Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/125874
the assemb ly line. it can be improved if you go the righ t way . Screw up and you ca n throwaway the cylinder and sta rt ove r again. But the 125 Zundapp cylinder is very expensive. so be ca reful. A t .298.96 each (yo u get a new piston with it), you don't want to be ruining too many of them. There's a reason. You get just what you pay for and these days if you get 50t for a buck you're lu cky. A Zundapp cylinder and piston will last almos t forever if you take care of them. The cylinder never has to be bored (it 's chrome plated ), and the piston is a forged piston by ~laul, the ' best there is. I've got a 125 ISDT cylinder, 1965 model that had a 56mm bore and a 50mm stroke. This cylinder went through one Six Days, that's 1.000 miles the hard way, Then I raced motocross with it for three and a half years here in Cali fornia in the 125cc Expert class and it still only has a .0 0 2 clearance. You're may be saying, "Yeah , hut that was you, E.C ., and before you a factory rider who knew j ust what he was doing. What about me, I 'm j ust Joe ° Average?" T his is E.C . · Birt 's new co lumn , Test Bench. Here 's how it wo r ks. Someb od y mak <:> s~ m~t h i ng they say. makes you go fast. O K, E.C. gets his han ds on wh at ever It IS , tests the bccjecsus out of it, and t he n deli vers the verdict - it either works or it do esn't. If it doesn't , he confers wi th the manu facturer to sec what they 'll do about it, tests it a second ti me after they've had a chance to fix it right, and then tells yo u what he learned about the prod uct or service tested . Companies wan tinc stuff tes ted can • send it to E.C., c/o E.C. and Walker, 509 In dustrial Avenue. Bo yn t o n Beach, F lori da . . . Ed. Part 2 - Zundapp 125s ....... I In the Pro class, it takes a good rider and faast motor to win. Here's h ow to build a quick Zundapp. How much horsepower a two stroke engine will put out (or how we ll it does its job) is co n t ro lled by three factors: A) where t he ports are , B) port 'size and shape, C) sp ark and g'.JS. The magic o f a two stroke is that no matter how it co me s off Well, if you 're J oe Average, may be a Zundapp is the hest bike for you. I've seen hun dreds of Zundapps belonging to Joe Averages with t housands of mi les on them that were just as good as they were the day they were new. Two things to remember are to keep t he dirt out of it (clean t he air filter and the air box after eac h race) and if you have an Enduro, check the ring end gap every 500 miles. When th e end gap gets up around .0 15 to .0 16 replace t hem. On the Zundapp MX model you have to be a little mo re careful because it has on ly one d y ke ring. I check mine after every th ree races. Replace this ring if it has more than .010 clearance. I run an Enduro piston in my racers because 1 may not have time to pull th e engine down and lo o k at it when I should. The horsepower between the two pistons is o nly a co uple of tenths so don't worry abou t detuni ng your 1\IX by using an Enduro pis ton. You will never know the difference. Alright, so t he cylinder is chrome plated and it cant be bored. What do I do if I break a ri ng and score the $298 .96 cylinder? I've made sleeves fo r the Zundapp that start at 125cc engine size and go up to 150cc if you wan t to, Sleeves cost $26.95, piston $34,88, and installation (if you do n't have a mach ine shop to do it for you) , $60.0 0, wh ich is also the finish bore . Total $ 121.83. Let's take a q ui ck look at the spark maker . The new Zundapp ~IX bike has a Bosch pointless magne to . Now , in Ge rmany they m ust have so me pre tt y bad gas. because you can't adva nce the timing more t han just a hair over 2.0mm and the engine would like to have about 2.5 or 3.0mm, so you will have to no tch the backing plate a hit more to get a selling somewhere in th is range. If you have taken the ligh ts off your Enduro a nd started racing it, set the points at .013. and the t imi ng at 2.95 to 2. 5 m m BT DC fo r best performance. Note of warning: With the ne w pointless mag don't kick the engine over witho ut the spark p lug wir e grounded. You m igh t b low either t he elec tronic box or the pul se em itter. Si nce this is the first yea r the Zundap p has use d t he p o int less magn eto in th eir bike I th ink it 's wort h while to · sh o w you how it works. Also just fo r th e kno wledge in th e bac k of yo ur he ad most all poi ntless ignition sys te ms wo rk o n this principle. See Diagr am No . 