, probably because the distributor is worn. However, it doesn't appear to retard as rapidly, and as conistantly as the MSD. I checked the battery voltage with the engine running and it was 14.45 volts. So it appears there is plenty of voltage for thr MSD.I called an accociate in the San Diego Roadster club for help. He is coming over tomorrow with a tool to check valve spring pressure, and several known boxs and distributors off his dyno. Hopefully I'll have some more news in the afternoon.

[johnneilson]

Sounds like maybe time to find an old Sun Distributor machine and test system.

John

[RansomT]

It almost sounds like a false rev limiter.  In the bike world, that happens when the crank/cam sensor signal gets outside of the normal expected range (e.g. Pushing the cams too high).  OR. When the ingnition box is reading the incorrect number of cylinders, then you get a 1/2 rev limiter.

[SPARKY]

have you checked the whether the MSD and the tach will work with each other-- some tachs you have to run a "black box" with them and some MSD systems---go online a check if they will work with each other.