Automatic Mixture Control

[Termite]

I noticed yesterday in the 2007 Dolmar catalog that it states "Automatic mixture compensation for clogged air filter".
This is the feature that my dealer eliminated when he notched the foam gasket under the air filter. I believe CuttinScott said that the diaphram pump in the carburetor needed such little air that the fuel volume should not be affected. Apparently Dolmar is saying it will affect the air/fuel mixture. Could this be some of the problems some people are having?

 

[Nailgunner]

well, damaging a gasket under an air filter is basically wrong.

If the carburettor sets fuel pressure based on what it can see at the intake, then if the filter is clogged fuel pressure will be reduced accordingly and all you'll get is a gradual reduction in power as the filter clogs up. On the other hand if it can see the atmosphere at large, as the filter clogs it will run rich and you'll see a rapid drop in power.

More to the point, the damaged gasket will be letting in all sorts of grot and nastiness.

 

[Termite]

Nailgunner, the air filter is used to do two things on this saw (1)filter air to the intake and (2)filter or more accuately limit air to the diaphram "fuel pump". Less air to the diaphram less fuel to the carburetor. No other saw that I know of does the latter.
I personally do not see how it could work, but that is Dolmar's theory.

 

[Nailgunner]

There's two things at work - the pump set, driven off impulse puffs, and the metering diaphragm which allows fuel in from the pump until it reaches a certain level of overpressure relative to the air behind the diaphragm. Usually the back of the diaphragm is open to atmosphere, but the Dolmar must have a passage leading from the chamber behind the diaphragm to inside the intake tract, ahead of the butterfly and inside the space within the air filter assembly.

it adds a bit of cost, but it's a cleverer system. Provided nobody stops it working by hacking the gasket about.

 

[Termite]

"but the Dolmar must have a passage leading from the chamber behind the diaphragm to inside the intake tract"
No, not the intake track, through the air filter.

 

[kevin j]

Isn't stihl intelli carb the same?
Just referencing fule pressures to intake air pressue after the filter, instead of atmospheric pressure?
So that at least generally the filter pressure drop is applied to both fuel and air systems, not just air restriction.

dealer maintains that it 'keeps the same power as the filter plugs'.
If I understand correctly, bs. It attempts to maintain the same FA ratio as filter plugs, but reduced flow will reduce total power.

andy?

kcj

 

[Termite]

The Stihl [IEM] intelligent engine management system is more complex. It incorporates a microprocessor and a magnetic valve to control fuel feed. More complex less reliable? More expensive? The way of the future? Yes.
I do not know how well the IEM system works. However, the system on the 5100 seems to be crude and unnecessary and best done away with.
I like my 5100.

 

[brages]

I have a Stihl MS290 that uses a setup similar to what you are describing in the Dolmar. The top side of the metering diapragm is not exposed to the atmosphere, but rather to the inside of the air filter.

 

[kevin j]

IEM is different than intellicarb. the carb thing was around quite a few years back. does it help? I've not had one. Sounds like dolmar is the same.