So you want to make your MB5 faster?

Well first of all, I would like to state that if you live in the USA, there are very little aftermarket parts available for the MB5 in the USA. Perk LLC is the only one I know of in the USA (Indiana) that is importing parts, he has some stuff in stock.
I would recommend starting with an aftermarket exhaust pipe, it is the best bang for your buck. After that, the 70cc kit, bigger carb, reeds, changing the sprockets, etc, the sky is the limit. Talk to Perk LLC and let him know how much you want to spend and he will get you going as fast as you can with any budget.
Some people transplant larger engines for other motorcycles, usually it is a lot of work to do, but if you have the skills, then bigger is always better, right?
The Honda H100 engine has been talked about before, I have heard it may be close to bolting into the MB5 frame, but I cannot confirm this. The USA did not get the H100, so I am not able to try this.
If you have done any engine conversion, please email me about it.
But how fast can you really get a 50cc bike going? Go as big as you want. Have fun!

Making your MB5 better!
Upgrade with OEM Honda 1980-1982 CR80 parts


- 6-speed transmission - The CR80 (1980-82) 6-speed transmission will fit right into your MB5 cases, you must use the all the gears, shafts, washers, shift drum, shift forks, & shift drum star. 
Take a look at the pictures to help you identify which parts you need from the CR80 to perform this upgrade.

Here are some photos of the 2 transmissions side-by-side.

  Now that all of you want to put a 6 speed into your MB5 :D, we need to look at the sprocket gearing. We all know that MB5’s don’t make a lot of horsepower, so gearing may be critical, especially if your MB5 is not very modified.

  Here are the gear ratios for the 2 motorcycles.

          MB5   CR80
1st   3.083   2.500
2nd   1.882   1.778
3rd   1.400   1.400
4th   1.130   1.130
5th   0.960   0.960
6th      ----   0.815

For this article I will use the term “short” and “tall” gearing. Some people call short, gearing down, or low. Example would be 4.56 gears in your drag race Ford Mustang. Some people call tall, gearing up, or wide. Tall gears would be your 3.70 gears in your daily-driver pickup.

You should have seen the taller ratios in the 1st, 2nd and 6th gears. This would mean you will need to shorten your final drive (sprockets), or gear down for it to work correctly, unless you have A LOT more horsepower. The sprocket selection will be need to be done on your own, because everyone has different modifications and horsepower. I will give you a guide, but you may end up buying more than 1 set of sprockets after you test. The math can get a little confusing, so I have done my best to try and simplify it.

On the 6 speed: 
For 1st gear to have a similar ratio to an original MB5 ratio, you would need 11/45 sprockets (4.09 ratio).
But then your 6th gear would be shorter than an original MB5.
For 6th gear to have a similar ratio to an original MB5 ratio, you would need 11/43 sprockets (3.91 ratio).
This will make the 1st gear a little taller.

Most modified MB5’s have a real short 1st gear, so a little taller would be ok. A little taller 6th gear is probably what most people are after, so maybe something like a 12/45 (3.75 ratio) would be a good place to start. If you have a lot more horsepower and want a tall 6th gear for some top speed, a 12/43 may work. I don't think I would advise this on a completely stock MB5.

- Big Clutch - The CR80 (1980-82) clutch that is the same as the MB5, except the CR80 has 1 more plate. When you create more horsepower with a MB5, you could upgrade the clutch with the CR80 clutch.

From the CR80, you only need to use the outer hub and one more fiber and steel plate, but I highly recommend you replace all the plates as a set. This outer hub you need is still available new from Honda, I have seen them priced on the internet for as little as $38. The MB5 and CR80 use the same inner hub, basket, springs, and clutch plates. See the diagram

The inner hub is part # 22121-168-000 or 22121-436-000
It is found on the following Honda's:
1980-83 CR80R
1979-85 XL100S
1985-new XR/CRF100

Whole engine swap - The CR80 engine will NOT bolt directly into the frame of the MB5, but there have been people do it, including new liquid cooled ones.