Stan Stephens gives a masterclass in this dark art: from the bot­tom of the crankcases to the top of the head!

Classic Motorcycle Mechanics - - CONTENTS -

Stan Stephens with nine pages of tips!

I’ve been do­ing mas­ter­classes in the var­i­ous models I spe­cialise in and won­dered whether I should do one on tun­ing, but one reader wrote in to say: “Would Stan keep it sim­ple and start from the ba­sics!” What is ba­sic for some read­ers is com­pli­cated for oth­ers! This re­minds me of a few years back when I de­cided to learn to play a mu­si­cal in­stru­ment. I fig­ured I’d choose the bass as it would be eas­ier than a ‘nor­mal’ gui­tar (four strings not six, y’see?). So I booked some lessons with a bass gui­tar tu­tor. When we sat down for my first les­son, he said: ‘Where do you want to start?’ And I replied: ‘From the very be­gin­ning.’ He started talk­ing about 12-bar blues but when he re­alised I didn’t even know how to hold the thing he said: “Ah­hhh from the VERY be­gin­ning then!” So, with that in mind, I will cover the ba­sics of what tools are needed and how to use them and the tun­ing will be a gen­er­al­i­sa­tion, not of any par­tic­u­lar model. I will not be go­ing into how a two-stroke works since I am as­sum­ing you have some de­gree of knowl­edge! I have to say that or this ar­ti­cle would be longer still! And Ber­tie doesn’t pay me that much! There are two types of port­ing equip­ment ei­ther air or elec­tric. I have al­ways used elec­tric as when I started tun­ing it was in a room over a shop and I couldn’t have a com­pres­sor ham­mer­ing away and elec­tric tools are so much smaller and eas­ier to use in con­fined spa­ces. I get all my equip­ment from CC Spe­cial­ity in Amer­ica. You will need a lathe or ac­cess to one or know a small lo­cal en­gi­neers that can help with machining head and bar­rel faces. To mea­sure port heights and widths you will need in­ter­nal di­viders and vernier calipers and a depth gauge. The ba­sic port­ing tools

needed are for the dif­fer­ent ports. If you think of all the tools that a den­tist uses it will give you some idea. Which tool to use de­pends on the port that you are work­ing on. For in­stance if you are rais­ing the top of an ex­haust port and the port is fairly long you will ob­vi­ously need a cut­ter with a long shank. Whereas if you are work­ing on the in­let port, which usu­ally is short, you can use a shorter cut­ter, which is eas­ier to con­trol. To raise or wi­den the trans­fer ports you need right-an­gle cut­ting tools. I use a va­ri­ety of cut­ters, a coarse one to shift the main part of the metal, a thin flat one to make the roof of the port flat, a small round one for get­ting around cor­ners and a di­a­mond im­preg­nated stone for fin­ish­ing off and vari­a­tions of these for par­tic­u­lar jobs. The en­gines we are deal­ing with in CMM are clas­sic road bike en­gines so you are not look­ing at get­ting the ut­most from the engine, just liven­ing it up, so it’s best to be con­ser­va­tive with the port­ing: re­mem­ber you can’t put metal back on. There are lim­its to which ports can be widened or raised be­fore giv­ing prob­lems, I will deal with these lim­its later. I will give any mea­sure­ments of port heights in mm from the top face of the bar­rel.


