Cycle News - Archive Issues - 1970's

. "" co ' l>. ,... N PUTTING THE Z(und)APP INTO RICKMANS Cl ,... N e " ..., " ~ w Z W ..J U > U '. s.c , Birt surrou nded by his work. STAGE II. The next stage of tuning, is y o u 're such a shoe th at you' re n ot satisfie d with th e above res u lts (done righ t), involves b oth piston and cy linder modification . This set-up includes a cylinder p o rtin g j o b . But be warned, it's a gamble to risk ruining your cylinder, but the result is about a 50% horsepower inc re ase from the 125. A word of cau ti o n on trying to c u t the ch ro me-lined Zundapp cy lind er: it's hard, and requires a ca rb id e cu tle r. If it slips or jumps, forget it. The edges of the enlarged cylinder ports have to be polished with Cratex to blend the edges smoothly since you h ave now cut away the overlapping chrome flashings which previously ran in to the ports and prevented ring ca tching, etc. E.C. roughens the intake ports to help better atomize the incoming fuel charge. The exhaust ports, in con trast , are polished for a smooth outflow. Refer to Fig. 3 for dimensions on the port wo rk . If you do both the piston and cylinder porting, a very rapid , high ho rsepower, p ip ey engine" is the usu al resu lt. It's no t so me th ing that amate u rs can rid e very eas ily. If you d o o ne o r th e o th er (F ro m a poc ke tbook standpoint, stick with the p isto n .) you have an e ngi ne that wo n't lo ad up and isn' t pip ey; a good compromise for most amateur rid e rs. REED VALVE MOD . • By E.C. Birt with JohnHuetter In the last issu e of CNW, we described the Rickman Zundapp as a ready to go racer, but in the current upward-spiralling 125 horsepower race, being ready to race does not alw ay s make: a bike a consisten t winner. The man who has done the most for the ligh tweigh t racers from E.ngland in terms of performance is E.C. Birt, Precision Cycle's tuner emeritus and grand guru. ' • People even adve rtise that their Rickmans have been tuned by him; he does the Team Rickman machines for BSA, all of the speediest blue fibergl ass scooters in the West have the E .C. Birt imprin t somewhere on the engine. There is almost a mystique about E.C. and his s uperp repared R ickmans. Close inspection and conversations reveal that th e mystique is nothing really magical but, like m o st things in life that are good, a mixture , of clear th o ugh t , hard wo rk , and an h onest dedication not to do a h alfw ay jo b o nce you've figured out the right way to d o it . The stock Ric km a n Zundapp is a good ligh rweigh t racer. E.C . Birt makes the m better, and very, very rapid. We were offered for test and inspe ction a IOOcc Zundapp piston-ported engine in a Rickman frame an d a I25cc mill with a Stage III setup, which includes a reworked head, extensive porting and a reed valve co nd uc t ing t he flow from a newl y d esigned 3 0m m Bin g, essentially a reduced sca le version o f the 36m m Bings o n the Hu skies and Maicos. The 125 had th e Grand Prix engine which is a b asic 15 HP un it with a heavier du ty gearbox and beefed up cran k , thereb y elimina ting the cra n k fail u re sustained by the highl y tu ne d 12 HP (stoc k) m o tors which were b asicall y designed as "boy racers" in En gland . This engine is co n tai ne d in the new Rickman, de sign ated Code 'S, wh ich also has a number o f o th er ch anges in cl ud in g a new re ar brake assembly with a snail cam adj u stm e n t . cu taway engine case to allow for easi er gea rin g ch an ges and a shift lever and kickstarter which lay inside the cutaway casc o in stead of sticking out as d o the previous models , including the CNW test bike . E.C. is not dogmatic ab out any piece of equipment where performance is By itself, E.C. figures that installing a reed valve on the Zundapp is good for ab o u t 20'7 on the top end. T hat's 0 without making any other modifications to the stock engine . Two styles of piston porting are possible to give a comparable increase in lower end with the reed valve : I) Remove .5 5 0 inch off the intakes side of the piston skirt. Put the piston back in and install the reed valve with the chosen performance carb o See Fig . 