what would your ideal cr125 mod motor be built with?

[matt vassallo]

Hello,

I will keep it short and sweet.

I built a 125cc bike using shifter kart technology, swedetech stock moto 99 cylinder, 6speed transmission, but with a 38mm TMX mikuni carburetor, and a genuine honda RS125 pipe.

If you were not limited by the rules, what sort of parts combinations would you do in terms of internals on the bike?

I am running a flat top piston right now but plan on switching to a domed piston at some point, and I was thinking of adding the powervalve back in.

Rather than go with honda rs125 parts and spares as they are super expensive, I decided to go the moto route after seeing the power levels and reliability the honda moto kart guys seem to be getting.

Thanks so much for everyones contributions on this site. i've been quite a lurker, and appreciate everyones helpfulness out here.





204 lbs bike, hoping 35 horsepower! Have a mychron4 on the way, and then it's time for tuning.[/img]
_________________
Shifter kart technology on two wheels?

[Bill Schmidt]

I know that porting the hell out of everything will bring more hp. Cases, intake, exhaust, piston all get carved up. I have seen some aftermarket heads that are an improvement, too. Dont forget programmed ignition. I bet that 45hp is obtainable.
_________________
Bill Schmidt

'95 Trackmagic 125 shifter (Kawi)
'88 Red Devil F500 4-link rear (Rotax)

[matt vassallo]

Bill,

thanks for the reply!

I would like to start by saying I do not want to have to replace a crankshaft every 8 hours and a piston every 4. haha.

I love how some stock moto guys run an Entire season and I would think that some form of in-between would be best.

an example i have in my ideal world:

For riding around on the street, at lower rpm, a piston would last maybe 40 hours (with the ring replaced sooner), but would be replaced the weekend before a track day.

the crank would be gone through every three track days / track weekends

Thank you for the tip on the ignition. I'll look into seeing what's available as well.
_________________
Shifter kart technology on two wheels?

[Jim McMahon]

So to be clear, you want to run a Mod honda on a bike on the road or the track? What fuel do you plan using? Almost everything you do from this point to improve the peak power will narrow the power band, thats probably not want you want. Even on the track on a nike you will want a smoother delivery than a shifter kart. Power vavle addition will help some, but you'll still be pretty peaky. A 45HP mod moto as a daily rider is just asking for trouble IMO.
You could widen the ports, which is preferable to changing the timing for your application. CDI will help, but would need to be built in harmony with your pipe/cylinder and chosen fuel. You could try picking up a known combo from a kart, but honestly I'm not sure if it's going to work well on a motorcycle. You could experiment with squish band percentages (not clearance) and see what you like.

Personally I'd drop to a smaller carb venturi for more greater intake velocity at lower RPMs, 30-34mm. Dual smaller carbs if you have the room and go totally nuts could net gains. If you plain making 40-45 out out this you better expect to have to jet it too, so you could explore the pumper carb route for that, rather than having to change jets all the time.
Really though a 45HP mod is going to have power delivery akin to a lightswitch and will take high compression so you are looking at running racing fuel all the time.
_________________
GPI Racing | WildKart | Maxter | Hoosier
Karting Festival @ Blackhawk Farms June 1st-2nd.

[matt vassallo]

The idea would be that when I wanted to ride it on the street once in a while, I would be able to by, say, switching out the cylinder for a stock moto or something.

Right now with the bike untuned it picks up the front wheel in first and second, but luckily the throttle is adjustable, so it's not bad at all. It's actually a hell of a fun rush and sound.


When I wanted to get some more power out of it I could put the 'hot' barrel/head/piston in together, and jet accordingly.

I agree that riding around on the street with a barrel that is tuned to the hilt would be not that smart for the amount of teardown time it takes and just overall maintenance schedule.

I am adding a power jet / dial a jet that will allow me to do fine tuning easier without changing the mains out, but obviously if there is a significant temperature and humidity change, The whole thing would come apart regardless. The jetting will be based off what my EGT's read.

Regarding the squishband area, I will look into an aftermarket cylinder head that has replaceable inserts when the time comes to go down that avenue. In a perfect world I would get an insert that is based off of the latest drawings that frits overmars provided to everyone regarding two stroke gp cylinder head profiles.

I have been riding my CR125 dirt bike with this exact carburetor and have become quite used to its power delivery on dirt and pavement so I will be keeping with this size for this year at least. We will see how it ends up being.

Thanks everyone for your responses.
_________________
Shifter kart technology on two wheels?

[matt vassallo]

I'd like to apologize for not answering the question re: fuel.

The ultimate fuel to me is motorsport grade ethanol or e85 blend, because it has such an incredible cooling capacity built into it, and the larger flow amounts work harmoniously with the larger sized carburetors. The additional mass flow works wonders inside the exhaust pipe, but the lengths of the pipe may need to be changed as the EGT is often lower by a significant margin to normal racing gasoline.

For the meantime however I will probably just be running VP CHP or an equivalent ~100 motor octane fuel that is oxygenated 5-10% when tracking the bike.
_________________
Shifter kart technology on two wheels?

