Too advanced?

[Dom]

I've got my 650b points set using the 12v light bulb method and have tested it with my timing light but having read on here about holing pistons just want to check I've not got it just a bit too advanced. It starts like a dream on the kick-start now however.

 

[Dom]

My video won't load as its too large, any ideas?

 

[Wingedwheel]

You have to create a YouTube channel and load the videos as a link.

 

[Dom]

I'll do that then, although timed with a lightbulb when I bought a timing light it's crept just fractionally before the F mark on idle

 

[gggGary]

Most important is not exceeding the mark at full advance....
But rule of thumb; if it doesn't "kick back" during starting, you're not over advanced.

 

[Dom]

It used to kickback badly which is why I delved into it, still got dodgy ankle nearly a month later

 

[xjwmx]

If you'd like some cushion, to me the engine seems like there's no perceptible loss of performance if it's retarded to the max but still within spec. IOW, nothing to be gained by having it up where preignition detonation could occur.

 

[Jim]

....by having it up where preignition could occur.

Just so we're all reading from the same script...

Pre-ignition is caused by hot spots in the combustion chamber and is independent of ignition timing..... hence the prefix "pre." Changing the timing isn't helpful 'cause it isn't the spark plug causing ignition... it's a "hot spot."
Ever have an engine get so hot that it "diesels on" or continues to run after you turn the ignition off? That's pre-ignition. It happens independent of the spark plugs wants and wishes.

The killer of pistons is "detonation." The fuel/air mix should be a controlled "burn" when ignition occurs.... a flame front spreads across the piston as the mix is consumed.... pressure rise in the cylinder is gradual.
With detonation we have an explosion of the mix instead of the gradual burn. Pressure rise is instantaneous instead of gradual.
Pistons will eventually say fuck it... I quit.

 

[xjwmx]

^Why do you hear pinging from preigntion when the timing is too advanced? And does that preigntion not kill pistons?

 

[Dom]

Thanks Jim but just so I'm clear if my timing is slightly too advanced this is still not a good thing?

 

[jpdevol]

Too advanced spark timing (simplified) results in combustion too early while the piston is still travelling upward, creating excess pressure and, therefore heat that can damage pistons, etc. - that's detonation.

 

[xjwmx]

^Whatever it's called, I'm pretty sure it's not good for your motor and my experience shows there's nothing to be gained by having your XS650 timing near the region where it can start

 

[jpdevol]

Fuel octane rating is actually an indication of resistance to burning, so the combustion completes more slowly and is therefore more tolerant of early spark timing - particularly at lower rpm when time from spark to TDC is longer.

In practical terms of the XS engine and ~90 Octane fuel, consensus is that idle timing be @ F (or a bit to the right) at 1100-1200rpm and no further left than the right side of the cover drain groove @ 3500rpm or higher

 

[Jim]

Why do you hear pinging from preigntion when the timing is too advanced?

As I said above, ignition and "pre-ignition" are 2 different events. You can't control pre-ignition with timing because it's independent of ignition.

And does that preigntion not kill pistons?

No, pre-ignition by itself won't hurt a piston. Just anecdotally but I've had pre-ignition (dieseling) many times on an overheated engine. Never had that lead to damaged pistons.

 

[Jim]

Thanks Jim but just so I'm clear if my timing is slightly too advanced this is still not a good thing?

Correct. See JP's (simplified) explanation above.

 

[xjwmx]

If you'd like some cushion, to me the engine seems like there's no perceptible loss of performance if it's retarded to the max but still within spec. IOW, nothing to be gained by having it up where preignition detonation could occur.

 

[Jim]

Let's try a different tack....

Pre-ignition is generally caused by hot spots. So, let's say we have a piece of carbon buildup where the edges start to curl up. Can be a piston, valve... whatever. This piece of carbon can get hot enough where it starts to glow... in effect turning itself into a glow plug. This can cause the fuel/air mix to ignite. It generally (not always) occurs prior to spark plug ignition.... which is why we call it "pre-ignition." Everyone clear? Moving on.....

Let's look at what happens after pre-ignition...

Pre-ignition will ignite the F/A mix. This can cause either a normal or abnormal combustion event. In the normal one, we get a slow burning (relative) flame front that propagates throughout the cylinder exactly as you would a spark plug initiated event. Since it happens early, that cylinder will tend to run hot, but it's still a normal... slow burning flame. This running hot can (not always) lead to detonation.... which is an explosion of the mixture instead of a slow burn... an abnormal event... and the one that's dangerous.

So, pre-ignition "can" lead to detonation, but the two are still different, independent events.... detonation can occur without pre-ignition.... with lean mixtures and too much ignition advance being the prime culprits.... not pre-ignition.

 

[xjwmx]

Let's try a different tack....

How about let's just let it go?

 

[Dom]

How about let's just let it go?

Or I retard my timing a touch?

 

[xjwmx]

I'm going for a retarded ride now. See yas.