Cycle News - Archive Issues - 1970's

ASKE.C. ( ~ Go t a technical problem that need s ex pe rt advice? Send qu e ries to "Ask E.G. " c/o Cycle News. Your rep ly will a ppear in the pap er within a few week s. Hard qu est ions may tak e a little longe r to answer. By E.C. Birt If it don't go, chrome it! E. C. co rrec t m e if I'm wro ng; bu t co uld n 't a p erson w ho ow ne d a 250 Y.l . Yamaha (w ith chro me bo re) ge t an endu ro or MX cy linder and have it po rted to Y. l . specs then they wouldn't hane to buy a whole new cy linder once it seized. sucked in d irt. or just wore out. Just oller bore it. Rule o f thum b w ould b e th e h igh er th e altitude th e mo re co mp re ssio n y ou can use a nd m o st of th e t ime you w ill need larger main je ts not smaller. Why ? Because the air is ligh ter a n d tha t b eing th e cas e since the air is wh at pulls th e gas u p th e mai n je t it needs a bigger h ol e to ge t th e righ t mixture of gas a nd air in . Fo r years I h a ve used a special te x ture of d iamond gri n d ing in the tra n sfers and some in ta ke tracts, b u t now I h av e co me up wi th a special process of glass beading that I use , in th ese ports. Sorry the kind I use is a trade secre t, Dr . G o rdon Blair has not written a book th at I know o f. (Ed. note: /Ie has written a couple of SAE tec hnical papers.] G ord on J enn in gs has a good o ne ca lled Two Stro ke T u ners Hand Boo k , and Car l Sh ipman's Motorc ycl e Tuning for Perfo rm a n ce : Th ese boo ks can be b o ught a t yo ur lo ca l d eale r. The Cycle News Chamb er test was run over th ree ye ars ago I th in k a nd wo uld be alm ost impo ssible to get n o w. What is a Q. U. B.? It's an Eng lish Greeves 3 80 th a t was d es ign ed by Dr . Blair an d the sta ff a t th e Q ueens Univers ity o f Belfast. All th e n ew Gree ves a re now call ed Q .U .B.s th at are racers. pr ess-fixed cen te r a nd 1 have see n th ese cen te rs tu m in th e o uter p art o f the flywh eel. The fir st Z undapp 12 5 e ngines had th e same flywh eel a n d thi s ha p p ened o n th e m once in a while. In a sh ort time Zundapp riv e ted their flywheels to ke ep th is fro m h a ppen ing. Now th e t eas on th at I thin k th e cente r is turn ed is b ecau se you state (I) w he n you kic k the engine over slow you get spark. (2) Kick it over fast. no spark. Now le t's look at this p ro b lem fro m this d ir ectio n . Your flyw heel h as two magn ets that a re rive t ed to the flywh eel h o usi n g. Any magnet has two poles. North o n one end and South on the o ther. When these t wo poles , Nand S pas! over th e prim a ry coil, it crea tes energy in the fonn of electrici ty. In a mag neto th e spark is store d up 't il the lobe of th e flywheel b reaks th e co ntac t of the poin ts. Now if th e ce n te r is turned in the outer h o using, th is is w h at could b e h appening : (I) If the cen te r is t urned a little an d yo u kic k it o ver slo w it will gen era te enough electricity to ma ke the sp ark plug sp ark . (2) Ki ck it o ver fast an d the points don 't st ay closed long enough at th e proper tim e th a t the N a nd S poles (rav el p ast th e primary coil so th a t it can st ore e no ugh ele ctrical ene rgy in th e Righ t on. Bu t new cylinders and porting are costly, so w hy no t run your Y.Z . ti ll you d o wear it out then resleeve it with a Y.Z. sleeve of cast iro n . then if you h ave an other problem yo u can rebore th e new cast ir o n sleeve; also your pi pe would h ave to be modified to fi t the en duro or MX cylinder because the exhaus t flange on th e Y.Z. cylin de r is of a differ ent style. Circumcized and o iled / own a '72 A T2 Yamaha MX and was wond ering if cutting fiue inc hes off a 7.5 inc h st inger would ge t m e m ore low end witho ut losi ng much on top? / usually run Full Bo re 50 S.A.E. racin g oil; b ut / didn't ha ve enough