GSXR250R (GJ73A) Race Bike Build

Well the CBR is now sold and the WP shock has found its way into the GSXR. It wasn't a straightforward swap though, there was a bit of cutting and grinding involved as the photos show.
Had to remove a small amout of material from the mount to clear the body of the shock.
Also had to enlarge the hole to clear the preload adjuster. I didn't have to remove a massive amount and its only a thin plate so not worried at all about weakening it. I fitted a 105nm spring as the 110nm was already a bit on the stiff side on the 600. Really looking forward to testing this out in a couple weeks. No progress has been made with the forks yet, I really need to pull one apart and see if my spare cartridges have any sort of chance to fit but it's not a job I've been looking forward to so its on the back burner. I know this shock is really gunna highlight the harshness of the front fork so something's gunna have to be done there soon I know.

 

Back again after a pretty successful but somewhat frustrating day at the track. Since installng the shock and getting the bike back together I realised something must be done about the soft front end sooner rather than later. I haven't had that many bikes over the years but realised I actually had a few different sets of forks and springs kicking around, so I rummaged all about the workshop and was really bummed when a set of ZXR750 springs which looked perfect were actually .5mm too big a diameter to fit. Then I came across the stock springs out of my ER6n. These are 1.2kg/mm, which is just crazy stiff, gawd knows what Kawasaki were thinking putting such heavy springs on a road bike. But size wise they were very close to the RGV, and then I had a brain wave... I ran some calculations on the stock springs with the most basic online spring rate calculator I could find which gave a rating of .4kg/mm. The Race Tech online spring rate recommendor for the `89 RGV gave the optimal rate for racing as around .7 (can't remember exactly). As the GSXR is a bit heavier and I do like to nail the brakes hard and late I figured .8kg/mm would be about right and 1 RGV spring + 1 ER6n spring would give me exactly that! I was a little concerned that this might put some lateral stress on the axle and triple clamps etc. so I did some internet research. Seems its fairly common for racers to use say a .9 and .95 combo to get it to thier liking, but no first hand accounts from anyone doing what I was trying (massively different rates). So I thought stuff it, we'll give it a go and just take it easy to begin with...

And it worked great! The weather was perfect and I could feel the difference in the front end as soon as I rolled into the first corner. I stuck to the plan and took it easy for a few laps, just testing the brakes harder and harder, feeling for any wobbles or shimmies. But it felt fine and the excessive brake dive was a thing of the past. I mainly focussed on the two corners where this was causing problems previously, trying different lines and braking points to see how it responded. It all felt good so I tried for a couple quick laps late in the session and through the bumpiest corners I could feel that the extra spring had thrown the rebound out. With 10w oil it was way too fast and the front was skipping over the bumps on corner exit and more worringly past the apex on one of the more front-endy corners. Not a nice feeling having the front move as any rider will know, at probably 8/10ths pace I got a good warning not to push the limit here, this will be a problem when racing I thought straight away. The compression damping however was fine, on corner entry with the suspension now in the proper part of the stroke the bike was well balanced entering every corner and for the first time I was getting feedback somewhere near what my 600 gives, I was happy happy! But the joy was short lived as I chucked it hard into turn 1 next lap. The springs had done nothing to help the high speed compression being harsh, the pounding over square edge bumps was the same as before. There is a huge bump on the apex of the fast turn 1 (which I have not really enjoyed from day 1 with this bike), I couldn't just pound over it like I do on the 600. Now not as scary as before due to the better chassis balance, but it demanded the same respect as before and I didn't feel much if any time could be gained here. So I headed back to the pits, an emotional first session for sure lol but there was a grin under my helmet when it came off. Its only now I'm being more analytical and therefore focussing more on the problems rather than the fact I had fun. Remember, my yardstick to measure to is a well set up modern 600, I want this bike to exceed its cornering speed. And its already pretty close...

So I sat in the pits and mentally debriefed myself afterwards and realised the problem with the fork is actually a fairly major one. On a modern cartridge system this could be fixed by probably half a turn of a screwdriver on each adjuster. With my basic 80's technology, changing oil weight is the only adjustment to the damping I can make apart from drilling bigger holes, but I get the feeling this could and probably would go horribly wrong if I try. Not to mention a lot of time and effort stripping forks constatnly. I don't have the time or patience for that so I've decided to get serious on a cartridge conversion. Anyway more on that later, but lets just say I have already set things in motion to make this happen. I set the laptimer up and got pumped for the next session to try and set a PB...

