GP125 Seat

To be perfectly honest I had completely given up on this frame. With the engine in pieces and being worked on for my RS125/GP125 swap, this bike no longer has a heart.

I’ve always loved the simplicity of classic flat top frame bikes and after my usual internet trawl of delicious bikes this week, things started to brew again. I cut the end off the frame to shorten up the working area and started playing with a template for a steel seat. With no real intention to build something permanent I got to this point and I’m wondering if I should progress with it, I kind of like it. It’s not perfect as I didn’t intend it to be used and I don’t have any real sheet metal working tools (folder, band saw, roller etc) so I had to be a bit creative with some thick steel and some g-clamps.

I do have a gripe on the GP125s styling. The tank side profile is fairly nice on the eye but it’s chubby. If I was really keen/wreckless I’d cut about 80mm out of it’s width and follow suit with the tail piece but… I’m not to fond of the idea of cutting and welding a fuel tank.

On the RS/GP front, I have ordered a small dentist drill and some carbine tip burrs to port the 2 stroke cylinder. Otherwise, I’m just waiting really.

RSGP125 BUILD3

Suzuki GP125 engine strip down

Here are some more internal photos of the GP engine as I pull it down. I ended up drilling most of the phillips head bolts which hold the casings together as they were corroded down the thread. What a stupid idea it is to use a phillips head, honestly.

This last photo is of the tiny GP gearbox assembly. I wanted to show it to give you an idea on how a motorcycle gearbox works (although similar to a car) in translating the up and down movement of the gear changer to a change in gear. When you push up or down on the changer, a mechanism spins the grooved barrel in the top of the photos. In those grooves are the gear selectors which move left and right depending on the groove. The selectors move the gears on the input and output shafts which engage with each other to drive different combinations of gears and therefore ratios.

ZARCO DERBI 125

A thing of true beauty

RSGP125 BUILD2

I swapped the Honda NX4 RS125 tailpiece from my 4 stroke onto its rightful home on the RS125 chassis.

A few NX4 RS125 expansion chambers also turned up on my door this morning, thanks Jesse! I know they’re not matched perfectly to what I will be doing to the GP engine but Rob has shown that they work well enough to get some great power figures from them. It’s also easier to use an RS one to get it going and rideable instead of stuffing around building one that is likely to be crap. I do plan to build a few in the future though…

RSGP125 BUILD1

I spent some time at Tim’s (a fellow F4 racer) place on Anzac day talking crap about bikes and figuring out how we can get some economies of scale going on with our GP125 builds. Being the good bastard he is, he gave me some… ummmm… used GP125 cases and a test bed GP100 cylinder for some time on the grinder before jumping into mine. THANKS TIM!
I started undressing her the other night and took some photos along the way:

Factory Suzuki GP125 crank case as provided by Tim

Tim’s degree wheel for port timing

Port side

Factory timing marks

Starboard side

UPPING THE GAME – ENTER RS125

This happened about two weeks ago and I can’t get it off my mind. I dream of building and racing motorcycles (hopefully for a living one day) and this is just the first step forward in many that are required. It is partly why I’m selling of my Datsun gear. This is a NF4 Honda RS125, a GP bike of the days when 2 stroke 125cc bikes were raced on the world stage. I’m unsure about the exact year of this bike but it’ll be approx the mid 1990’s.

The Honda RS125 was a bike that could be purchased directly from Honda by any club racer or enthusiast, they’re the peoples racer of sorts. The later model variants have a revised rear suspension system and a thicker main frame (basically overall better), but that does not detract from how awesome these little bikes are.

The plan: take the engine from my Suzuki GP125 and with some guidance from Team ESE and Tim modify/tune it to produce a chubby power curve (approx 20hp would be fine) to tackle the F4 kart tracks. Then I will squeeze it into the tiny frame and learn to ride a real race bike. I am not planning to sell my Suzuki FXR150, rather build a new exhaust and keep it serviced as a backup if the unthinkable should happen and the ‘stink wheel’ (as they call them) fails. Last week I bought all new fasteners for the frame as the original ones were looking pretty shot. I also woke up to a fairly new set of Dunlop slicks at my door. There are many things I still need to get for this bike but I’m working through them all slowly. Here’s to the beginning of a life of 2 strokes!

JANUARY 2013 MT WELLINGTON

Tim on a borrowed RS125 frame with GP125 engine

I spent my Saturday and Sunday at Mt Wellington for a practice round and a non points race day. Having not raced since November, I was eager to get as much track time as I could handle which consequently ended in my body now being wrecked!

It was a really good weekend of bikes with clear skies and a breeze to keep us slightly cooled. I let a few of my mates have a go on my FXR which should hopefully kick them into gear and mean they’ll soon be out there. During the Saturday practice I made the most of my opportunities to follow Brian and others where I could in the hopes their riding ability may rub off on myself. Brian gave me a few pointers on braking which helped hugely and lead to setting a new PB of 32.61s anticlockwise. On Sunday I improved on this with a 32.20s anticlockwise and I’m eager to keep the ball rolling into the 31s bracket! Thanks guys, I appreciate the hell out of any guidance I can get.

Races run:
5 lap sprint anticlockwise with no grid order (first on the grid gets pole) then straight after returning to the pits, back out for a 5 lap clockwise sprint
10 lap race around ‘the ring’ (midfield coned off) anticlockwise then straight after, clockwise 10 laps. Again no grid placing.
Borrowed bike race – anyone can enter as long as it’s not your bike – 10 laps clockwise then 10 laps anticlockwise around the ring.

I swapped bikes with Rick’s son Will who rides a CBR150 with cams, exhaust, carb, which was really fun! It had much better throttle response and seemed as fast as my FXR (which has more work done to it) although I need to work on the jetting of my bike still. The CBR150 hasn’t been on the dyno before but I’d love to over lap the curves between bikes for a comparison.

Thanks to all that helped out and organised the day, great fun!

Tim on his own Aprilia RS50

Cully’s FZR250 frame with GP125 engine

Will on the CBR150

and again hustling with John in the midfield

Andrew slipping out of first position during an anticlockwise ring race

More to come

GP BARS

I made some clip on bars this week after work for the GP125. I’ve very stoked to have them at a more suitable height! Now I just have to cut off the front and rear foot pegs and make some new ones to correct the seating position.

The bars are currently at quite a shallow angle due to the bulk of the factory tank. I’m not about to chop and weld it but man I wish I could! I’m toying with the idea of making one from scratch in the search for less mass and better fit. I suppose I’ll see how I get on.

The above are two reference images including the bars. Note the frame shape, I’m hoping to change that…