So, we will start at the bot­tom and work our way up, so first of all we will have a look at the crankcases and see what im­prove­ments that can be made. There is not any worth­while work that can be done in the lower part of the cases; the most im­por­tant part is where the bar­rels meet the cases: they never seem to match each other! I have used a YPVS 350 crank­case as an ex­am­ple but the the­ory ap­plies to all two-stroke en­gines. Not only is it im­por­tant that the bar­rel matches the cases but also that the gas­ket matches them both. The area we are talk­ing about is where the gases exit the crank­case and go up the bar­rel to the trans­fer ports, this area is not the trans­fer port it­self, we will call this the catch­ment area. The gases ob­vi­ously have a very short time to travel up from the catch­ment area to the trans­fer port and out into the com­bus­tion cham­ber, there­fore any help we can give by re­mov­ing ob­struc­tions, the bet­ter. On the YPVS the catch­ment area in the base of the bar­rel is con­sid­er­ably larger than in the cases, also the base gas­ket matches the cases not the bar­rel. First of all with a sharp blade cut the gas­kets to match the base of the bar­rels, next with a felt-tip pen mark the top of the cases and place the matched gas­kets on the cases and with a scriber mark around the gas­ket. This will give you a line to work to, don’t just match the edge of the cases to the line, try to repli­cate the shape and the flow of the orig­i­nal. The more ac­cu­rate you are the bet­ter the match and the flow will be. Ob­vi­ously be care­ful not to dam­age the gas­ket face on the cases. There is a lot of metal to re­move from the crankcases to make them match the bar­rels. On other makes or models it may be the other way around and the metal will have to be re­moved from the bar­rels in­stead but the prin­ci­ples are the same. The other im­por­tant part of the catch­ment area is the cylin­der liner and the bridges be­tween the bot­toms of the trans­fer ports. Any flow­ing work in this area will be ben­e­fi­cial. The liner it­self can be cut and raised to en­large the catch­ment area also. On most Yama­has you can raise the cut­aways in the liner by up to 8mm but don’t do it on 500 or 750 Kawasaki triples be­cause they have a habit of crack­ing there any­way! On later en­gines with crank­case reeds

(e.g. DT125R, TZR250, KR1S and RGV) flow­ing the crankcases is even more ben­e­fi­cial. I am sure if the engine’s de­signer ever saw what the pro­duc­tion line did to the cases he would have a fit! If you re­move the reed block and look down the in­let port on one of these en­gines you will see that the spig­ots on the bot­tom of the bar­rels pro­trude into the ports. On the DT125R there is 12mm to be re­moved from the spig­ots and about 6mm on the TZR250 and KR1-S. The main rea­son that my pro­duc­tion race KR1S were so fast in the late 1980s was the amount of work I did on the in­lets in the crankcases. There are huge pieces of cast­ing mask­ing the reed valves and there are two crank­case bolts which have need­lessly large cast­ings around them in each in­let. With all the ob­struc­tions re­moved and the in­lets flowed around to the trans­fer catch­ment ar­eas I used to gain up to 8bhp! All the work to the crankcases we have cov­ered here is free power, I am sure it was in the orig­i­nal de­sign of the en­gines. It can all be car­ried out with the sim­plest of equip­ment and there is not a lot of chance that you can do any dam­age.

In­let port

Next we move on to the in­let port. There are three main types of in­lets, pis­ton con­trolled, reed-valve and disc-valve, I will just be cov­er­ing the first two here. The ear­lier two-strokes (i.e. pre Yamaha RD se­ries) had pis­ton-con­trolled in­lets. This means that the in­let port is opened and closed by the pis­ton as it goes up and down the cylin­der bore. At first you would think that the ear­lier the in­let opens the more fuel/air would be drawn into the engine but the prob­lem with a pis­ton-con­trolled in­let is that with an ear­lier open­ing in­let port as the pis­ton goes up the bore it means that the in­let closes later as the pis­ton comes back down the bore. The lim­it­ing fac­tor with this type of in­let is that if the port closes too late it will blow back through the carb and also make it hard or im­pos­si­ble to start. My ad­vice would be if you want to ex­per­i­ment with in­let port tim­ing do it by short­en­ing the in­let side of the pis­ton, not by low­er­ing the bot­tom of the port, then if you have gone too far it is easy to re­place the pis­ton. The in­let port should be smooth but not pol­ished, with no ob­struc­tions. Any in­let stubs or gas­kets should be matched to the port. As we have seen, the limit of the bot­tom of the in­let port is the gases blow­ing back out. The limit of the top of the port is that when the pis­ton is at bot­tom dead cen­tre the lower pis­ton ring must not show in the port, oth­er­wise the ring will snag and break. I like to al­low the top of the port to be 5mm lower than where the ring comes down to; this al­lows there to be a large cham­fer at the top of the port. The cham­fer is for when you fit the bar­rel to stop the ring snag­ging. There needs to be a cham­fer at the bot­tom of the in­let to stop the bot­tom of the pis­ton catch­ing. Kawasaki’s way around the prob­lem is by us­ing a ‘lar­ynx’ at the top of the port to sup­port the ring at BDC. Other en­gines – like the YPVS – use a bridged in­let. We have cov­ered the lim­its of the top and bot­tom of the in­let port. To ob­tain more

