GSXR250R (GJ73A) Race Bike Build

Been slaving away all day today and after heaps of trimming and sanding with the multi tool, the fairings now fit nicely. My neighbour helped me heat up the VJ21 fairing stay with a propane torch and we managed to bend the ends around so they fitted the VJ22 upper fairing and I was able to drill some holes and secure the top fairing. It will need one more center support bracket and although the fairing is now perfectly positioned it is still held on with masking tape so I need to fabricate some mounts before the job can be scratched off the list. The best bit is the fairing center piece fits perfectly after a bit of trimming, very surprising as it was made for a V-twin 2 stroke not an inline 4 4-stroke! The fairing does hug the frame close but the tank is so narrow there is still enough room to keep legs out of the wind blast.

I also mounted the seat unit properly. Had to remove the vinyl-covered foam, trim the front of the base a bit and made a new seat pad out of sticky back high density foam. The tail does sit a bit lower at the back now which doesn't flow so well with the rest of the bike's lines and there is a bit of a gap around the tank mount. But at least its solidly mounted and feels good. It will have to do for now but getting a proper GJ73a tail is definitely higher up the priority list.

I took the bike for another ride and the jetting definitely feels a bit off on the top end but I'm not sure if rich or lean. I'll have to do some research on the subject to see if I can figure it out. The midrange throttle response is awesome, its snappy like no other carb'd bike I've ridden. Almost as good as fuel injection, instant torque as soon as you crack it open. I wasn't expecting a 4cyl 250 to feel torquey at all! Bodes well if I can get the mainjet sorted... Still blowing a bit of white smoke, starting to look like the valve stem seals mich have hardened. Hopefully a couple more heat cycles might soften them back up a bit.

I've really been on a roll lately, so stay tuned for more progress!

 

OK been a while since an update, just been so busy with getting this bike finished, my 600RR prepped for the resumption or the race season, not to mention my work being mental! The good news is the bike has now been on track. The bad news is it was only a very small amount of time, none of it with the bike running very well

I pretty much got all the race-prep done, inc the lock wiring and catch tanks. The only thing I haven't done to make it MA legal is an enclosed bellypan, but this will be OK for club racing. My first run on track the bike was running OK, but would not accept full throttle above 10k, it just bogged and stumbled. After 2 laps I started loosing power badly so I came back in. We pulled the airbox and carbs out (the first of many, many times...) to check the float bowls were all full and decided to try richer jets. Floats checked out OK, and next session the bike just would not start. After a couple of bump start attempts it roared into life and off I went. Same problem, bike ran OKish for a lap then just lost all power. So back to the pits for another teardown of the carbs and inspection. Long story short, it turned out to be a dodgy fuel filter In between issues with sticking carb slides and float valve needles, we tired two stages of richer-than-stock jets and the problem got progressively worse, so it was decided to put everything back to stock and take the air screws out to two turns as the bike still would not start without being bumped. In a frantic race against time I got 4 laps in right at the end of the day, with the bike running ok, but I was unable to use full throttle or it would just bog down.

With time so short until the big race I have decided to fork out the dosh to get the bike set up on a dyno. The local operator is a very knowledgeable mechanic and has already tested my coils which were fine and advised using a new set of plugs, taking another turn out on the air screw and go from there. With the help of a friend I have also gutted the stock exhaust and made a straight thru 'silencer' with looks like it will be the business. If nothing else at least it will now sound like a proper race bike! The only good news is the bike did feel great in the corners, although I wasn't able to attack them with much speed. Will post up results of the dyno session next week.

Radiator catch tank installed

Installed an hour meter to keep track of maintenance

New home-made muffler core

Did some graphics to entertain the people stuck behind me on track!

My bikes at the track.

The Honda had been under a blanket for 3 months, all I did was take it out there and put fuel in it, it ran like clockwork. Days like I had with the GSXR sure make you appreciate modern bikes and fuel injection!

Oh and the Honda doesn't look so pretty now, I lowsided it at Symmons Plains last weekend. More work to do GRRRR!!!!

