Unusual Ignition Problem

Superman · 11-29-2001, 10:22 PM

The strangest thing happened Sunday. My '73 Datsun wouldn't start. New cap and rotor. Points have a gap and I filed them a little. Plugs look okay and I cleaned them. I even wire brushed the threads. Checked them and there's a nice spark.

When I put the timing light on to just rough-check the timing, it wouldn't work. Then I noticed that the spark plugs and the timing light work fine laying on the valve cover for ground. Timing is roughly okay. The head is indeed nicely grounded in two places but it looks like the spark plugs are not firing when they are in the cylinder head. I guess I've never seen something like this before.

Early_S_Man · 11-29-2001, 11:35 PM

Check the ballast resistor ... it may be open.

Superman · 11-30-2001, 07:14 AM

Thanks Warrren. I did check voltage after the resistor (the truck has a Chrysler ballast resistor), and found under 8 volts. After wiggling it a bit, I got 10 or 11. If I bypass the ballast resistor, the engine still does not start. I'll fiddle with it this weekend.

Superman · 12-02-2001, 05:59 PM

The guy at the parts counter says a spark under compression is very different from one at 14.7 psi. He said the plugs I was buying wold work fine and that the old ones are probably fouled. the non-firing problem has nothing to do with conductivity he says. They're only six or seven years old. Anyway, they worked like a charm.

RoninLB · 12-02-2001, 06:10 PM

Great thread, so far. How can plugs be " probably fouled"? You removed them and they looked good enough to not run out and buy new ones. Is there more to this?

911pcars · 12-03-2001, 12:39 AM

Why do you assume the installed plugs are not firing just from what's not coming out of your timing light? Your test with a spark plug showed you have spark. Do this with each plug wire to confirm all cylinders are "firing". The spark, BTW, should be healthy. You should hear a loud snap. If not, you might have some contact point/condenser/primary ign. circuit issues.

You need three things for combustion: Air, Fuel and Spark. We'll assume we have enough air. You've checked for a good spark (although we don't know if it's happening at the right time). Check for fuel by removing the air filter and taking a peek into the carburetor throat. With the engine OFF, open the throttle. You should see the accelerator pump squirt a shot of fuel into the throat. If so, you have enough fuel to start the engine, although not necessarily enough to keep it running (that's a function of the fuel pump).

Back to the spark. If it happens at the right time and you have the other two essentials, it should start. If it backfires, the spark is happening but at the wrong time. Maybe a wrong spark cable connection, but this is leading us away from the basic symptom. Let us know what you find out.

Sherwood Lee

Superman · 12-03-2001, 06:16 AM

I'm not sure I explained this properly, Sherwood, so here goes. The carb has a glass window on the float bowl (should be required on all carbs) and there is plenty of gas. Yes, I can hear the accelerator pump squirting. I also have a can of starting fluid and I'm not afraid to use it. for these reasons, I knew I had fuel.

the spark plugs are rusty on the outside and sooty on the inside, about six years old and have given between 30K and 60K miles of service on a VERY OLD Datsun engine. Not hard to believe they could be fouled.

the engine does not even act like it might start. Nothing. I attached the timing light to each ignition wire and the light simply does not flash. This is a new experience for me. I have never had a timing light not flash. So, I removed all four plugs, reconnected them to the ignition wires and laid them on the valve cover. They produced a blue spark that did not snap audibly. I hooked the timing light to each one and the timing light flashes just fine, as long as the spark plugs are not in the cylinder.

So you see, this is how I can be so certain that the sspark plugs are not firing in the cylinder, but they are firing outside the cylinder. Plus, when I installed new plugs, the truck started and ran like a champ. Case closed, I feel.

Early_S_Man · 12-03-2001, 09:47 AM

Interesting ... I neglected to anticipate the possibility of six-year old, carboned-up spark plugs!

Reminds me of an adventure I had over Memorial Day ... 30 years ago on my uncle's farm in East Texas. He had a 1950's model John Deere 2-cylinder diesel tractor with external flywheel and V-4 gas starter-motor ... that they had been having to pull to get started. Seems that the starter motor had the original spark plugs in it! Took the better part of a day to get at the plugs, had to take most of the cowling off ... rusted bolts, etc. The next day, after installing new points and a set of Champion spark plugs, it started up on the battery for the first time in several months! Those old plugs looked like black moss was growing out the end of the plug, and it extended past the ground electrode! Rather amazing what can happen to spark plugs after 15-20 years of use, having never been heated up to operating temperature!

BK911 · BK911's Garage

Gentlemen!

I just wanted to make sure I read something correctly. You can check for spark by using an inductive timing light? You don't need to remove and ground a plug? If the light flashes you have spark, if it doesn't, you don't?

Is this correct?

BK

Superman · 12-03-2001, 02:02 PM

Yep. It seems as though a timing light will tell you whether the electricity is arcing inside the cylinder. I'd agree that this is a new learning for me and I'm likely to try and use this knowledge at some future point.

Warren, you shouldn't have to consider the possibility that an (occasionally respected) backyard mechanic would try to retain visibly gummed spark plugs. That would be idiotic.