area, if you are us­ing larger carbs, you will need to wi­den the port. I wi­den it to a max­i­mum of 60% of the cylin­der bore di­am­e­ter, un­less the width of the bot­tom of the pis­ton is less than that. Mov­ing on to the reed-valve in­let, the pur­pose of the reed-valve was to cure the fail­ings of the pis­ton-con­trolled in­let of blow-back. It was thought at first that the ex­tra ob­struc­tions of the reed assem­bly would limit power but this proved ground­less. The only prob­lem was that on the ear­lier reed en­gines the Ja­panese were far too con­ser­va­tive with reed block size. When we used the RD400 and later the LCS in pro­duc­tion rac­ing we had to use the orig­i­nal reed blocks and I spent hun­dreds of hours on the dyno test­ing dif­fer­ent ideas of flow­ing the reed blocks and dif­fer­ent reed ma­te­ri­als and thick­ness. The best re­sults I achieved were by re­mov­ing the bridges in the blocks and the di­vider at the end and us­ing longer reeds which seated against each other at their tips. The think­ing there was that at high revs when the reeds don’t have time to open and close there was ab­so­lutely no ob­struc­tion in the reed block. Ob­vi­ously that would not be sen­si­ble for the road but would be an idea for some­one pre­par­ing an engine for sprint­ing or hill-climbs. When not be­ing used for pro­duc­tion rac­ing it was eas­ier to use larger reed blocks from other models, e.g. TZ750 reed blocks. To see how the Ja­panese cor­rected the prob­lems of too small reed blocks, the 350 YPVS came fit­ted with reed blocks the same size as the TZ750! The other ob­struc­tion to the in­let is the pis­ton. For rac­ing we re­moved the back of the pis­ton com­pletely but that would be too rad­i­cal for the road. How­ever, en­larg­ing the holes in the pis­ton to the width of the port will help. Don’t cut away the back of the pis­tons in a 350YPVS; the bridged in­let is too wide and it needs the pis­ton to bear against the bridge. You can see that in­let port on the tuned reed-valve engine is open for 360 de­grees. The limit of the top of the port is the same as the pis­ton con­trolled in­let and I have found there is very lit­tle to be gained from low­er­ing the port.

Ex­haust ports

Now on to the ex­haust port. Up un­til now most of the work has been to im­prove the ef­fi­ciency of the engine with­out re­ally al­ter­ing its char­ac­ter­is­tics. The ex­haust port is where all that changes. Nat­u­rally as with most things in life you don’t get some­thing for noth­ing. By al­ter­ing the ex­haust port height and to a de­gree the width, you will raise the peak power (more bhp at higher revs), the trade-off be­ing that you will lose bot­tom-end power. Al­ter­ing the ex­haust port width will give sim­i­lar re­sults with­out los­ing as much bot­tom-end power. The down­side here is that you can’t wi­den the ex port too far or the pis­ton rings will suf­fer as they try to

