Topic: Strange sound from my CB750K...

[Yaquina]

Recently (the last couple of days or so) I have noticed something strange about my bike. Sometimes, but not always, when I turn the key to 'ON' I hear a fairly loud 'pop' from somewhere around the engine. It sounds almost like a gas valve or similiar, venting pressure. It doesn't sound like a bang or a mechanical clank or anything like that. More like pressure being released from something?

It has been very hot here lately and at first I was thinking it was some kind of release valve on the gas tank or something, but it only happens when I first turn the key to 'ON' and it doesn't last or hiss out, just kind of pops.

I thought I knew this bike pretty well, but I can't think of what it could be.

Thoughts?

[Burnboy]

sounds like a short. check your fuses. It may be a bare wire contacting something and once you power the bare wire arcs and makes a small popping noise.

[technojock]

It could be there is some fuel in the cylinders and the ignition is sparking when powering up.  It wouldn't hurt to tun the gas off when you park and check to see if the carbs are flooding.

Tony

[gane]

I like Tony's thought. Engines like to come to rest at or near tdc. if a bit of un ignited mix is still present and spark occurs when key is cycled. pop is result. I've seen it occasionally on cdi bikes where ignitor was on the way out. unfortunatly can think of no way to test theory. luck G

[Yaquina]

Interesting. 

I like the fuel idea as well although I agree, there is no way to test it. I have never heard of the spark kicking on when the key is turned, you think that could be the igniter faulting?

[gane]

Tony, No, I believe your' K (if SOHC) still has points. , since on shut down engine often stops at a resistance point. (compression)this is also where ignition timing is close to sparking. on our bikes spark occurs when points are closed, and and continues until points open. a time and distance which increases with worn phenolic block. An aniquated method of setting points was with a "dwell meter" which measured the amount of time/ distance traveled points opened, as opposed to "points gap" which measured distance contacts opened  at  new/optimum  height of breaker block. This was/is part 1. Part 2 is/was  setting breaker plate  for optimum  ignition timing. IE spark occuring in proper relation to piston position.and is measured w/ timing light. consult your manual. and throw a new set of points  at the old girl, chances prob will dissappear. luck g

[eurban]

Does your 750 have a Dyna electronic ignition?  Some of my bikes with the Dyna ignition would occasionally fire a plug (or 2 or 3 or 4?) when the ignition switch was first turned on.

[Gordon]

on our bikes spark occurs when points are closed, and and continues until points open.

This is wrong.  The spark occurs the instant the points OPEN, not close. 

[754]

There is a way to test it.


 To shut down flick off KILL switch first, then key.


 On startup, turn key and see if it still happens.

[BlackMax]

It could be there is some fuel in the cylinders and the ignition is sparking when powering up.  It wouldn't hurt to tun the gas off when you park and check to see if the carbs are flooding.

Tony

Almost surly, exactly correct.    one plug is firing while gasses have seeped into the cylinder. Mine does the same thing every once in a while. 

[GammaFlat]

on our bikes spark occurs when points are closed, and and continues until points open.

This is wrong.  The spark occurs the instant the points OPEN, not close. 

I realize this sounds counter-intuitive... but Gordon's got this right (of course ).  It makes perfect logical sense to think that closing the points would energize the coil causing it to make a spark - but alas no.  A coil gets "charged up" when you apply voltage (points are closed)... when you turn off the source of voltage (points open), the magnetic field in the coil rapidly collapses which induces the nearby coiled wires (other side of the coil) to make a "boomy" or high voltage charge which is what wants to jump across your spark plug gap.  We can thank Nikola Tesla for the invention of spark plugs.  It was his birthday about a week or so ago.