But it wasn't to be. I missed the next session thanks to a spate of crashes, the first involving 4 bikes going down. There was also a seperate incident with my friend who I was pitted with. He bought my old 600RR and crashed in in his first session We ended up fixing the damage to get it rideable, but before he got a chance to take it out again, crashed his FZR250 under brakes on a cold tire! Poor bugga, he wasn't hurt but I felt so bad for him, the track can be a curel place sometimes... Then the wind picked up in the afternoon which really effects the little 250 and killed any chance of trying for a laptime as it was a headwind on the straight and I couldn't even hook 6th gear.

This was also the first time out with the WP 4618 shock and no complaints there apart from a little too much compression. I backed off the low speed adjuster 3 clicks which helped but the high speed comp needs backing off too. Due to the hectic day I forgot and left it too late do this before both my last two sessions, I was spewin the second time but the anger helped, as I thought if I went really fast it would make the shock work better lol. The amazing feedback this top quality shock gives should make it easy to sort next time anyway, such a great bit of kit I'm so glad I kept it! I had installed it with 3mm added ride height but its still running a little wide in the most important corner onto the straight. It is an improvement on before and there's no headshake so a couple more mm should sort it out.

For my last sesson I stopped analysing the bike so much and just tried to ride it hard and hopefully get at least one flyer in so I would have some good data to look at. But the blustery wind made it really difficult to be consistant and killed top speed so even with some mistake free laps I was about a second off my best times. I've never seen the wind blow so hard form that direction before, it was a direct headwind and was probably costing most of the 1 sec just on the straight. It might have given me a little push up the hill tho... In all I can be happy with the progress made as the laptimes did come easily considering the conditions and the bike is more definitely more fun and conficence inspiring. To be honest I've known well before this bike came to me that the damper rod fork design dictates damping will always will be a compromise, especially on the compresson side. I placed more faith than I should have in simply having the correct spring rate but one session was all I needed to see the light. Getting them to work properly for racing is a dead end route. I can't even be bothered trying heavier oil, as I know it will just fix one problem then create another one! I don't want to go the emulator route either as these seem expensve for what they are, very basic compared to a proper piston and shim stack. Plus you still need to muck around getting the oil right for rebound and no external adjustment would mean that you would simply never bother because its too much stuffing around on a race day. As a budget option I guess they should be considered as some people really rate them, I've read they work better for lighter riders but that may have been written by a fat bloke who wears winter gloves year round! Maybe I would be of a different opinion if I didn't have these cartridges hanging around. Making them fit is my top priority, will post back here with any progress I make good or bad. In the meanwhile I will be preaching my gospel of modern suspension to my CBR250 riding competitors to take this approach, or any approach to improve their forks (ZXR riders I'll let off because their's are half decent). If any FZR's turn up I'll give them a serve too. I want people to race with and I don't like their chances of keeping up running saggy old springs and dirty damper rods! They will be fighting a losing battle. Plus I don't want someone trying to go up the inside of me at turn one on the bump stops...!

Sorry I waffle on so much sometimes for those that made it to the end!

 

Thanks Simon, I'll be chasing some more HP in the future for sure and would love to know some of the specifics you did to your hot motor to get the power up. I have a spare Across engine and a Bandit head to bolt to it. I'm not really sure if I should hot this one right up or just bolt it together with cleaned up ports and focus my efforts fully tuning the original engine as it's got lower Ks and I know it goes well. Think I'd like to keep one motor close to stock so I don't have to stress about breaking both of em. The Across motor has done 40,000kms but the leakdown readings on it were quite good, slightly lower than the 8000kms stock motor but well within the green zone. (The Bandit head has done about 34000ks). So I guess new rings aren't necessary, but I dunno how it will hold up to the rigours of racing in the long term, with the price of head gaskets I don't want to pull it apart too often! Any tuning tips greatly appreciated, please PM me if you don't want it made public.

I've already spent too much so she is definitely a keeper but no regrets and the Island Classic in 2016 is definitely the target now. May as well go all out as I'm not going all that way to be blown away down the straight by the well sorted CBR's. There is weight to come off still, the steel spacers for the coils and the wheel spacers weigh a ton and a few steel brackets need to be made in alloy inc. the fairing stay. I've got exactly a year so will do this stuff bit by bit as the money comes in. Will be great to represent this awesome little Suzuki at the biggest historics meeting in Aus at the best race track in the world

 

Yep that's the plan, I just can't decide which one to hot up! With the price of head gaskets I am leaning towards leaving the original engine alone and focussing on the spare. At 40000ks I'm not sure if I should go the whole hog with new rings and maybe bottom end bearings, the leakdown numbers were fine though so maybe just a port and polish and compression raise and see how she goes... But more important things have been and are happening first!

