GPX250 resurrection

[Midnight_tinkerer]

Hi everyone,
I was given a GPX250 by a mate who realised he did not have the time that was needed to restore it. It was in very poor condition, but unfortunately I do not have any photos of it before I started. I also bought a ZZR250 to use as spare parts. I plan to get the GPX back into running rideable registered condition, and perhaps do a few mods along the way.

This is a picture of how it looks at the moment after some painting. I've included a photo of the original fuel tank and the ZZR seat to give you an idea of what condition it was in when I got it.

Some of What's been done so far;
- Removed all original stickers.
- Polished or painted all plastics so that they were matching in colour. Some had been painted matt black. Some were stock gloss black with faded stickers. Original fuel tank was badly dented, rusty inside, and at some point someone had fitted a fairing mount bolt which was too long and it had punched a hole into the tank. Found another second hand one on ebay real cheap, which was straight with no rust or holes, but had the paint stripped off it. This was fine with me as I was repainting it anyway.
- Had seat recovered.
- Was missing key, so ordered a blank one from china for $2 and got a locksmith to cut it to fit ignition.

[Midnight_tinkerer]

Here is the ZZR that I bought to use as spares. Have already robbed the chain guard, gear lever & linkage assembly, and CDI box, to use on the GPX. The CDI box on the GPX had been pulled of its mounting tangs and was left hanging down by the swingarm when I got it. The bike had then been left out in the weather, so the CDI box had then filled up with water. Tried pulling apart the CDI and cleaning & drying it out. But when I stuck a battery in it & cranked it over it still had no spark. Plugged the ZZR CDI in and it now has good spark.

[Midnight_tinkerer]

Here is the front wheel after painting & polishing. I have a friend who is doing the paintwork for me, as he is better at that sort of stuff than what I am, and he is only charging me for materials, so can't complain. It was originally purple. I can do all the rest of the mechanical work no problems. The original disc was worn under the minimum thickness so I ordered a Metal Gear disc & pads. The disc mounting bolts were rusty, so I buffed them on a wire wheel & painted them with high temp engine paint. Turned out ok. The wheel bearngs still seem ok, so I'm not sure whether I want to refit them, or replace with some new low friction bearings. Fitted a new Pirelli MT75 tyre.

[Midnight_tinkerer]

Look what arrived in the post today. The original forks were bent and were creased just below the lower mounts, and had a little rust pitting. I could have straightened the old forks in a press, and sanded the pitting smooth. But they would still have the crease & pitting which bugged me. So because I got the bike for nothing, I thought I could afford to spend a bit on some new forks. While I was at it, I thought I may as well make them better than stock so I also ordered some race tech fork emulators. I have buffed all the paint off the lower legs, but they still need some more preparation before they are ready for painting.

[csmith12]

Nice!

[Motofool]

Originally Posted by Midnight_tinkerer

Hi everyone,
I was given a GPX250 by a mate who realised he did not have the time that was needed to restore it. It was in very poor condition, but unfortunately I do not have any photos of it before I started. I also bought a ZZR250 to use as spare parts. I plan to get the GPX back into running rideable registered condition, and perhaps do a few mods along the way............

Excellent project, Glenn

It hurts looking at those bikes

You can drop some oil inside the wheel bearings if you carefully make a gap between the seal and the inner ring with a needle.

[Midnight_tinkerer]

Originally Posted by Motofool

You can drop some oil inside the wheel bearings if you carefully make a gap between the seal and the inner ring with a needle.

Thanks motofool, for the tip. I did think about completely removing the side seals and cleaning & repacking with grease. However oiling it might work better, as in lower friction & better lubrication. Though my concern is whether the oil will stay in there or leak out all over the wheel. What has been your experience with this?

Also I've heard of people fitting low friction bearings. I think they may be ceramic. The reason I ask is for performance & fuel economy. Reduced friction means I could achieve a higher top speed with the same power when playing, or use less fuel when commuting.

[Midnight_tinkerer]

This upper fairing which came with the bike is not original. It is an aftermarket fibreglass one piece unit. I tried to find an original plastic fairing, because the fibreglass one is not a good fit, not lining up with all the mounts, but good ones are rare & expensive. So I'll make do with what I got. I thought about leaving it off and making the GPX naked for aesthetics & lighter weight, but I kind of like the fairing for practical reasons, such as highway trips, top speed aerodynamics and fuel economy.

