I'm running the 14 tooth front sprocket, and I still think it is geared too high. Time to look into the rear sprocket.

Problem is, I'm drawing a blank... don't I need to go with a larger rear sprocket to lower the gearing? Wouldn't a smaller rear sprocket make it "harder" for the engine to turn the rear axle?

I think it comes with a 41 tooth, and if I'm thinking right[img]i/expressions/face-icon-small-confused.gif[/img] I believe maybe a 42 or 43 tooth might be a good place to start. I'm going to try to find a sprocket that has a bore close to the hub size, and a friend can chuck it up in a lathe and fit the bore to the hub. At least that's the plan.

Anyone have a spare sprocket hub laying around, and can give me the dimention of the hub where it supports the sprocket?

 

The rear sprocket is much bigger, which means a change in the rear sprocket would not do nearly as much as the front. I've been told going lower than a 13 would probably be too small (would be hard for the chain or something.) Of course this would have to be custom made, as no one seems to know of a 13 that would fit.

If you are going to go up on the rear sprocket, you are going to have to go much bigger to get the same result as you got from 15 to 14 front. 41/15 is 2.73:1 and 41/14 is 2.93:1. going from a 41 rear to a 43 rear is 3.07:1 which is only 70% as much of a change in gearing as was going from 15 to a 14 front.

Damn, that was confusing even for me. I think checking out gear ratios would be the best way to figure out what rear to go to.

Check out <a target=_blank class=ftalternatingbarlinklarge href="http://www.sprocketcenter.com/g/4156/gear-ratio-chart.html
">http://www.sprocketcenter.com/...atio-chart.html
</a>

Front sprocket gearing changes are not linear. You probably noticed going from a 15 to a 14 (I think you did 15 before going 14?) was about 60% as much as going from a 17 to a 15 was. Going from a 14 to a 13 will be much more than 15 to 14 was.

*THE IMPORTANT PART*
Going from a 15 to a 14 was a 7% increase in gear ratio, so you would have to go to a 44 rear to replicate the same percent increase that you got from going from 15 to 14, which would almost be as much as going from a 14 to a 13 front.

What you have to ask yourself is how much more low end do you want? Bigger rear sprockets offer nearly linear change to the drive train (each tooth gives about the same increase, and almost the same percent increase.) Smaller front sprockets give a non linear increase with each tooth (being every tooth gives much more than the last.)

There is (if I understand propperly) a point of dimenishing returns because of motor loading. The less you load a motor, the less efficient it gets. IE, more power to the wheels, but the decrease in RPMs on the drivetrain isn't linear to the percent of change in power to the wheels. Some ATVs I've seen in low range where the gear ratio on the chain has actually been modified with *THREE* sprockets instead of the stock two sprockets, have a top speed of 5mph, but not an increase in 1200% of power to the wheels. Can you get me? They use it for mud exclusively if you were wondering.

-JNY

 

OOXJ, what are you finding that leads you to believe you want lower. I often wish I had more power to scale some pretty steep hills. My feelings is that the gearing with the 14t is the best compromise between speed and climbing power, I think it needs more raw Horse Power. I rode a Viva 250 (the nice one) the other day at a dealership and I truly think that their Honda copy motor had more power then our Yamaha copy, and it seemed to be geared correctly from the factory.
What about is there a high preformance CDI box that could help us?

 

I'm Not An Engineer, so take this with a grain of salt.

I'm with you outofline. There are two ways to go with the engine to get more power without loosing top speed. One is to go even lower on the gearing and increase it's rpm capability which requires intense engine work (valves, seats, springs, liners, cam/exhaust/intake work, to only name what I know of) or increase the ammount of fuel that it is able to burn at one time. So, to make that power, you need air and fuel and spark. Going to a better plug with aftermarket CDI would definately give us more power. I'm thinking in the 15-20% range. Of course, airflow from a port and polish on the intake and exhaust port modification (and maybe even a custom pipe with a different airflow/backpresure signature) would be useful in this as well. Unfortunately, finding a CDI that will work on this will be a bit of an issue, and it should be a computer programable, live-tuning model so we could experiment with the right timings and such for the best power. Plus it will allow you to tune your low rpms to have more grunt (slightly advanced from normal) which will give use wheelie popping, hill climbing low end as well as slightly more retarded timings for top end (like beginning a climb with peak HP output.) This is if Jetmoto does not offer a bigger bore/stroke piston/sleave/rod/cam upgrade (which I really really really hope they do.)

