[karasman]

Guys,

I have rebiuld 80' Z50R and fit stock CT70 Engine with stock carb there. Then I have installed tb88 cylinder with high volume oil pump.

Two questions :1zhelp:

- right now, due to 8" wheels (instead of CT70's 10") and CT70 gearbox it runs max 20 mph.
Which sprockets you suggest? I have brand new 13T for front already - what should be at rear (tell me if front 13T doesn't make any sense here)?

- should I use CR7HS spark plug for tb88 or a different one?

Best regards,
Adam

 

[karasman]

Guys,

I need a hand here. Any ideas?

BR,
Adam

 

[T Smith]

Is the rear a stock 37? I'd start out with a 16-17 in the front and adjust from there.

 

[Red Dwarf]

I'm just doing the same to my Z50R, although I'm only running 75cc.
Mine runs out of legs at about 30mph (hard to tell an accurate speed).
I'm running a 12 tooth front and am just changing the 37 tooth rear for a 31.
Gotta shorten the chain which is a PITA, should've played around with the front sprocket instead as Thomas suggests.

I'll let you know how I get on.

 

[Red Dwarf]

Just remembered this was posted a while ago on a UK forum.

Honda50.com Gear Ratio - Speed Calculator

Having run the calculator, my new 31 tooth rear isn't gonna make much difference to the top end.
Looks like a 16 tooth front is needed too.
I'll hold off chopping the chain!

 

[karasman]

Guys,

Is gearbox the same in CT70 and Z50? Can I use Honda50.com Gear Ratio - Speed Calculator as proper example?

Stock is 37T on rear but at this step I do not have one.
I will start with 16T on front. What you suggest to buy for rear?

Thnx,
Adam

 

[HondaFiddyMan]

16 over a 13 will make a HUGE difference.

-HFM

 

[Red Dwarf]

It does list CT70 gearboxes on the calculator, both 3 speed semi & 4 speed.
I'd take the readings with a pinch of salt, but it is a good indicator to show if your'e heading in the right direction.

 

[karasman]

Thanks for advice!
I will try with 16T and 37T.

How about a spark plug? Should I use CR7HS or a different one?
Maybe NGK CR7HIX is a better idea?

 

[Z]

It does list CT70 gearboxes on the calculator, both 3 speed semi & 4 speed.
I'd take the readings with a pinch of salt, but it is a good indicator to show if your'e heading in the right direction.

Correct. I developed the calculator. It generates mechanical speeds if you have enough horsepower to pull the given speeds, but is dead on accurate in regards to engine rpms at each given speed.

However, if the rider is 300lbs, and is running a stock 50cc engine, you won't be able to pull 75mph even if you are geared for 75mph.

Here's my rule of thumb.

88cc with race head, good exhaust, 20mm carb and all the goodies will net you 50-55mph if properly geared on 10" wheels.
108cc's with all the fixins will net you 60-65mph if properly geared
117cc's with all the fixins will net you 65 mph if properly geared

The gear ratio calculator keeps people honest on what sprockets they should run, not what speeds they will actually reach as there are too many unknown variables.

 

[Carlo C.]

On that combo I would think 16/30 would get you pretty close. Should be a good mix of acceleration and top speed.

Carlo

 

[Red Dwarf]

I'll be trying 16-31 as son as the new sprocket arrives, no mail here at the weekends!

Z, well done for making the calculator. Funny that a guy from over the pond posts it back to you!

 

[karasman]

Hi Guys,

Thanks for advice! It is really priceless for me.

My new 16T just arrived.
I have bought JT Sprockets product (#220 0327) - it is listed as Z50J part, so it should fit to my Z50R...
Unfortunately it is not :/ OEM honda fixing plate has the same pattern as sprocket when in mounting position... The distance between the second pair of holes isn't correct...
Any suggestions how to mount it?

Best Regards,
Adam

 

[T Smith]

Flip the plate over??

 

[karasman]

It is symmetrical - it won't help...

 

[Red Dwarf]

Dunno what you got there mate. I bought a 420 one for a Stomp pitbike & it fitted fine.

Just had a blast with the 16-31 set up and it's great! Much more range in each gear and a top speed around 45 ish.

Our speed limit is only 35mph on open roads and 25 in town so I'll have to be careful!

Had to brake so hard for a junction I was reminded I need to do a fork bush job as the front wheel was rubbing the skidplate!

 

[karasman]

Ok, there is a hope
I had fixing plate for Z50J, part #23802-045-670 – now I will try with #23802-040-000. It should arrive tommorow

 

[Red Dwarf]

Ok, there is a hope
I had fixing plate for Z50J, part #23802-045-670 – now I will try with #23802-040-000. It should arrive tommorow
View attachment 94969

That should sort it mate. After all, it only has to line up with the holes on the sprocket.
Takes me a month to get stuff from CMSNL though!

 

[karasman]

OEM fixing plates - two sizes

For all those looking for answer - there are two dimensions of OEM fixing plate for Z50 and CT70.
I bought the second plate at Honda Dealer for round 2$ - available in two days

I am goint to test 16/37 sprockets together

 

[fatcaaat]

I run a 16 on the front and a 37 on the back on my z50 hardtail. The motor is the original z50 that I've converted to a high compression 75cc with cam and 19mm carb, but stock exhaust. With this combination I max out at 46mph on 8 inch wheels. I weigh 180lbs.