Are Mikuni VM carbs really finicky??

2strokerfun said:

I'll bet that humidity and altitude will also play a part in it.

Click to expand...

The effects of humidty tend to vary with temperature. High humidity with low to moderate temperatures (say below about 50) will tend to make the engine run a bit rich, but the effect is pretty minimal. While high humidity at high temperatures will cause rich running and a loss of power.

Why a difference? It has to do with the saturation pressure associated with specific temperatures. Basically when water is a vapor in the air it exerts a pressure that becomes part of the total atmospheric pressure. So lets say it's your basic Chicago day in July it's 100 degrees (F) and it's 100% relative humidity . Because we are at 100% humidity it means we've reached the point where no more water will evaporate into the surrounding air (i.e the saturation point. That water vapor in the air will exert pressure (it's a gas now) of 1.93" hg. So if we have a barometric pressue reading of 29.00, only 27.07" hg will be exerted as AIR pressure. The lower density air would require less fuel to hold our air/fuel ratio at best power 12.5:1

If we left every thing else the same but dropped the temperature to 60 degrees, the water vapor would only exert a pressure of about 0.5" hg, so our actual air pressure would be 28.5" hg. The available air density would be MUCH higher at this temp. Higher density air means we need more fuel to hold our air/fuel ratio at best power 12.5:1

For a given temperature, as the relative humidty goes up the air pressure we can use to make horsepower goes down.
Basic rules HOT and HUMID lean it out a lot, Cool and humid MAYBE lean it a little. But keep in mind as the air temp goes up the ignition delay period is reduced and engines become more knock prone, so tread lightly.

Keep in mind I generalized the hell out of this for the sake of clarity, but hopefully you see the basic connection. I really think this is easier to understand than it is to explain
There are standard charts for relating relative humidity and temperature to pressure for those of you who want to play around with this stuff. A lot of good drag and road race tuners use baro, temp, & humidity to form a density altitude (i.e pressure) value to tune from. I use the PerformAire computer from Altronics rather doing the calculations by hand with a little pocket computer. The Altronics unit calculates density altitude based on temperature, true barometric pressure, humidity, vapor pressure and dew point. It's a slick little piece and easier to handle than a fragile barometer and psychrometer.

The ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) is a good starting point if you want to really learn the science of this stuff. http://www.ashrae.org/ . Aviation sources will have similar info.

I hope this helps.

If your bike is really as finicky as you are describing I'd be inclined to think you have a scavenging issue because the transfer ports are poorly matched. Two-strokes from the 70's are notorious for having scabby transfer ports that cause all kinds of weird tuning issues. Just cleaning up the transfer windows with a right angle porting tool makes those old cylinders really come to life.

The VM carbs just aren't that sensitive to weather changes.

You can easily reshape even oversized transfers with epoxy. A lot of 465 and 490 Yamahas never would have run right with out a good bit of epoxy in the transfers. If they screwed up the exhaust port, well..... that's another story.

Yep. As long as there have been two-strokes there have been nitwits screwing up the exhaust ports looking for more power.