Symptoms of too much timing advance

[Moparmyway]

Just guessing!! Lol! Is it possible to have too much raw fuel in and not be able to combust it because of the advanced timing and cause the piston to slap the skirt?

I would think that it would exhibit rich running characteristics before any hammer sound

 

[Keith Gandy]

I would think that it would exhibit rich running characteristics before any hammer sound

Ya I would think so too. Just kinda was thinkin the piston was fighting against combustion somehow

 

[Marshy]

I hear you, but it didnt misfire or pop, it just did a little metal to metal hammering mid throttle.
Filing less off of a new key cured the problem

The sharp metal ping sound is from detonation. When two flame fronts in the combustion chamber collide a pressure pulse travels through the engine and the sound you hear is the resonant of the cylinder fins.

Pre ignition and Detonation some say never happens in a chainsaw?
Stock Aussie ms660 -only clue to something was not quite right was it seemed to be running VERY hot for the work it was doing..
Old stale fuel was the culprit combustion chamber looked bout the same, fuel quality and shelf life seems to go down yearly in my parts back when we had good old super and never had to worry bout this crap fuel going off! WTF has it all come to? lol.. After having a look and bit of a head scratch working out as to why this happened put it all back together (no new parts). well saw is still running 5-6 years later..
View attachment 20546 View attachment 20547

When you have detonation it can causes small crators in the piston head. Some engines can tolerate a lot of detonation for a long time without any permanent damage.

 

[jmssaws]

Only saw I've ever seen that was detonating was a bone stock ms460.

 

[Moparmyway]

The sharp metal ping sound is from detonation. When two flame fronts in the combustion chamber collide a pressure pulse travels through the engine and the sound you hear is the resonant of the cylinder fins

I thought detonation happened after the spark ?

It wasn't a sharp metal ping, more like a click, but only at mid throttle while cutting.

 

[Marshy]

I thought detonation happened after the spark ?

It wasn't a sharp metal ping, more like a click, but only at mid throttle while cutting.

Yes, detonation is after the spark and pre-ignition is combustion before the spark ignition.

You said metal to metal hammering. Depending on the engine it could sound like a dull click. My father's old Dodge slant 6 would ping hard sometimes if we were working it hard. It sounded like marbles in the combustion chamber. Another good analogy is an ignition wire grounding on a valve cover, that arc snap is close just not as large.

 

[Moparmyway]

Detonation and pre-ignition would have continued to happen, the noise was alleviated by less advance

 

[Moparmyway]

Not too sure about that theory, but I do know that this 661 is stronger in the cut with a filed key than it is with a stock key, and she usually works with full comp 36" 404 chain

 

[Marshy]

Anyone try the opposite retarding the timing? Say if ones goal was only holding the highest revs in the cut like your 661 holding 13k in the cut. In theory you will lose some torque and snap but overall peak HP ruffly stays the same, the plus being you gain over rev holding more revs in the cut?..
A saw from the factery that likes to rev high would have less advance timing than a saw that holds back up top but is very "torquey" as such is one way to look at it yes/no? just thinking out loud..

By adjusting your ignition timing you are changing what is called "location of peak pressure (LPP)." It can measured by an in-cylinder pressure transducer and it's the part of the combustion cycle where pressure is the highest. Large diesel motors measure LLP to determine if the fuel injection is timed correctly. Ideally, the LPP should occur at 14 degrees after top dead center. Depending on the chamber design and the burn rate, if one would initiate the spark at its optimum timing for a given RPM the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees after top dead center. LPP is a mechanical factor just as an engine is a mechanical device. If you peak the pressure too soon or too late in the cycle, you won't have optimum work. Therefore, LPP is always 14 degrees ATDC for any engine. So depending on several other factors like fuel, combution chamber shape, rpm, temperature ect, will affect how fast/slow the fuel charge reaches LLP.

Engines are optimized for a type or grade of fuel. Engine designers use the term called MBT ( Minimum spark for Best Torque) for efficiency and maximum power; it is desirable to operate at MBT at all times. There is going to be a point where the power is the greatest. Less spark than that, the power falls off, more spark advance than that, you don't get any additional power.

I won't take full credit, I paraphrased the above info from a few good sources. The rabbit hole runs deeper and it starts relating compression ratio to advance and the trade off between one another. It's interesting if your into that sort of thing.

 

[Brush Ape]

Good God, the slant six......push button tranny or three on the tree. I had one in a '64 Belvedere 4 door. My brother had one that was manual, and when you pulled the old trick of running out of gas on a dirt road with a girl but really was out of gas, you could drive it back to town in gear by cranking the starter.

 

[Marshy]

Good God, the slant six......push button tranny or three on the tree. I had one in a '64 Belvedere 4 door. My brother had one that was manual, and when you pulled the old trick of running out of gas on a dirt road with a girl but really was out of gas, you could drive it back to town in gear by cranking the starter.

Lol it was in a van and was an auto. One motor came out of a valiant with push button trans.

 

[Chainganger]

OK, how many of you are also indexing your spark plugs?

Now the Dodge slant 6 was my first car, did all my old fashioned tune ups, it was fun, but if it was damp and rainy for awhile I had to take off the distributor cap and wipe off the moisture from the inside of the cap.

 

[Moparmyway]

OK, how many of you are also indexing your spark plugs?

I'll call your indexxing and raise you some custom butcherizing

 

[Chainganger]

I'll call your indexxing and raise you some custom butcherizing

I fold!

 

[ammoaddict]

Y

If to far it will spark well before top dead center. And the kick back is the fuel igniting forcing the piston down before it has revolved the crank to go down correctly. It could even spark and give you the backfire through the exhaust. Anyone ever put a distributor in backwards in a small block Chevy? Firing with the valves open

Yes I have. It split one of the mufflers apart

 

[Al Smith]

It's been said on lower compression engines a slight more advance could help but not so much on higher compression engines .Which I'm about certain has to do with optimum highest pressure build up on ignition .
I also think it's just something you just need to figure out because although they might all be sisters they are not identical twins .There's a lot of things ,ignition and burn rate of the fuel,temperature ,altitude ,shape of the combustion chamber etc .etc .