m oney fo r some so / got Bardahl motorc ycle "Oil and m y bik e runs good. Will it hurt it? Ed N ielse n Long B each , Calif Yes, Virginia there is a Dr. Gordon Blair / was wond erin g if there is any rule of th umb or so me formula for determining carb ure to r size. Also is there a ru le fo r je tting fo r altitude th at w ill get yo u close? What primary co mpression ratios do yo u fin d that works best and for what displacemen t ? What texture of p or t su rface do you feel offers the b est flow ? On several occasions y ou ref er to a Dr. Gord on Blair. Cou ld y ou please give m e th e titl es of his publications? A re th ere an y other bo oks you recomm end? I'm afra id / missed th e Cy cle News cham ber d esign co mparison test. Could you pleas e se nd m e a co py of th e test ? P.S. What is a Q. U.B.? Doug Brown Canoga Park , Calif There is n o rul e o f th u mb th a t I know of. Each tuner a nd engine th a t is b uilt ca n use anywbere from a 16 to 44 mm car b oWh a t I use is: 26 to 28 m m Bings on 80cc en gin es 30mm Bings o n 100cc to 125 cc engines 3 6 to 38m m Bings for engines from 250 cc to 4 50cc. Why a Bin g? They 're ea sier to tu ne and develop m o re h orsep ower for me . J e tting fo r a ltit u d e, that's a go od one. Doug Br ow n Canoga Park. Calif (I) Do ug, to my kn owled ge there is no set formula for detenni ning leng th o f in ta ke t racts . From my o w n e xp erienc e on th e dyno, the shorter t rac t will de vel op more horsepower above 7,0 00 R . P .~1. Whereas a longer intake tract wo uld be better fo r fuel economy , sm o o th er lo w-e nd o pera tion a n d torque. But w hy figh t it? Install a reed valve an d you h ave apple p ie with ice cream o n it . (2) The m o st foo l-proof m e thod h ere is hell-arc wel d in g a nd a h and gri nder o n allo y cyli nde rs a n d lo w-h eat silver so lder o n cas t iron cylinders. (3) Th e m ain reason fo r m o re tra vel up front is, you d on ' t wa n t to bott om out th e fork s firs t. If yo u d id it w o uld pr obab ly b e endo t ime . (4) Yes. th e main reas on fo r a level swingann p o si ti on is yo ur cen te r of gravi ty. Also , yo u r c ha in a nd sprocke t wea r is mi nimized. Zoon! d ap-dap-dap / happen to ow n a / 972 R ick m an Zundapp and / was w ondering if a larger carb and a d iff er ent p ip e would h elp ? Wo uld a 30mm B ing carb clamp to the original intake manifold or would it have to be modified? Paul /lod ges Glendale, Calif Larry Byward Costa Mesa. Calif Cutting o ff yo ur sti nger wo n't h elp your problem; b u t I h ave a Reed cag e m ad e b y Noguch i th a t will ' cure th e pr oblem. Nothing wro ng w ith Bar dahl. Use it if yo u like it . It wo n't hurt y o u r engine, makes a li ttle carb o n a n d y o u will ha ve to d e-carbon m o re often, is all. because it keeps a low C.G. or is there an other reaso n ? Rickman MK 2 fairing Latest offering in the Rickman lineup is this street/cafe fairing for CB500 Hondas that accepts the stock head lamp and instrument cluster. The unit comes complete with mounts, fittings, instructions, and a pair of plated drop barsdrilled for the Honda controlsets. It comes in Virgin White or Widow Black'>- an' make sure you're woman's neither, mate. Good hunting. The doctor does prescribe a 30mm Bing and pipe or p ipe mo d ifica tion wh ich is good for a 30% increase in horsepower. Fa ctors that c rea te horsepower incl ude how much a ir and gas m ix ture we ca n get into the engine and how a nd when we pull the exhaust out. (I) Stock air filter is too small a nd shou ld be replaced with a la te m odel 1974 Ric km a n 1\IX air filter. (2) Use a 30mm Bing with 51·595 ato mizer, 273 or 276 needle jet. 25 to 35 p ilo t jet and 135= 140 main jet. Air screw , one quarte r to o ne- half turn o u t. Yo u 'll n eed a carb uretion b ushi ng to fit th e Z unda p p man ifo ld ; we h av e the m