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.However, phenolic petalsrespond to the air requirement of the engine more quickly and they do not flutter andrebound off the cage to the same extent as stainless steel petals.To increase the life ofphenolic petals they should be carefully sanded around the edges, using 600 grit wetand dry carborundum paper, before being fitted.Some petals are smooth on one sideonly, so be sure to fit these with the smooth side sealing against the reed cage.When a large increase in high rpm air flow is required, a larger reed valve assemblywill be necessary.There are special assemblies available for some engines but often itwill be necessary to adapt a reed valve from a larger, or more potent, engine.This canbe quite frustrating, as you won't find many dealers with reed valves in their parts binsfor you to inspect for size.The best way is to go to a motorcycle wrecker, cylinder inhand, and look through his range of reed valves.Don't look for a reed valve which willdrop straight into the inlet tract of your cylinder, as it probably won't be much biggerthan the standard assembly.Instead, look for a valve which is a little wider and perhapsa little higher than standard.The fixing holes probably will not line up with the holes inyour cylinder, but that is not a great problem providing the wrecker has the inletmanifold which matches the reed valve.Check that the inlet manifold has a hole of thecorrect size to suit your carburettor; if it does, you are in business.The next problem is enlarging the inlet cavity to suit the bigger reed cage.Toaccomplish this, you will have to use your judgement.Start by measuring the reedassembly and comparing its size with the reed cavity in the inlet port.If it's 4mm wider,then grind 2mm off each side of the cavity and soWhen the reed valve fits the cavity, you can then decide what has to be done to fixthe reed valve and manifold to the cylinder.If the fixing holes are close, then it may bepossible to elongate the holes in the reed cage and manifold to align with the cylinder.In some cases, it will be a matter of filling the stud holes in the cylinder and thendrilling and tapping new fixing holes.Probably the most extreme case is when theYamaha RD350 or 400 is fitted with TZ 750 reeds.In this instance, an aluminium platewith fixing holes to suit the TZ reeds is welded to the RD cylinder face.Before some special replacement reed assemblies are fitted, they have to be68 modified in ways different to that outlined on preceding pages.One such reed which Port and Cylinder Scavengingcomes to mind is the R & R reed for RM and PE model Suzukis.This reedflows very well but it falls short in two which could easily catch the unsuspectingtuner out.The first problem is that the screw heads on the lower side of the reedprevent the cage from seating properly against the base of the Thus an air leakcan develop and spoil engine performance.What must be done is file the edges of thescrew heads flush the cage mounting face, so that the cage can seal against thecylinder base.The other problem involves the reed stop for the main bigger) reedpetal.The stop is too flexible and actually rebounds the reed petal when it comesagainst the This sends the petal into flutter and reduces high speed To curethis, the R & R stop should be removed and the standard Suzuki reed stop Youwill note that the Suzuki stop is much thicker, so longer fixing screws are required.Ifthese are unobtainable, the holes in the stop can be countersunk to give the screws morebite.Before we close the subject of reed valve there are a couple of don'tswhich you should keep in mind.Don't ever bend the reed stops to increase reed lift anddon't ever fit a spacer under the stop to increase reed lift.Either practice will causepetal flutter at higher because the reed becomes unstable (i.e., out of control).Onthe average 125 motocrosser, increasing reed lift by just 0.7mm will knock 2 hp off thetop end between 9000 and 10,500rpm.The other don't is this: don't waste your time cutting the back out of the piston orenlarging the skirt windows.This weakens the piston and there is little or no gain in hpanywhere in the power The only exceptions to this rule would be in the case ofdesert racers, or bikes which are very pipey, if they don't have any holes high up on thepiston skirt.Drilling a pair of round holes just below the ring land will help cool thepiston crown and little end, or if the bike is very peaky and nothing else has tamed thepush of power, maybe a pair of holes will help.The holes shouldn't be too large: 10 to13mm is plenty big enough for a 125 or 175, and larger engines could use holes about 14to 16mm in diameter.After the holes are drilled, carefully chamfer them on the insideand outside of the piston, then dress the holes with 180 grit wet and dry.Theseprecautions will help prevent premature cracking of the piston skirt.69 Chapter 4The ExhaustIT IS TRUE to say that the largest contribution towards achieving the current highpower levels from the two-stroke engine has come from increasing knowledge in thearea of exhaust system design.Originally, the exhaust pipe was designed to get burntgases out of the engine as quickly as possible.Then, as designers learned more aboutpressure waves, they attempted to make use of them to scavenge the cylinder of exhaustgases.The basic theory of pressure waves is quite easy to understand, but the practicalapplication of pressure wave phenomenon is very difficult to Fortunately,the experimental work done by two-stroke engineers during the past twenty years hasmade the task of building an effective exhaust well within the grasp of any two-strokeengine tuner [ Pobierz całość w formacie PDF ]

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