Yamaha TZR250 TDR250 Hoverizing
Mounting and frames and pullys
Electrics
Fuel supply
Starting options
Mixture
Pipes
Blue Printing
If you are able to obtain a complete bike this is the way to go .You get all the bits and can see it running ,check that the power valve opens up at about 6000 rpm watch the small arm between the cylinders and the power valve motor open and close .When the power is turned on the servo motor should move about 10 degrees forward then 10 degrees back .Ive had loads of faulty controllers which either turn 2 full revolutions on power on or dont turn at all .Fred Wilson had a controller last year which worked fine until it started to drain the spark from the CDI unit .Disconect it and the CDI worked fine .
If you have a compresion tester check both bores ,remember its a piston ported two stroke so the compression reading will not be true as its only from the point that the transfer port closes .If they are even its a good sign .
Check the smoke out of the exhausts both should be even .I saw a TZR go through an auchtion for £800 last year with smoke out of one cylinder only .
If you collect from a breakers yard the engine only insist he removes the head infront of you check the pistons for signs of detonation ,Sand blasted and melted on the top ,check the bores for bad scores and the edges of the exhaust ports for damage ,this is an area prone to rings snagging and can be the end of the barrels or a rebore .Check the rebore size on top of the piston 0.025 is one size up .You may find it hard to find pistons over 0.200 ie 8 sizes up ,you may have to rebore 2 or three sizes in one go to remove bad damage so be aware of how much lift is left in them .The plugs should be dark brown however this can be missleading unless the engine has been run up to temperature at high revs and then stopped dead .If they are ash white or show signs of splatered aluminium on them take care .Having said all this these engines are cheep and easy to rebuild as far as engine rebuilds go .The cast bores are not as good at cooling as the KR1s with its nickoseal plated bores but are much cheeper to rebore when things go wrong .
Check that you get the following they all cost money afterwards
Engine complete with magnito and generator ,Coil ,Radiator or use a mini one ,carbs ,powervalve servo , powervalve controller ,Regulator rectifier ,battery may be better to buy new ,loom optional ,rev counter optional but nice ,temp guage optional but nice.Pipes get TZR not TDR pipes even if you get a TDR engine . Dented ones will be ok for the hovercraft but may have no value to a bike breaker .The less dented the better .two of the same hand will do .
Try Partfinder in our suppliers list ring em and they fax 100 breakers in the uk (dont take the first one to ring back, write down the prices and wait a few days ,Check for postage costs and VAT they usually exclude it .All take credit cards so you have some form of gaurentee)
Strip down and split the engine and gearbox ,be sure to keep the nuts bolts etc. marked up ,watch out for the copper washers 4 no. on the head .Gearbox and clutch housings are full of oil this drains out overnight if you remove the bolts even if you don't split the case .Be warned don't leave it on the lounge carpet overnight .
Remove the magneto with a puller from any motorcycle shop ,(M & P accessories £5.00 )
its a left-hand thread with a bolt in the middle .Don't loose the key on the shaft and the nut is an odd Japanese thread .
If you can manage it pull off the clutch if not there is a metal check strap which bolts to both halves of the housing , if not - if you crack the case and work the halves apart the strap will break with a bit of fatigue .
Remove the head and barrels ,base nuts can be difficult to get to on the inside .
Remove the shafts from the gearbox and discard them
Remove the crank take care not to catch the pistons and don't loose the main bearing retaining shims in the groves around the bearings
The water system is sealed by 2 rubber washers where it passes between the halves ,keep them safe .
Clean everything and inspect it, replace the crank oil seals and check the bearings .clean any debris from the oil feed holes into the bearings .Check the bores and the piston rings .They go a specific way up so take care if you remove them .
There are two main options cut off the gearbox and reduce weight and bulk ,the down side to this is that you loose the original mounting points
Or remove the cogs and shafts from the gearbox but leave the case in place so retaining the mountings ,a bit more weight and bulk.
I have only seen one craft which uses the drive via the gearbox this was a french craft with a counter rotating fan setup and he needed the clutch disengaged to get it started .Four strokes usually retain the gearbox drive as the two are not easily split .In a two stroke it is no problem yam RDs TZR TDR YPVS Kaw KR1S all run fine as a crank drive only .The suz RGV takes oil into a crank bearing from the gearbox and therefore has not been used with much success .
Its a matter of choice our section on The Cometition in F3 gives an indication of what others use .
If you cut the gearbox off mounting is onto the 8 crankcase studds and the original front mounting bush .some use all these others use the four outer crankcase studds only .
There are two Methods
1 Direct drive with a pully on the crankshaft and the engine mounted rigidly onto the frame usually the belt is tensioned by adjusting the engine downward ,this is the lightest and easiest method but has the down side of placing a side load on the crank bearings and introducing fatigue into the frame .
The fatigue should not be underestimated the engine resonates at 10,000 rpm the fan resonates at 3000 rpm ,somewhere within the frame these meet .This effect will cut through a steel frame in minutes as if it was made of butter .Last season I made a rigid frame solid and triangulated with tubular ties from the uprights down to the base tubes .The ties broke clean in half after one meeting .I adapted the ties to a bolted setup which had some give in it and built in some rubber bushes .So far so good .This is why the flimsy battered old frames last ,and are seen on the winning craft .Make it too rigid and it WILL break and all frames will break at some time .Two seasons ago Ed Hurn Broke his frame on the last junior race of the season Last year Luke did exactly the same thing .Fatigue ! dont forget it !
Pully Option 1 buy a ready made pully from K & M Products ,I saw a number of these fail last season as the back flange came off .I understand Ken Rigley was made aware of this and may have improved the design .If not a few bolbs of weld on the back flange would be a good idea
Option 2 make your own .Purchase a blank pilot bore pully from BSL ,Steel 16 tooth H section 1/2 " pitch .2" wide ,Remove the nut and two cogs from the end of the crank .Bore a hole through the pully to the same diameter as the outer cog ID ,then open up the bore to take the nut and a socket inside the pully leave the back wall of the bore to the same thickness as the outer pully .You should now be able to bolt on the inner cog then the pully should replace the outer cog and the nut sits within the pully .If all fits ok remove the pully and inner cog ,Grind the teeth off the inner cog and clean it up in the lathe .Machine a good chamfer on the outer corner of the now round cog as a weld prep .Now machine a mandrel identical to the end of the crank but without the keyway.Clamp the inner cog and the pully onto the mandrel and weld the cog to the pully in short sections until you have a continuous weld all the way round remove the mandrel clean up the weld in the lathe and the job is completed .You have a keyed 16 tooth pully for the end of the crank cost about £15 with BSL trade discount. as against £50 from K&M I checked mine with the dial guage and it is about two thou. out which i put down to weld distortion arround the keyway .On the belt drive two thousands of an inch is nothing to worry about.