hey guys a 300ex tore me up(stock ex)

[PA_Banshee]

Compression ratio has nothing to do with atmospheric pressure. The actual compression does but not the ratio. The compression ratio is just the volume of the cylinder at the bottom of the stroke compared to the volume at top dead center. I'm trying to do the math right now, hahahaha. And yes before you ask, I am at work


***edit***: the ratio is top dead center until the exhaust port/valve starts to open. Some other things may apply, but in general this is it.

[PA_Banshee]

And school starts.... now..

I'm going to assume no knowledge of terms here so I'm sorry if I explain things already known. I'm getting the stock bore and stroke from yamaha's website (64 x 54mm). The 64mm is the cylinders diameter so to get the volume of the stroke we'll convert to centimeters and do the math.

3.1416 * 3.2^2 = 32.17 (pi * radius squared)

32.17 * 5.4 = 173.718 cc's/cylinder hence the 347cc's

If we add to that the dome size we get the total volume when the piston is at the bottom its stroke. I think the stock size is 23cc.
173.718 + 23 =196.718cc

196.718cc / 23cc = 8.5:1 hmmm..... did I do something wrong? nope it's because the actual compression ratio is calculated from the top of the exhaust port in the cylinder to top dead center. If we take the 23 ( the "1" in 6.5:1) times 6.5 we'll get the total volume once the exhaust port is cleared.
23*6.5=149.5 (149.5:23 = 6.5:1) Then if we subtract the dome volume and divide that by the initial area of the cylinder's flat circle we can get how far the stroke is in the cylinder after the exhaust port.

149.5-23=126.5
126.5/32.17=3.93 cm This tells me that the exhaust port opening in the stroke is 5.4-3.93= 1.47cm in height. Can anyone confirm that? I can't remember what it looked like when we put the cool head on my friend's banshee, but it sound about right.

Yeah, my mother always told me I should've been a math teacher!

[1 bad shee]

ok i took everything apart carbs head off reed cages ....everything looks good...it wouldnt just have been the new oil and mix....would it?

[370banshee]

well anyway i was right about the PSI in the cylinder being compared to 14.7:1 at sea level. anyway at 119 if your at higher elevation that coudl be right

[go-blow-or roll]

well brett that's pretty impressive!! if you ask me, wish i had a 1/10 of that math skill, but then again i work on polaris "stuff" so i guess it doesn't apply to often lol. how did your needel and seat look 1 bad shee? are they the ones that came in the quad?

[370banshee]

hey jim i never thought about needle settings

[PA_Banshee]

That's one area I haven't had any experience in. I've changed mains and crap like that but I've never touched needles or really had to adjust the carbs much.

[go-blow-or roll]

(here's that damn spelling again, bear with me lol) i would check your needel and seat the "ones" under the carb floats. with the banshee being high reving and well a 2-stroke at that. if they are the ones that came with the bike i would almost bet that they were wore out. the seat is more often then not made of brass and the needel aluminium(sp?)they wear into eachother from vibration. this will cause the needel to "drop down" in the seat, not allowing the floats to meter the fuel properly, causing it to dump way too much fuel into the cylinders. if you do get new ones i would recamend the ones with the black rubber nippels, they seal and last longer then the others.

[go-blow-or roll]

one more thing (1 bad shee) was it the 300ex that was piped or was it your quad?

[1 bad shee]

ok...like i said everything looked good...but im never messed with carbs beside taking apart and lookin for dirt in the bowl,,,so as for as settings im lost...i tried a 42:1 spectro mix it seem to have ran better but still no smoke at all so now im goin to try36 or 32:1...shouldnt there have been alittle? should i try 36:1 or go to 32:1? thanks for all the help