GN250 questions...

[spannermonkey91]

Right, I'm just going to bombard you guys with random questions.
My GN currently reaches an indicated 75mph on the flat on a decent day at about 8,000 rpm in 5th, on downhill of M54, it will hit 85-90mph nearing 9,500-10,000rpm. Thing is, this high revving just does not appeal to me, so if I was to gear the bike upto bring the revs down, say by 2 teeth down on the rear and one up on the front, Reckon I'd lose my ability to cruise the motorway with a steady 70?
Also, I have a good understanding of 4 stroke engines and how to increase performance, but is there any viable way of increasing my low down torque? as my engine sounds amazing to me at full throttle between 3-4000rpm and I'd like to stay there with taller gearing for as long as possible

What I'm basically trying to ask is "Guys, are there any ways to make my GN into a torquier, beefier, lower revving bike? I'm not after more speed, Just after keeping in the low revs for as long as possible"

Sorry if I sound a lil immature, I am only 19

[guest7]

I reckon with such (relatively) limited power available you'd be likely to stop the bike pulling a decent top speed. Single tooth increments on the rear sprocket might be the way to go until you're happy with a good balance between revs and speed, but that could be expensive.

Don't forget, if you gear it for lower revs in top you may end up using a lot more fuel going through the gears. Use the bicycle analogy: if you are using a higher gear you need more energy to accelerate to your cruising speed.

GC

[beeman]

Not immature at all. Its a question that most of us have pondered on at some time.

Henry Ford had the answer just go bigger with the engine, simply said not so simply done.

The usual trade off with more power is the loss of low down torque. which is not what you want.
Gearing it up to much, can produce a bike, which has not enough power to pull higher gearing so making it potentially slower.
The main problem in my experience is that small jap singles need to be reved to get the best out of them. As opposed to brit singles with more of a flywheel effect which can plug away from lower revs.
No doubt others on this site will put me right and fill me  in with the tech details.

beeman

[cloggy]

 Generally when you tune you get more top end. Mid range torque is much harder; so not cheaply or easily had, so no.

[spannermonkey91]

Not immature at all. Its a question that most of us have pondered on at some time.

Henry Ford had the answer just go bigger with the engine, simply said not so simply done.

The usual trade off with more power is the loss of low down torque. which is not what you want.
Gearing it up to much, can produce a bike, which has not enough power to pull higher gearing so making it potentially slower.
The main problem in my experience is that small jap singles need to be reved to get the best out of them. As opposed to brit singles with more of a flywheel effect which can plug away from lower revs.
No doubt others on this site will put me right and fill me  in with the tech details.

beeman

Well I noticed that the chinese copy this engine a LOT for its simple design, but also offer the engine in a 300cc which just uses a larger piston and barrel, Which is also sold as a kit for around 120 quid... I've done the math with the bore differences and it checks out right... Do you think this would aid my situation?
I've also noticed that the carb on the bike is actually pretty small, maybe a slightly larger carb is in order?

[SteveC#222]

Hi There,

I ran a GN250 for 3 years daily to work. To be honest I never got mine over 80 even downhill!! There isn't that much you can do to tune them - I have heard of the 300 convertion but bear in mind that the bottom end is marginal on the 250!   I always thought the Gn was pretty good on bottom end torque for a 250, mine would pull away in 3rd no probs and even 4th once when the gearlever stripped the splines and left me stuck in 4th all the way home.

If you are riding it at those sort of speeds and revs you really REALLY need to keep a close eye on the oil level..GN250's drink oil especially at high speeds/revs, I hardly ever did a full oil change on mine as it used so much oil I treated it as a total loss system..remember the bottom end can be a bit fragile.

if you find the headlamp is crap - mine was really crap to the point of being dangerous - don't bother with a bigger bulb, you'll just get a brighter defused glow as the problem is a crap reflector design. Best bet is to fit the complete light from an XBR500 or CB350S - instant daylight! 
...oh yes they eat chains and sprockets
...the front brake will quietly eat the pads until one day the brake will seize on

..Fasten the side panels on with cable ties cos they fall off and cost an arm and leg to replace

If you really want to go quicker you might be better off spending your cash on something a bit bigger.

Oh yes , look after the front downpipes as they are hard to find and expensive if they rot through.

On the upside, they handle really well and once you get up to speed you don't need to slow down much for corners!

Welcome the the Thumperclub!

[SteveC#222]

Oh yes and my back wheel used to spit out 4 or 5 spokes a month too

and if it gets really icy the carb will ice up so it will cut out below 4000rpm

[spannermonkey91]

Thanks steve... All the speeds I quoted are indicated, so I wouldnt trust their accuracy.
I do feel that the GN has some good low end torque already which is what's got me wanting more =P
In your opinion steve, Is the GN worth fixing up and keeping? =)
I havent actually noticed any oil loss, but then again, I'm used to oil-tight rings... got me paranoid now so I'll check tomorrow. Bike never smokes, not even slightly =)

[SteveC#222]

Don't get me wrong, the Gn was a fun little bike and pretty reliable but a bit limited.

Personally I wouldn't spend a fortune on it, if you can afford it I would look for something a bit bigger maybe a GN400 or similar. Have a look on Ebay or in the for sale section.

[spannermonkey91]

GN400's have severely limited parts... I keep considering a *Whispers shyly* Royal enfield...
As a lot of big singles seem to be offroad based and I just cannot ride enduros and the like very comfortably

[SteveC#222]

Nowt wrong with Enfields! I always fancied one myself. Several in the club have them and they can probably tell you more than me. Trouble is the 'nostalgia factor' always seems to force the price up. You sometimes see the Armstrong/Harley Davidson  350/600 army bikes going fairly cheap.

If you like slow revving/ torquey you'll love Bruce's Norton Big4!!

[johnr]

but if a gn250 isnt quick enough at 75mph, perhaps a brit sidevalve isnt the best thing for the daily commute. i love my m21, but i wouldnt want to commute on the motorway on it. before you start tuning the gn, remember that you could end up spending a lot of money and time to make your bike minutely faster but much less reliable. i think if you do the maths and work out the financial difference between tuning the gn versus selling it and putting the money you would have spent tuning to the cash and buying something better. i recon you can easily spend 400 quid on a gn250 and not gain more than 2 0r 3 bhp and a nicer sounding pipe. whereas an xbr or srx would give much higher speeds but still be stock and thus more likely to be reliable.
as attractive as tuning your gn might sound, i think you will only get what you want if you buy a bigger bike.