So I made a compromise. I removed the lower belly pan & cut a few inches off the bottom of the upper fairing, so that it had roughly half of the original fairing. I like to be able to see the engine but also have a little wind protection. So hopefully this should do the trick.

The new screen is a dark tint, fitting in with the whole black stealth look.

[Motofool]

Originally Posted by Midnight_tinkerer

Thanks motofool, for the tip. I did think about completely removing the side seals and cleaning & repacking with grease. However oiling it might work better, as in lower friction & better lubrication. Though my concern is whether the oil will stay in there or leak out all over the wheel. What has been your experience with this?

Also I've heard of people fitting low friction bearings. I think they may be ceramic. The reason I ask is for performance & fuel economy. Reduced friction means I could achieve a higher top speed with the same power when playing, or use less fuel when commuting.

You are welcome

You could do either way.
I have been doing that several times with no leak out of oil.
Grease dissolves the oil after a while; being the idea to keep the original grease fluid and wetting all metal surfaces.
The needle that I use is dull, so it does not perforate the seal.

I don't know about ceramic bearings, but I do know that the original ones are designed for sustained 10K rpm and a lot more of weight that these ones support.
I also know that as long as they don't have water penetration (reason for the two-side seals) and proper internal lubrication, they will last almost forever.
If yours have no play or evident restriction to roll, I would keep them.

The friction that kills us is aerodynamic, which grows with the square of speed.
The second one is from the many O or X rings of our chains.
You may gain a fraction of % with better bearings, which will be killed by a couple more of mph or a dry chain.

[Tough Noogies]

Great work, so far! Keep it going

[Ish]

Very nice work. I am learning quite a bit here.

[JohnnyBravo]

Keep up the good work

[Midnight_tinkerer]

Originally Posted by Motofool

The friction that kills us is aerodynamic, which grows with the square of speed.
The second one is from the many O or X rings of our chains.
You may gain a fraction of % with better bearings, which will be killed by a couple more of mph or a dry chain.

Thanks for that. I will keep the original bearings & try oiling them. I think your right, the ceramic bearings might be very little improvement for lots of coin.

Yeah motorcycles are about as aerodynamic as the side of a barn. I frequent the Ecomodder website, and there are a couple of guys there with Ninja 250's with major streamlining, that have made big gains in fuel economy. But they look a bit.... well unconventional. I like the idea, but did not want to go to that extent just yet.

I was going to swap the o'ring chain with a plain roller chain, go to taller gearing, and add an auto chain oiler. I was also going to overhaul the brake calipers and see if I could modify to eliminate brake pad drag. Also planning to run higher tyre pressures. All this in an attempt to reduce rolling resistance as much as possible. Basically making the most of what little power it had.

[Midnight_tinkerer]

Originally Posted by Motofool

It hurts looking at those bikes

Yes I also feel dismay when seeing bikes in such poor condition, especially when many problems are preventable with a little love, soap & oil.

[Motofool]

Originally Posted by Midnight_tinkerer

Thanks for that..........

You are welcome

[Midnight_tinkerer]

Below is a photo of my own homemade tail tidy. I hacksawed off the original rear fender assembly so it was flush with the underside of the tail light. I then bent up a new numberplate / blinker mount out of alloy plate. The blinkers came off a mates triumph 675, who replaced his originals with LED's. I have not wired them in yet.
The underside of the GPX taillight lense appears to be blacked out so no light comes through. I hope it is just a matter of removing lense and removing whatever is coating it, so that the taillight will light up the numberplate. The original rear fender had a separate number plate light mounted, but I wanted to simplify the setup.

[Midnight_tinkerer]

Finally got around to painting the lower forklegs. I have ordered a fork seal / dust seal kit from pyramid parts. They were half the price of other brands, so I hope the quality is ok.

[Motofool]

Originally Posted by Midnight_tinkerer

Finally got around to painting the lower forklegs...........

Good progress !!!

What type of paint did you use?

I need to paint my fork-legs as well because original paint has been damaged by road's staff.

[Midnight_tinkerer]

Originally Posted by Motofool

Good progress !!!
What type of paint did you use?
I need to paint my fork-legs as well because original paint has been damaged by road's staff.

I had this paint left over from painting a CT110 engine that's going in my 79' CT90 frame. I liked the satin finish. There is so much gloss black on the GPX already, so I thought the forks would look better satin. Before I sprayed them I heated the forks up first with a heat gun. Seems to give it a better finish. The same paint sprayed onto a cold surface seems to be more glossy.

[fishdip]

How hard is it to do the race tec emulators?