[Jim McMahon]

Yes, ethanol or even better methanol will make more power with the correct adjustments. Of course that throws the squish shape and sizes from ftiz out the window.
Yes larger venturi is good with alky, dual barrel even better. A set of buller duals would make it fly I'd say, especially on alky. What about punching it out some too? You are talking some serious power then, just not many laps per tank! You could have an alky setup for the track with carb(s), cylinder, head, pipe and CDI ready to rock. Add some static advance and go.

Riley Will is a smart cookie that you might be able to bounce some idea off. Oxygenated fuels are mostly outlawed for kart racing so by going that route you are introducing a variable there when it comes to comparing notes from kart mod motos and what's going on your bike. There's some smart guys on the 2cycle section of the karting.4cycle.com that are willing to share knowlege too. They know how to make "usable" power as they mostly race on dirt.
_________________
GPI Racing | WildKart | Maxter | Hoosier
Karting Festival @ Blackhawk Farms June 1st-2nd.

[matt vassallo]

I have thought about larger displacement, but it depends what they say going against the 250 four strokes. I believe you are allowed 150 cc against the 250cc four strokes.

It's easy to change a ton of variables and not have any idea where you went wrong!! This is why i have started with a pretty close to stock moto setup.

After the mychron gets here and i'm all set up with EGT and water temp it will be dyno time, and i'd like to ask, what kind of temperatures do you guys tune your EGT to? 700C ?
_________________
Shifter kart technology on two wheels?

[Chris Reinhardt]

Matt, real simple here... Stock moto top end for the street, call Swedetech and get a mod cylinder and head for the track. You can use an 05 CR125 piston for the mod setup instead of an RS. I wouldn't go with a programmed ignition, unless your chasing that last hp, they are deto monsters, you have to chase the tune all the time.

If you don't care about the resale on the stock moto, get the cases cut for a straight intake, and have them ported. Ditch the Mikuni and get a PWM Keihin, Swedetech straight intake.

Next thing that would be limiting you is the pipe. Not sure if you could modify a kart pipe to fit your application or not? Otherwise look for something like an RS125 Samba pipe.

Or by the time you spend all that money to build a RS125 clone, you probably buy an actual RS125 motor, have a cassette gearbox, 40hp+, pipes, ignitions, heads, pistons, etc, etc, bolt onto it....

CR
_________________
East Coast Super Kart Series
"This is how we roll!"
www.eastcoastsuperkart.webs.com

CR2 Motorsports
"Home of Cobalt Superkarts"
www.CR2MotorSports.webs.com

[matt vassallo]

thank you everyone,

I will provide some dyno info when i get back from my latest work deployment, and we can go from there!
_________________
Shifter kart technology on two wheels?

[Curtis Cooksey]

Get a detonation ring in the head and put a V-force reed cage on it. http://www.accelerationkarting.com/v-force3reedcage.aspx
_________________
www.accelerationkarting.com

[Enzo Iadevaia]

I say use an RS125 motor or get yourself a TM KZ10 and put that in there..

[Chris Reinhardt]

[Joey Guyon]

i would listen to Curtis and purchase a Vforce reed cage. It's the best bang for the buck.

You can/should also cut your head to change the squish clearance down to the .032" range. I have no experience with the E85 fuel, but the compression ratio at that squish setting is still tame.

If you get into cylinder porting, you need to expand alot of variables in order to optimize the port work. Pipe design and ignition mapping (over-rev) will come into play. Swedetech or Riley Will at BRC are your guys if you want to lose your mind. Case porting will also be in order.

Take your time and do your research first. What is your total budget? Chris wasn't kidding when he suggested the RS. It might be cheaper?!?

[matt vassallo]

I appreciate all of the comments.

I am not after every last bit of horsepower, and the bike is already done and running.

I am not going to switch the mounts and everything around for an RS125 motor.



I have far less than 2000 into the bike now total, considering I was able to sell all of the OE parts off that I didn't use, as well as spares I didn't need. I have more in safety gear than I do into all of the bike stuff. RS don't come at that price just yet. A good example on USGPRU is around the 6000 range.

For what I have into it I feel it is a good deal.

I think that If i were to equate the budget, the performance would be good enough to where If I were a better rider, it would show.

I am thinking to tune in my stock moto cylinder I will just use the domed piston and adjust the squish accordingly.

For a 'mod' cylinder I wouldn't mind spending some more to have someone make a nice bolt on solution, designed around a domed piston and better head.

I can use the ccspecialty tool to increase the time area and angle area on the transfers by very small amounts without damaging the angles..

A rebuild on a CR is around the 700 dollar range, vs around 1200 from rscycles for an RS. That extra money can go towards a hospital bill trip deductible, new ceramic nikasil plating from the uk, or simply more track time and/or newer tires.

If I were trying to win, you're right. I would have an RS. However, because I travel with my job, I am just trying to have something nice and fun for the 2 months a year when I'm home, and hopefully get it out to a track day then as well.

If i get 35 horsepower to the rear wheel, i'll be very happy. The bike only weighs 204 lbs with everything on it. With race plastics and all the lights/horn/blah removed I am hoping it will be 190.
_________________
Shifter kart technology on two wheels?