There are many good reasons to tune a Jawa.
Ignition
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). The thyristor unit is prone to overheating, resulting in erratic or absent spark. A replacement unit can be purchased or soldered together from component parts. Some experimentation has been conducted http://www.mopedarmy.com/forums/discuss/1/537594/499438/ and a thorough review of thyristor replacement units is found here
Carburetor
The Jawa comes typically with a Jikov 12mm carburetor. If you find a jet that fits, it can be soldered closed and drilled or reamed to the appropriate size.
A Puch E50 tall SHA intake can be modified to fit a Dellorto SHA, which are available from common 14/12 to 15 and 16mm sizes, and are easy to tune and find jets.
Above is a modified and the as-received Puch intake. Note the mount has to be ground down to fit into recessed platform in the cylinder and the holes have to be extended inward to match mounting holes. In a similar way other intakes may also be used, or even a small diameter stock exhaust header can be cut to fit as an intake. Because of the tight fit you will need to use allen head bolts, and any washers will need to be ground down on one side to a "D" shape. You can cut your own gaskets from gasket paper.
Jawa 210 cylinder with puch tall SHA intake and SHA 15:15 installed
A Piece of Pergo flooring can be used as a shim to fill the depression in the cylinder. Aluminum is good too. This avoids grinding the edges off the intake and allows it to look more finished.
Exhaust
There are no bolt-on pipes for Jawa.
If you match the Header diameter, you can install a clamp on such as the Jamarcol Fuego 28mm clamp-on chamber.
Any pipe can be hacked and welded to make work. If you have access to a welder and the skill to make it nearly air tight, you can weld any chamber to the stock JAWA header.
The stock exhaust port is quite small and you will have to relocate and install larger studs but Puch or tomos or other flat port exhausts can be also made to fit. You need to cut it to fit the recess around the JAWA cylinder's exhaust port, and drill out the stud holes to the correct location to match the JAWA. Don't forget to make a mount for the exhaust in the rear, and never mount to the swingarm.
Jawa exhaust, ported - note the edges are widened and 2 larger and wider bolt holes have been drilled and tapped to fit a larger Tomos header, and the exhaust bore has been opened to match.
Cylinders
Jawa cylinders consist of an aluminium jug with with an iron sleeve pressed into it. This sleeve can be pressed out to give better access to the ports for modification. See thread here for elaborate cylinder sleeve porting: http://www.mopedarmy.com/forums/read.php?1,4034618. The 210 cylinder is superior to the 207 cylinder due to its two additional angled transfer ports. It also has a larger flange around the exhaust port making it easier to modify for a larger header. This cylinder can be bolted to the 207 engine with no modifications.
There is a commercial rototiller that has an engine which is a derivative of the 210 engine, allowing access to 80cc cylinders. Not much info but there is a video on Youtube here
Additionally you could always weld up the cases and move the studs to use a different cylinder, considering the small end bearing size and stroke. Puch cylinders may be close, and have been adapted to the JAWA 210 by Euro tuners. The JAWA has an atypical 14mm wrist pin making cylinder swaps very difficult. JAWA also have a shorter 41mm stroke vs the much more common 43 among major moped brands.
Porting
Stock Jawa port timing is pretty okay but the ports are really small. The stock cylinder has a cast iron liner so it should be pretty easy to modify the ports. For more information read about Calculating port timing or seach porting. The safest conservative porting is to open up the intake and exhaust and widen both by ~2mm, but do not raise or lower the top or bottom edges. This keeps the timing the same but allows much better flow. If you're installing new intake or exhaust you'll want to match those to the cylinder.
Overboring
There are several piston sizes for the 207 and 210 engines. As the stock cylinder was worn out it would be bored. 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) can be found from eastern Europe.
Other
Additional options to get a little more from your JAWA include:
- Milling the head - shaving down and reshaping the head; even just removing some material (smoothly and flat) increases compression and power.
- Regearing - you can improve top speed at the cost of low end
- Clutch - The clutch is not easy to modify but lightening the clutch (such as evenly drilling it out) could provide additional stall allowing you to obtain higher RPM power right off the line. This is not trivial and balance and strength must be considered.
much of this info contributed by user Vlado