[Motofool]

Originally Posted by Midnight_tinkerer

I had this paint left over from painting a CT110 engine that's going in my 79' CT90 frame..............The same paint sprayed onto a cold surface seems to be more glossy.

Thank you very much.

[TnNinjaGirl]

Originally Posted by fishdip

How hard is it to do the race tec emulators?

Not really that hard. Having a drill press or a bench vise is very handy but not completely necessary.

[Midnight_tinkerer]

Originally Posted by fishdip

How hard is it to do the race tec emulators?

Easy. You could fit them with the forks still in the bike. The only difficulty would be in refitting the top cap, as the emulators sit underneath the springs acting as a spacer, so there would be more tension on the standard springs. Unless of course you cut down the spring spacer tube by the same amount.

[fishdip]

Originally Posted by Midnight_tinkerer

Easy. You could fit them with the forks still in the bike. The only difficulty would be in refitting the top cap, as the emulators sit underneath the springs acting as a spacer, so there would be more tension on the standard springs. Unless of course you cut down the spring spacer tube by the same amount.

What are you going to do?

[Midnight_tinkerer]

Originally Posted by fishdip

What are you going to do?

I was thinking of just using the standard springs & spacers, and see if I can still get the top cap on. My thinking is that the standard springs are probably on the soft side, and considering its age, may have sagged a bit. So a bit more preload might help. I was going to ride it like this first & see how it handles, kind of trial & error.

[Midnight_tinkerer]

Just some more info on what's been done to the bike.

I have had it running. It required another CDI box as I've already mentioned. The carbs were stripped & cleaned. The original airbox rubbers which attach to the back of the carbs had shrunk & gone hard, so I replaced with new ones. Also fitted a new air filter foam, just a genuine one for now. Also the breather pipe from the top of the engine to the bottom of the airbox had gone hard & shrunk, so replaced that also. While I was at it I decided to do the airbox mod(which I saw on another thread), to make it easier to get the carbs in & out, which entailed cutting the battery box off.

I used an auxiliary fuel tank to fill up the carbs, but they initially flooded & fouled the plugs. Sometimes with empty fuel bowls, the float/needle assembly hangs down & jams in the open position. So I tapped the fuel bowls a few times with a screwdriver handle, and the shocks free up the needles & they then seal. I fitted new plugs & changed the oil & filter, and she then fired up ok.

It blew a bit of smoke initially, but the longer it ran the smoke gradually diminished. I suspect the rings might just be gummed up from sitting for a long time. The odometer only shows a few thousand kilometres, which would be good if it were the original speedo, but not so good if it has gone over 100,000km, as the odometer goes back to zero after 99,999km.

It was sweating a little fuel from the fuel bowl o'rings, so will have to remove the carbs again & replace them. Also it did not run well with the choke on, so I will have to check the choke circuit again. But otherwise it seemed to run good, with reasonably crisp throttle response.

[Midnight_tinkerer]

I finally received the fork seal kit, and was able to assemble the forks today. I was a little disappointed that I put a small mark in the new paint, but overall happy to finally have them together.

[jcgss77]

Nice thread, thank you for the detail of all the work you are doing.

Are you going to be rebuilding the ZZR?

[Midnight_tinkerer]

Originally Posted by jcgss77

Nice thread, thank you for the detail of all the work you are doing. Are you going to be rebuilding the ZZR?

I'm not sure what to do with the ZZR just yet. I thought about using the engine as a spare performance engine for the GPX, and experimenting on it with porting, decking, etc. This way if I stuff it up or blow it up, I still have a stock engine to put back in.

I have considered building the ZZR as a naked. They look better naked than the GPX because of the different frame & tank. Plus I don't have the fairing for it anyway. I can't afford to register 3 bikes, so if I built it, I would probably have to sell it.

[jcgss77]

I sure wish I had that dilemma...I would LOVE to have a zzr250 to work on. You could always ship it to me in Arkansas, USA...

either way, keep up the info!

[choneofakind]

Originally Posted by Midnight_tinkerer

Easy. You could fit them with the forks still in the bike. The only difficulty would be in refitting the top cap, as the emulators sit underneath the springs acting as a spacer, so there would be more tension on the standard springs. Unless of course you cut down the spring spacer tube by the same amount.

Wait, back up.

I thought the emulator install required drilling/chamfering 3 sets of holes in the damper rods to assure that the emulators were the only thing controlling the damping?

[Midnight_tinkerer]

Originally Posted by choneofakind

Wait, back up.