Also, nitrous is an option. Good wet+fuelpump kits are about $500 if I recall and they install pretty easily between the carb and the intake.
You could use it only when you needed the grunt to climb, and even remove the bottle when you don't want someone to be able to hit the magic button.

We all know nitrous can be dangerous and bad, so it should probably be a last resort, huh?

What about drivetrain lightening? Could we remove mass from the rod, flywheel, etc to help out with double rotating mass? I can't remember the formula, but there is a formula for X grams removed from the drivetrain = Y percent increase in power. X is grams/rotations per powerstroke so weight removed from the flywheel, for instance, would provide more power (per gram) than weight removed from the wheels.

Don't underestimate wheel weight/size either. If you can go to a 18" wheel, it will lower your rollout from 62.8 to 56.5 which is like an 11% increase in the numberical gear ratio. This ALSO has the effect of lightening the drivetrain which increases power (and 4th gear max rpm) so you will only loose top speed because of bearing resistance. The bearings on the axle will be spinning 11% faster, and bearing resistance goes up fast after a point. I'll bet 18" tires would actually only lower the top speed by 1-2mph but give rougly 10% more power/torque at the wheel. Eventually, traction becomes an issue =/

Man, I love science. If anything here is utter nonsence, it's because I'm in a hurry. Hope it helps somehow.

-JNY

 

<div class="FTQUOTE"><begin quote>Originally posted by: outofline

OOXJ, what are you finding that leads you to believe you want lower. I often wish I had more power to scale some pretty steep hills. My feelings is that the gearing with the 14t is the best compromise between speed and climbing power, I think it needs more raw Horse Power. I rode a Viva 250 (the nice one) the other day at a dealership and I truly think that their Honda copy motor had more power then our Yamaha copy, and it seemed to be geared correctly from the factory.

What about is there a high preformance CDI box that could help us?</end quote></div>

Whew... Thanks for all the info pimpsmurf. It will take awhile to digest all of that. I just think a 250 should have more grunt. I had a '79 Honda XR-185, and loved it. I don't think I ever loaded it down to where it wouldn't pull itself. I could do almost anything in second gear.

I agree about a port and polish on the intake, but the intake is such a snake I wonder if that would do much good.

Maybe I should just leave it the way it is. Just a lack of horsepower might just be the cause. The engine runs so good, I don't want to start tearing it down and risk screwing things up.

Honda's take has always been just spin it faster to gain horsepower. That is what they did for years with their 4 stroke Grand Prix bikes, racing against Yamaha, and Suzuki's 2 stroke bikes. That is where the 6 cylinder CBX came from. Only problem with that is you have to start making parts very light, and you loose torque. I had a '79 CBX and while it was FAST, it had NO torque. Less rotating mass. Do you remember Honda's oval pistoned Grand Prix bikes? UnGodly amounts of horsepower, it simply would fry back tires. They gave up after they couldn't find any tire that could put the power to the ground.

 

Sorry about going off. Once I get going, it's hard to stop me! lmao. I had a dinner to attend, so I actually cut it short if you can believe it.

I noticed what you were saying about the intake. Maybe something can be done, but where is the carb? I'm looking at the side view and it seems like the carb is not visible from the side. That is weird. Space must have really been an issue. (fitting a 250 into a 200 frame?)

As far as it being snaked around, P&P actually works to our advantage even more then because there is more resistance to be overcome bia polishing

If airflow is really such an issue, there is always forced induction! I know people with turbos on thier lawn mowers (not even kidding, I'm talking bored with real truck suspensions and the mowing decks removed), so getting a super charger on this guy couldn't be that hard. The problem would be finding a supercharger that would be small enough, and figuring a way to drive it on the engine, as opposed to the chain. I don't think this would have enough airflow to support even a small turbo.

We aren't looking for all that power are we? See what I mean about ranting?

Anyway, on the more practicle side, porting and exhaust port modification, a more advanced plug (like the e3 sparkplug that should be out this year) and a ignition would be sweet. An aftermarket ignition will sort of shoot a shower of sparks instead of just an arc of sparks. It's pretty impressive to see it fire. It will definately allow us to burn more fuel (more power) but I doubt we would find one that would work.

