CSC TT250 - Page 71

TT250Owner’s andServiceManual
www.CSCMotorcycles.com
70
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 Apairofpliers
 14mmsocketwithratchetandextension
 Sparkplugwrench
 10mmwrench
 LonghandledPhillipsheadscrewdriver
 Largebladedflatheadscrewdriver
 6mmAllenwrench
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TheTT250hasatwo‐valveengine.Theintakevalveopenstoadmitthefuel/airmixture,andthe
exhaustvalveopenstoexpeltheexhaust.Whentheengineisatthetopofitscompressionstroke,we
wantbothvalvesclosed.That’sbecausewewanttocompressthe
fuelairmixture,igniteit,andthen
allowtheresultinghighcombustionpressurestodrivethepistondown.Ifanyleakageoccursaround
anyofthevalveswhilethisisoccurring,theenginewilllosepoweranditcould“burn”avalveifthe
combustingfuel/airmixescapesaroundthe
valvewhileitisstillburning.
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Whenengineersdesignanengine,theywantittodotheabove,buttheyhavetoaccountforthe
thermalexpansionthatoccursasenginetemperatureincreasesduringnormaloperation.Inorderto
compensateforthisthermalexpansion,theengineersdesigninagap
intherockerarm/valvetrain.As
theenginewarms,thisgapapproacheszero,andeverythingworksthewayitissupposedto.
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OntheTT250,thevalvetrainlookslikeyouseeinthephoto
(thephotoshowstheexhaustvalve,butboththeintakeand
theexhaustvalvehave
similarvalvetrains).Thevalvegap
(alsoreferredtoasthevalveclearance)iswhatthelowerred
arrowpointstointhephoto.
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Astheweardescribedaboveincreases,ithastheeffectofreducingthevalvegap(i.e.,theclearance
builtintothevalvetraintoaccountfor
thethermalexpansionastheenginewarmsup).What
happensisthatasthiswearoccurs,thevalveactuallymoveshigherintothecylinderheadandthe
valvegapdecreases.Ifthisweargoesbeyondacceptablelimitswithoutadjustingthevalves,thevalve
gapgrowssmallerandsmaller.Ultimately,thiswear
willresultinthevalvebeingheldofftheseat
whencombustionoccurs.Thisisbad,becausewhenthisconditionexists,hotburninggasesescape
aroundthevalvesealingarea.Ultimately,theseburninggaseswilldestroythevalveandthe
seat.That’swhathappenswhenwe“burnavalve.”
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The
abovescenarioisalsobadbecausethevalveneedstocool,anditiscooledprimarilywhenitis
closedagainstthevalveseat.Thatallowsheattoescapefromthevalveandflowintothecylinder
head.Ifthevalveneverfullycloses,thevalvewillcontinuetoheat,and
thevalvestemwillexpand
diametricallysomuchthatitseizesinthevalveguide.That’sbad,too,becausewhenthathappens,
thevalvewillstick,thepistonwillhitit,andyou’vejustboughtyourselfanewengine(oryou’llhaveto
payforexpensiverepairsonthecurrent
engine).