Big bang while trying to start.

[K9Sicem]

I have a 76 LTD that I just got a week or two ago. I have driven it 50 to 75 miles without issues. I tried to start it last night without luck. When I shut off the key it back fired loud. I tired again and it did the same think. This bike has an electronic ignition. This is my first bike so I have little knowledge of what to look for. I have restored several old trucks. Based on that it is my thought that it is loading up with fuel and only getting spark when I tried to shut off the key. Any help would be appreciated.

[rcmatt007]

First of all. Have you changed the timing belts?

[K9Sicem]

I have not. It was changed when the bike was rebuild. It should be almost new.

[dontwantapickle]

I have had that happen on a Norton I had with an electronic ignition.
It turned out to be a bad key switch that only worked when in the perfect position or made contact when the key was turned.

It may not be your issue, but it might be worth checking out.

[Sagebrush]

Seems to me I remember something about a low voltage situation where the coils didn't have enough juice to fire while the starter was turning over but just as you released the starter button removing the starter amperage the coil fired one time and started the bike. Seems to me it was a classic symptom on a 1500 of a weakening battery. I'm not very clear on what the final cause was but I would check for spark while turning the engine over to see if it there our just too weak to fire.

[ericheath]

My 77 will kick back when I turn the key off sometimes, but not a Big Bang, and not when just releasing the starter button. The motor is dead still when switched off. It has Dyna Ignition.

Looking forward to theories.

I have wondered about the Hall effect of the Dyna triggering a signal to a coil as a result of the power drop as the key is switched off. The bike is supposed to fire ahead of TDC and with the piston stopped ahead of TDC, it would have to backfire.

The loud bangs could be from the waste spark touching off unburnt fuel in the opposite cylinder on the exhaust stroke.

[robin1731]

snip...…... thought that it is loading up with fuel and only getting spark when I tried to shut off the key.

First step. Verify if you have spark or not while cranking the engine. Pull all the plug. Place one plug in a plug wire and ground it to the engine. Crank engine over. You should have spark. And yes, installing just one plug will give you spark. You do not need two plugs installed to check spark with a Dyna.

Actually do this first. Before doing that turn the engine over with the starter with the run switch in the off position. Just to make sure there is no fuel in the cylinders. Have the fuel petcock turned off too.

.

[K9Sicem]

I took the plugs out and checked for spark. No spark while turn over the engine, but after I shut off the key I got one spark. I'm think the bang is coming from the burn off of unburnt fuel. How do I check the switch?

[Shadowjack]

Do you know if it ever ran with the Dyna? This is exactly what a stock ignition does if the ballast resistor gets unplugged. Some Dynas bypass the resistor, I think, but do you know if it was installed correctly?

[robin1731]

Do you know if it ever ran with the Dyna? This is exactly what a stock ignition does if the ballast resistor gets unplugged. Some Dynas bypass the resistor, I think, but do you know if it was installed correctly?

Depending on how it was wired though it should still have spark while cranking. But that is a good thing to check.

And yes, the ballast resistor should be removed if using both Dyna ignition and coils. But if you are still using the stock coils you still need the resistor. A new one comes with the Dyna.

.

[gltriker]

I have a 76 LTD that I just got a week or two ago. I have driven it 50 to 75 miles without issues. I tried to start it last night without luck. When I shut off the key it back fired loud. I tired again and it did the same think. This bike has an electronic ignition. This is my first bike so I have little knowledge of what to look for. I have restored several old trucks. Based on that it is my thought that it is loading up with fuel and only getting spark when I tried to shut off the key. Any help would be appreciated.

As this Topic progresses, there are quite a few mentions of Dyna.

Has this LTD's 'electronic ignition' actually been visually identified as a Dyna unit?

[gltriker]

I have not. It was changed when the bike was rebuild. It should be almost new.

Two timing belts. Not just a single, It .

It's a best practice to remove both timing belts' covers, now, and check for correct camshaft pulleys' static timing alignments.

Timing belts are very inexpensive.

See this Timing Belt Replacement tutorial from NGW Club's Shop Talk.
https://www.ngwclub.com/forum/viewtopic.php?t=3544

[K9Sicem]

Both timing belts are good. Yes, it is a Dyna unit. Ran fine. Next night I wanted to ride and that is when it would not start. I checked for spark. Will not spark while turning over. It will give off one spark when I shut off the key. Battery is near new and has good voltage.

[robin1731]

Have you done any of the suggested tests/procedures/diagnostics?

.

[ericheath]

Could be a worn ignition switch. The one of mine that exhibits this had a bad switch and I pulled it apart and tightened up the rivets inside.

Have you probed the wires at the Dyna to see if they are getting power with the switch on?