The Link Be­tween Throt­tle Re­sponse, Horse­power Loss, and Reed Petal Per­for­mance.

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Boyesen Reeds

An Ob­served Trial ma­chine’s over­all en­gine per­for­mance (it’s abil­ity to achieve con­sis­tent ac­cel­er­a­tion re­sponse and peak horse­power), is di­rectly linked to the per­for­mance of its reed petals. Twostroke Ob­served Trial en­gines re­ceive air and fuel mix­tures through a me­chan­i­cal in­take sys­tem that is com­monly re­ferred to as a Reed Valve.

Reed Valves are al­most al­ways lo­cated be­tween the car­bu­re­tor/throt­tle body and your ma­chine’s en­gine, and are de­signed to work in con­junc­tion with a sys­tem of del­i­cate, pre-mounted reed petals. To­gether, the Reed Valve and the Reed Petals cre­ate the heart of an Ob­served Trial ma­chine’s in­take as­sem­bly. The small, flex­i­ble Reed Petals are pri­mar­ily re­spon­si­ble for con­trol­ling the inlet of air and fuel by the sim­ple process of open­ing and clos­ing against the sur­face of the Reed Valve. Be­cause Reed Petals are re­spon­si­ble for the on­go­ing reg­u­la­tion of air and fuel into the en­gine, they are the sin­gle-most cru­cial com­po­nent within your ma­chine’s air-in­take sys­tem. They are the flex­i­ble com­po­nent of the Reed Valve that “re­acts” to the en­gine’s cy­cling pis­ton – this en­gi­neer­ing fact makes them… “The Gate­keep­ers of Power”.


To reg­u­late in­com­ing air and fuel in the proper ra­tios re­quired for per­for­mance, Reed Petals are de­signed to “re­ac­tively pulse” as the en­gine’s pis­tons cy­cle through their in­take and ex­haust strokes. This reed petal pul­sa­tion, or “FLEX CY­CLE” oc­curs at a one-to-one ra­tio in uni­son with the up­ward and down­ward move­ments of the en­gine’s pis­ton. Within each reed petal flex-cy­cle, the petals are de­signed to flex open in uni­son with the pis­ton to al­low air and fuel into the en­gine. The petals then briefly close, pre­cisely “kiss­ing” against the sur­face of the Reed Valve to stop the in­ter­nal­ized air and fuel from back­wash­ing, or “re­vert­ing” out of the en­gine. This process is com­monly re­ferred to as “Reed Petal Trap­ping” and is one of the per­for­mance tun­ing ar­eas that in­flu­ences a Reed Valve’s abil­ity to sup­ply the en­gine with the proper air and fuel mix­tures needed to op­ti­mize power and ac­cel­er­a­tion re­sponse.

Through­out their life span, Reed Petal flexes mil­lions of times. Be­cause of this con­tin­ual strain, all Reed Petals in­di­vid­ual com­pos­ite fibers start to lose their mem­ory, de­form, and/or ul­ti­mately chip at their edges mak­ing them slow to re­act in uni­son with the en­gine. When reed petal de­gen­er­a­tion oc­curs, most, if not all reed petal de­signs, re­gard­less of ma­te­ri­als used, are un­able to prop­erly trap the air and fuel charges in uni­son with the en­gine’s pis­tons. In this re­gard, fail­ing to man­age air and fuel “re­ver­sion” is a known cause of de­creased en­gine per­for­mance and can eas­ily be reme­died by chang­ing to a new set of Reed Petals.


An Ob­served Trial ma­chine’s Reed Petals should be con­sid­ered a main­te­nance item that needs pe­ri­odic at­ten­tion in or­der to keep your ma­chine’s en­gine per­form­ing to op­ti­mal lev­els. If they are not pe­ri­od­i­cally in­spected and re­placed, they will be­gin to con­trib­ute to a num­ber of poor per­for­mance symp­toms in­clud­ing hard-start­ing, eas­ier en­gine stalling, loss of re­sponse and power, and will also pro­duce in­con­sis­tent power de­liv­ery. In­spect­ing your ma­chine’s Reed Petals pe­ri­od­i­cally is a quick, cost ef­fec­tive, and easy do-it-yourself main­te­nance project that en­sures your en­gine is re­ceiv­ing the proper air and fuel mix­tures re­quired to achieve con­sis­tent power de­liv­ery and ob­tain peak horse­power out­put time-and-time again. Reed petal life ex­pectancy varies dra­mat­i­cally with your ma­chine’s en­gine and in­take de­sign and also varies greatly depend­ing on how hard or how of­ten you ride. Some­one who is al­ways on the throt­tle, al­low­ing the en­gine to con­stantly run at high RPMs can ex­pect less life from their reeds petals. If you ride for long hours, then it is a good idea to check your Reed Petals 2-3 times a sea­son to en­sure max­i­mum horse­power out­put is main­tained.


When check­ing your reed petals, look for edge chip­ping, sur­face tears, cracking, or a peel­ing of the sur­face ma­te­rial. Also, check to see if the Reed Petals are main­tain­ing their static gap against the Reed Valve sur­face.


In­spect­ing the Reed Petals on an Ob­served Trial ma­chine first in­volves get­ting ac­cess to the Reed Valve unit. This will re­quire you to re­move the car­bu­re­tor and the Reed Valve As­sem­bly from the en­gine. The process is fairly straight­for­ward and can be per­formed with ba­sic tools. If done cor­rectly, you should be able to fin­ish the job in about an hour. Un­bolt the car­bu­re­tor or throt­tle body, re­move the Reed Valve from the en­gine and in­spect the en­tire as­sem­bly. If the as­sem­bly’s petals are pulled away from the cage, cracked or have rough edges, the petals should be re­placed.