in stoc k because we make them . (3) If yo u're a desert rider I reo mmend that yo u usc a ' 74 Ric kman h igh pipe w ith a p ip e m odificatio n d o ne to it . Sin ce you are in Califo rni a , Precision Cy cle in Lawndal e or Flyi ng Mach in e Factory in H arbo r City can ha ndl e th is fo r yo u. The h igh p ipe w ill pr ev en t ro ck rash. Fo r MX o r T'I' a low pipe wou ld wo rk we ll an d wo uld prod uce mo re h orsep o wer th an the h igh pipe. All the heat that fits Why should / use et hyl gas in my BS A MX and DKW MX (a co uple of wi nners / know). B ut why no t unleade d or regular? Terry /la nd elin San Francisco, Calif Treeky Deek / have a DKW with the "A" engine. When / put the plug on the head to test it fo r spa rk it looks good when / kick it over slow. When / kick it fast it doesn't spark at all. Th e po in ts, primary coil and coil are all go od. The flyw heel is off anoth er DKW ignitio n but look s alike and h as the sam e num ber "402" stamped on it. Please h elp. Ed Nelson Pleasant lIi11, Calif First thing firs t , where d id your flywheel go? Ne x t yo u say that th e flywheel yo u h ave ca me off a D KW ign ition . May be it di dn' t. Co uld b e a Zu ndapp flywh eel (th ey lo o k ali ke a nd may even have th is same nu m be r o n them b ut won't interch ange). Th e o nly th ing I can come up w it h in my mind is th a t th e m agnets o f th e fly wh eel are o u t o f ph as e with the primary co il. The ce n t er p art of a Bosch o ld style flywhee l (the o n es th at wer e used on the first Sach s then later o n the D KWs th at d id n 't have a Mo te -p la t ] was a condenser to nrc the p lug. Cure fo r the problem, replace the fly wheel w ith a new one an d spot h eli -arc wel d the cen te r to the o utsid e of th e flywheel h ousing so it ca n't slip in th e ce n ter aga in . Point, match and set (I) / ha ve b een playing around wit h different length inlet tracts and / was wondering if th ere is any form ula f or det ermining length fo r any given R .P.M. power peak. (2) A lso / have found it dif f icult t o keep things matched real well and was wondering if t here is some sort of putty or filler ma terial o n th e mar k et that wou ld be suitab le fo r fi lling in th e misma tc hed areas. (3) Aft e r reading so me adue rtisements for shocks a q uest ion arose in m y mind. Th ey say uih en moving shocks fo rwar d you sho uld keep rear wheel travel one inc h less then the front . Why? (4) A nd finally I have always heard a bik e w ill handl e bett er with th e swingarm parallel to the wo un d. Is this Gasolin e, in a 54-dollar wo rd, means "A refin ed p etrol e um, a vo la til e liq uid hydro -carb on, highly flammable and whe n vaporized and m ix ed with a ir. explos ive" says 1\Ir. Web st er . Gasoline li ke unlead ed, re gular o r et hyl gets its octane ra ting due m a inly fro m th e amo un t o f lead th at it has in it. The hi gh er th e octane rating the slowe r an d mo re e ven that it burns. Th e lower the octan e th e fast er it will b u rn . S ince m ost moto rc ycle engines re ly o n air p ass ing o ver th e h ead a nd cy lin d er to ke ep fro m over-heating, we don 't w an t to c rea te an y m o re h eat than possible. ( Yo u gotta read that tw ice . . . Ed.) Regu lar gas w ith its fast a nd une ven burn will ge n era t e m ore h ea t. Now this qu estion o p en s up ano th er quest ion th at I ge t . Wh ich is, h ow a bou t Avia t io n fuel in my b ike ? A ll I can say to th is is d on 't us e it. T h e octane rating is h igher th an ethyl ; but it a lso has de-icer an d o ther ad d it ives that a re not go od for y o ur e ng in e. Some riders th a t I've ta lked to fe lt th at they have b etter horsepower wi th av ia tio n fuel. I've foun d it n o t so . Moral o f the story , use ethyl. In the long ru n it w ill pa y off. •

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