I am looking for some advice on what the best carb for my 250R would be. I want as much low end and throttle response as I can get. I am a B level motocrosser and do quite a bit of trail riding with it. It is an '85 motor with Wiseco Piston, Delta V force Reed Block, Bill's Pipe & K&N Filter wit stock airbox opened up on top. Pretty stock other than that. I currently run the stock '85 stock 250R carb with the round throttle bore. I am wondering if just going to the flat side carbs will help a lot or should I try to get a 38mm or something else altogether. Will a 38mm benefit me if I am not looking for top end? The other thing to consider is that I have been running good race gas through the quad lately, so I am thinking of upping the compression. Is this a wise move for low and midrange considerations? Is a Cool Head the best way to get this done? Also would consider porting if I was convinced it would give me what I am after. I guess what I am getting at is for my riding style, I don't go wide open and carry a lot of speed, I like the low end grunt and snap of a 4 stroke quad, but I like my 250R and really don't want to go to a 4 stroke yet.
Any advice, experiences or thoughts are greatly appreciated.

 

If you are going to be consistently running high octane through your motor, you should have your head decked to do so. An aftermarket head would be the way to go. As far as which one you should talk with your builder to see what they recommend. You can gain a lot of low end by getting your cylinder ported. That is something you should look into. Whether you decide to get it ported or not, you should still look at getting a new carb. Some good ones would be a 38 or 39mm A/S or PWK. I am not really bought on what an A/S is supposed to do, but whatever.

Really it comes down to setup. Port work, head work, pipe, carb, fuel, and gearing are all the factors you should look into when it comes down to it.

 

First thing to do is to get rid of that pipe and go for something smooth. ESR makes a nice smooth pipe and so does FMF, but your bills pipe is way too peaky for low end grunt- I know because I used to have one and then I got an FMF Gold Series, Huge difference. If you got a bigger carb before porting it, you will just be feeding too much fuel to your motor and you would not see the gains that you should. Get it ported and then go for the carb. 38mm TMX Mikuni will offer great low end and so will the 38mm PWK Keihen. Your reeds are horible for low end, you want stock reed cage with stock or Boysen reeds. High compression gives you low end. Tell your builder you want your head milled to a 19cc dome. The best thing to do is to buy the cool head so that you can swap out domes, but that is a lot of extra cash.
Good luck,
JP

 

Yes liek boogerbob said get an ESR pipe, get boyseen reeds, and get a 19cc dome. BUT! Dont get a 38mm carb! Smaller the carb more bottom end you will have. I would get a 36mm A/S (air stryker) The air stryker helps get more air in on bottom end, and has more venting. You wont need a 38mm carb, beleave me. Then you can have it ported For low-mid range from who ever you would like. If it was me I would go to ESR because they have real low prices real good work and real good costumer service.

 

ESR pipes are fine. If you decide to have your motor ported, you should find out what pipe your builder recommends to go with your setup. I would stay away from their cylinders since Eddie no longer does the porting. I have seen a few cylinders compared side-by-side that were "supposedly" ported to the same generic style. Needless to say, there was a visible difference. I am not trying to start something about ESR, I just would not buy a ported ESR cylinder.

As far as reeds go, I have used V-Force, Pyramid, Boyseen, and of course the stock cage. These were all used on a stock-jug 250R with no porting. It had a Paul Turner Eliminator 98 Pipe and an open airbox with a K&N. This was only ridden in the sand. I felt that the Pyramid took away more bottom end than the V-Force, and the Boyseen was about equal to the V-Force.

An aftermarket head is not only nice for swapping domes, but it offers a much better cooling jacket. There are a few on the market to choose from.

You should call a few different MX builders and see what they would do with your motor to get it to where YOU want it to run. Then go from there.

 

If I were to mill the head to get it down to say 19 mm, do I just take it flat off of the surface? I know this is kind of giving me more squish band, which I am understanding is what reallly helps the low end. Is this a decent route to go, or is the combustion chamber shape most important. I am a machinist and I have thought about doing this for a long time. I do intend to stay with good race gas from now on, probably 110 octane minimum. How high on the comp ratio can I go and still see positive results?
If I do go for a port job, do I need to coordinate comp ratio with it or is higher always going to be better?
Is it possible to go to far on a 250 R motor and make it such a low end beast that it isn't fun? Have you ever heard of anyone doing this?

 

Oops! Sorry about that! When looking to buy a carb, try ebay. A lot of guys with 125's will be looking to upgrade their carbs and there are a lot of 125's from the '90's that came with really nice 36mm carbs. Also, consider boring out your carb. Just remove all of the brass, slide valve, and anything else that is in the way and bore it out. Just DO NOT go as big as the slide valve. You don't want any chance of sucking air around the sides of the slide. My machinist got mine to 35.75 mm. It is not as good as one of the nice flatside carbs but I could deffinately tell a difference, plus it was only 20 bucks- for you it would be free.
JP