top speed ???

[turbomatt1]

what kind of top speed are you hot rod guys getting out of your bikes ? ...& what gear combo gets you there ???
mine is a K2 with a 125 Zonger manual clutch, big exhaust & 18X33 gears. I weigh 230ish & can get about 55 indicated if I tuck in, & 50 setting straight up. (havent checked it with a GPS yet, but flow of traffic indicates it's at least close)
my jetting is still pritty lean up top, as I havent been able to come up with a #90ish main.
(anybody got one?)
I'm thinking that a 31 tooth rear & the right main Jet will get me to the 60mph mark. (?)

 

[69ST]

Weight is a factor but, at least in the adult weight range - above 170, it doesn't affect top end much as you might think. I hit the scales at 190 and adding 120lbs of wife to that knocks off less than 5mph on a flat road. The biggest effect is how the bike ascends long hills; that's where any extra weight is really felt. Wind resistance is what soaks-up the lion's share of engine output at max speeds.

Most stock Chinese engines have low compression, mild cams and the ports don't flow all that well. That's not all bad, it allows the engine to live happily on crap gasoline and keeping revs relatively low is easier on the rotating & reciprocating assemblies. Torque, especially in the midrange remains unaffected and you get decent throttle response but peak hp occurs at fairly low rpm. In a nutshell, top speed equals horsepower and as you've seen, your actual top end is less than expected. What I've noticed with these engines in stock tune is that the power curve falls off quickly; the bike accelerates well up to a point and quickly runs out of breath.

A stock Z108 motor, properly setup, is good for high 50s. I would expect your Z125 to be a bit faster than that. 60-ish top speed seems a reasonable expectation. Three factors need to be addressed:

• 
  A new motor takes a good 500 miles to start loosening up, 1000 miles before you can even begin to think of it as being broken-in and will keep getting better for at least 2000-3000 miles. It's not uncommon to pick up 5mph after full break-in. There's a lot of friction in a new engine.

• 
  Jetting needs to be optimized before you begin thrashing the motor. Too rich and you'll lose some top end power, but as long as it's not so rich that it won't rev-out cleanly you're not giving up much. More critical is running lean. As any old school racer knows, "lean is fast". But, lean is also risky. Burnt pistons/valves will result from hard running with a lean mixture

• 
  In order to reach full speed potential, gearing has to be dead-on. The best way to approach this is to select a sprocket combo that has the engine right at peak hp at top speed. To approach this scientifically, you need to know 3 things: rpm at which peak hp occurs, how fast the engine is capable of pulling you, overall gearing from the crankshaft through the transmission countershaft. It's still possible to figure out optimum gearing without that data, but it's a trial-and-error proposition.

 

[turbomatt1]

Well said. & your description of the power falling off rapidly up top, is dead on with mine. perhaps even worse with my jetting issues. with a 100 main jet, it was stronger in the upper mid range, but would break up a bit at high rpm WOT. with an 82 main (all I could find) it dosent seam quite as strong in the upper mid range, but revs out cleanly, & actually pulls harder if you back off the throttle just a touch.

So, how fast is your ride, & what is your setup ??

 

[69ST]

That's sharpens the focus a bit more. Some educated guesses...

The #100 main jet was slightly too large. That's why it pulled harder in the midrange and wouldn't rev-out cleanly. Rich mixtures are optimal for acceleration; that's why power valves, accelerator pumps and EFI tables/functions that recreate those functions are used with more spohisticated induction systems. The carburetors we use are much simpler devices and don't have as many tuning parameters. Lacking fuel enrichment circuits to cover transient lean spots in the overall fuel curve, such as exist under acceleration, the only option is to get the WOT jetting dead-on, followed by the idle mixture then find the best compromise to get through the largest section of the fuel curve, the midrange. That usually means running the richest mixture that allows clean rev-out and the richest idle mixture you can get away with (doesn't load-up the plug).

You've found the rich & lean limits, a process known as "bracketing". Next thing I'd do is source a #92 main jet and go from there. If it's still too rich, try a #87, if it revs-out, then substitute a #96. One the jet size range has been bracketed, you split the difference (midpoint jet size), recheck & repeat. By the third go around, you should be close enough to make single # jet changes until you find the best one.

These carbs function pretty well under steady-state conditions, particularly at idle & WOT. You have to tune around the inevitable lean spots at part throttle and the transient lean spikes that occur under acceleration when the throttle opening is first increased. That's why running at the rich limits at both ends usually produces the best overall compromise.

