Mikuni BN carb on MX engine.

M

Morton21

New member
Looking to get away from the tillotson that we have on it. With the no restart rule we haven't had much luck tuning it to where it won't die under sudden stops or spin outs. Although it doesn't happen often still want to eliminate the issue all together. I know on our big block 420 we ran the 38 super BN carb and had excellent luck with it. Looking to do a 40mm on our built yz450 engine.

Anybody have experience with a super BN mikuni on a mx style engine?
 
Why not run the keihin that comes on it, that would be a much better carb. Or you could find a crf450 carb they're 42mm I think.
 
Actually the driver should be able to keep the engine running. That's a big part of his job. EC never intended the engines to run at idle.
 
For all intensive purposes your right... But if you get slammed into the wall and then bounced around by other karts hitting you while parked... Your thoughts are probably not going to be in keeping the kart running. On top of that, your foot may or may not be on the pedal at that point in time or even be able to hold the foot on the pedal while going through that stuff. If the kart is good after all that but the motor dies... your done. Why take the chance?

On top of all that, the tillotson has been very finicky and does not idle down enough to work well with the recluse clutch that's in the engine. It will not idle no matter how tuned below 2500. It over heats the clutch because it is in a constant state of "slight" engagement. We've fell out of a race because of that too, losing the clutch due to heat. Trying to get something that will idle down and stay running.

Flash, I think that's what we are gonna do. We have talked to some carb guys that do them for MX racers and decided for reliability that is probably gonna be the best option.
 
If your hung up on running alcohol, try getting a FCR41-43 converted. Micro sprint guys probably have some.
 
That's very expensive, I wouldn't give that much. I bought a 41mm fcr off of eBay about 3 yrs ago for $150 maybe even less.
 
No plans to run alcohol... Maybe e85. But as of now we are gonna stick with the race gas we have been running.
 
Sorry. I assumed when you said tilly that you meant dual tillys on alcohol. Personally I think a slide is much more driveable and user friendly once set up. But I haven't ran a tilly on a big thumper either, just industrials. FWIW, You can find FCR's all day on ebay and other forums ranging from $150- $400 straight off of bikes. You get what you pay for. I've had the pleasure of meeting and chatting with Kent and his equipment is good stuff. I would assume these carbs were from one of his karts so they are properly set up. I wouldn't hesitate to throw 300 down for his carb. I took the cheap way out and bought a $150 FCR from ebay with no hot start. I spent 2 weeks testing and jetting and testing some more. I also filled the hot start with epoxy, bought a cable, bought some jets and needles, and built a carb-head adapter. All said and done, I got around 300 in mine plus my time.
 
http://www.tillotson-racing.com/collections/racing-1/products/hm-1-unlimited-all-stars-jr-drag

This is the carb we have been using. EXCELLENT hp and a nice power band for big tracks. Killed the low end torque a little much for the smaller tracks. With the nationals going to be at an average size track and low bite we are gonna need to bring the power band back down in the RPM range. We'll hang onto this carb and maybe retry at a later date but for now we are looking for some reliability. We boxed up the stock keihen carb and sent it to Dicks Racing, seemed to have a lot of knowledge about them over some of the others we've talked too. We'll see how it all shakes down. If we don't like it, then mechanical injection it will be...
 
For what it's worth, the fcr on my trx doesn't like being stopped from wide open. Seemed like we had to really lean the needle but stay big on main jet. So, we wired a second battery and hit the starter with 24v when Ive run out of talent and am sitting quietly. I think you are paying for an extra .039" of bore on Kents 41. Fcr is a good carb. Just have to understand all 3 fuel circuits. I probably dont. Good luck.
 
The super Bns are made primarily for jet skis, snowmobiles and the big side by side atvs of 900cc or more, usually in pairs or 3s configurations with a big mikuni pump to feed them, popoffs set in the 20 to 60 range, they do flips and jumps ects and don't shut off, so i'm sure they'd work fine on uas karts with thumpers in the 450/500 range, when i get off my lazy butt and finish my jawa, i't'll be sporting on of 42mm since i have 3 39s that came my way by freebies, but need the choke removed and rejetted for alky, i've did some research and i think they'll do fine, you can get new ones from summit for about $250, we'll see how they work out in the near future.
 
Might try one on my yz450f or stay with it's fcr 39 and bore it a bit and do away with the float using a double pump around. That's a whole other project lol. Tied up with looking for 35 hp in 14.5 inch alky clones and animals atm.
 
I run a44mm super bn on my Honda 450 and is everything I could ask for. My opinion, and it's just that, My opinion seems to be the opposite from everybody else, I prefer the super bn carb over the slide carbs.. in my experience, the butterfly carb is more controllable, and smoother. I have run the super bn for a lot of years and I know them so that's why I went that route, no other reason then that. The BN carb is so tunable it's not even funny. But like I said, it's my opinion and nothing more then just that.
 
I run a 38 super bn on that, I run the 44 on my crf. Its big bore on alcohol with throttle position sensor hooked up and core exp rekluse with heavy wedges, blue pressure plate springs, and white springs in the clutch disc. No stalling problems as of yet and I've spun out a few times lol
 
You must log in or register to reply here.
Back
Top