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 only a little bit aftermarket parts available for the MB5 in the USA. Perk LLC is one of the few 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 so I can post it up for others to see.
But how fast can you really get a 50cc bike going? Go as big as you want. Have fun!
Upgrade with OEM Honda 1980-1982 CR80 parts
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 , we need to look at the sprocket gearing. We all know that MB5’s don’t make a lot of horsepower, so gearing needs proper attention.
Here are the transmission gear ratios for the 2 motorcycles.
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
Original MB5 sprockets are 13 tooth front & 43 tooth rear.
Seeing as how those of you who are doing this modification most likely have other modifications that add horsepower, the original gearing may not be too bad, as you can see that you get a geared down 6th gear, as compared to the original 5th gear. Even dropping a tooth on the front sprocket will still result in a lower ratio in 6th as compared to the original 5th, so a 12/43 may not be a bad option either.
If you did this mod to a mostly stock MB5, you would need to lower the ratio on your sprockets, as the 6th gear would be taller than the original 5th. Now you could leave it and 6th would result in more mpg, but probably not have much pulling power. So if you wanted a similar to original top gear, you would want approx. 11/43 sprockets.
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:
Whole engine swap
The CR80 engine will NOT bolt directly into the frame of the MB5, but there have been people do it, including newer liquid cooled engines.
The CR80 cylinder will not bolt directly on, but it can be done. To fit the 80-82 CR80 cylinder, you will have to bore the cases, weld up additional aluminum material onto the cases for transfer port matching and base gasket sealing. After that you can use the cylinder, head, carb and a custom exhaust. In my opinion, this amount of work and machining is not worth it compared to a bolt on 70cc big bore kit that is readily available