I thought the emulator install required drilling/chamfering 3 sets of holes in the damper rods to assure that the emulators were the only thing controlling the damping?

Doh. Yes sorry about that brain fade. You do have to drill the damping rod initially. I had even started to assemble the forks before I remembered they needed to be drilled, and had to pull them apart again.

I don't know why, but when answering the post I was thinking about removing & replacing the emulator for adjustment purposes, which can be done with the forks in the bike.

Sorry for the misinformation.

Regarding the 3 sets of holes. The damping rod originally has a pair of holes. I drilled two more pairs of holes 90 degrees from the originals as per the instructions. One pair 10mm above, and one 10mm below the originals. However I found that when assembling the forks, the lower pair of holes were completely covered by the alloy collar that sits on the bottom of the damping rod. The instructions are not model specific, but are more of a generalised guide for a few different types of forks that the emulators might be used in.

[Midnight_tinkerer]

Originally Posted by jcgss77

I sure wish I had that dilemma...I would LOVE to have a zzr250 to work on. You could always ship it to me in Arkansas, USA...

either way, keep up the info!

I like having this dilemma It only cost me $200 too .
I'm sure the freight to Arkansas would cost more than it would to buy a running registered ninja in Arkansas. But anyway, I'm not inclined to part with it anytime soon.

[jcgss77]

Originally Posted by Midnight_tinkerer

I'm sure the freight to Arkansas would cost more than it would to buy a running registered ninja in Arkansas. But anyway, I'm not inclined to part with it anytime soon.

You are too right. Shipping would be a nightmare, and I would not want to part with one either. However, the only way we can get them here is either be stupidly lucky or import one.

[Midnight_tinkerer]

Hi guys & gals,
Sorry I havn't posted for a while, as I've been busy setting up a new workshop, starting my own motorcycle shop. Have not done much to the GPX except putting it together so I could transport it to my new shop. I cleaned the front brake calliper, fitted new o'rings & pads. When I fitted the new forks I noticed that they did not line up perfectly with the clamps, suggesting that the lower clamp is bent. Also when fitting the front wheel, there was a reasonable gap between the wheel & the forks. Tightening the axle pulled them together, but it will cause extra stress on the bushes. So will have to pull the front end apart again to fix these issues. Some photo's of the bike at the moment and my new shop.

[Midnight_tinkerer]

Got a chance to do some more work on the bikes. The GPX carbs kept flooding despite having been previously cleaned. So I pulled them again and found that once the fuel had dried off, I could see some deposites on the float valve seats. This could be causing the needles / valves to stick. Tried a number of different solutions, Acetone, CLR, Thinners, Aerosol carb cleaner. Nothing worked really well, but they did eventually get cleaner, mostly from lots of scrubbing with a tooth brush. Yamaha's carb cleaner works, but I didn't have any at the time. I think I will also have to replace the needles as I suspect that the rubber tip has hardened with age. I also pulled the tops off the carbs this time & found one of the diaphrams was torn. Oh well, lucky I had a ZZR250 for spares. I robbed a diaphragm from the ZZR carbs but noticed something different between them.
I don't know if this has been covered in another thread. I searched but could not find anything. But the Australian model GPX250 appears to have an extra thick carb diaphragm cap which prevents the slide opening completely. However the Australian ZZR250 Carb cap is a more normal thickness and the slide opens all the way. Also the ZZR250 has one size larger main jets.

Does the American model Ninja have the restricted slides? If so, do you replace with the unresricted ones or just grind them out? I was going to use the Gpx carbs with the stock motor, but use the ZZR carbs with the modded one.

[Midnight_tinkerer]

I also got a chance to pull the engine out of the ZZR250 and pull the top end off. Found a few interesting things. Both the intake & exhaust cams appear to have the same lift of approx. 4.8mm, So from reading 'Racer X' this is the more powerful engine. There is damage to the crank & rods from something that had been dancing around in there. I have not split the bottom end yet, so I haven't found out yet what part could have caused this. The pistons appear undamaged.
The valve seats appear to be recessed quite deeply into the head, so that there is a decent lip around the edge of each seat. It looked like this would be shrouding the valve, impeding air flow. So as this is to be my 'Hot' motor, I decided to try and remove this step / lip. I had to remove a fair amount of metal, so I was a little concerned that it might lower the compression too much. I guess I will have to get the head skimmed to get some compression back.
Looking at pictures of the newgen heads, they do not appear to have the valve seats as deeply recessed as my pregen.

[dukeswift]

Looks good keep up the work!