 

The results from the dyno are in... The bike was running horribly rich and only making 28hp! It wouldn't even pull to redline at full throttle which is exactly what happened at the track. The issues were numerous which we worked through one by one. The operator was really awesome and let me spend the day there doing all the work of removing and adjusting the carbs which kept the cost down heaps for me I learnt SOOOO much too, there is waay more to jetting a carb properly that simply adjusting pilots, needles and main jets. The dyno sheet below shows we ended up at near 37hp which is what I was hoping for. Its not perfect but that is competitive HP as its the same as what one of my rival's ZXR250 made and only 2-3hp off the CBR250RR's that were tuned on the same dyno.


Here's some of the things we did to fix it; O rings to seal the diaphragms better, drilled one of slides as it was slightly different to the others, lowered float height, removed some corrosion from the float needle passages, removed the air filter, took the intake flange off the filter and fitted just that, 100 main jets, these were too lean so went to 102 outer and 105 inner, changed needle clip position to richest setting. That got it pretty close. There is still a lean spot at about 9k RPM but I should be above this most of the time at the track. More of a concern is the wobble in AFR curve at the top end making the HP fluctuate a bit. This happened on every run right from the start so difficult to say what is affecting it but it's probably not jetting. Mechanic thinks it could be a harmonic issue with the exhaust or possibly the cam timing being slightly out. I've got an Across exhaust header which has a longer mid section before the pipes go 4 into 1 so will eventually get this modified to fit and try it. Race day is now only 5 days away and feeling much more positive about things now...

 

Well I'm pleased to report my first race weekend was a huge success!

Apart from having some fuel starvation issues in qualifying I managed to get a decent lap in before it struck to get 7th on the grid out of 20 riders. This was a historics only field but had everything from a Honda RS125 up to a GSXR1100 so I was pretty pleased considering it was my first proper ride on the bike. The fuel issue isn't fully resolved, I have to run the tank nearly full or risk it starving and bogging down, something that will have to be sorted in time. But the dyno work was worth every cent, every single HP matters when you have so little to begin with!

It was tough getting the bike off the line cleanly, the clutch is very grabby making a smooth launch very hard, I managed decent starts in our 5 races considering but the big bikes swamped me every time at the start. Luckily the lil bike is an absolute demon in the corners and on the brakes, so I was able to make some nice passes in all of the races. I probably could have battled harder at times but as it was basically a for-fun meeting I didn't want to get too aggressive, sure is frustrating getting blasted down the straight then held up in the corners tho! I finished with I think 6-7-7-8-7 results for the weekend and was 2nd in the up to 500cc class, beating a few 400's and only bettered by the 125GP bike. I managed to record a couple of the races with my el-cheapo eBay camera, WARNING though the volume on these is really loud so turn down the speakers before hitting play!





Will be back out on track this weekend at our final inter-club round, doing double duty on the 600 and the 250. Just hope my body can survive the punishment who knows I might even have to start fitness training if I keep this up

 

Thanks guys I've taken the bike out a few more times now and it's still going great. Our ride day last weekend was my first chance to go up against the MC22 Honda that made 40hp on the same dyno (mine made 37). We had an absolute blast chasing each other around, us being similar size and weight made things pretty even. The CBR is slightly quicker in a straight line as expected, but not by much. I can hang in the slipstream down the straight but not gain on him, he can do the same and pull out and pass, so a couple more HP would be nice. The GSXR initially gets off the turns a bit quicker, then the CBR takes the advantage back as the revs get right up into the top end. Braking was about even, the limiting factor on both bikes being too soft fork springs we concurred.

The bike will be getting an upgrade soon, I'm selling my `04 CBR600RR and swapping the standard shock back into it and keeping my WP 4618 for the GSXR. I'm going to have to do something about the forks too. I have a spare set of 600RR forks that I will hang onto too and see if the cartridges can be made to fit the RGV fork legs. If not some Racetech cartridge emulators will be the go I guess. Stiffer springs will definitely be on the shopping list too once I have this sorted. The bike is so much fun to ride hard, but it feels a bit sketchy over some of the bumps, improved suspension will definitely help with my confidence to really see what this thing can do

 

Just the standard RGV springs which are linear. I've heard the emulators work well but would rather try and use what I have rather than buy new stuff. The CBR forks are a lot longer so the cartridges will need to be shortened, plus I'm not sure how the compression damping will work or if I can get an adjuster mounted in the RGV lowers. I have a friend who's a highly skilled machinist so hopefully it can work, if so it should make a massive improvement, and be fully adjustable.