Anyone here doing trick stuff to XR 75's

[vmxrider]

Spurred on by the seemingly fictitious XR 75 world championship    I'm having a go at putting together a world beater engine . on a budget.
So for have sleeved the cylinder (SL100 sleeve) and bored to 54mm using a chinese piston. Bored the cases for the bigger sleeve.
Stroked the crank to 51mm. Waiting on new rod kit before pressing together. Have a manifold for 22mm carb. Will be fitting an oil cooler from some chinese bike, and have a Tighe cam I had made years ago.
Anyone else here doing this sort of stuff? any tips suggestions, experience.
 
     

[Hardex]

I think a good pipe is a must and this guy sure has a pipe that might have the goods and i bet it sounds sweet

http://www.ebay.com.au/itm/Honda-1973-XR75-K0-Fully-Restored-Rare-/250841152968?pt=AU_Motorcycles&hash=item3a674c3dc8

[vmxrider]

Got a pipe ready to go, bought it off one of those XR mad Victorians 10 years or so ago when we joined them a few times for some vintage minibike GP races. 
Thats a nice looking bike on ebay.

[Hardex]

Also suggest needle roller bearings in the cam tower and if your real keen do the same on the rockers.A wild cam is a must with Bigger valves and don't forget you will need a CDi .Wont be cheap but man it will hammer.

[BETRIC]

It,s a waste of time having bigger valves in a XR75, test have been done and it has been found the standard valves produce just as mush HP, it helps to have a good port job, not much room in the head anyway for bigger valves on an XR75, for a neeedle bearing cam shaft you have to use a billet cam that has been machined on the ends to suit the bearings in the cam tower, Tige makes them but big $$$$, use a S/H lockhart oil cooler as used on a Harly Sportster, that what they used back in the day, I use PVL CDI's on my XR75's, should have my Monoshock XR75 finished soon, spent nearly $8k on this one then I have 2 more trick XR75's to complete.  

[vmxrider]

Have heard that 5 speeders and 80's have a slightly bigger intake valve, which may help a bit on a 4 speeder with the big bore mods and bigger carb.
In 2 minds whether to space the cylinder for the extra stroke, which presents issues with stud lengths and cam chain, or go with a shrunk rod. Made a fixture the other day and experimented with a press and some heat was able to shrink an old rod with some success. It all being done an a budget, lots more time than money so far, enjoyable stuff.

[smed]

As far as I am aware The valves are all the same diameter for 4 speed,5speed, XR80 & 100   

[smed]

Have a look at www.vintagefactory.com             there is an xr section on there with scanned hot up articles from back in the day,you can even print them off,The powroll website is also worth looking at,they roughly explain how they do stroker cranks,Back in the day they would take the 4spd's out to 116 or 118cc, Megacycle cams i think it is list big valves for xr's

[vmxrider]

Here's a work in progress photo.
http://s1084.photobucket.com/albums/j413/vmxrider/?action=view&current=IMGP4892.jpg

[Swiss]

vmxrider, be careful doing shortened rods for stroker cranks.  Years ago when Powroll started doing cranks they used electrical current to heat the area of the rod that they wanted shortened.  Paul Olmstead saw and article about the Bill Bell Honda bikes and in the article they mentioned heating the rods with a torch and squashing them.  Paul told me that the Honda rods outgassed Cyanide gas when heated with a torch and it was a dangerous way to do it.

Swiss

[vmxrider]

Thanks Swiss, will check into that, although only I'm only doing one as a trial and one to use (hopefully). Must be something in the copper coating to mask the areas of the rod that dont need to be case hardened that contains the dangerous stuff.
Powroll would, I imagine, use induction to heat their rods which would be a much more precise and controlled process. 
I've read some articles on the Bill Bell Hondas on XLINT or OLD RICE and some of the stuff you have done Swiss is also very interesting.
My other project is a '74 XL 250 with a Yamaha 350 big bear sleeve and piston.
   
 

 

[Swiss]

Different topic but what is the bore on the Yamaha piston?

Swiss

[vmxrider]

83mm, pin dia. same as XL and deck height very close

[Swiss]

The TT350 Yamaha also used a 19mm piston with a deck height of .9825"/24.95mm from pin center and an 86mm bore.  The DR350 Suzuki used a 19mm pin also but don't have the deck height for it, 79mm bore if I remember correctly.  In face the early DR350 was a virtual copy spec wise for the old Honda XL350.  Had to dig out some old notes!

Swiss

Sounds like a good project beyond your XR75.  I have an XR75 that needs cleaning up and freshening for the next generation of rider in the family.  Taught my now 24 year old son how to ride on it when he was about 5 and could only touch the ground with his toes.  It is one of the early big bore engines at just about 100cc with cam/porting etc...  Of course the new bikes have better suspension, less weight and run like there is someone chasing them!  HA!HA!