50 mph on a 2007 Suzuki GN125?

[timg]

Hi Everyone

I'm looking for a bit of advice on my bike. In a nutshell, is it wrong to expect a comfortable 50 mph out of a Suzuki GN125 and would it be worth looking at a slightly smaller rear sprocket (to drop the revs rather than to go faster) when it needs replacing?

I've been riding my GN125 for about 3 months and overall I really like it. Most of my riding is in 30 and 40 mph limits. However I do occasionally stray onto a couple of 50 mph dual carriageways near home and this is where I get the "problems", if that's what they are.

Once I get above about 45 mph the engine seems to vibrate a lot. Get to 50 mph (somewhere between 8000 and 9000 rpm I think) and it's unpleasant to keep my feet on the pegs to the point where I back the speed off to 45 which isn't ideal in traffic. It's only showing 3000km on the clock so it doesn't seem like anything should have worn out and returns around 100 mpg which suggests that the engine is performing well.

I was happy that this was the limit of the bike until I did some training on a Yamaha YBR125 at the weekend. I was paired up with someone preparing for their DAS and so I was working hard to keep up with the big boys in 50 and 60 limits. But I couldn't help but notice how easily the YBR got to 60 at much lower revs and the engine was a lot less vibey. This got me wondering if there was something wrong with my GN after all.

I'm guessing that I'm comparing two very different bikes and probably being unfair on my GN. But looking at reviews of both bikes it seems they should both do 70. I'm sure the YBR would but really doubt the GN could without a downhill and a strong tailwind.

So, is there anything worth checking or getting checked? Is it worth looking at a smaller rear sprocket to drop the revs at 50 mph and make things more comfortable when it's time for a replacement? I'll be keeping the bike for the forseeable future (£££) and just want it to be a bit more usable at 50 mph.

Any advice appreciated, but please be gentle on my little GN and lack of purchasing power .

Thanks

Tim.

[eastanglianbiker]

are you sure you are getting into top gear for it to be revving that high at 50mph?

[mattycoops43]

Doesn't sound right to me either

[polecat]

wonder if its got a gear missing ?

Make sure your next bike is a YAMAHA !

[timg]

Hi All

Thanks for the replies. You had me doubting my sanity over the possibility of a missing gear to the point where I had to go out for a spin before I could say for certain . All five gears seem to be working. Beginning to think it must just be a limitation of the bike, but 50 mph does seem quite low.

One thing I have noticed is a lot of hesitation from the engine at around 7000 rpm. Smooth acceleration up to around 7000 rpm, then very hesitant. It will work through it if you keep the throttle on and it's fine if you change down from a higher gear and the revs are higher than 7000. Hope this makes sense, I can't think of a better way to describe it. Wondering if this is something to do with the carburetor.

I got the bike serviced as soon as I bought it (it had been stood, apart from doing a few miles, for 12 or 18 months). So it has a fairly fresh spark plug, oil, oil filter and air filter if that has any bearing on things.

Tim.

[Tango]

You'd need to go for a larger rear sprocket to drop the revs....but to be honest a tooth or 2 extra on the rear sprocket won't make a lot of difference. You'd be better off increasing the front sprocket by a tooth....that would make a bigger difference.......reduce acceleration, but increase top speed...or at least drop the revs at 50mph....

[timg]

Thanks for the input, Tango. I've had a play about on http://www.sprocketcalculator.com/ (haven't counted the teeth but according to a spares website the stock sprockets are 14 teeth front and 42 teeth rear. Sprocket Calculator reckons that changing to +1 tooth at the front and -1 tooth at the rear would give about a 10% increase in speed with a similar drop in torque. I also had a look at +2 teeth on the front sprocket but the decrease in torque would be going on 15% which seems like it could compromise the acceleration a bit much given the GN's general lack of guts to start with. Wondering if I made such a big change would the engine still be able to pull the bike up to 50 mph in 5th gear or would I end up stuck in 4th and revving the bits off it again? Not sure what to do for the best, might just live with things as they are and have another think about it when one of the sprockets needs replacing .

[Tango]

To be honest Tim I wouldn't go more than up 1 tooth on the front sprocket on that bike.....it won't have the guts to pull too higher gearing. -1 tooth on the rear sprocket?.....wouldn't bother if I were you..... it wouldn't have any significant affect on the overall gearing.... May be worth having a count of the teeth on your current sprockets as someone may have already tinkered with it....

Don't forget that altering the gearing by changing the number of teeth on the sprockets will affect your speedo readings unless the speedo is driven off the front wheel (don't know on a GN125 if this is the case).....

[timg]

Thanks Tango and sorry for the slow reply. I roughly figured out that the larger front sprocket would be like being a gear up all the time so I tried a few (gentle) hill starts in second. Engine says no . At best it feels like I'm going to knacker the clutch if I go down this route. So I'm going to leave the Suzuki as it is, enjoy it for what it is and for what I use it for most.

Since ditching the L-plates a couple of weeks ago I've been on the motorway for a couple of very short runs (500 rpm below the red line gives between 55 and 60 mph indicated). So I know it can do it if needed, on the very odd occasion that I might need it. Picking a slow lorry to sit behind isn't ideal but it's all paid for .

Thanks to you and everyone else who responded to my initial query, I appreciate it.

Best wishes

Tim.

[Fozzie]

I'm good with kitting up 125's so they go quite a bit quicker

I rode a GN on my CBT before I called shotgun on the CG.

It only did 50-55mph so was only used for CBT's.

Try this setup!

1. Lighten it anyway possible, take off rear pegs and replace parts around the bike for lighter alternatives.

2. Give it a iridium plug as I found they took off the edge of the vibration from the engine as the combustion is cleaner.

3. Raise the front sprocket by 1 tooth and no more.

4. Service it, make sure the air filter is a good quality one, the oil is the right type for the bike and so on. As it will get the best out of what it's got.

Then perhaps quickly check the valve clearances as these go out fast on a 125 but are easy to do.

Reckon less than £100 is all you need to get a good 5-10mph out of it.

Oh and P.S. Check chain tension, keep it bang on and well lubed.

If there are any kinks or tight spots replace it

[kevy d]

Hi everyone, been read your posts with interest .I've just run in my Mutt Mongrel 125

which has this engine and managed 62mph at 8000rpm,..didn't seem to want to go any further. I'm quite happy with that.

I changed to an lridium plug which seems to help...definitely not any worse. 

By the way I'm only 9 1/2 stone with all my gear on.