the great oil debate

[bwalker]

Everything was jetted crisp except the main. The key to a fast racing sled is clutch setup. Poor clutching and a ton of hp gets your tail kicked between your legs. Especially when hilliclimbing as back shift is so important.

 

[Pilsnaman]

I am new to this forum so take it easy on my guys .
Reading this thread a few questions that have come up and I hope someone can help me with this. I know dirt bikes and snow mobiles have jets for the gas and oil mix but from what I have seen in chainsaws (which I believe is what the main topic is) there are no jets. Instead the engine uses its vacuum to suck the gas and oil mix into the cyclinder. Also, there isn't an exaust valve in a chainsaw but a port opened when the cyclinder moves down and the intake valve is a a simple one way valve, I think it is called a reed? I agree that too little oil is bad but wont too much oil foul plugs and create coke, what they call it in the navy at least? Please help me to clear up my confusion. Thanks to all, this seems like a great site.

 

[jimbo1490]

Chainsaws use diaphragm type carbeurators instead of fuel bowl carbeurators like bikes use, but they most certainly do have jets. The 'vacuum' line you referred to is actually a pulse line which communicates the fluctuating crankcase pulse to the carb diaphragm, thus pumping the fuel to the jets as needed. Think of the diaphragm/pulse line as an 'on-time' fuel delivery system for the jets. The bowl type carb uses a 'warehouse' system where much more fuel than neeeded at any given time is stored in the fuel bowl, and the fuel delivery force is usually gravity. Each type has pros and cons, of course.

WRT your comments on the exhaust valve, I'm not quite sure if you are talking about two or four stroke engines. All two stroke engines use the piston as a sort of valve as it covers and uncovers exhaust and/or intake ports at the correct times. Some two-stroke bikes use an additional adjustable valve in the exhaust port which accomplishes variable port timing. But the main valve is still the piston/port interface.

I think all current production dirt bikes are using reed intake valves, though this was not always the case. Reeds use to be in vogue for chainsaws, but not any more. Piston port and rotary valves are other possible arrangements. The book BWalker referred to earlier in the thread is a great read if you are looking to know. In fact I need to re-read my copy; there was no internet back when I read that book the first time

Jimbo

 

[Pilsnaman]

I will have to look up that book and see what else I can find. Thanks for the info.

 

[begleytree]

Pilsnaman, Welcome to the site.
This oil debate is like argueing religion. Both sides can prove to themselves that what they do is right, but can't prove the other side is wrong.
I think too much is bad, and too little is worse.
Anything between 20:1 and 50:1 is acceptable for most modern 2 strokes. The minute variations within this range is for the comfort of the saw owner only, based upon their experiences, and types of mix oil used. Everyone has their reasons regardless of logic, scientific analysis, beliefs, and rumors.
Any more than that is argueing for the sake of argueing.
-Ralph

 

[Pilsnaman]

Here is a great explination for dumby's, or myself either way , on how a chainsaw works. http://home.howstuffworks.com/chainsaw.htm
The section on carbs is really good and explains everything well. I didn't see any valves on there though, don't know if the saw they took apart was old or something, and all ports are opened using the cyclinder. The only thing I cant figure out is how the air/fuel/oil mix gets from the bottom of the piston to the top. Thank you.

 

[bwalker]

Begley, Oil ratio usage is not like religion or some other unscientific thing. Its very mechanical in nature and science based. Its the people that interject the religion so to speak. BTW I am right,lol.

 

[Jackman61]

Here is a great explination for dumby's, or myself either way , on how a chainsaw works. http://home.howstuffworks.com/chainsaw.htm
The section on carbs is really good and explains everything well. I didn't see any valves on there though, don't know if the saw they took apart was old or something, and all ports are opened using the cyclinder. The only thing I cant figure out is how the air/fuel/oil mix gets from the bottom of the piston to the top. Thank you.

The air/fuel mix is "transfered" to the combustion chamber by aptly named transfer ports. If the exhaust is at 9 O'clock, the carb is at 3 O'clock, there are transfer ports located at 12 and 6 O'clock on the cylinder. There are positive and negative pressures below the piston. The negative pulse is timed to coincide with the opening to the carb, drawing in air/fuel. The positive is timed to coincide with exposing the transfer ports which loop from the crankcase to the combustion side (top) of the piston. Hope this helps.

 

[jimbo1490]

Stated another way, the downgoing piston forces the fuel/air charge up through the transfer ports and into the cylinder above the piston. The transfer ports connect the crankcase to the portion of the cylinder that is above the piston when the piston is down. In some cheaper engines, these ports are merely vertical grooves cut into the lower part of the cylinder.

For a newbie, it's at first hard to grasp that the intake port on a two-stroke engine primarily just charges the crankcase; the crankcase then charges the cylinder via the transfer ports.

The above statement deliberately ignores the direct inertial charging and scavenging that happens in a well-designed modern two-stroke engine; the newbie should focus on understanding the basics first, IMHO, just like we all did.

Jimbo