Jake's 1973 XL285-Single "Brat-Tracker?¿" Build - UPDATED Nov. 28, 2011(1stpage)

[Sonreir]

34 is too big dude
go with a 30 it will be way easier to jet as well
good luck

I agree.

According to Mikuni, the VM30 is good for up to around 30 horsepower. VM34 is overkill.

 

[xb33bsa]

....

I did some research and 32mm & 34mm vm mikuni is what is recommended for my 76 xl250

if racing and with a performance exhaust and wide open throttle near redline all the time that is true but if you want a pleasant MUCH more ridable machine get a 30 bigger is only better for serious racing

 

[Turkeyfellow91]

The problem here is that I already have the 34mm and air filter and just need to tune it, as well as it has a big bore kit so is not just a 250

 

[Sonreir]

The problem here is that I already have the 34mm and air filter and just need to tune it, as well as it has a big bore kit so is not just a 250

In stock form, your bike displaces 248cc. Even going +2mm on the bore (which is about as much as the sleeves and jugs can handle without some serious machining and resleeving), you're still only looking at 262cc. That's a five and a half percent increase. Assuming you also see five and a half increase in power, you'll be going from 20 horsepower to 21.1 horsepower. You really think you need a carb on your bike that's rated for 40 horsepower?

I'm not trying to be a dick, but the argument you're making seems a bit strange to me. What I'm hearing is, "I have the wrong parts already, so I might as well use them". You could probably sell that VM34 and air filter and pick up a VM30 and air filter without taking too much of a loss.

The problem you're going to be facing when putting a carb that large onto an engine that small is the tuning portion. Can it be done? Sure. Are you going to be happy with it? Probably not.

Just trying to save you some heartache, man.

 

[Turkeyfellow91]

I understand that you are trying to help, the part I'm confused about is why is the mikuni vm 32 and 34mm carbs the most popular carbs on xl250 if they are plainly to big

This is just 1 of many resources I found
http://www.oldrice.com/CarbStuff.htm

 

[Sonreir]

Because this is the Internet and so people can post whatever they want. Then other people read it and think it has merit. If you really want to get me going, ask me about the benefits of backpressure. ;D

This is the manual put out my Mikuni specifically for tuning their VM carburetors:
http://www.mikuni.com/pdf/vmmanual.pdf

Skip to page seven. In the upper right is a chart that specifies the designed-for ranges for the carbs. I prefer to use the upper range because I'm happy to give up just a little on the bottom end for a bit more top end.

At 21PS, (which is about 20.5 horsepower) you're falling into the VM28 range.

 

[Turkeyfellow91]

Well thanx for the info, I guess I have to wait to get a vm 30

 

[xb33bsa]

i can probably fix you up with a 30mm VM mikuni core,spigot mount, for cheap
that is not why i tried to steer you but it just so happens i have a ton of used carbs