bend into the port. When rais­ing the ex port to the re­quired height for max­i­mum power the two-stroke tuner would also raise the trans­fer ports to re­tain or im­prove the mid-range power but I am go­ing to as­sume here that you don’t pos­sess the equip­ment to raise the trans­fers. Bear­ing that in mind, it is best to be a lit­tle con­ser­va­tive in rais­ing the ex port height. On older clas­sic two-strokes the ex ports are fairly low but the en­gines are likely to be­come peakier sooner. With later reed-valve en­gines you can be more dar­ing with height be­fore you start mak­ing it too peaky and with en­gines with pow­er­valves you can throw cau­tion to the winds (al­most!). There are a few pa­ram­e­ters to keep in mind as the ab­so­lute max­i­mum amounts you can mod­ify the port by. Never ex­ceed 70% of the bore size when widen­ing the port and that is when us­ing steel pis­ton rings. With the old cast-iron rings the fig­ure is much lower, in fact I wouldn’t wi­den the port at all with cast rings. Come to think of it, I wouldn’t use cast rings at all! Never ex­ceed 50% of the stroke when rais­ing the port, 70% width and 50% height is what you would ex­pect in a top-end power race engine. The shape of the ex­haust port is very im­por­tant; it is the shape which per­suades the rings to stay in their grooves. If the port is too square at its top edge, the rings will try to bulge into the port so the port has to have a curved top edge, but the more curved it is the lower the top-end power is. The wider the port is, the more curved the top of the port must be. Also the port must have a cham­fer to help ring life and to stop them snag­ging in the port. Once again the larger the cham­fer the eas­ier on the rings but there is a loss in power with wider cham­fers, so as you can see ev­ery­thing is a com­pro­mise. With a clas­sic engine, espe­cially for street use, I would rec­om­mend you keep ba­si­cally to the ex­ist­ing shape, The shape must be sym­met­ri­cal, i.e the curve of the top edge and the ra­dius of where the top of the port meets the sides of the port must be the same on both sides. If the shape of the port is lop-sided or the ra­dius is dif­fer­ent from one side to the other the pis­ton rings will be pushed around one way in the grooves more than the other. This is the main cause of ring pegs com­ing adrift. If you com­pare the port­ing on a tri­als engine to port­ing of a high per­for­mance engine you will see that the tri­als engine has a low, nar­row ex port with a very curved top to the port. This pro­duces its best power at very low revs. Cou­pled to a low com­pres­sion, the tri­als engine has soft tractable power with a smooth spread of power but with lit­tle top-end per­for­mance. The ex port of the tuned sports engine has a wide, high ex­haust port with a much flat­ter top to the port, this engine pro­duces

its peak power much fur­ther up the rev-range: more revs, more power and more speed! Some­thing else to con­sider when rais­ing the ex port is that as you raise the port you are low­er­ing the com­pres­sion. On a two-stroke the com­pres­sion does not start un­til the ex port closes, so the higher the port, the lower the com­pres­sion. You must al­ways work out how much you have raised the port and how much it has low­ered the com­pres­sion and ma­chine the head to raise the com­pres­sion to com­pen­sate. So far we have cov­ered the shape of the port in the cylin­der wall. With the port now higher and wider there will be a ‘neck’ in the shape of the port as it goes through the bar­rel to­wards the ex­haust pipe. As much as is pos­si­ble there must not be any nar­row­ing of the port from where it leaves the cylin­der to where it meets the ex­haust pipe. If the engine has a flange or stub fit­ting for the pipe make sure there is no step or re­stric­tion, in­clud­ing the gas­ket. If the engine has pow­er­valves, flow them to the new shape of the port. Once again make sure there is no re­stric­tion and when fin­ished en­sure that the pow­er­valve ca­bles are ad­justed so that the pow­er­valve fully opens the port. With the other ports the the­ory is that the ports should not be pol­ished be­cause the slightly rougher fin­ish helps the fuel and air to mix as it goes through the engine, un­like the ex­haust port where the fuel mix has done its job and needs to exit the cylin­der asap; so polish the ex­haust port as much as you like!

