Here is a pic of a factory Batavus Grand Prix:
A Batavus G.P. is a rare bike in the U.S., and some would say it is a shame to do what I did to mine. They originally came with a variated Peugeot engine, which would make a really cool platform for a racer, as there are tons of aftermarket go-fast goodies available for those engines. You purists can rest easy though, my bike came to me thoroughly redneck engineered with a yamaha ttr90 motorcycle engine bodged onto the bottom of it, as well as a ridiculously extended home-made swingarm... among other things. The original motor mounts were removed. Having been been desecrated already, it seemed a good candidate for my racer project.
After asking some experts, I decided I wanted to use a Derbi flat reed engine for it, and started my search for one. (Little did I know the cuperteens moped gang had already basically hoarded every Derbi on the west coast!) After months of searching numerous posts on www.mopedarmy.com , I found one out in Florida, bought it and had it shipped here. (Thanks mattology) It was in running condition and had a 70cc metrakit and a pipe on it already. At the time, I didn't know enough about them to know that the Variant revolution engines were different from the standard variant engine. There are no pedals, (fine with me) and the engine has a few subtle differences that make it an excellent platform for a race engine. I won't bore you with those details.
Here is the first build out of the bike:
I fabbed a custom motor mount for it, then modified the front end for a set of EBR hydraulic forks. I wanted to run mags, but the derbi wheels are hard to come by, and hard to find sprockets for. I found a Puch Hero wheel at Treats
( www.treatland.tv ) that had close to the right spacing, and matched it with a standard 16 Puch 5 star front wheel with a custom made disc brake adapter. (Made by Mike Thomas and also sold at Treats.)
I got the bike together and ran it around, it was pretty fast.. but I wasn't satisfied with it. It seemed like something was still not right, like it just had a lot more potential.
I brought the bike down to the shop, www.ptownscooters.com , and my mechanic Jesse Stevenson (fellow racer and moped guru) was kind enough to go over it with me. First thing Jesse helped me figure out was that the port timings were off.. too low to get into the powerband and let either the pipe or transmission do its job.
On top of that, the cylinder was leaking a bit around the base gasket (shimmed up with 8 beer can shims!) Next, the Motomatic pipe (which are typically flawless) was poorly modified to try to make it fit the bike, and the header tube was a mess.
It also had the wrong belt on it, it was slipping badly and not variating fully.
Jesse worked a little magic helping me port the cylinder for better port timings, and we made a proper shim to raise the barrel up to height.
I then modified a Minarelli scooter pipe to fit by cutting and turning it. I also added a delta fly clutch to help it run in the rpm range we were aiming for. This clutch is amazing, and highly adjustable!
With these changes the bike was running pretty well, and I put quite a few trouble free miles on it at the track, as well as terrorizing the neighborhood near the shop!
I guess I could have left well enough alone at that point.. but no. I had a couple of conversations with my friend Mike Nazaroff about further improvements that could be made to the bike, and next thing you know he drove down from Seattle and whisked my bike away to his machine shop!
One thing we both felt the bike really needed was a proper ignition. On a typical typical derbi engine there are readily available performance ignition options. The revolution has a proprietary setup, and the only option is to modify one to fit.
I decided on to run the HPI Mini rotor designed for a Puch. It is very lightweight, and has a great lighting system.
I am not sure what all work is involved in getting it to fit properly, but I am confident Naz can do it.
It just so happens that a Minarelli scooter and a Derbi share a similar bore and stroke. Minarelli engines are commonly found in many different brands of scooters, so there are tons awesome cylinder kits available. It also just so happens that over at the shop we had a Malossi MHR water-cooled 70cc race cylinder on the shelf.
To get the MHR kit to bolt up he filled the cylinder stud holes, then drilled and tapped them for the Malossi bolt pattern. Next he welded extra material onto the cases so the transfers could be properly ported, and milled the cylinder mating surface flat. He also had to trim up the piston skirt for bottom clearance.
Here are some pics of Naz modifying everything.
Piston skirt modified for clearance:
Milling the transfer ports:
Boring the cases:
Cases Ported:
Fitting the cyylinder:
Malossi rates this cylinder at 24 hp out of the box, and it creates most of its power around 12,500 rpms! The intake on the Derbi engine is configured better than the Minarelli engine the kit was intended for. It is closer to the crank, and has a larger opening. I will be running it on a Tassinari 8 petal reed, with a 28 mm Mikuni flat slide carb. I would love to get it on a dyno and see if it indeed meets Malossi's claim, but if it puts out anywhere near 20 hp that is still a ton of power for a bike with only 70 cc's and weighing in at a mere 120 pounds!
Naz is on the right track with a few other mods that seemed prudent, re-doing the motor mounts, and modifying the front fork/ headset tube to fit much better quality bearings and hardware.
New engine mounts:
Engine mount with tubes brazed in for bolt to pass through:
Testing the new mounts for fitment:
Here he is reaming the headset for the new bearings and races:
New races installed:
He is also making a rear motor mount that the revolution has on the frame to eliminate sway in the rear end.
Another thing he did which is cool, machined a custom adapter for my Hero wheel, making it so I can use readily available Puch sprockets.
Again, big thanks to Naz, he has gone above and beyond my expectations to take my original build to the next level. That's why they call him the Wizard! I couldn't be more excited. We are hoping it is done by the end of the month, perhaps in time to debut at "The One" motorcycle show in Feb, and then also to hit the track some time in march. I intend to put it up against some really fast scooters in the open and auto classes in this year's race season.
