GN250 transmission noise?

[aju]

Hi,

I have a US spec 1982 GN250. This is my first bike and I'm trying to get a handle on some issues with it. First, I noticed a noise that I think is coming from the transmission. It starts at about 30 mph and occurs from there up to top speed (at about 50mph it gets drowned out by the motor). It sounds like whirring, but a bit like metal against metal - kind of like a speedometer cable spinning inside unlined housing. The sound is completely independant of engine speed (I've shifted gears, coasted with the clutch in and allowed the revs to drop, etc.), however the severity of the sound changes with throttle position. Anyone out there experience this noise in their 250? How concerned with it should I be? I had a car many years ago with a differential that made a somewhat similar sound - I was advised by the mechanic to just ignore it and drive.

Second, the bike seems to suffer from fuel starvation (or what feels like it) at wide open throttle. I mostly notice this when on the highway and trying to maintain speed up a hill. 3/4 throttle will sometimes give me better acceleration than wide open. Any ideas here?

I've not been getting the gas mileage that I expect from this bike. I've been averaging about 45 mpg over about 4 tank fulls. I was expecting about 70. Is it entirely due to my right hand, or am I correct in thinking that due to the small size of the engine I should be able to run it at WOT all day and still come away with very good mileage? I think something's amiss here and I have a couple things I'm planning to do. Any suggestions are welcome.

Clean air filter
Clean petcock screen
Gap/clean plug (possibly replace)
Check valve clearances
Tension drive chain

Thanks all. So far this bike has been a blast to own. Just enough get up and go to get out of its own way. Improving the gas mileage will make it great.

[Lucy]

Hi,



I've not been getting the gas mileage that I expect from this bike. I've been averaging about 45 mpg over about 4 tank fulls. I was expecting about 70. Is it entirely due to my right hand, or am I correct in thinking that due to the small size of the engine I should be able to run it at WOT all day and still come away with very good mileage? I think something's amiss here and I have a couple things I'm planning to do. Any suggestions are welcome.
.

When I lived in Auckland and did a lot of commuting at medium to slow speeds I got about 143 miles before switching to reserve, but now that my riding is on the open highways (mostly) I only get about 111 miles before having to switch. This is with a ten litre tank.

Hope that helps.

[Katman]

Regarding the noise, have you checked the condition and adjustment of the drive chain?

[aju]

Good question. I noticed last time I oiled the chain that the inside of the side plates on the chain are wearing. I wondered when I saw this whether this is a sign that the chain is not properly matched to the sprocket, or one/both of the sprockets are the wrong size. The chain is far from uniform in stretching - when running the bike on the center stand the chain moves in such a way as to make the sprocket appear eliptical. I am under the impression that this is to be expected on all chains to a certain degree - not sure if mine falls in the normal category or not. The chain does appear to be within tension limits.

One additional comment: the bike backfires when it's hot and I chop the throttle shut - most often when going from WOT to closed. Not sure whether this indicates a lean or rich condition (logically it would seem that unburned gas is getting forced into the exhaust which would indicate rich, but this would happen if there was so much air flow it blew out the spark too).

[FilthyLuka]

I wondered when I saw this whether this is a sign that the chain is not properly matched to the sprocket, or one/both of the sprockets are the wrong size. .

Nah, its just worn. Bung on a new chain and sprocket kit :-) If you wanna check the sizes of the sprockets, it should be 15 front and 42 (or was it 41?) rear. I've got a 16 front on my GN and there is not problems.

Get a decent chain and some good sprockets if its worn to buggery

[Katman]

And regarding the backfiring - make sure you have no exhaust leaks at the header pipe or the muffler. On overrun air can be sucked in through any leak hole causing backfiring.