Trans­fer ports

There are many ar­ti­cles out there con­cern­ing port tim­ing; all of them as­sume you are tun­ing the engine for max­i­mum power with race ex­hausts and large carbs. They never take into ac­count bikes that don’t have the op­ti­mum ex­hausts, carbs and ig­ni­tion and run an air­box. If you have tuned the engine to race spec, there is a spe­cific height and area that the trans­fer ports should be when matched to an ex­haust port that is at its max height and width, but we aren’t talk­ing about fully de­vel­oped race en­gines here. Ba­si­cally, as the ex­haust port is raised you lose some mid-range power. To fill in that mid-range is the job of the trans­fers; the higher the ex­haust port rel­a­tively the higher the trans­fers must be, but on your clas­sic engine you have only raised the ex­haust port a con­ser­va­tive amount. On most clas­sic en­gines the match­ing of the cast­ings to the lin­ers is very poor, espe­cially in the trans­fer win­dows. If you have a right-an­gle drive port­ing tool my ad­vice would be to make a re­ally good job of match­ing the bar­rel cast­ing with the liner and en­sure all the trans­fers are ex­actly the same height. Usu­ally that will have raised the trans­fers as well as mak­ing them more ef­fi­cient. When all the port­ing work has been done cham­fer the ports. Espe­cially make sure there is a gen­er­ous cham­fer top and bot­tom of the ex­haust port.

Heads and com­pres­sion ra­tios

With all the port­ing to the bar­rels fin­ished, it is time to turn our at­ten­tion to the heads. The first steps are to work out how much you are go­ing to ma­chine off the head to raise the com­pres­sion and what squish clear­ance to have. Un­less you are go­ing to just skim 0.5mm off the head to slightly raise the com­pres­sion you are go­ing to have to work it out us­ing that hor­ri­ble word from school: maths! I am go­ing to start with com­pres­sion ra­tios: on most clas­sic two-stroke road bikes there is a power in­crease to be had by in­creas­ing the com­pres­sion. There is al­ways an op­ti­mum com­pres­sion above

