MS440 idle issue.

[Kyler Monares]

Our department has an MS440 with a 20" bar running .404 Rapco chamfer chain.
i have spent the past few days trying to figure out this idle issue. After running it WOT in a cut or with no load it takes around 10 to 15 seconds for the RPMs to drop down to idle. I have fiddled with the carb adjustments and nothing i do seems to work. I do have a Fast Tach on the way to make sure i start tuning my saws correctly. i have the H side tuned so it 4 strokes intermittently under no load at WOT. Do i have the L and the LA screws way out of whack?

 

[Joel D'Angelo]

Department has an MS440 with a 20" bar running .404 Rapco chamfer chain.
i have spent the past few days trying to figure out this idle issue. After running it WOT in a cut or with no load it takes around 10 to 15 seconds for the RPMs to drop down to idle. I have fiddled with the carb adjustments and nothing i do seems to work. I do have a Fast Tach on the way to make sure i start tuning saw correctly. i have the H side tuned so it 4 strokes intermittently under no load at WOT. Do i have the L and the LA screws way out of whack?

If turning the L screw this way or that doesn't solve the problem I would rebuild the carb. The kits are like $10 and it only takes a few minutes. If that doesn't work you might want to bite the bullet and have a pro look at it.

 

[ccronin]

it is almost certainly a small air leak. Does it have a decomp?

 

[Kyler Monares]

yes it has a decomp, how do i check if it leaks?

thanks Joel, im on a first name basis with my local Stihl Saw shop owner, he can probably set me up with a carb kit...

 

[ccronin]

There are several threads on here about pressure and vacuum testing. Do a search and you can get detailed instructions.

 

[Kyler Monares]

Thanks Cronin, i will check them out, Thanks again everyone!!!!!!

 

[Trx250r180]

saw should start and idle good at 1 turn out on h and l screws ,i always start there ,then tweak if want better performance ,if does not idle good there ,an air leak somewhere like was said .

 

[Kyler Monares]

I tried the one turn out on both L and H and the saw wouldn't even start. 1/4 on the L and 3/4 on the H and I can get it started

 

[SteveSr]

I tried the one turn out on both L and H and the saw wouldn't even start. 1/4 on the L and 3/4 on the H and I can get it started

What are the initial carb settings on the side of the air filter housing? A stock saw should run correctly at these settings. If not you may have an air leak somewhere or a carb problem.

 

[SEAM]

Could also be a slightly restricted tank vent or fuel filter, or a crack in the fuel/impulse line...

 

[spudulike]

It sounds like it has an air leak under vacuum to me as holding on to revs when the throttle is snapped shut is one of those signs. I would have also expected the carb to need more than one turn out on the H & L screws to stabilize the idle and flat out running.
I would check the impulse line and seals as it will have a leak somewhere and very much doubt a carb kit will sort this one out!

 

[SteveSr]

It sounds like it has an air leak under vacuum to me as holding on to revs when the throttle is snapped shut is one of those signs.

Very good explanation as to what is actually happening to keep the idle up.

I would have also expected the carb to need more than one turn out on the H & L screws to stabilize the idle and flat out running.

Another good indication for an air leak... One or both screws have to be richened to get the saw to (almost) run right. You can't really tune an air leak!

 

[Kyler Monares]

What are the initial carb settings on the side of the air filter housing? A stock saw should run correctly at these settings. If not you may have an air leak somewhere or a carb problem.

Side of the housing says 1/4 on the L and 3/4 on the H

 

[Kyler Monares]

It sounds like it has an air leak under vacuum to me as holding on to revs when the throttle is snapped shut is one of those signs. I would have also expected the carb to need more than one turn out on the H & L screws to stabilize the idle and flat out running.
I would check the impulse line and seals as it will have a leak somewhere and very much doubt a carb kit will sort this one out!

Excellent way to explain it! Thanks
I'll be able to check it on Tuesday when I'm on shift again. I have torn apart a 345 following directions here on AS and its symptoms make me think that it has a leak also.
I'm getting more confident about working on saws. I hope on this 440 it's as simple as an impulse line

 

[spudulike]

Side of the housing says 1/4 on the L and 3/4 on the H

Beware, this is with limit caps fitted! if they have been removed, these settings may well seize your saw!

 

[SteveSr]

Beware, this is with limit caps fitted! if they have been removed, these settings may well seize your saw!

Absolutely! There is usually a turn/partial turn bias incorporated into the screws *before* the caps are fitted. This is because the caps limit rotation to only about 270 degrees (3/4 turn). This is the main reason that I reinstall limiter caps on saws that I send out. The limited setting sets the MAX richness for correct operation at sea level. The main reason for any adjustment at all is to lean the saw out for operation at higher elevations than sea level.

 

[Kyler Monares]

Beware, this is with limit caps fitted! if they have been removed, these settings may well seize your saw!

Thanks for the warning.
Can you describe what those caps will look like?
I don't have the saw with me right now so I can't look at it
1 - 1 1/8 turns on the H screw and the saw noticibly 4 strokes at Full throttle IIRC

 

[ccronin]

If you see orange or red plastic you have limiters . If you see metal they are removed. Also, if you can turn the H and L screws a full turn in or out limiters are removed.

 

[Kyler Monares]

replaced d-comp. saw runs perfect now. thanks guys!