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To quote one of the best aero experts in land speed racing, Jack Costella "How you open the air is important but it is more important how you close it." Why do you think streamliners all taper at the rear? To close the hole in the air as smoothly as possible. Its not a concept and it is very valid.
To quote one of the best aero experts in land speed racing, Jack Costella "How you open the air is important but iet is more important how you close it." Why do you think streamliners all taper at the rear? To close the hole in the air as smoothly as possible. Its not a concept and it is very valid.
Well, it is not uncommon for my rear wheel to lock going from 3 to 2 or just letting off the throttle in 2, usually in a roundabout, so if I can avoid feeling it more than now, I will be happy.
Clip on are mopped item too since a Yamaha TZR 50 has a 32mm fork these days (when I only have 33mm).
I found a set tonight for 20$ or so and will get them tomorrow hopefully.
I also found an RGV 250 fairing wich can be a good start for further investigations.
Long story short : funky rattling noise + power loss led me to stopping the bike before terminal failure.
I first thought it was the cam chain tensionner that was shot (I heard it is not uncommon to have to replace the chain @ 40K).
I ended up opening the engine and am now massaging the head to make it more amenable to swallow the ton.
I had a look at it and felt the exhaust port were very lame compared to the inlet so I started modding one, applying techniques written by David Vizard, the Austin Mini wizard.
I worked on one of the port only to try and compare result with the untouched port.
Here is my home made flowbench :
Poor water on top, open valve / start the timer, stop the timer when water reaches a given level.
I putted straps on the valve stem on the spring side to lift them the same amount (half lift to have more time and offset the small amount of water)
Anyway I checked the modded exhaust port vs standard and coked exhaust port (there are 2 ports) and at a similar lift with a given quantity of water, the modded port takes 14% less time to evacuate the water.
I had kept on modding the port because the long side radius to the guide is very not streamlined.
At some point I will have to 3 angle the valve seat because Suzuki is only doing a 2 angle jobbie and the way in is some kind of radius that takes up room.
Also I am well under way at polishing the chamber.
I was thinking about swapping the 72 mm piston with a 79 mm piston from a DR350 but I would need some fancy stuff either drilling the rod little end 2mm bigger (wich can not be done in situ) or sleeving the piston.
Also I know nothing about the eventual crown deck height difference but that could be an issue.
Maybe I need to get a quote from JE or the like for a custom piston, GN scaled up to DR size ...
That being said, increasing capacity would be cheating in the 250 race to the ton ...
To quote one of the best aero experts in land speed racing, Jack Costella "How you open the air is important but it is more important how you close it." Why do you think streamliners all taper at the rear? To close the hole in the air as smoothly as possible. Its not a concept and it is very valid.
Don't listen to me! I just spent a week at Bonneville chasing my tail. Should have killed a soft record instead I just managed to bump it by .511 mph. But we did make it onto French TV.
http://videos.tf1.fr/auto-moto/plein-phare-sur-la-speed-week-de-bonneville-2013-8248733.html
Her's an oldie but goodie. Makes your brain think.
So the head job is somehow on hold since I decoked the reference exhaust port only to find out it does flow the same if not more than my modded port.
I did find motivation to remove the cylinder and the news were not too good :
I have been told it does need a rebore.
But today I found some parts new from China since the GN 250 is still produced over there.
So I ordered a cylinder and piston amongst other things for basically the price of the rebore (machining is expensive over here).
I even get to get a nickazyl coated cylinder.
Parts are due in a month time wich allows for buying some proper Vesrah valves (no, I will not buy chinese valves ...) and have the head properly done (check guides with new valves, renew if necessary, maybe have the seats properly cut ...)
The head is still at the polishing state.
It would need a picture though ..
I have been out to buy a replacement 250 today, the newer version of the GN which is called a TUx or Volty someplace.
The bike was advertised for 20K miles and at 300€ but with a bent fork and basically pretty tired trim.
Turns out she has 50K and the engine can't be started prior to buying !
WTF !
And the seller is a bloody shop !
Chinese parts, here I com !
IMPORTANT QUESTION :
Are the rings for nickazyl plated cylinder identical to rings for standard cast iron cylinders ?
Rings for Nicasil are not the same as for cast iron cylinders.
TU250X is only designed to do about 65~70mph (14bhp at wheel)
The only 'advantage is the fuel injection
You could get your original cylinder bored for less than 300 Euro
Overbore of a cast iron cylinder + piston and gasket set, say 300€ (sounds fair)
New nickazyl cylinder + piston + gasket set + delivery = 60€ ...
The euro Tux as of 1997 is same power as GN and does not have FI.
I don't know how much is it @ the wheel but it is 22hp @ the crank (when new, on a good day)
That's the kind of power I am planning to use to hit the ton.
I asked a fellow engineer some feedback about the piston and cylinder with the old japanese part in hand and the opinion was the parts look OK.
Cylinder is sand casted so should be better as far as heat dissipation is concerned.
He pointed out some areas that needed work on the piston wich I did.
I then polished the piston crown (work is still in progress).
Valve cutouts were there from casting and the piston top had been flatted in a lathe as there was circular markings wich got away.
The piston alloy is quite tought compared to some other alloy stuff I have been polishing (SU carb for instance) so I quite feel reassured.
Here is a view of the polished chamber too :
You might have notice the quench / squish area on the exhaust side had a hard time pinging, it does point out the cooling must be improved if I want to run decent CR.
Overbore of a cast iron cylinder + piston and gasket set, say 300€ (sounds fair)
New nickazyl cylinder + piston + gasket set + delivery = 60€ ...
The euro Tux as of 1997 is same power as GN and does not have FI.
I don't know how much is it @ the wheel but it is 22hp @ the crank (when new, on a good day)
That's the kind of power I am planning to use to hit the ton.
You need to realise Japanese horses are smaller than 'real' ones.
No way did a GN 250 ever make 22bhp, SAE or DIN and the TUX has even less power
Your working with 14~16bhp on a good motor, plus, you need to rev it to get that (probably 8,500rpm or more)
Worn motor will be far less, maybe 10~12bhp (still enough to do 65~70mph in still air or with tail wind)
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