GSXR250R (GJ73A) Race Bike Build

Hi All,

Just found this great forum and thought it would be a good place to share my current project bike! I've lucked out and picked up a rare GJ73A which I'm going to fix up as a track / race project. Its in great condition with only 7600 Kms on the clock, only downside is it's missing the tank and side panels but I've already picked up a tank from the wreckers plus I already have a set of RGV fibreglass race fairings that I'm going to try and make fit. So far has only cost me $500 for the bike, $330 for the tank and $300 for the RGV I picked up a couple years ago (minus a motor) which will provide a few spares as its missing too many bits to be a viable re-build project. My aim is to keep the budget as low as possible, as my main form of fun is racing 600 Supersport which chews a large hole in my wallet! The aim with the 250 will be to get maximum track time at race meetings and hopefully teach me to ride the 600 faster, as I still think sometimes I ride it too much like a 1000! Anyway here we go, starting with the day I picked it up....

 

Didn't take long to get it stripped down. First task was to get the rocker cover off and check the valve clearances and condition of the camshafts. The bike was obviously stored outside for quite some time as the head was covered in dirt and grime. The plug holes were the worst, caked in crud and took much solvent and compressed air to clean them out before risking taking the plugs out. The plug in cyl 2 was badly rusted but came out OK. The cams are in perfect condition but 8 of the 16 valves needed adjustment. One of the intake valves on cyl 2 I couldn't even get my smallest feeler gauge into, but I guessed it correctly as all the replacement shims brought it back into spec on first attempt The radiator was also removed and flushed at this stage, but no rust at all came out. Very pleased, I have no reason to doubt the bikes Ks are anything but legit. I found an old spare set of spark plugs to put in which will do for now, and put the rest of the engine and cooling system back together.

 

Next job was to strip and inspect the carbs. Upon taking off the float bowls I was greeted with massive amounts of rust in there, I guess this explains what happened to the original tank. The bike doesn't have a compliance plate so has probably never been ridden outside of Japan. My guess is it sat for a long time as it wasn't a straightforward job to get it registered, water has sat in the bottom of the tank and at some point a previous owner has tried to get it running, sucked a heap of rust from the tank and that is the result. Anyway, I rushed in so fast I didn't take before photos, but here is one after all the float bowls and jets were cleaned.

Apart from the rust there was very little varnish and sludge in there so hopefully blowing carb cleaner and compressed air through all the jets and passages will be enough for it to run right.

 

Only big problem with the carbs was the choke slide on cyl2 was stuck fast. I tried pushing, pulling and twisting it loose along with doeses of WD40 but it wouldn't budge. I've circled cyl 3 in the photo as #2 isn't visible, but this is the culprit.
I had to remove carb #1 from the bank in order to get at it properly to try and loosen it. I am always a bit nervous separating carb banks as there are little seals, springs and connector parts that can be tricky to spot. I tried to be extra careful but sure enough the spring on the throttle adjuster link shot off down the back of the workbench and became a massive operation of removing tools and parts which took the better part of an hour before I finally found the little sucker! :-/

But once access to the slide was good, a bit more WD40, a sharp hit with a hammer and it was loose again. Gave it a light sand with 800 wet and dry and was sliding like new again. Reassembling the carbs went well, one of the seals on the fuel feed pipe was badly deteriorated but I smeared it all over with RTV and will keep an eye out for any leaks once running. Next the slides and diaphragms were cleaned and re-installed. Carbs back on the bike, throttle and choke cables lubed and it was time to try and fire the little beastie up for the first time...

 

After refitting and filling the cooling system, I put some fresh oil in and rigged up a car battery. It took a fair bit of cranking, but the bike is going pretty well for a first run...



After warming up I set about balancing the carbs.


The position of the adjusters is a bit tricky on this bike so I pre-placed a screwdriver on each adjuster.


Balancing was not the hardest bike I have done, but I did find matching the two pairs a bit tricky as it seemed to greatly effect the idle speed with only small adjustments of the center screw. I got it pretty close, it will need doing again once the bike has had a proper run and I will need to finally invest in a workshop tachometer as this bike doesn't start reading until 3k lol

After the carbs were done I performed a leakdown test, which provided excellent results, again backing up the low Ks this bike has done. Cylinders 1,3 & 4 were perfect with virtually no leakage. Cylinder 2 was leaking a bit past the rings but was still well within the acceptable zone. Again I will re-check this after the bikes first ride. More updates to come as I continue with the project...!

 

Thanks for the warm welcome guys and thanks for the tips on the carb seals. I have found the Litetek seal kit on their website and it looks impressive with some great info on installation / setup. Seeing as the bike is 25 years old I think I will go ahead and order the kit and do the job properly. From what I have heard viton is far superior to the nitride used in old seals so it will be a good thing to do.

I live and race in Tasmania, Australia. We have two racetracks here, Symmons Plains (a real horsepower circuit) and Baskerville which is a real riders circuit with a great variety of corners and elevation changes. We have an up to 250cc class here and there are a few 4cyl 250's that race against a couple of Honda RS125's, Aprilia RS250's and a modded NSR150, so a good mix and I can't wait to get out there and dice with them!

I found the GJ73A wiring diagram on this site and a workmate of mine who is studying Japanese has translated it for me. I will photoshop it into English and put it up in the next few days. It would be great to get a hold of the workshop manual as he could probably translate bits of that for us all too. I wouldn't ask him to do the whole thing though might be stretching the friendship a bit lol

I didn't take compression readings, the leakdown tester works differently in that you put about 100psi into the cylinder and measure the amount of air that escapes. The benefit of this over compression testing is you know exactly where the air is escaping from i.e. from the valves or past the piston rings. It can also diagnose a blown head gasket by sending air bubbles out the radiator cap, a handy tool to have with race bikes as you can measure engine wear periodically and fairly easily.

The only other work I have done to my bike so far is fit a spare rear shock I had from my `04 600RR. As luck would have it it's exactly the same length and a perfect fit!

I had to order a standard 600RR lower shock bolt as the original bolt is slightly too short with the new shock's wider alloy clevis. The reservoir location means I will have to relocate the igniter box but no major drama as I plan on stripping all unnecessary wiring from the harness and moving things around anyway.