Are Mikuni VM carbs really finicky??

[2strokerfun]

Replaced the Keihin on my '76 Honda elsinore 250 couple of years ago with 36mm Mikuni because the needle and needle valve were just worn out and all the jetting in the world wouldn't stop the old carb from running blubbery rich any longer. Been generally pleased with it, but have been chasing the perfect jetting off and on for two years. Finally tried a double taper 6DH7 needle on Sunday and thought I had found the perfect "magical needle" for this application. Started on middle notch of needle. Smooth power delivery, not a hint of either rich or lean. Perfect plug chop and no hint of detonation no matter what I tried to do. It was a crisp 68-69 degrees when I did this. I was happy and smug!!
So I go to take it out yesterday, at 88 degrees and once again I have a hiccup in the power delivery. A rich spot, I believe. I didn't have time to try lowering the needle a notch and see if that cured it. I'm hoping it would.
Maybe a 20-degree difference is more significant with this carb than what I am used to dealing with. Not particularly humid either of the two days. I've been told the round-slide Mikunis were more sensitive to temp/humidity changes than the old Keihins. I expect to make a main jet and/or needle change between 90+ degree weather and 50- weather, but never really thought I'd need to make a change for 20 degree difference. Not a big deal to make a needle setting change (on this bike, I don't even have to rotate the carb to do it). But I'm still miffed whether I'm still chasing the perfect carb setup or not.
Anyone have any experience with the old VM Mikunis that could let me know if that is what I'm looking at or am I still maybe chasing the perfect jetting and Sunday was a fluke? Thanks!

 

[2strokerfun]

Guess I just answered my own question this afternoon. It's 86 degrees. Dropped needle a clip and back to perfect jetting. I'm guessing about one clip position for every 15-20 degrees, but if it is this persnickity, I'll bet that humidity and altitude will also play a part in it.
So, as I've been told (but really didn't believe) the Mikuni VM IS very finicky when it comes to temperature--at least on my bike.

 

[dirt bike dave]

Another variable you did not mention is barometric pressure.

It can change quite a bit. To see the relative effect on power of various weather and altitude conditions, you can play around with an online calcualtor like at this link http://www.csgnetwork.com/relhumhpcalc.html

It appears a difference between a barometric pressure of 30 inches and 29 inches can make a bigger difference than a change from 50*F to 80*F.

 

[2strokerfun]

Neat site! It's a keeper

 

[Rich Rohrich]

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

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.

 

[2strokerfun]

It does help. Thanks.! I know about density altitude from flying, but it never crossed my mind at all from a jetting viewpoint. So it appears conditions resulting in less lift for flying conditions would also create similar situation in carb venturi.....
So I did a search for density altitude calculator and found another neat little engine tuners calc:

http://wahiduddin.net/calc/calc_hp_dp.htm

 

[Rich Rohrich]

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.

 

[Uchytil]

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.QUOTE]

And some idiots "hogged out" the ports rendering the cylinders useless. I just bought a "good condition - never ported - jug for my 440 Maico (on-line), it's going back. Some knucklehead got a little crazy with his die grinder at some point! Some of the over zealous (moronic) port jobs will cause hard starting also. A nice slow grind to just remove the casting flaws goes a long way.

 

[Rich Rohrich]

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.

 

[whenfoxforks-ruled]

Too bad the exhaust port is so easy to get to!

 

[Rich Rohrich]

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