1. Re­move the in­te­grated Air­box/Sub­frame As­sem­bly by lo­cat­ing and re­mov­ing all Sub­frame bolts as well as the rear brake light wiring har­ness plug. Lo­cate and loosen the Air­box to Car­bu­re­tor in­take clamp bolt(s). At this mo­ment the Air­box/ Sub­frame should be ready to re­move from the ma­chine.

2. Lo­cate and loosen the Car­bu­re­tor to Reed Valve in­take clamp bolt(s). While not­ing the ca­ble rout­ing of the throt­tle ca­ble, gen­tly pull the car­bu­re­tor to­wards the back of the bike. The Carb will re­lease from the Reed Valve As­sem­bly’s in­take boot. You do not have to re­move ANY throt­tle ca­ble con­nec­tions to the car­bu­re­tor; just set it aside.


3. Lo­cate and re­move the 4 Reed Valve As­sem­bly bolts. Re­move Reed Valve

As­sem­bly from en­gine. 4. Lo­cate and re­move the 4 small screws that hold the two Reed Petals and the Reed Stops on the Reed Valve As­sem­bly. Af­ter re­mov­ing the petals and the stop, in­spect the Reed Valve for for­eign ma­te­rial be­tween the Reed Petals and the Reed Valve’s petal sur­face junc­tion. Also in­spect the Reed Petals for ANY de­for­ma­tion or sur­face dam­age, look­ing pre­cisely for chip­ping at the tips of each petal.


5. In­stall new Boyesen Car­bon Tech or Boyesen Power Reeds and all re­quired com­po­nents to the Reed Valve As­sem­bly us­ing thread locker to en­sure that the screws stay in place dur­ing oper­a­tion. NOTE! If you are in­stalling Car­bon Tech Reeds you will use the stock Reed Stop. If in­stalling Power Reeds, you will re­place the stop Reed Stop with the in­cluded Boyesen REV Plate. Use of the stock Reed Stops with Boyesen Power Reeds negates the per­for­mance of the Power Reed de­sign. 6. Take care to in­stall the new Car­bon Tech Reed Petals and Reed Stop sym­met­ri­cally on the Reed Valve. All Reed Petals and Reed Stops should sym­met­ri­cally align on the valve, to­tally cov­er­ing the in­take ports, and main­tain align­ment with the Reed Stop. WARN­ING! MAKE SURE that all the screws have been cov­ered with thread locker and all have been tight­ened well. This will en­sure that the screws will not back out and be sucked into your ma­chine’s en­gine caus­ing it to be dam­aged.


7. Re­in­stall the Reed Valve As­sem­bly. Tighten Reed Valve As­sem­bly bolts in a criss-cross pat­tern to your par­tic­u­lar ma­chine’s man­u­fac­tur­ers spec­i­fi­ca­tions. Op­ti­mally, new Reed Valve gas­kets should be used if the ex­ist­ing gas­kets show tear­ing or de­lam­i­na­tion. In­spect all gas­kets care­fully to con­firm proper con­di­tion. 8. Re­in­stall Car­bu­re­tor/Throt­tle Body. Make sure that the Car­bu­re­tor/Throt­tle Body is ver­ti­cally ori­en­tated and is set at the 12 o’clock po­si­tion rel­a­tive to the en­gine. WARN­ING! MAKE SURE that the car­bu­re­tor is set well in­side the in­take boot. MAKE SURE that the clamp fits evenly and se­curely around the en­tire boot sur­face. Fail­ure to in­stall the Car­bu­re­tor/Throt­tle Body proper into the in­take boot can re­sult in an air leak, ul­ti­mately cre­at­ing a lean con­di­tion in your ma­chine’s en­gine. 9. Re­in­stall the Air­box/Sub­frame on the ma­chine tak­ing care to en­sure a proper fit of the air­box boot on the Car­bu­re­tor/Throt­tle body. If you are run­ning the rear brake light, re­mem­ber to plug in the wiring har­ness. WARN­ING 1! MAKE SURE that the car­bu­re­tor is set well in­side the Air­box In­take Boot. MAKE SURE that the clamp fits evenly and se­curely around the en­tire boot sur­face. Fail­ure to in­stall the Car­bu­re­tor/ Throt­tle Body proper into the in­take boot can re­sult in an air leak, ul­ti­mately cre­at­ing a lean con­di­tion in your ma­chine’s en­gine. WARN­ING 2! MAKE SURE that the Throt­tle Ca­ble rout­ing is the same as when you re­moved the Air­box/Sub­frame. Check to make sure that the ca­ble did not par­tially re­lease from the car­bu­re­tor’s ca­ble hous­ing lo­cated at the top of the Car­bu­re­tor. Trial Mag­a­zine and Boyesen would like to thank Gas Gas Mo­tos UK for help­ing with the gen­er­a­tion of this ar­ti­cle

2013 Bri­tish Cham­pion Michael Brown (300cc JST Gas Gas) “To com­pete at the cut­ting edge of the sport it’s es­sen­tial to me that any af­ter­mar­ket parts fit­ted are tested be­fore us­ing them. None of the parts make a huge in­crease in the ma­chine’s...

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