1994 XR80R carb conversion to XR100

Hello, I have a XR80r that I have had since it first came out. Now as I am older I am getting into working on bikes and have heard that a cheap way to increase power to a 80 is to replace the carb with a XR100 carb. I took apart the bike and did a complete cleaning of it and it still seems to run a bit rough when I open the throttle all the way, it cuts off when its full open. I hear that its mostly the needle or carb, and since its the original carb I think its probably it as well. So instead of digging in and replacing parts I am just going to throw on a XR100 carb to fix it and give it a upper hand. So my question is what are the steps needed for this. I know I need a xr100 airboot and throttle cable. Ive been all over eBay and have seen xr100 carbs for $20-$80. My stock carb and cylinder head has a 22mm intake opening and the xr100 carbs range from 25-30mm intake openings. So there is a 3-8mm difference. What do I need to do here? can I just install them as is, buy a intake manifold converter or do I need to mod the cylinder head. Also is there a preferred carb? I have seen Mikunis (with a 25 or a 30mm intake opening) which I know are awesome as I have one on my harley, but have also seen aftermarket and OEM carbs (which range from 24-28mm openings). Also do the years of the Carbs play any performance difference, I.E a 1986-2000 XR100 carb to a 2001-2011 XR100 carb. Please advise...also I am curious on a bbr or other rev box as mine is a stock one and really bogs my bike down.

Edited by Nethawk

I'd suggest you check your valve adjustment, condition of your air filter and points before going any further.

Hello there, I am not worried about it. I am wanting to do a upgrade to the 80 by putting on the XR100 carb and airbox. I was just wondering if I have to dremel out the cylinder head intake to make it compatible with the bigger 28mm carbs as it is a 22mm right now stock. Or atleast what I could do.

Hello there, I am not worried about it. I am wanting to do a upgrade to the 80 by putting on the XR100 carb and airbox. I was just wondering if I have to dremel out the cylinder head intake to make it compatible with the bigger 28mm carbs as it is a 22mm right now stock. Or atleast what I could do.

Intake valve is 24mm. Installing a larger carb than 24mm is a waist unless you've had oversize valve installed. Extracting more power out of the XR80 is even more expensive than the 100; however, the 80 is the better platform.

The 100 carb is a direct bolt on for the XR80 head; you could try to port match the gasket if you so choose but there is no 'step' in diameter. Performance gain would be marginal if any; however the 100 carb seems to have more readily available jets. Free flowing exhaust, intake and Jetting will net you some usable power for under $300.

Beyond that the costs are exponential. A nicely used CR80 will provide way more bang for your buck than a built XR/CRF80.

Thanks for the input! though I am alittle confused by what you mean by there is no 'step' in diameter? Right now my intake opening on the cylinder head is 22mm, if I put a xr100 carb that has a 28mm opening on it, I would also need a 28mm intake manifold right? but even then its like sucking a small straw through a big straw and that your only going to get what the little straws putting out right? So I would have to dremel out the cylinder head intake opening if this were to work? or just bolt on the 28mm to the 22mm cylinder intake and that should work?

Iv'e done a little porting in my time and you are on the right track. Don't let the size comparison of your intake valve be confused with the size of the carb bore. The two are not directly related. A 28mm carb does not flow as much as a 28mm piece of pipe. If you look through the carb, do you see a perfect round hole with nothing poking or sticking out? All these protrusions in the carb reduce its flow. Also, you DO want to blend in the intake to match the carb, otherwise that step will cause turbulance from hell, and it can cause all kinds of tuning grief :ride:.

Now you might not see a lot of performance from just a bigger carb. Remember, what goes in must come out, so you would need a better exhaust as a minimum. Then of course, cam, porting and higher compression. On the other side of the coin, you are going to change your powerband. With a larger carb you can gain some top end power, but you are going to lose some low end and maybe even some throttle responce.

For those who think I am wrong, do some research. An engine is an engine no matter what it is used in. Take a hopped up VW. A lot of builders use 48mm webers, but very few run 48mm valves. the more common sizes are 44mm & 46mm. Yet for some "mysterious" reason it works.

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