Okay, this is my issue. I bought an 86 250r from this guy that put on an bigger Mikuni carb and set it up to run 50/50 pump/100LL avgas. I wanted to return it back to stock and run pump gas, so I bought a used stock carb from a board member. Problem is, the Keihin carb that I got is too small. I really don't know anything about carbs. Can you guys take a look at the following pics and let me know which carb is better? If I use the smaller Keihin, then I'll need to get a new boot for the airbox to make it fit. If you guys think the Mikuni is a better carb then I'll use that.

The 250r is pretty much stock except it has a DG silencer and it has 4 holes in the airbox with a unifilter. It also came with a twist throttle, so I'm going to change it out to a thumb..not used to twist on a quad. I'm not going to do anything else to it. Keep it as stock as possible.

Thanks for any help. I'm gonna try to get this running myself without bringing it into a shop.

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That appears to be an ESR TMXX carb... If it is, it's worth quite a bit of money. The one on the right is a stock 34 mm 250R carb. They work well, but nowhere near the TMXX. It is the newest thing from ESR see it here:

http://www.esr250.com/carburetors.htm#TMX

 

Another thing I have to add is that the Mikuni carb is as old as the quad itself. The guy bought it brand new and put the carb on right away. Would it still be an ESR carb? Would there be any benefit to using that carb with a basically stock bike?

 

Well, maybe I was wrong... It does appear to be some sort of Motorcycle carb. A larger carb will help even a stock 250. My TRX is basically stock other than the pipe and reeds, it has a 38mm A/S and runs very well. The carb would have very little to do with what fuel the machines runs on, as long as it is a gas carb. Check the compression, as long as it is below 175 or so it should run fine on 91 octane fuels, just use a good pre-mix at a proper ratio. I like Castor 927 at 40:1.

 

If all you have is a silencer and a drilled air box, I would just try and make the stock carb work. I put a 38mm on my R (and I even have motor work) and it was just too difficult to get it dialed in perfect. Most people run the 38mm carb with big bores and power-valved motors.A 36mm would be perfect to wake it up, but if you already have a stock 34mm, I would try and make it work.