which the in­crease in heat negates the pro­posed in­crease in power. As we are not deal­ing with race en­gines here, it is wise to be con­ser­va­tive. Small in­creases in com­pres­sion will be ben­e­fi­cial through­out the rev-range; larger in­creases in com­pres­sion will ben­e­fit per­for­mance at lower revs but be detri­men­tal to power at higher revs. For ex­am­ple, a mo­tocross engine would run a higher com­pres­sion than a road race engine if they were both run­ning on the same fuel. On a clas­sic two-stroke I would not rec­om­mend over a 7-1 com­pres­sion ra­tio and you would need to use the best un­leaded fuel with the high­est oc­tane rat­ing. The ‘com­pres­sion ra­tio’ is the ra­tio be­tween the vol­ume of the cylin­der and vol­ume of the com­bus­tion cham­ber. To cal­cu­late the com­pres­sion ra­tio you need to mea­sure both these vol­umes. At school I must have been very se­lec­tive about what I re­mem­bered: how to mea­sure vol­umes of cylin­ders was ob­vi­ously deemed im­por­tant to re­mem­ber! Pi R squared x height! I al­ways take Pi to be 3.142, R is the ra­dius, the two means squared (which means times it­self). In sim­ple terms, for ex­am­ple on a 350LC with a bore of 64mm, the ra­dius is half the bore, half 64 is 32. The stroke (or height) is 54mm. So our lit­tle sum is 3.142 x 32 x 32 x 54 = 173.7cc, this is the cylin­der vol­ume but if the cylin­ders have been re­bored to (say 1mm over­size) that will al­ter the sum to 3.142 x 32.5 x 32.5 x 54 = 179.2. We now have to mea­sure the other part of the ra­tio, the com­bus­tion cham­ber vol­ume: this can’t be cal­cu­lated, it has to be mea­sured. To do so you will need a bu­rette or cc mea­sur­ing tube. Ide­ally the engine will be on the bench, not in the bike. Re­move the cylin­der head, clean-up the head and bar­rel faces and re­move the spark plug. Po­si­tion the pis­ton at top dead cen­tre (TDC) and smear some grease around the edges of the pis­ton to seal it and wipe off the ex­cess. Use a new head gas­ket, fit the head and torque it down. Po­si­tion the engine so that the cylin­ders are up­right and check that the pis­ton is still at TDC. Fill your bu­rette with paraf­fin to the zero mark and in­sert it into the plug-hole, re­lease the paraf­fin un­til it comes level with the top of the plug-hole and read off the bu­rette how much paraf­fin it has taken. If for ex­am­ple it shows 18cc, deduct 2cc for the plug-hole, which gives 16cc which is the vol­ume of the com­bus­tion cham­ber. To work out the com­pres­sion ra­tio, take the vol­ume of the cylin­der, ADD the vol­ume of the com­bus­tion cham­ber and DI­VIDE by the vol­ume of the com­bus­tion cham­ber. Which is: 173.7 + 16 di­vide by 16 = 11.85-1 CR. If we were talk­ing about four-strokes that would be the com­pres­sion ra­tio, but with a two-stroke the com­pres­sion does not start un­til the pis­ton closes the ex­haust port. The cor­rect way to cal­cu­late the com­pres­sion ra­tio on a two-stroke would be to mea­sure from the top of the ex­haust port to the top of the bar­rel, then when work­ing out the vol­ume of the cylin­der in­stead of the stroke of 54mm in the equa­tion, you would in­sert the port height, but for the pur­pose we are in­tend­ing it is fair to gen­er­alise that the ex­haust port is 50% of the way up the stroke (i.e. 27mm from the top of the 54mm stroke). In this way the

“Two-stroke tun­ing is an art – but one that can be learned. It’s of­ten about get­ting the best out of the ma­chine as the orig­i­nal de­signer in­tended, be­fore the needs of mass-man­u­fac­ture came into play.”

com­pres­sion ra­tio we ar­rived at ear­lier would be halved, so in­stead of about 12-1 it would be 6-1. This on a two-stroke is called the ‘cor­rected com­pres­sion ra­tio.‘

Squish bands

Not all clas­sic two-strokes have squish heads: the GT750 doesn’t. The squish area of a cylin­der head is the area around the out­side of the domed com­bus­tion cham­ber. The pur­pose is to squish (squeeze) the petrol/air mix­ture into the cen­tre of the com­bus­tion cham­ber where the spark-plug is. With­out a squish head it would be very dif­fi­cult to get a rea­son­able com­pres­sion with­out a flat in­ef­fi­cient-shaped com­bus­tion cham­ber. The squish clear­ance is the dis­tance be­tween the pis­ton at TDC and the squish area of the head. The tighter the clear­ance the more ef­fi­cient the squish. The idea is not to have any mix­ture trapped in the squish area, it all needs to be squished into the com­bus­tion area. Any mix­ture trapped in the squish area is wasted, if 10% is trapped that is 10% power wasted. Any mix­ture trapped will cause det­o­na­tion or pre-ig­ni­tion, known as pink­ing, which will cause dam­age and seizures. The rea­son for hav­ing any clear­ance is to al­low for any ex­pan­sion or stretch­ing of parts. On a clas­sic engine you need at least 1mm of clear­ance be­tween pis­ton and head to al­low for any stretch­ing of heavy con-rods and pis­tons. Ide­ally the an­gle of the squish band should be the same as the an­gle of the dome of the pis­ton top. The squish area should be 50% of the area of the com­bus­tion cham­ber, here comes Pi again! So 50% of a 64mm di­am­e­ter com­bus­tion cham­ber would be 3.142 x 32 x 32 di­vided by 2. Mea­sure the squish clear­ance be­fore strip­ping the engine. You will need