The most awesomest news is I have successfully converted the rather rubbish RGV damper rod forks to use 2004 CBR600RR cartridge internals Once I had the forks apart it didn't take long to figure out it was going to work. The stroke of the cartridge was not an issue as the CBR is longer travel and the total fork length so similar that no mods to the cartridges were required at all. I had to use the RGV top out springs and while not a perfect fit where it comes into play at full extension it should work fine. The only other original parts inside there now are the washers for the springs.

There was maching required to the lower sliders in order to remove the damper rod guide and cut it down so the cartridge extended the same length as the original setup. This was tig welded back into place, the only other machining required was to enlarge the drain bolt holes and probably the most difficult job, turning the fork cap down and cutting a new thread to suit the 41mm diameter. Luckily my friend is an absolute master on the lathe and had the whole job done in a couple hours. I was worried about the compression being too stiff as with no adjustment in the fork lower they were effectively on max, but using 5w oil and pumping the cartridge by hand it felt OK. I ended up scoring some Race Tech .85 springs and cut my own spacers from PVC pipe. Base preload was an educated guess at 35mm but I cut some 45mm spacers too in case it's not enough. The beauty of these carts is its really easy to change these out at the track. No spring compressor is required as there is no tension on the spring once the fork cap is removed unlike most other cartridge designs. I'm racing this Sunday so seriously can't wait to try these out, its gunna be like a new bike I recon!

Another win was making my spare Across rear wheel fit. All I had to do was swap all the spacers and disc from an 18" RGV wheel (shave 1mm off center spacer) and fit new bearings. The wheel bolted in and lined up perfectly. I have a set of wets and a sprocket on order and will have them for round two at Symmons Plains next weekend.

Also started work making a proper bellypan. I won't have it done for round 1 but will need it to pass scrutineering at round 2. So far so good, but it will need a lot of filling and sanding before I can make the final piece. It's looking like we will have the biggest ever grid of Lites 125-300 class bikes this weekend and weather report is looking perfect. Will post back with race report of how I go and hopefully some good pics

 

Yep the bike is already a lightweight, but I'm not exactly anorexic so every gram will help. I haven't weighed it yet as my digital scales didn't work, I need to find some old fashioned ones to use...

So after last weekends racing, there is good and bad news. The good news is the suspension changes have absolutely transformed the bike's handling as suspected. It took a bit of fiddling with the adjusters but after two sessions I left it alone as it felt close to perfect. I was able to push the front end really hard with great feedback and the extra braking stability meant I was braking about 20m later than before at the the end of the straight. I had two good races, finishing 3rd and 4th out of field of 19 bikes. There was issues with the timing software so I haven't seen any laptimes yet but I don't think I went massively quicker. The big difference is the bike feels much safer at the limit so I can be more confident and consistent and making passes is so much easier. The clutch has loosened up a bit and I got it off the line really well both times, in race 1 I very nearly beat the 2 strokes into turn 1. Unfortunately there was a bad crash in the 3rd race with two riders colliding at the end of the straight which brought racing to a finish as one of them needed a ride to the hospital (broken collarbone it turned out). The bike had given trouble starting more so than usual, it was a warm day and we had to bump start it each time. The low speed fueling seemed worse too...

I thought it best to check the valve clearances before taking it to the high speed, hard braking Symmons Plains circuit as they hadn't been checked since before the bike was run and it now had about 9 hours run time. Turned out cylinder #2 had the same inlet valve as before close right up and leakdown tester revealed the adjacent inlet valve was leaking to about 38% (cold engine). I changed to the smallest shim available and there was still no clearance. So looks like I have one, maybe two knackered valves, there will be no racing for me this weekend

I will remove the cylinder head over the weekend and see what's going on. At least I have a spare head so I should be able to salvage a couple good valves. I've taking a gamble that an Across head gasket will fit as from pictures I could find it looked identical to a GJ73a gasket out of Japan and is less than half the price ordered from the dealer. At least now I have an excuse to start the performance mods, so Si I eagerly away your tuning tips!!!

 

Here is a video of race 2. Watch me get absolutely slaughtered by the Honda and Aprillia RS250's up the hill!


And here is a few pics of the bikes that were entered. It was a great turn out, a few first time racers and still a few work-in-progress bikes but nearly everyone had a great day. Looks like its gunna be a big year for 125/250/300's racing in Tassie! Would be great to see a few mainlanders come down and mix it up in our ranks too..!

