Classic Dirt Bike — Autumn 2017

at 35 degreesafterTDC.Usinghis owncam

grindingjig Goddenalteredthesetimingsto 60,

85, 90 and 55, vastly increasingthe overlap, but

this on its ownwasn’tenoughand modifying

one part of an enginehas an effect on other

areas.Whatwaslikely to happenwith such

modification to thecam timingwould be

the majorityof the increasedfreshcharge

cominginto the cylinderwould be pushed

straightout the exhaustvalveunlessthe

port shapewaschanged.Willoughby

reportedthat Godden’shandiwork in the

inlet port prevented this fromhappening.

Otherwork, aimedat stoppingvalvefloat

at high revs, includedprogressive rate valve

springsand alloypushrodsratherthan steel.

With amotor nowsafeto7000rpmratherthan

6000rpmit wasausefulincreasein performance

for the expert riderwho it wasreckoned

couldquiteeasilycopewith the slightloss of

flex ibilityfromthe modifiedunit.

Even this performance-enhancingwork

wasn’tenoughfor Godden’s1000-metre

engines, andthe work neededto makethe

venerableJAPcut it with the oppositionin

Europeamountedto majorsurgery.Gone was

anysemblanceof power below4000rpm,but

in its placewasreliablepower waybeyond the

7000rpmof the ‘GoldTop’ motor.Toachieve

this, Goddenhas aseriouslywild cam profile

that increasedthe valvelift by 50%and needed

flat-grindingonthe valves’edgesor theywould

tangleat high speed.Alliedto this extra lift,

the inlet port wasopenedout from 11 ⁄ 8 to 1^3 ⁄ 8

whichneedsan AmalGP carb with twin-float

chambersto feed in the fuel. Naturally,Godden

felt the standardDuralcon rod wasnot quite

rightfor this wild engineand usedasteel one.

At this levelofcompetitionthe search

xtra performanceis intense;any

le increasein power is welcomeand

nythingwhichprevents power loss

worth investigating.Forinstance,

houghimportantfor providingsparks

othe plug amagnetocan absorbpower.

Okay,only alittle power,but farbetter

be availablefor race winningthan

t. Apparentlyrunningthe magnetoat

ABOVE:Thesepoorthingshaveatoughlifeinaraceengine.

BELOW:Lininguptheflywheelsbeforetighteningthecrankpinnuts.

Withspreaderbolts

openedouttocreate

sometension,asharp

taponthecrankpinwill

splittheflywheels.

hereisoftenalimitonhowmuchliftabigvalve

beforesomethingtouches.Theflatontherim

valvesmiss.

ABOVE:Inanultra-highcompressionenginethereisthe

ddangerofvalveshittingthepistoncrown.Thisis bad,so

thhosethatknowwhatthey’redoingrelievethevalvepockets

soif it doestangleit’ssquareonanddoesn’tbendthevalve.

Atthis

for ex

littl

an

is

th

to

O

it

not

LEFT:Th

canhave

meansthev

ABOVE:Goddenmadehisownspecialcogsforvarious

applications.

quarterenginespeedratherthan half engine

speedreleasesthat absorbedpower.

JAPWRINKLES

When interviewing Godden forthe earlier

feature,Willoughbyrealisedtherewasavast

amountof untappedinformationon making

aJAP enginework to the best of its ability.

He also,in conversationwith Don,realised

that whilenot everyonewanted to super-tune

an engineto howl it roundthe Europeanlong

trackstherewerean awfullot of enthusiasts

who wanted to go grasstracking,second-half

speedway riding,hill climbingand sprinting

with the engineand theywould benefitfrom

knowing howtoscrewthe unit together.So,

the pair collaboratedand produceda‘howto’

featurefor the 500ccJAPmotor.

Alot of the work wasunder the heading

‘commonsense’,butevensoitdoesn’thurt

to go backto basicswith suchthings.Forthe

basisof this, and indeedall suchfeatures,it has

to be assumedthe readerattemptingto follow

the advicehas awork space,toolsand some

knowledge.Taking eachpart of the enginein

turn,Goddenrelatedhis experiencesof the

robust unit and howitholds up in service.

Taking the crankcasesfirst the mainbearings

042-049mbzBroadslide_044.indd 42 02/08/201712:39:14