Bw'S ( Zuma) 100cc and Jog MVT SC90

[kevin55950]

Hi! I'm new on the forum and I want to talk about my project: converting a 50cc Yamaha Zuma scooter to a 103cc.
I've been working on it for all summer, I've a 42mm stroker crankshaft on it and a Motoritec 54mm Big Bore kit.
I'm only 17 years old and I have very little experience on scooters over 70cc engines. Also, I'm french native (living in province of Québec in Canada) so there might be misspelled words and grammatical errors.

At the beginning of the summer, I've made my engine case machined at a machine shop . The guy did a bad job on it:
-Cylinder fitting too tight
-Crankshaft reduced too much and badly balanced
-Crankshaft was moving from left to right, as the machinist does bore out the crankshaft bearing ''holes'' .
-He installed crankshaft and bearings with a press, thus he doesn't do it well, the engine case bent and gears always come loose. I can pull and push them, like if it is missing something.
I've changed cylinder twice and piston 3 times because of all this.


Now, I'm starting over, with a ''new'' engine case as the other one is good for junk.

The parts I'll be using for the engine are:
-54mm Motoritec Big Bore kit. ( from Autotech355)
-44,8mm stroker, 85mm conrod MVT Carrera crankshaft
-15/50 primary gears
-24mm Polini PWK carburetor
-Polini Evolution reed block, or Malossi VL14 if I receive them.
-Intake pipe from Autotech355
-Stage6 Over range variator
-Prodigy clutch and bell ( from Autotech355)
-Harri'S Kevlar belt.
-Polaris Scrambler 90cc air shroud (because stock one is way too small)

I will post some pics in the next post.

[kevin55950]

This is the new engine case with a mod we call ''open port''


54mm cylinder that I've paint black and that I polished the cooling fins side.


Cylinder with cut stock 50cc cooling shroud ( with my old engine case)


Intake from Autotech355

Others images are uploading.

[kevin55950]

The story of this scooter:

(That's when I receved it, except the exhaust pipe)

On January 1st, 2012, my best friend had a atv accident and died.



This scooter was on project for a Polini Evoultion engine and his parents (he was 16) doesn't know what to do with the dismounted scooter that missed a lot of parts. It was at a shop to have the engine build during the winter, but the guy that owns the small scooter shop just didn't worked on it.
I talked to his mother about it, and she decided to give me as heritage. So I sold my almost stock Aprilia SR50 2007.
The polini engine ( the Evolution) was way too expensive for me, as it needs a lot of ''only Polini'' components to work well, and these parts are expensive. So I decided to do the build that I was supposed to do on a 2000 Yamaha jog 50 on the Zuma ( we call them Bw's in Canada).
And now, I'm here to show the project, that will be done as a kind of tribute to my friend. That's not exactly what he wanted, but if he was alive, I think he'll been proud of it.

[teddy554]

Sounds like you have your work cut out for you, I wish you all the luck and cant wait to see the build come along

[90GTVert]

:welcome: to the forum! I like the project.

Sorry to hear about your friend, but I think any of us gearheads would be happy to know someone was carrying on one of our projects in any form.

[kevin55950]

Yet, I have some problems that were unexpected. Soft seizure points on piston (has 150 miles on the old engine case), the nut on the variator side of the crankshaft is 12mm (normally 10mm) and it was not included with the crankshaft. One day or two and I'll install everything on it.


First start at the beginning of this summer, sounds like a weed eater with the small K&N I used. ;D

[kevin55950]

Today I found a problem:
There is a little spot on the bottom of the cylinder. Like a little bump that is higher than the rest of the inside of the cylinder. I can't sand it well because it is too big to be sand well. It'S like it has been a factory error or something.
I'm looking to buy another cylinder:
I have the choice of a 54mm cast iron with a single exhaust port ( like I haved) or a ''more powerful'' 54mm cast iron cylinder kit with booster exhaust port. It has the single port in the middle and 2 little ports a little higher. Which one is the best according to you?

[jmkjr72]

i have the lc version of that kit and brent has used the ac version of that kit

i personaly like it as if you want to raise the exhaust port to get a few more revs you still have the smaller ports to help on your low end power

[jmkjr72]

are you talking about a hump in the head
if so that is a factory low compresion head
a photo would help

[90GTVert]

It's easier to go more wild with the single port I think, but the aux ported kits do well for the average build. The aux ports are nice, they are just hard to open up with the tools most may have around. I run the aux port cylinder on my 103cc daily rider type of engine and it does well, making power across a wide range.

[kevin55950]

I'm looking for a 54mm cylinder but in aluminium niskali and I don't know if somebody manufacture them. As the Autotech guy told me, I may hone my cylinder but piston is scratched anyway. I might go to a MVT 90cc engine ( 50,6mm piston) but they are liquid-cooled and makes the scooter easy to see that is tuned a lot (LC conversion). It is also a very revving cylinder that needs the MVT exhaust to run well.
Did you took your cylinder kits from autotech355 or else?

I only have a YMS V8 exhaust on it that looks like a Piaggio stock exhaust but is bigger. Well performer but way better on 70cc engine, not perfect for 100cc, it makes the power band at only around 6.5-7K rpm.

I'll take a picture of the cylinder today. I tried to sand it on 400 to 1000 but it doesn't disapear. The Autotech guy told me that I might ''cooked'' the cylinder. Only done 150 miles on it with short rides of max. 2 miles while engine isn't running lean. 25:1 oil-gas as he told me too.

Edit: I've heard that NCY made 54mm aluminium cylinders. Not sure of it, I can't find it on their website.

[90GTVert]

My aux ported cylinder is from PFS . My others are just generic stuff from kaseaparts.com or eBay. There is a 52mm aluminum cylinder made...

www.maximumrpm.com/atv/parts/zoom/AL052045.html

[kevin55950]

I've repaired the cylinder. Not the best way to do it, but it will let more flow for my open port mod.


If you look close, you'll see that the side of the bottom of the cylinder is thicker at one place. that was the ''bump'' I was talking about. (the bright spot, I couldn't take a better picture.)


While doing it...


And how it looked finally. Piston is not too loose in the bottom of the cylinder, not more than with the normal sides.

Now, what can I use to do a good, but thick base gasket? With 3 paper base gaskets I haved, piston merely hit the cylinder head.

[90GTVert]

You can use a sheet of aluminum to make a spacer. Experiment and find the thickness you need with washers below the cylinder. Then either make a spacer that size and use case sealant on both sides for assembly, or make it a thickness that allows a gasket on each side of it.

[kevin55950]

I can't send a message to Maximum RPM, I tried but it's impossible. Did you found another place that has the aluminium cylinder ?