I sold the Zonger 4 seasons ago, replacing it with a 110 Honda Nice and building a second bike with a tuned version taken out to 137. The stocker is capable of a consistent 67mph, the 137 about 10mph faster (but gets there a helluva lot quicker and doesn't slow down when ascending hills). Both took some experimentation with gearing & jetting, about 700 miles to really loosen up and keep getting better as the miles roll on. Can't really give specific gearing recommendations for your engine as rev range, power curve and primary/trany gearing may be quite different from your Z125. The stock motor, being a known quantity, was easier to setup. It only took three sprocket combos to find the best overall...the first one, which was based on calculations. The tuned motor was a lot trickier. At one point, it was so overgeared that the bike was no faster in fourth than it was in third.

Best gearing with the Z108 motor was 17/33. Judging by results of others in this forum with your motor, that may be overgearing for your Z125. Unfortunately, there doesn't seem to be any transmission gear ratio or power curve info available for the current crop of Chinese motors.

 

[turbomatt1]

sounds like you've tuned a few Thers no substitute for experiance. thanks again for the valuable info.
Living on the painfully Flat sandbar they call Florida, my seat O the pants tuner tells me that if I get this thing jetted right, it should be able to efficantly pull a 18/31 gear combo.
(I'm 18/33 now)
Your 70ish MPH speeds are impressive! Is the Nice motor auto clutch, or manual? Are the very pricey ? what are you running for exhaust system?
I would love to invest in a large jet assortment to tune with, but have been unable to find a set.

 

[MSZ]

Your 70ish MPH speeds are impressive! Is the Nice motor auto clutch, or manual? Are the very pricey ? what are you running for exhaust system?

Bob,

Why don't you document the total costs for your 137cc build.
Expenses are all relative.

 

[Dan_Lorenze]

With 17/35 gearing my Lifan big bore quickly hits mid 60's, any taller and I would start to loose bottom end. The cheapy Oko 26mm works like a champ but feel it's still fat, I'm gonna go with a 90 and see how she does.. My build cost? I'm right at a grand and run crap gas.. Pretty cheap considering I hung with a Ducati in the twisties for a few miles on Sunday, the poor guy tried to lose me.. The power,gearing and handling is just where I want it, but I could use a steering stabilizer right about now..

Go ahead RX, rain on my parade... What am I doing wrong this time?

 

[69ST]

Top speed is a secondary issue, good for tuning and the occasional banzai blast. Coming up with a combo the you like is primary. If you know the top speed potential, then you know what a reasonable cruising speed is (about 80% of top speed). How the engine does its job makes more of a difference. Some of us like high-revving screamers, others prefer stompers; both could potentially have the same top speed. What's best is a balance of what you want and what you're willing to invest.

The cost of getting a Nice engine these days ranges from about $700-1800, depending on whether you opt for new, used or rebuilt. That's about going rate for other small Honda motors, new from Honda. Both manual and semi-auto versions are available. It's a quality unit with a bulletproof lower end, in stock form they seem to run forever. Taken from that perspective, it's a lot of bang for the buck. Compared to the current crop of Chinese engines being sold for $350 on ebay the price seems like the national debt. As you've seen with your carb, Chinese parts can be difficult to source. You pays your money and takes your pick. The 137 or other tuned versions are a can of worms, with regard to this thread.

Note to DL: Neither rain, nor sleet nor snow...let the parade continue Your comments on gearing and speeds bear out what I mentioned in an earlier post. If you're happy with the results, that's all that matters. You're the one who's riding it.

I'll go out on a limb and guess that you'll probably be happier with the carb jetted on the fat side. Part-throttle power/response almost always takes a hit somewhere in the curve with leaner jetting and I've never seen much difference in WOT power/top speed as long as the motor will rev out. Sounds to me like you have everything pretty well dialed-in, as-is. Still, with thoughts of performance improvements in mind, I can't resist tweaking things either.

 

[reynolds_brad]

Expenses are all relative.

Sure are Dan, and are not something to brag about. I tell a few people costs and others ive learned to keep my mouth shut. When ive got the monkey parked at a coffee shop or somewhere else here in the City ide say about 80% of the time the $$ question is the first or second thing that comes out of someones mouth that approaches me. I never give any one on the street a straight answer. For a few reasons: One, i really dont care for strangers criticisms on what things should be worth. Two, there are quiet a few shady characters in the city around here and ide hate for one of them to try and jack my bike.

im pretty close to getting a final total on my 137-Monkey and im also pretty close to finishing the 124+R-ZB.

Take a guess as to which of the two has a total build cost of just a little over about half of the other total build cost...

If i don't kill the budget with paint and exhuast that is.


None the less back to the topic. I cant give any of you any numbers just yet as im not finished with my ZB. Soon though ill give you all some good speed numbers between my two builds. First things first im getting my ACL reconstructed on the 23rd. Once the Doc gives me the OK ill be back at it.

Oh one last thing. use GPS!

