In theory you would be correct but in real world testing are the viscosities different enough that you would ever know? Makes me curious, but at the same instance if one saw in my fleet only ran well on MotoMix I would just use that and not worry about it, to each his ownGiven 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...
Anybody else think the exhaust of a saw running motomix smells a bit like burning plastic?
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...
You mentioned "carb rebuilt previously", was it rebuilt with OEM parts or aftermarket? Further it would have been interesting to me to see how the saw would run after ~ half a tank full? Maybe it would only require a little "run in time" with the new fuel?
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