some 1.5mm or 2mm multi-core sol­der. Re­move the spark-plug and bend a length of sol­der into an ‘L’ shape and put it through the plug-hole so that the end of the ‘L’ touches the edge of the cylin­der. With a span­ner on the fly-wheel nut, turn the engine over back­wards and for­wards over TDC a few times to com­press the sol­der. Re­move the sol­der and mea­sure with a vernier. This will tell you the squish clear­ance. You will then be able to work out how much to skim off the head or bar­rel. Squish bands and com­pres­sion ra­tios are in­ter­linked be­cause if you al­ter one you will need to al­ter the other. From the above cal­cu­la­tions you will be able to work out how much to take off the head. Next we move on to the lathe to do the machining.

Machining heads

To ma­chine sin­gle cylin­der heads, I have a man­drel fixed to an old chuck with a thread which screws into the head’s plug­hole. First of all I ma­chine the face of the plug­hole on the mill to make sure the head runs true when screwed onto the man­drel. To ma­chine twin cylin­der heads I have a face-plate fixed to a chuck for the lathe. The face-plate is drilled and tapped for all the many dif­fer­ent heads that I do. Again I ma­chine the faces of the plug­holes but this time it is im­por­tant to make sure that both plug­holes are ex­actly the same depth. I bolt the head to the face-plate us­ing two domed spac­ers in­side the com­bus­tion cham­bers with long bolts go­ing through the plug­holes and through two spac­ers ex­actly the same length and into the threaded holes in the face-plate. The head should then run true ready to skim the re­quired amount off. With the head-face ma­chined to raise the com­pres­sion it’s time to ma­chine the squish bands. I re­move the face-plate from the lathe and fit the man­drel that I use for the sin­gle cylin­der heads. With a twin cylin­der head I screw the plug thread of one com­bus­tion cham­ber onto the man­drel. I set the tool post cut­ter to the re­quired an­gle. Not all squish bands are the same an­gle (most Yams are 14˚ while an RG500 is 21˚). With the lathe on a slow speed I ma­chine the squish band, I then ma­chine the other side the same. So, there you go! Now you know a lit­tle about two-stroke tun­ing. Me? Well, since learn­ing to play the bass gui­tar I’m now in a band called Two-stroke Smoke and we play some of the lo­cal pubs. See? Never too old to learn some­thing new – so get tun­ing!

A tweaked and tuned Elsie is a hoot out on the road.

Wanna port a Pow­er­valve?

Here we are match­ing the cases and rough­ing out with a coarse cut­ter.

You’ll need these port­ing tools.

Here’s the gas­ket cut to the base of the bar­rel.

Fi­nally, the fin­ished cases!

Here we are flow­ing the crank­case reed cases.

Yet more tools for the job!

Wanna tease more from your TZR?

And now flow­ing the bot­toms of the bar­rels.

Check­ing for highs or lows.

It’s time for match­ing the ex­haust port to the gas­ket and flange.

Et voila! The fin­ished ex­haust port.

Fin­ish­ing the port all off.

And now check­ing the di­am­e­ter.

Here’s a tuned ex­haust port win­dow.

Wanna keep your Ket­tle on the boil?

In com­par­i­son, here’s a stan­dard and tuned in­let.

And now race tuned and stan­dard pis­tons.

Here are both tuned and stan­dard reed-blocks.

This is the Kawasaki ‘lar­ynx’ in­let.

Here the tops of the trans­fers are all the same height, area and an­gles.

LEFT: Wanna Kwik Kwak?

LEFT: Mea­sur­ing head vol­ume with a bu­rette.

Here we are machining the squish band.

Wanna kwikken your KR-1S?

A tuned head and a stan­dard head.

The head is bolted to a face plate to skim head face.

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