So, after being out on her a couple of times I found that she was only hitting the powerband between about 60-80% then dropping off. This had to be rectified. After some research online, there were a couple of mods that could be done.
1. Drill out the airbox to get more air in
2. Substitute the airbox intake boot from a 2005 CR 125
3. Put in a V Force reed block
4. Machine a pin that stopped the exhaust valve from closing some of the way
5. Get it ported
6. Buy some exhaust spacers
7. Change the gearing
As I wanted to keep costs down and this is information that I want others to be able to use, this was the plan
1. Drill out 6 x 1/2 inch holes on each side of the airbox
2. Couldn’t get hold of a 2005 air intake
3. Install some Boyesen power reeds
4. Machine a 7mm stop pin
5. Clean up as many of the casting flaws that we could with the tools we had and polish up the exhaust ports
6. Make some exhaust spacers which were 5mm thickness(combined)
7. The gearing already had a 52T on back as opposed to standard 51T
After stripping her down, I came across some interesting things!
The standard exhaust valve stop pin was not even there and then when I took the valve rod out, half of the bearing was missing(Which was interesting as the engine had just been rebuilt by a shop in the South West(if you want to know what the shop is, you can email me) for the owner that I had bought the bike from. He had only ridden it for 1 hour since that rebuild!)
In these pics you can see the gunk on exhaust valves
This pic shows heavy scoring on valve
Here you can see casting flaws in the exhaust port
Looking at the side of the reed block, you can see where the reeds are open
Work carried out by Ace mechanic(Dad) and myself
Various stages of transfer port work. Just cleaning up the casting flaws as porting/tuning is a science in itself
Cleaning up of exhaust valves
Cleaning up and polishing of exhaust port and manifold
Here we have the exhaust valve stop pins
The top one is the original size, which was made from a 4mm drill bit
The one below that, was made from the shaft of an old clutch lever bolt. The diameter of the stop is 7mm
Here is the modded pin(7mm)in place
These are the Boyesen power reeds
There are two layers, the top one opens up at low to mid throttle and the lower one opens up after that
These are both sides of the airbox with 1/2 inch diameter holes
The last mod was some spacers that go between exhaust and exhaust manifold. You can make a 2mm and a 3mm spacer then double them up for 5mm. We had some material that was about 1.25mm thick, so we made 4 and put them all in.
Pics coming later
Power has a lot to do with temperatures in the exhaust.
Therefore I have a theory that the paint on the exhaust helps to retain or maybe add to temperatures within pipe
So, I stripped the paint off of mine(I think it was bare to start with, but someone sprayed it).
We were running the fuel ratio at 32:1, which I was told by Bernie Richards(my Uncle)is quite old school. “Hey Bernie, Im an old school guy”. So we changed the ratio to 40:1, then I dropped the needle jet clip down to 4th, from 3rd(make it richer)position just to be safe. We also changed from Silkolene(which is supposed to be quite smokey)to Castrols’ latest 2T performance oil and super unleaded petrol.
A bit too much headshake at speeds, so tightened up headset to compensate.
So The Verdict?
Wow, what a difference!
The usable powerband increased to between 45%-90% without exhaust spacers
Then when we dropped those in, it incresed the low end to about 40%
In some places I was able to use a higher gear and use torque of bike and also roll on the throttle on some corners to keep up momentum before flicking clutch for launch time.
All in all, great gains for about £52(reeds & gasket set)and labour time.
Until the next Bike Tech, happy DIYing