4. This is relatively simple and reliable. 2j Re move .100 inch of the p ist on ski rt, as above. Drill two holes of th e dimensions specified in Fig. 5 with their centers .050 inch off the center line of the piston. T hese ports (newly created) n ow fee d directly into the reed valve . This is a very h igh pe rfo rmance modification which should be inspected after each race, since cracks tend to develop be tween t he holes an d the cu t-off sk ir t. Me thod one is, ' obviously, less hassle .. · following: 1. Scribe a d istance of I.S50 inches, centered on the middle of the exhaust port on the top of the piston . 2. Run the piston down to BDC, and mark the transfer ports as diagrammed. 3. Remove the barrel. 4. Take a convex file and file off the leading edge where you've marked the ports to a depth of .030 to .04 0 inch. The edge of the piston should be tapered as pictured in Fig. 2. 5. If you want m o re top end, just work concerned. Reed valve setups have been done. with the Bings or with 2Smm Mik unis. lIe was the first to implement the reed valve setup on the Zundapp engine. It does make building an engine more expensive, in addition to the added main te nance req uired . The Precision Cycle R ick mans have mo re than just engine work to improve perfo rm an ce , if you want it and are willing to pay for it, b ut more about that later. T he re is o ne ru le in prep aring a comple te racer from the Rickman Zundapp: You get what you p ay for. There doesn't seem to be any way o f escaping that h ard fact of life . on the exhaust side. If more top end and mid-range is desi red, file down the areas above the exhaust and transfer ports, as described. 6. If you already have a piston ported engine, take another .100 inch off the leading edge of th e piston. If not, leave it alone. If you do have an engine that is ---- ~- METHODS OF PERFORMANCE ST AG E I. The most basic performan ce m odifi cations and the safest for the home tuner involve the p iston only . Le ave the cy lind er alone if y o u wan t in creased p erformance th at you can prob ab ly do y o u rsel f, with out comp ro m isin g re liability or in creasing m a in ten ance requirements. The Zundap p b arrel is hard ch ro m ed and it' s no t ea sy to avo id c hip p ing o r peeling the ch ro me , unless y o u h ave spe cific kno wledg e an d to ol s. Refer to Fig. I for the Stage I m ods. The fo llo wi ng w ork, alo ne, sh ould result in about a 10% in crease in h orsepower. Remove th e he ad but leave the piston in for the ~ ~. 01o-. 040. 080" o f f Bz...se alr ead y p ist on p orted , you can ta ke the he ad to a good machin ist an d have .OSO inch t aken o ff the b ase of the cy lind er head. This ap p lie s to th e o lde r 12 .5 HP en gines only an d raises the co mp res sion ra tio t o about 13: 1. If yo u have one o f the newer 15 HP engines, take off o n ly .020 fo r th e same results. There's no great gain in doing the ab ove unless you have alrea d y opened up the ports, o r had it d one righ t, .r Cyl J/e ad ST AGE III. The St age III ZLindapp engine involves p u tting together all of the above in a total high performance package that will produ ce one o f the zap p iest 125 machines o n any tr ack . You're talking about: I ) Porting, piston and cy linder. 2) .OSO or .020 in ch off the head, as ap p ro p riate for you r mill. 3 ) Installation o f th e reed valve with the "right" car b u re t o r. Zowie! The Complete Competition .., One of the t rick set-ups is a Yamaha rear hub to cushion the final dr ive and take a load off the CB gearbox.. ... I I I <\ ' .... ' J' J •f IJ. . 1. 1 .. . · The GP engine has cutaway for easier gear changes. Levers are inside line of engine . Fin configu~~i~n ~s t,,!o,re aligned w ith air ~fl ow .

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