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/141039
Everybody's Got Somethin' to Hide, Almost .., '" Vl ;: w Z w -' U > U T he reed valve itself di sclosed. T he par t sticking out is the stop fo r th e reed. b y Dav e S choonmak er T o almo st the extent of paint schemes, moto rcycle me chanical s seem to b e subjec t to fa d s. With out a d oubt the fa d o f th e e arly 1970s is the reed valve . In th e pa s t fiv e years it wa s the rota ry valve an d " 9W th e re ed valve is sw ee p in g th e in d us try. If yo u w ant to go fas t you go t to h ave a reed valv e. \\'ant to kno w wh y and be tter ye t how ? Re ad on . Ever w o nde r wh v the air. filter on y o ur p iston-port t~...o-st rokc gels all slimev? Mayb e yo u th ough t it was p ol u tio n , well it w asn ' t. The pi ston-po rt 2-cycle engi ne draws gas/air mi xt ure int o its crankcase by the rise o f the p is ton and traps it wh en the p is to n closes . the door o n th e in ta k e port. Obvi ou sly as the p isto n starts its way down so me of th e mi xture w ill es ca pe b efore th e pisto n fu lly covers the int a ke port. Il en ce, sli me y a ir filt er . Th e first fun cti on of a reed valve is to prohibit t h is loss. Th e re ed is kind of lik e a flap bl owin g in the bre eze. Wh en th e breeze stops (pi ston ) re a ch es T op De ad Ce n te r) th e reed falls shut an d then c ran kcase pressure h olds it tha t way. Wh a t d o es tha t d o ? Well, yo ur air filter mi ght be a littl e cleane r but probabl y n o t. Hut, think o f that gas th at didn't ge t away . Oh boy, m ore cran kcase pre ssur e, ' o h boy. mor e torque, o h boy, better mil e age. Se ems like there cou ld be m ore benefi ts ac ru ed for a ll t ha t hassle, doesn' t it ? Well , th ere ar e. Just th in k o f how wild yo u c o u ld make th e in ta ke timing with o u t losing crankcase pressure. Wh y y ou c o u ld d o awa y with piston skirts and c arve up th e wh ol e side o f cy lin de r. Cept , the p iston' s still go t to go up an d d own and h ave some th in g to guide it. So ma ybe you shou ld just drill a couple of h o les in t he p ist on ski rt and extend the in take p o r t 10 m e e t the hol e s. However, tha t s till o ffers less ru n ning s urface fo r th e p is ton . ~ Th e m ore it jets the less you ge ts ( in l o n g e d ty ) . Th e ot h er ma jor dis advantage to th is m e th o d is th at there is more than a re m o te possibility that you m igh t slip a nd c a rve unwant ed holes in your cy linder. an unhappy cir cumstance. D o ing' it yo ursel f is not recommen ded . Wan t to have your cake and ea t it too ? N o t o nlv vou do it vourself, b u t yo u c an' also ' keep your piston perf e c tly h ap p y in its surroundings. Dale He rb ran d son of D H Enterprises has d e vel o p ed a j ig fo r porting cy linders for reed valves . The obvio us so lution to the pi st on align me nt pro b lem is to c re ate a wh o le n ew p o rt wh ich c o n nec ts to the int a ke p o r t in the b ody of the cy lin d e r. Th e m eth od fo r accomp lish ing thi s is no t quite so o bv ious. Hc rb ran dso n' s to ol is a cy linder of carbonized steel wh ich has ali gning slots for a one quart er in ch d rill bi t. I t d ro p s in t o th e cy lin d er and h as a n in dex ma rk for align ing th e j ib t o th e cylinder. T h en y o u take a n e lectric dri ll an d drill away . The h ei gh t and an gle have already been d etermin ed for yo u. Just m o ve the jig bac k an d for t slightly to bring ea ch b oo st port ou t to the p ro p e r size . Clean up the b u rrs an d the porting is done. It ' s so fool p ro o f tha t it almos t takes the fea r ou t of carving o n a cy linder. T h e wh o le operation . c an b e com pleted in about fif teen min ut e s. Jigs are availab le fro m He rb randson fo r 125 and 250 C Zs, 12 5 o r 250 REED VALVES: Gas/ atr passes on through the reeds, throug h those fou r holes yo u see and int o the crankcase. Th en up through the holes in the bott om of cylin der and to the com bustion cham ber. Hul tacos . 12 5 ~I onle ss a Ca p pras a n d 250s, 12 5 H od a k a an d th e 12 5 Zun d ap p . The cost is S·W .OO. He su re to mention whether or not yo ur cy lin der is o ve rsize as th is will re qui te a d if fer ent size . Writ e: D .H . En te rprise s, 1605 l.ynngr o ve D r.. Manh a ttan Be ach , C al ., 90266 or call 2 13 /372 ·6-l2 7. ca n T his is the H erbrandson jig and the two holes you see correspond to bo ost parts. What they do and how they got into this Wombat Th e ji g fits in like so. A tt ack those two little holes w ith an electri c dril l. Th e results.