Modded 394 lean tuning issue WTF...

[XP_Slinger]

Good thread. But why did the saw run fine then progessively get worse? I get changes in temp etc...But by what he described it went from happening after a few cuts to happening after half a cut. That's a pretty extreme swing.

 

[RIDE-RED 350r]

Gaining compression, more performance and thus more fuel demand as the rings broke in?

Or possibly carbon starting to build up in the chamber and on the piston crown giving a bit more compression and thus a bit higher combustion temps thus increasing fuel demand??


Or...a little of both??

Either way, head scratcher for certain! I'll be watching for that with my modded 394 that blows 210psi cold!

 

[wcorey]

The carb needing some work to catch up with the work you did to the saw was my first thought when I read the title. I wouldn't suggest carb modding as a troubleshooting step so I didn't say anything about it.

I would agree except as I said in the original post that it didn't need any carb work initially and ran just fine as it was. No additional mod work had been done as it began to gradually deteriorate.

 

[wcorey]

Good thread. But why did the saw run fine then progessively get worse? I get changes in temp etc...But by what he described it went from happening after a few cuts to happening after half a cut. That's a pretty extreme swing.

Really it's much more extreme than that, went from running fine to going lean before it even started the cut, over a fairly long period of time.

 

[wcorey]

Gaining compression, more performance and thus more fuel demand...

This is pretty much what I'm figuring it to be.
An example of a saw getting stronger as it broke in and needing more fuel.
Just seems a little extreme but I can't think of anything else that fits this kind of problem that occurred in such a linear progression.
I usually bring 20 or so mostly play saws to gtg's and not always the same ones so takes me a while to get a number of tanks through all of them.

It actually had some very light transfer on the ex side that didn't even affect the comp. After all that lean running I'm surprised it wasn't worse.
P&c had been previously cleaned up from moderate scoring/transfer from when I originally aquired the saw.

 

[wcorey]

More ethanol content now in your fuel than there was then

And/or

Different 2 cycle oil being used

Or colder temps now compared to then, so she needs more fuel

Same old 10 percent 87 octane i run in everything.
Semi random oil, 40:1, whatever synth i get a deal on.
Thing has probably seen 5 different oils over 4 years. Just got through a gal of ultra I got free with a 460, now onto a quart of Saber. I generally don't notice much difference...

Don't forget that this happened in very measured fashion, not bad one tank/day/week/year and good the next.

 

[XP_Slinger]

And the light bulb has turned on. Never would've thought of that.

 

[RIDE-RED 350r]

This is pretty much what I'm figuring it to be.
An example of a saw getting stronger as it broke in and needing more fuel.
Just seems a little extreme but I can't think of anything else that fits this kind of problem that occurred in such a linear progression.
I usually bring 20 or so mostly play saws to gtg's and not always the same ones so takes me a while to get a number of tanks through all of them.

It actually had some very light transfer on the ex side that didn't even affect the comp. After all that lean running I'm surprised it wasn't worse.
P&c had been previously cleaned up from moderate scoring/transfer from when I originally aquired the saw.

A bump in compression does generate more heat and higher fuel demand. You can have some compression increase due to carbon in the chamber and on the piston crown as well. I think that compression gaining whatever the cause may be the best theory.

Playing a little more into that, you may be in the territory where 87 is on the borderline of not being enough octane. That will generate alot of excess heat too which leans the combustion mix because some of the fuel mix is burning off early.

 

[wcorey]

A bump in compression does generate more heat and higher fuel demand. You can have some compression increase due to carbon in the chamber and on the piston crown as well. I think that compression gaining whatever the cause may be the best theory.

Playing a little more into that, you may be in the territory where 87 is on the borderline of not being enough octane. That will generate alot of excess heat too which leans the combustion mix because some of the fuel mix is burning off early.

Really not much carbon on the piston crown or chamber.

As for 87 octane, I and many others run it in saws with more comp than this one and I've yet to see or experience any signs of detonation or other issues with it.

 

[huskyboy]

Are you going to bring this saw to Dan’s? I don’t think I have ran your 394.

 

[Moparmyway]

Are you going to bring this saw to Dan’s? I don’t think I have ran your 394.

There’s going to be a few more 90cc studs there this year ........ better bring them too

50cc and 95cc showdowns

 

[huskyboy]

There’s going to be a few more 90cc studs there this year ........ better bring them too

50cc and 95cc showdowns

Awesome!

 

[paragonbuilder]

Ding ding ding! Dan you da man!
Why didn't you fix it for me at your gtg two years ago?
Would have saved me all the harassment and derision from the stihl fan club, lol...

Seat started at .037", drilled out to .046, big improvement but not enough, finally went to .059.
Checked the feeds into the main jet also but they're huge. The H needle seat hole isn't real big either but it shares the total input capacity with the idle/transition jets and fixed main bypass.

So now that that's solved, why did it take so long for this issue to manifest?
Seems obvious but maybe there are other options?

You gave enough clues Bill, that there wasn’t much left. What is interesting is that you have increased jet size exponentially to make it run. Does this mean you have maxed out the cylinder? Mine doesn’t need that... I’m very intrigued...[emoji41]

 

[huskyboy]

You gave enough clues Bill, that there wasn’t much left. What is interesting is that you have increased jet size exponentially to make it run. Does this mean you have maxed out the cylinder? Mine doesn’t need that... I’m very intrigued...[emoji41]

Does more fuel usage mean more power... or can a saw make better usage of less fuel to make more power? That’s my question.

 

[Chainsaw Jim]

I would agree except as I said in the original post that it didn't need any carb work initially and ran just fine as it was. No additional mod work had been done as it began to gradually deteriorate.

I see that all the time.

 

[Chainsaw Jim]

Does more fuel usage mean more power or can a saw make better usage of less fuel to make more power?

More fuel equals more horsepower.

 

[Deets066]

More combusted fuel equals more horsepower.

Fixed

 

[Deets066]

If you have a lot of intake duration it's very common to have to drill the carb. Where are you at on the intake here @wcorey

 

[RIDE-RED 350r]

If you have a lot of intake duration it's very common to have to drill the carb. Where are you at on the intake here @wcorey

Where you are going with this makes sense, but being that the issue came up and worsened over time kind of takes porting put of the game doesn't it?

 

[Deets066]

Where you are going with this makes sense, but being that the issue came up and worsened over time kind of takes porting put of the game doesn't it?

What was it that caused the carb to need drilling?