There are many good reasons to tune a Jawa. The 207 frame is extremely light weight and solid; it's also very compact and can be set up to have a very low profile. The 207 engine is simple in operation and one of the easier units to work on. Despite its lack of bolt-on performance, the 207 engine provides a great platform for experimentation and is thus an excellent learning tool. The Jawa 207 and 210 engines both employ a simple CDI with an internal rotor and external stator setup which greatly cuts down on the engine's rotating mass. The best part of all is that the engines are available readily and at a very low cost, so one can completely screw up and be back on the road for very little money. The following is a list of what has been done to improve performance on these bikes. These things may or may not work for your setup and have not yet been proven on a large scale yet. Jawa tuning is in its infancy, and these procedures come with no guarantees with regards to performance, reliability, or safety. TUNE AT YOUR OWN RISK! If you have anything to add PLEASE DO SO!
Lets get started then!
The Jawa comes typically with a Jikov 12mm carburetor. Though this unit is an efficient design, the lack replacement parts will be a limiting factor, particularly in the area of jets. The answer to this problem is one of the simpler things to improve. A Puch E50 intake can be modified to fit by grinding down the base, then grinding away the sides of the pipe above the base to allow room for the screws. This can be done easily with a bench grinder and maybe a drill to re-position the mounting holes if need be. The end result is a very standard intake allowing one to choose between a wide variety of carburetors. Many have chosen to run a larger intake and a Dellorto SHA 15.15 on this setup. The Jawa runs well with SHA carburetors and can be tuned to not only perform nice on the top end but also idle beautifully.
There are no bolt-on pipes for Jawa. At this point, two things have been done with some success. First off, one can remove the stock baffle and open up some new passages to improve flow. This is of course cheap and keeps the bike looking stock. More recently, people have been getting Biturbo pipes welded to stock headers. This typically gives a significant speed increase and if a second-hand pipe is used, the setup can be extremely affordable.
Recently the porting on the Jawa 207 has been addressed. The cylinder ports in this engine are very small, even for a stock moped engine. To start with, the exhaust port is tiny and can be widened without changing the timing. 24mm is about the limit in width before there becomes a danger of catching a ring. Widening the port to 22mm will be a significant change over the stock 18mm and leave a nice margin of error. When widening the exhaust port, you will want to continue the passage all the way down to the port exit where the exhaust header bolts up, this is a lot of metal to remove, but a large port going into a narrow passage will not do.
The second area to address is the minuscule transfers. They're not only small but they also do not open all the way. Generally it's not worth messing with transfer ports themselves, but in this case the procedure is easy and does not risk damaging the cylinder. The piston crown on the 207 is 4mm thick which leaves some room for grinding. With the cylinder head off the engine, and the piston at BDC mark the edges of the transfers on the top of the piston. Now remove the piston and draw 2 parallel lines across the piston connecting the transfer marks. Next, on each side of the piston directly between the transfer marks measure inward 6mm from the edge of the piston and make a line perpendicular to the parallel lines forming a little box where the transfer would line up with the piston. Do this on the other side of the piston as well. Next you want to grind out that little area to a depth of 2mm at the edge of the piston tapering all the way up to the perpendicular line you made where. Measuring 2mm at the edge of the piston is easy because it amounts to half way up the crown. Great care should be taken to make sure that the inlets ground in the piston crown are symmetrical to each other so as not to upset the cylinder's scavenging. the end result is that the ports are now opened all the way and the duration is increased. Porting a stock engine has shown small but noticeable performance gains with a 12mm carb and mildly de-restricted pipe. A larger carb or expansion exhaust might take even more advantage of this.