Engine give weird "puff" when turning off ignition

For some reason, I couldn't get my engine started tonight. It cranked and cranked, and sounded like it almost caught a couple times, but would never start. Here's the weird part. When I turned off the ignition switch, there was this odd "puff" from the engine. I was able to repeat this a couple times. I would turn the key on, crank it for a few seconds, then turn off the ignition and "puff". And one time, I got a full backfire *BANG*.

Any idea what is going on here?

Post edited by: Skyman, at: 2007/09/21 22:48

The '84 GPz550 I once owned did that when the ignitor failed. So if it's your '82 CSR1000, then that would be my guess as to the prob - bad connection or failed/failing ignitor.

RetroRiceRocketRider wrote:

The '84 GPz550 I once owned did that when the ignitor failed. So if it's your '82 CSR1000, then that would be my guess as to the prob - bad connection or failed/failing ignitor.

It's my '78 LTD. Does that make a difference?

I should probably add that I have a Dyna S installed. But the '78s didn't have ignitors, right?

I will also add that I took a spark plug tester (the light that plugs in between the plug and the wire). I had spark on all four plugs when cranking. And the plugs fire again when I turn off the ignition. That's what is causing the backfire.

Any idea what would cause this?

Post edited by: Skyman, at: 2007/09/22 01:35

Actually, it's gas building up in the cylinders when you are cranking it and it isn't starting. When you turn the ingition off it is like opening the points, and causes the coils to fire the plugs, if a valve is open you will get a puff like that. Check that your vaccum caps didn't get blown off your intake boots when it happened.

The Milkman wrote:

Actually, it's gas building up in the cylinders when you are cranking it and it isn't starting. When you turn the ingition off it is like opening the points, and causes the coils to fire the plugs, if a valve is open you will get a puff like that. Check that your vaccum caps didn't get blown off your intake boots when it happened.

Okay, then it is normal for the plugs to fire when the ignition is switched off?

If so, then I just need to figure out why the darn thing isn't starting up.

Thanks.

If not already, would use strobe-type timing light to check timing and operation of advancer unit.

On the Dyna S, it is normal to get a spark from one or both coils when turning off the ignition. If the engine had been running, it shouldn't cause a backfire because the exhaust system should have mostly spent charge in it. Since your bike won't start, you have fuel and air accumulating in the exhaust system. That pops when the final spark occurs.

Have you tried starting fluid to see if it's maybe a weak mixture?

Post edited by: loudhvx, at: 2007/09/22 09:56

My bike does that once in a while.(starts perfect) Dyna S installed about 6-7 months ago. I thought maybe the ignition switch was starting to short out? Only does it maybe 1 out of 5 shut downs.

Okay, problem solved.

On close inspection of the coils, one of the primary lead wires was broken where it connects to the coil. I resoldered it to the post.

But the bike still would not start. So I pulled my plugs. They were all fuel-fouled. All that time cranking with no spark fouled them real good.

I just happened to have a set of new plugs which I had originally bought to use on my CSR project. I put those on, and it fired right up.

Is there any way to recover fouled plugs? Before I put on the new plugs, I tried cleaning up the contacts with a piece of sandpaper. But that didn't seem to help. Can they be cleaned, or should I chuck them?

There are actually machines out there that clean up plugs. They look like little pencil sharpeners, but I don't know the details of how they operate. A friend of mine had one and it worked to some degree, but was no substitute for new plugs.

I think the little pencil sharpener is actually a tiny sandblaster. you can recover plugs with a sand or bead blaster, but like loud said, they are never quite like new. Also MAKE DAMNED SURE you blow/clean all those tiny bits of sand/glass out of the plug before it gets anywhere near a bike.