 

Well it's not good news since my last update I have removed the head and barrels and I have two badly mushroomed inlet valves and the cylinder liner in #2 is worn horribly due to what must have been a big rust spot. There were some rust marks in other cylinders too. The pistons are also stuffed with 1 - 3 showing wear marks above the rings which could have been caused by the rust but also could have collapsed skirts. Either way I won't be using them again... My main hope lies in finding some oversize pistons and getting it bored out, otherwise I'll have to track down a second hand cylinder which I'd really rather try and avoid. Some good news is I have also stripped the top end of my spare Across motor and Bandit head which all appears to be in usable condition which gives me options and enough parts to get one engine going again. A real disappointment was the Across valves having a larger diameter stem so they are useless to me unless they can be turned down, but probably cheaper to find some GJ73 valves than that option.

On a more positive note I have received all the good info from Si on how to wring some more hp out of these engines so I will take my time and build 2 proper race motors. I have already started the cleaning process for the first cylinder head, it's a painstaking job but the next stage will be some porting work and lapping in all the valves. Lots of work and $$$ ahead but if I can get up to 50hp I'm sure it will be totally worth it..!

Rusted cylinder bore, where the lines are has resulted in an uneven surface
1 cylinder done, valve seats had a lot of carbon mashed on em but came up well
Exhaust ports look horrible, I'll be giving these the full polishing treatment to resist carbon buildup

 

Murdo, the valve stems are approx .3mm larger in diameter. Reaming out the guides might be an option, but I would rather stick with the standard size if possible. It's starting to look to me like the valves are the weakest link in these motors and I'm seriously considering a set of Chinese valves. I know Chinese parts and bikes have a bad reputation for quality, but that has been changing of late from reports I've seen. Their motor industry is growing at a rapid rate and quality control is a lot better than it was just a few years ago. Perhaps even the quality of the material has improved since the originals were made 25 years ago, these might have better durability than OEM. . A full set of valves + stem seals is about $100, I think it's worth a punt...

Anyway not a whole lot of progress to report yet, I've been too busy with work and other stuff to get much time in the workshop. Luckily with extra work has come extra cash to splash, so that has led to the purchase of...

Two spare bikes!!!

I took a trip to Melbourne and Bendigo to buy these bikes. Both were going cheap as they were non runners. The black bike is hard to start and has low compression on 2 cylinders apparently but is otherwise in amazing condition. It has a few scratches and blemishes in the paintwork but is all original and otherwise in near perfect condition. It is spotlessly clean, could have just be a good detailing but it looks like this bike has been babied and stored inside all it's life. There is barely any rust on the exhaust or anywhere else, not even a spec inside the tank! The plastic and rubber parts look like the day it was built, not even a scratch around the ignition barrell. All the fairing bolts are original and shiny, it even has the odd shaped original mirrors. It has obviously been cared for and stored inside all it's life, 17000ks on the clock which looks legit. The yellow one is also in very good cosmetic condition, but is a bit grubby and rusty, looks like it had been sitting in the owners carport for a couple years. It has 12000ks on the clock but came with receipts for engine work where the whole bottom end was replaced (from an Across) along with new cam chain and tensioner. Not sure what went wrong there but after spending all that he had trouble getting it to run and lost interest in iy. I suspect it just has carb troubles, they look pretty grubby. The whole bike needs a good going over to get it ridable but should be a great source of parts. The black one is way too nice to break up, it won't be hard to restore back to showroom condition if the engine isn't majorly stuffed and I could potentially more than double my money on it. It will probably be hard to part with it though, I'm pretty attached already!
This sticker is sooooo coooool!!!
And here is the obligatory 80's/90's acronym that every Japanese sportsbike had to have back then


Back to the race bike. The oversize piston set has arrived, plus I bought some bore gauges and took measurements from both cylinder blocks. The Across barrels are in excellent shape, no taper issues and well within spec, these will get a hone and the best set of used rings I have. I'm pretty confident the original barrels will bore out OK to 49.5mm, but these are still out at the machine shop. I've done a fair bit of polishing in the exhaust ports, these are coming up well now that I have figured out a custom tool for my dremel using wet and dry sandpaper and WD40. This is one polished at 400 grit, I will eventually take them out to 1200 then polish with Autosol.


My Dad is a big supporter of my racing (I think he enjoys it almost as much as I do!) and has kindly paid for a set of FCR carburetors. These little marvels of engineering cost the same as I paid for each of the spare bikes! But they will be the icing of the cake of the other engine tuning, with no butterfly valves to disrupt flow they should release a good bit of power, especially when slamming it on out of the corners. I had to order them for the FZR250 as this is the closest match spacing wise, but check the last photo you can see they need to be brought together a bit. This is gunna be a tricky job but I have already started and it will be possible for me to do it which is a massive relief! So many jobs to do now not sure what to do next or when I will get a chance. Hopefully some more good progress updates will be forthcoming soon...