 

[crazycoop]

Sure are Dan, and are not something to brag about. I tell a few people costs and others ive learned to keep my mouth shut. When ive got the monkey parked at a coffee shop or somewhere else here in the City ide say about 80% of the time the $$ question is the first or second thing that comes out of someones mouth that approaches me. I never give any one on the street a straight answer. For a few reasons: One, i really dont care for strangers criticisms on what things should be worth. Two, there are quiet a few shady characters in the city around here and ide hate for one of them to try and jack my bike.

I know exactly what you mean!
Every couple of years I try to build something fun in between race cars. This year I'm building a 1971 CT70H just for kicks so that me and the family can have some fun this summer. Well three years ago I built a bar stool. All Moly tubing, Aluminum wheels/ slicks, 5.5 Honda engine, juice brakes etc.
I took it to a car show and some guys asked me "how much would I sell it for"? and I gave him a price that was $100.00 over what I had in it. He basically said I was stupid and that he could build it for less than a third of the price I told him.

He did shut his cake-hole pretty quick when I whipped out $500.00 and told him I would expect 2 of them delivered to the house by the end of the following week. Then I told him that they had to made out of Moly tubing and they had to have brand new Honda 5.5 engines on them. Basically the exact copy of the one I built....He just walked away...all of my buddies that were hanging around started laughing. I smiled and put my money back in my pocket.
Watching my friends ride that thing has been worth all of the money that I have invested. Totally priceless

 

[69ST]

Discussions centered on price alone are pointless. Imagine what the world would be like if that was the main, or only, consideration Purchase decisions are almost all based on fuzzy math. Were purchase price the overriding concern we'd all be driving Daewoos, living in houses the size of a 3-car garage and shopping mainly at Walmart, Big Lots and the occasional garage sale! Purchase price and value aren't the same thing. Perceived value is what it's about. Small bikes, as much as any other discretionary purchase, are like a multiple-choice test with many correct answers. If nothing else, come purchase time, no one's arm gets twisted. As I've said before..."you pays your money and takes your pick".

The key difference is the diminuitive size of our bikes. For example, when's the last time you walked up to an H.D. owner and asked, point-blank, how much the bike cost? A lot of people just assume that because it's a small bike that such Q&A aren't tacky and ill-advised. Of course, just because a question is asked doesn't mean that you're obligated to give an answer.

 

[justin1982z28]

I agree with not wanting to discuss cost with those who are just trying to be critical. On the other hand, it is nice for people who are thinking about taking on a similar project to have an idea of what they are getting into. For example, I once built a turbo 4 cylinder car that I got into mainly because the initial purchase price of the car was cheap relative to its potential speed and they were old enough that it was reputed to be a junkyard project. Unfortunately, this was not true, but the perception that was pushed by the community of owners of these cars was that they were cheap to buy/mod/drive. In every respect, I would have been far better off with a small block chevy in anything by the time it was over due to the little things that aren't mentioned but add up very quickly.

Unless you are a shop, hotrodding anything has to be done for the love of the project, not because it is going to be cheap or because you think you can make money. Hotrod projects are rarely cheap or profitable.

 

[turbomatt1]

LOL Turbo Dodges.... I have a bunch of those.

So... back to the Topic... what are you guys running for top crusing speed & with what mod's / gear combo ??

 

[milspec ct]

110cc bored and stroked ct 70, Monster?Ultra head,22mm Mikuni,D&L exhaust.I weigh 205 lbs bike is geared 16/36,pulled 58 MPH (GPS checked) on the flat.Just got finished rebuilding to 120 cc.Took it for a test ride,after some jet checking and breaking in the new piston and jug,Im confident to see 60 to 65 MPH for sure.....

 

[yedi]

ct70H, 4 speed Installed DR ATV's 85cc kit, 13front, 45 rear, seen 52mph (engine was screeming) I weigh 275lbs. 20 mins later, the piston hit the spark plug, fixed that, then the piston hit the Intake valve and broke the edge of the piston off. I stretched the rod (indicated by a caliper against the new rod/crank assembly and that the wrist pin was loose up and down, like an ovaled hole.) So.......I got a new piston, crank and rod, new bearings, gaskets, ordered an oil cooler, and a some new front sprockets (15, and 16tooth)

Put motor together and tried 15 tooth front and 45 rear, runs real strong, didn't run it to hard because motor is still fresh, seen a strong 40mph, not pushing hard at all, I say 50 for sure, easily.

Installed 16tooth w/ the same 45 in the rear, can cruise at 30-35 real easy, seen 45 pretty quick, I would guess 55 reving fairly good. could get 60 if I rev it like the first time, but that will never happen, sick of ripping these motors apart. Only problem is going up hills now, 4th gear bogs down, but its still quick on the low end, so I'm gonna keep it there, just for the cruising factor w/ my buddy and his moped that will do 52 easily.