1. As t he pol e p late ro tates, t he s to rag e ca pacitor in the elec t ro nic box is charged via the charging arrna ture (o ne) at the moment of igni tion, th e po le shoe tags off pole pl ate (t wo ) as it turns p ast the pulse emitt er (t hr ee) on the ar ma ture plate. This induces a vo ltag e pulse in the input co il which is rectified and then opens t he lo c ked thyristor via a diode. The potential of the st orage capaci tor c an now discharge across th e conducting th yri st or tr ac k (fo u r) and the prim ar y winding o f the ignitio n co il. The high voltage required f or ignition is generated in th e seco nda ry winding an d pr oduces th e spark at th e spar k plug. The ma in im portan t thing is you don 't have to , wo rry about the points goi ng closed on you or having to service th e mag. Even the Zun dapps that use po in ts are ne xt to trouble free because th ere is an 0 ring groove in the case where the m ag cover goes, making it dirt and water proof. and I have run one set of po in ts for mo re than a year of racing without replacing them. The biggest prohlem with a Zundapp is the fact that if you trea t it right you don't have anything to work on , and that takes all the fun out of playing mechanic. Every manufacturer of motorcycles looks for more horsepower in new models and Zundapp is no exception. They have been up late doing 'their homework and go one step further. that's putting a stress on dependability. First it has to be capableof winning the Si x Days because what you are b uying is what they won a Gold Medal with the year bef';re. Only after they know it's the best they can build will they let it leave the factory for you. T he first Zundapp 125cc engines that we saw in the United States h ad 12 .5 DIN ho rsepower, t hen came the 15 .0 DIN lI P version. and now we have the 19 .0 DIN HP engine. If you're any thing less th an an Expert l\IX rider, then all you need to do is lea rn to use the horsepower tha t the new ~ IX has a nd you won't have any problem winning races and seei ng you r trophy shelf fill up. If you're an Expert (or like me - can't keep your hands off the cylinder), then there's room for improvement. There are three kinds of horsepower. the kind you hear about, t he kind a manufacturer claims you have, and the kind that really counts: how much you get to the rear wheel. The days of a seat-of-the-pants type of building racing bikes is over with. :\ < dyno is. not the whole answer but it's a necessary tool if you want to get out that last bit of power. Then you prove it on the ra ce track. My dyno doesn't make you feel good because it's a rear wheel dyno. and what you see is what you get . It's no t the reading that you get t hat's . importan t, it's what percentage of increase you give the engine which makes the difference. The stock Zundapp engine will m easure out three to four horsepower less on my dy no than what the factory claims. wh ich isn 't too bad . No te Diagram No .2: curve number o ne is a stock 15.0 DIN Enduro e ngine. jus t the • way it came out of the box. Curve number two is the same engine afte r it had bee n tu ned properly on t he dy no. This goes to show that wha tever kind of bike you have. it must be tuned righ t to get out the m a xi m um number of ponies. T he 125co Zundapp wi ll pu ll a good solid 16 .5 horsepower at the rear wheel when se t up right. I've seen 250cc ' engines p roduce less, even though they claimed a lot more than any 19.0 ho rsep o wer. You can get horsepower in t h ree stages. Stage One, a little bit of sweat, reed val ve and a reed p ipe and you have a 30% inc rease. Stage two, 30mm Bing ca rb and a different air filte r, about a 4 0% increase. Stage three , 50 to 60%. No w you kin d of spend your life savings it seems because no ma tter ho w you c ut th e cake horsep ower costs cubic dollars an d yo u ca n go just as fas t as you want to, dependin g on how much yo u wa nt to spen d. This ho lds tru e fo r all bik es no ma tter wh at kind, an d o nc e yo u m ake th e gra d e to E xper t it gets expensive. There's n o suc h thing as a s to ck bike winning 125cc E xp ert class unless the rider is cherry picking. There's o ne last step, which is a works engine. What is a works engine? It means to build a motorcycle wi th one thought : to be the best for a certain type of raci ng, a ma chine built to fit on e rider, with o nly o ne go al, t o be a winner in whatever it was built for. I'm always being asked is a reed valve the way to go ? All I can say is that 95% of the ti me, yes. It will give you more horsepower t han possible with a regu lar pis ton port design, but m u st be done right or you w ill wind up worse than stock. (See the dyno chart. ) Any reed engine sh ould have a special reed p ipe built t o fit the needs of the re ed engine. The stock pipe on the 125cc Zundapp will work alright for you. If you have a Rickman with a 125 Zundapp in it yo u 27