There are many good reasons to tune a Jawa. The 207 frame is extremely light weight (95 lbs stock) 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, Jawa engines provides a great platform for experimentation and is thus an excellent learning tool. The best part of all is that engine parts are available readily, so one can completely screw up and be back on the road for very little money, taking into consideration the downtime sourcing potentially hard to find parts. 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!
One of the most common complaints about Jawa mopeds is the unreliability of the stock ignition system. The 207 and 210 engines both employ a simple contactless electronic ignition system (the forerunner to the modern CDI) with an internal rotor and external stator setup which greatly cuts down on the engine's rotating mass. The thyristor unit is prone to overheating, resulting in erratic or absent spark. A replacement unit can be soldered together from component parts. Some experimentation has been conducted with universal CDI kits but they are difficult to install and homemade replacement thyristor units are often cheaper and much more rugged.
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 Jikov carb takes uncommon larger size main jets, but are possibly compatible with other brands. If you find a jet that fits, it can be soldered closed and drilled or reamed to the appropriate size.
The mixture screw on the Jikov adjusts the mixture on the idle circuit only. The needle is adjustable for mid range mixture and has an effect on wide open throttle.
The Jikov carb can be successfully bored out, but performance gains are minimal due to the small diameter intake.
A Puch E50 intake can be modified to fit by using a spare cork riser and widening the mounting holes a few millimeters inwards. The end result is a very standard intake allowing one to choose between a wide variety of carburetors. A 14 mm square intake will match the cork risers and intake port perfectly. Many have chosen to run a 14-15mm intake and a Dellorto SHA 14.12 to 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.
One can remove the stock baffle and open up some new passages to improve flow. This is of course free and keeps the bike looking stock. 3-4 1/4" holes can be drilled into the stock baffle, equally spaced and oriented around the original opening. Alternatively, the original passages may be only slightly enlarged. Care should be taken to ensure all carbon is removed from the baffle passages.
There are no bolt-on pipes for Jawa, although the stock header may be reused with an aftermarket pipe and exhausts for other bikes may fit.
One can use the Tomos two-piece BiTurbo and a copper reducer fitting, 1-1/2" diameter, from a hardware store to mount the new muffler to the existing stock header on a stock 207 type Jawa. The original header will need to be trimmed some, but care must be used to not cut too much so that the muffler doesn't hits the pedals. If one heats up the header with a torch and grind the copper fitting so that it is flared slightly, the copper fitting can be tapped in with a mallet and will produce an interference fit. Additionally, copper expands more than steel when heated, so the fit will tighten when heated together. Then simply tighten down the muffler to the 1" copper fitting with the supplied bracket.
Of course, almost any performance exhaust can be forced to work. The most common issue is mounting the exhaust to the frame. Stock Jawa exhausts mount beneath the muffler while most mopeds mount above. "Straight" exhausts such as the Techno Boss can simply be flipped upside down but angled and non-symmetrical exhausts (such as the BiTurbo and Techno Estoril, respectively) require the stock mounting bracket to be modified. This can be as simple as using steel mending braces (available at any hardware store) or as complicated as relocating the stock mount.
The stock mount was not designed to carry the weight of larger performance exhausts and will flex under stress. This problem can be solved by either strengthening the stock mount or adding an additional brace.
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.
There are several piston sizes for the 207 and 210 engines. As the stock bore was worn out it would be rehoned to accept the 1st overbore piston, then the 2nd, etc. Each rebore increases by 0.25 mm. In the United States pistons were available up to the 3rd overbore, but pistons as large as the 8th overbore (an increase of 2 mm bore and 7 cc displacement) have been found in eastern Europe. This is relatively uncharted territory - early reports indicate higher low end torque due to increased compression but no substantial effect on top speed. Aggressive porting may be required to reap the full benefit of increased displacement.