What exactly is 4 stroking??

[edisto]

Wooooosh.......

That was the sound of some of that techno stuff flying right over my bald melon!

TWolf is saying the saw has the hiccups!

My take on what he said is this: because of high rpms, the piston moves past the exhaust port before the charge is completely burned, i.e., there is not enough time for complete burning. Because the burn is incomplete, pressure is lower, and so less of the the exhaust makes it way out.

Because there is more exhaust left, the next charge is diluted, and so takes less time to burn. In addition, the engine slows a hair, so that charge has more time to burn. The result is a complete burn, and maximum pressure for that charge, so the saw speeds up again, and a higher proportion of the exhaust leaves the cylinder, because the scavenging is more efficient.

This puts the engine back at square 1 with a dense charge that takes longer to burn than it takes for the piston to clear the exhaust port.

The load on the saw in the wood drops the rpms enough to prevent the overrun, giving more time for a complete burn.

 

[Adirondack]

Angelo,
See if you can find a chart with cylinder pressure related to EGT and A/F ratio. The engine is your chart is under the constant load of the prop, whereas chainsaws go from unloaded to fully loaded. A properly tuned unloaded 4 stroking chainsaw at WOT might have an A/F of say 11-12:1 which is much too rich for complete combustion so the EGT is low due to the cooling effect of the unburned fuel and the RPM's are off peak due to the inefficient combustion. Drop the saw into the wood (now the engine is loaded) and the combustion pressure rises dramatically. The 11 or 12 AF that wouldn't burn very well at the lower pressure now burns almost completely at the higher pressure and gives max HP. EGT goes up due to more complete combustion leaving less unburned fuel available for cooling.
A lot of folks think max unloaded rpms= max loaded rpms and therefore faster chain speed. So they tune their saw to scream at max unloaded rpm with an AF of say 14:1. Then they drop the saw into the wood, the pressure rises and near perfect combustion occurs so there's no unburned fuel left over to cool the cylinder, EGT soars, HP drops. After a while the saw starts running like crap and the poor guy is baffled as to why so he takes it to THALL who pops the muffler off and thinks KACHING! another scored piston, that's the 3rd one today as he shows the guy the damage.

Great. Thanks. This is why I stick around to learn more useful info. No wonder why so many people burn up so many GREAT saws. I have heard people talk about making their saw scream and then they know it is tuned. If only more people understood how to tune their own saws. :chainsawguy:

 

[Jtheo]

That's how I adjust carb! I also put a tach on it to see what the rpm's are. Finally to ease my mind I pull plug and check color. I do this after revving saw while at wot and shutting it off. If plug is a light brown I'm good to go.
Bob

I would be helpful if a member could post a good color picture of a plug that is the proper color of brown when the saw is properly tuned, for people like me that needs to learn more. Thanks

 

[angelo c]

here's a pretty decent one..

http://www.spark-plugs.co.uk/pages/technical/diagnosis.htm

some of them are not applicable obviously.

 

[Jtheo]

here's a pretty decent one..

http://www.spark-plugs.co.uk/pages/technical/diagnosis.htm

some of them are not applicable obviously.

Good deal, Thanks

 

[redunshee]

Good deal, Thanks

Here's another.

http://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm

 

[scotclayshooter]

Oww hot hot hot!

 

[Jtheo]

Oww hot hot hot!

That's a very good one, and a big help to me. Thanks for taking the time to do this, and thanks to all the other members who helped on this thread.

 

[Tzed250]

Order of importance:

1. Exhaust note

2. Max RPM reading

3. Plug insulator color