Topic: Loud bang. Misfire?

[felicious]

I noticed that my spark plug boot on cylinder #3 was not seated correctly. Spark would not happen reliably unless I pushed the cable over with my thumb. So I took the boot off and screwed it on tightly hoping that the spark would catch on. Started the bike but heard a loud bang, after about a minute, coming from my exhaust on the right side. Turned off the bike and pushed it back to its spot. Same situation again this morning.

Is that a misfire? Sounded like an old car from a 30s movie if that makes any sense?
Where could this come from? At first I thought it's unburnt fuel but now that it happened again I'm not quite sure.

1978 CB550K

[Vinhead1957]

Could be that plug is fouled and only fires intermittently ! Fuel is collecting in the exhaust and when it does fire the hot gases ignite what makes it into the exhaust.  Try a new plug and if you haven't already clip 1/4 inch  off the plug wire and reinstall the cap.  It helps to roll the rubber seal back up the wire to get that cap in tight and then roll it back into place

[felicious]

I ordered a new cap to be safe last week. Will try that and a new plug.
Makes for awesome sound effects though.

Does misfiring cause any damage?

[Vinhead1957]

It can but is usually in the exhaust

[felicious]

I see, so you are telling me to pull the trigger on the exhaust I have in my shopping cart for a couple of weeks?

[PeWe]

Loud bangs were fun when I was 18-25. 50mph, kill switch killed engine, continued to roll with gear in a few seconds, twist throttle helps. Kill switch on again and engine start with a loud bang. My RC 4-1 gave nice sound effects. A gang of naughty bikers with 4-1 had fun then.

Valves might not be very happy. I do not know if the explosion can force valve into wrong direction or if it was just worries.

[Nic]

Loud bangs were fun when I was 18-25. 50mph, kill switch killed engine, continued to roll with gear in a few seconds, twist throttle helps. Kill switch on again and engine start with a loud bang. My RC 4-1 gave nice sound effects. A gang of naughty bikers with 4-1 had fun then.

Valves might not be very happy. I do not know if the explosion can force valve into wrong direction or if it was just worries.

+1 I just started laughing thinking about all the people we scared doing this in cars to unsuspecting walkers. Sometimes it would blow the muffler to bits. You could feel it through the floor. Oh for the good ol days.

[felicious]

Just checked out the problem again. There is spark between the boot and the engine. No clue how this happens. The boot doesn't seem cracked.

Boot damaged?


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[felicious]

And the video that shows how/where it happens.

https://www.youtube.com/watch?v=henSJ88aFj8&feature=youtu.be

[Deltarider]

Just checked out the problem again. There is spark between the boot and the engine. No clue how this happens. The boot doesn't seem cracked.

Boot damaged?

Felicious, it is my belief that every owner of a CB500 or CB550 (and maybe of SOHC Fours in general) sooner or later will have this occur. It is very common. That's why I advice always to check (and doublecheck) the ignition before moving to other possible causes. Back then you had aftermarket 10.000km service sets that contained not only sparkplugs, filters but also 4 caps.
Thanks for posting that video. It shows clearly what is happening and why it so often goes unnoticed. I suggest a link in the FAQ.
A routine check should be done regularly, preferably in the dark. BTW I have good experience with these red silicon sparkplugcaps but I confess I never ride in the rain anymore.