A Modified cam from Jetmoto would be SWEET if it would help us put out more power (intake port is opened for longer than stock I believe) and with a P&P + exhaust mod it should push the carb to a couple sizes larger on the jets.

I would still like to know what the stock RWHP and torque results were on the dyno!!! I'll bet it was around 10hp.

-JNY

 

No one has ever answered me, and I still wonder if the carb is a little too big. Intake velocity slowing down too much.

The carb sits right under the front part of the seat. It and the filter is a tight fit in there. As far as an ignition system, why wouldn't any aftermarket system designed for a 250cc engine work? It might require splicing into the wiring harness somehow, but other than that what would be a problem? Something else I might look into.

Did someone actually run one of these on a dyno? I think one of the biggest hurdles is simply the motor is a pushrod design. Can't push it too hard.

 

<div class="FTQUOTE"><begin quote>No one has ever answered me, and I still wonder if the carb is a little too big. Intake velocity slowing down too much.</end quote></div>
All other 229cc engines that I can find use 23mm carbs. A 23mm japanese carb couldn't cost that much, so maybe I'll give it a try and see.
Have you modified your exhaust port like is described here? http://forums.atvconnection.co...id=519700&startpage=1


<div class="FTQUOTE"><begin quote>As far as an ignition system, why wouldn't any aftermarket system designed for a 250cc engine work? It might require splicing into the wiring harness somehow, but other than that what would be a problem? Something else I might look into.</end quote></div>
As far as I can tell, ignitions are different from model to model because of the difference in timings, coil design (points, dual points, etc) and maybe some other reasons I don't understand. A programmable ignition would work, but we would need to know how to install it (IE, the timings, etc of the stock ignition) then program it to stock and start tinkering. Even at stock timings it would provide more spark because of the way it pulses the spark in intervals with post-ignition "clean up" sparks after the main shower of sparks. An aftermarket plug that can handle the heat, and provide a better spark pathway (like the e3 does) would also add to this. After you got the ignition working, you could toy with programming it and see what works best. Some programable ignitions can even have different profiles that can be triggered by a button mounted on the handlebars. This would allow you to go into high torque mode for climbing and such with a simple press of a thumb button.

<div class="FTQUOTE"><begin quote>Did someone actually run one of these on a dyno? I think one of the biggest hurdles is simply the motor is a pushrod design. Can't push it too hard. </end quote></div>
I believe John@Raceway was actually present for a dyno testing where the top speed they list (64mph I think) was determined. I would expect they ran it on a loaded dyno with hp/torque graphs, but none of this has ever surfaced. I'll bet the torque was higher than expected, but the horsepower was lacking and they decided not to publish it.

-JNY

 

23mm? And the Jetmoto has a 30mm? IMO it is clear that "they" built this thing for top speed... bragging rights, etc... As far as a carb, it looks to be a Mukini clone. As I've said before the bike might run better low down if the intake velocity could be kept up higher. At low rev's, and a load on the engine it simply cannot suck enough air/fuel mixture into itself because the velocity is too low/slow. I'm no expert, and I might be talking out of my you know where, but it makes sense to me.

As far as ther exhaust mod's, no I haven't taken the pipe off yet.

 

The air velocity (I believe) controls how much suction is applied across the jets which pulls fuel into the carb. the size of the carb (again, I think) controls how much mass air is allowed in at high rpms. If the quad isn't running rich, then the air fuel mixture is right, but if a 23mm carb was installed, it would require smaller jets because the air would be more restricted, correct? The velocity that the air is drawn into the cylinder is only part of the equation. a larger carb should allow more air in at the same speed, so a lower velocity is needed to get the same amount of air that a smaller carb would need. Does that make sense? Still, maybe a smaller carb is worth a try? I just don't know.

I still think smaller rear tires, with exhaust work, K&N filter, new jets, and port and polish is going to give this thing all the power it needs to climb. If you read the link I sent, they describe a nasty design near the exhaust manifold where it is very restrictive. I plan on checking it out myself and maybe getting an exhaust shop here to cut out the blockage for me.

I really don't know enough about these kinds of carbs to provide much to the conversation. It would be great if someone from Jetmoto could step in and resolve some of these lingering questions for us.

-JNY