I have a stock 86 250R engine. I'm switching from an FMF fatty (junk!) to an ESR TRX6 for strong lowend and midrange. Topend isn't a big concern. Now, I'm at the intake and carb. A guy at ESR suggests a Mikuni TMX36 w/ a stock CR250 reed cage. He says that combo will improve all around power and Throttle response (<VERY important!) better than just about any other comparable mod like a RAD valve, RAM valve or PWK35 or 36 etc. etc.

My friend, today over the phone, said he doesn't feel that ANY bigger carb w/ that CR250 reed valve is going to help unless porting and head mods for for lowend and midrange are done along with it. he said port if I'm gonna do the Mikuni/CR250 Reed combo otherwise just put a RAD valve and stop there because the RAD is the best reed cage for low RPM torque and power>> I've heard that before too so I'll assume that of the RAD valve. Anyone disagree?

Now, I'm no mechanic. Everything I know is from reading and theory...not practice. So, with that in mind, I've ALWAYS read that a bigger carb is, without exception, a mod that favors upper mid and top end and actually WEAKENS throttle response because the velocity is lower in a bigger carb. That is, unless the head and ports are modified to handle the bigger carb. I know it gets tricky when you start combining different mods so I'm not sure what to make of all this.

I've also considered weights for the flywheel to make the power more tractable. Good idea or not? I also read on another "2 stroke expert" site that reed spacers (between head and reed cage) are bad for what I'm trying to accomplish for some a air-related reason and thast they are a gimmick

So, any mechanically knowledgeable people are more than welcome to shed some light on this project. I'd really appreciate it! Based on EVERTHING that I've taken the time to explain, what do you all think would be the best thing? Remember: Lowend, midrange and zappy throttle and strong acceleration. The hell with a 50 HP at 11,000 RPM screaming revver....not fun for the woods!

 

As two strokes are reed controlled, the low-end lost is not pronounced like it is on four strokes without head mods. Anytime you can get more air into a combustion chamber you can add to power in all rpm ranges, you just have to alter other parts of the engine to tune it into the rpm range that is most useful to you. The CR250 reedcage method works but not as well as the RAD or RAM valves. It also costs more but when a manufacturer suggests a certain setup for thier product, there is usually a bit of R+D behind the suggestion. It may be worth your while to check it out. My little brother runs an '85 ATC250R that is very mildly set up with aftermarket parts and he runs flywheel weights. We ride some serious tight trails in the mountains of Maine and he , while being a virtual expert with the 3-wheeler, had some trouble with the streambeds and reving the poor thing to the moon. The weights were an immediate and incredible benifit. Avoid the spacers. Excellant if the RPM is aiming high but worthless for those trails.

 

AKA ZTN

Well, the TRX6 pipe alone will give a huge low and mid gain. Now I'm looking for more. I'm gonna look into the flywheel weights. They seem like a good idea. The RAD valve has been for my first choice for modifying the intake. I'm skeptical about the bigger carb w/ out doing any porting. Thanks for the advice....

 

Mikuni 38TM. You will see low and top end gains with this carb even with stock ports. I have used them on my polaris, 3 atc250r's, trx250r of a friends, blasters, etc. I would also recommend FTZ's intake. Check it out at www.FTZracing.com.

REED SPACERS ONLY HELP A BAD PORT JOB! Really. You won't notice any difference.

 

AlkyBurninX,

How does a larger carb give more low and mid? Doesn't that go against conventional logic?? Please explain.

I'll go look at FTZ's intake. What benefit does it have for what I'm looking for?

 

Well, I personally feel the mikuni is a better carb then the Keihin. This comes from experience with many of each manufactures sizes/models (aside from the FCR for 4-stroke). 38 is really not much larger than stock. Isn't stock a 34 or 36mm? I feel the mikuni mixes air and fuel better at low end, which makes power. Also, you would think a larger carb is worse for lowend, but everyone claims stock 26mm banshee carbs make more topend than 32/34/35mm, and the larger are better for lowend. I have used the 38tm/tmx on 250r's (I now have a 41mmTM) and I have a friend that still runs one. They help power throught the RPMs. When I took the 34 off of my 350 polaris and put the 38tmx on it, the added low was amazing.

Intake/reed cage mods will help at all RPM's including lowend since you don't really alter port timing or air waves (the two things that make a "powerband").

Also, if you buy the 38 now and modify later, you won't have to purchase another carb because you will already have what you need. Same thing with the intake. FTZ is the king when it comes to 250r stuff. They use the R engine in ATV's and Micro Sprints. Both types of racing require lots of torque from a small engine. Ask them what they think. They are pretty good guys to talk to if you have a good knowledge of bikes.

I am not a fan of flywheel weights. Decrease acceleration and throttle response. I would rather feather the clutch.

 

i have a 310 mx (mildly ported) pro-x cylinder, i used to run the fmf fatty pipe and rad valves/with a 38 pwk airstriker. this produced a lot of low end power and instant throttle responce. (gearing is 13/38 with 18" tires). problem was it ran out of power quick and i was shifting alot to keep from running out of top end power. i have a sparks mx pipe now and i will tell you it does not have the low end power of the fmf fatty. not even close. the sparks pipe has more mid range and upper end power than the fatty. i just rebuilt the motor and have installed the sparks cr250 reed cage. i'll let you know soon how it compares to the rad valves after i break my motor in. the quad is faster with the sparks pipe on it, but it doesn't have the four stroke smooth power band like the fatty did.

 

I'll tell ya what. Buy a Keihin38mm Airstryker and a Moto Tassasnari V-force2 reed block and be done. This is a very potent combination and will grow along with your project nicely. Get some low-mid porting and you'll be set! Don't mess with the weights. Mikuni carbs are funky, don't listen to a salesman (although most good shops will have access to Keihin and Mikuni) try and pitch you a piece of MIkuni sh--. Our customers have used just about every combo, and the one suggested here works magic!