Saw runs champ with Motomix but terrible with mixed fuel....

[Rockjock]

I have a 024 AVS that runs great with Motomix but really stumbles with e-fee mixed fuel. Very odd indeed.
I have seen this with a MS 290 as well.. runs really well with the motomix, not so good on mixed fuel. Any ideas gents?

 

[Big_Wood]

tuning

 

[SEAM]

These two types of fuel have a different specific gravity - with the same setting a carburetor will deliver more mix (gasoline+2-cycle oil) than Motomix. If the carburetor is set for Motomix the engine is likely to run too rich with regular mix - when set for regular mix it will run on the lean side (or not at all) with Motomix...

Also, I would avoid using both alternately as there will always be some fuel left in the tank - they don't mix well.

 

[mountainlake]

Learn how to tune a saw. Steve

 

[Rockjock]

Saw is tuned correctly. I can return it to the base settings and this still happens. Once the mixed fule is removed and motomix is put back in it runs correctly. It is a most interesting problem! SEAM saw had maybe a thimble of motomix when I topped it off.. ran great until the premix got into the carb... I have a few spare carbs for it so I will throw in a kit and have a go at it again.

 

[blsnelling]

Base settings are just that...a starting point.

 

[astnmacgto]

Oil ratio/type? That will definitely change a tune

 

[alexcagle]

Base settings are just that...a starting point.

I agree.
The basic standard settings are in the middle of the road.

Dealers pull a new saw out of the box, fuel it, start it up, and don't realize how many variables that saw may encounter, depending on where they are shipped to on earth.
Temperature, humidity, altitude, and not to mention what formulation of gasoline is produced in that area. Not to mention what kind of oil mix was used, in what ratio, how long it's been stored, and what condition the gas can was in, plus how long the gas sat in the storage tank at the quickie mart.
I've seen many a burnt up piston from people trying to save a few cents by using some cheap chinese 2-stroke oil they bought at the Dollar store, or boat oil mix.
It amazes me how some of my customers will go buy a $1200 saw and use a gas can they found at the dump that has a spout that points upward, leaks, and sets in the back of their truck 24/7, rain or shine.

 

[Khntr85]

Wow, very strange problem indeed.... I guess this means you should send me the saw imeadiatly lol.... Wierd problem none the less!!!!

 

[Rockjock]

Base settings are just that...a starting point.

Oh I agree with you but even at that point it runs very poorly.. same settings, add mixed fuel and it runs great..

 

[mdavlee]

Big difference in pump gas and canned fuel for my saws. 1/4 turn on some.

 

[Rockjock]

Big difference in pump gas and canned fuel for my saws. 1/4 turn on some.

still no joy... now this carb was rebuilt and the previous owner used only motomix in it.. I dunno very odd.. this is the second saw I have had do this.. tune is correct. just runs and stumbles very badly..

 

[SalahHH83]

I've never used the canned fuel but maybe this explains why my MS180C seems to run rich. It has that damn non adjustable carb and I use the same 50:1 non-E mix in it I use in everything else but it never seems fully happy. Maybe i'll try a can of that motomix stuff. I just cant bring myself to pay those prices.

 

[Boomer 87]

From what I've seen motomix is a heavier fuel, so your carb will be tuned leaner, if you go to pump gas, it's typically a lighter fuel, you would in theory have to fatten it up some.

 

[Boomer 87]

I know when I went to avgas over pump I had to lean all my saws some to get the correct tuning. I run canned fuel in a couple saws and comparing that to avgas the carb settings will not need adjusted... but if I ran pump gas again it was way off. I hope I'm relaying the info to where it's understandable.

 

[mountainlake]

If the mix your using is good it's a case of bad tuning plain and simple. Steve

 

[Rockjock]

If the mix your using is good it's a case of bad tuning plain and simple. Steve

Mix was fresh less than 5 hours old.
Tuning was correct as well.
saw seemed weak and anemic..
pressure vac test good
carb held pressure good
impulse hose, intake boot all in good nick

removed the mixed fuel and ran it dry added the motomix and running like a champ..
NOW ran that fuel in a 026, ms440 and a 361 no issues at all.. made some test cuts all was good..

will try it again in the am.. if all else fails carb kit and go from there.

 

[SEAM]

If the saw runs badly, tuning is not OK. The tuning needs to be adjusted to the operational conditions of the saw (carburetor performance, air intake, exhaust performance, etc.), hence the word "tuning", and one of the factors there is the quality of the fuel. Pump fuel has a different viscosity when compared to Motomix, so when changing from Motomix to pump mix with the engine adjusted for Motomix the carburetor will need to be re-adjusted to compensate.
So, tuning was correct for Motomix but definitely not correct for pump mix. Some saws (carburetors) can handle both without adjustment but probably do not run at 100% with either...

[I edited this one - one always should think and then type ]

It is not only viscosity but also the way the fuel burns under compression, the amount of air needed for combustion and the temperatures produced. Generally, Motomix and the like burn hotter (leaner) than pump fuel...

 

[trboxman]

Given that description I'd speculate that a stiff diaphragm would push motomix better than it would pump mix at the same mix ratio, but that would be a hair split difference...

 

[alexcagle]

Anybody else think the exhaust of a saw running motomix smells a bit like burning plastic?