Old GN250 help wanted

[Zerollax]

Hello everyone,

I'm new to riding have recently purchased my first motorcycle. I bought a average condition suzuki GN250 83' to get me started. What a change this has made to my life. Even just commuting to and from work which is the main reason I got the bike has been a blast. I can understand now why people start riding and absolutely love it. The bike as a whole is very enjoyable to ride although the shortened exhaust is a bit loud and does backfire on down shifting even more with rev matching.

Anyway back to the issue at hand. I went for a ride Sunday night to a friends house. On the way back I came to a set of lights and as I was slowing down the bike started to run really rough for about 5 seconds till I came to a stop and it stalled. I tried to start it but no luck. I sat on it for about 5 minutes before I tried again and it started but it still felt really rough at low rpm. I decided it would be best to try get home or at least closer to it. Travelling along at about 55kph in 5th it felt really rough but when in say 3rd at 50kph it was revving higher and didn't feel rough at all.

Made it about 10 minutes down the road before I had to slow down and it cut out again. I pulled over and stood in front of the light and tried to start it whilst looking at the plug. Straight away I noticed that the plug was shorting out to the engine casing through the boot of the plug. I could see the blue spark very easily so I had a little play with it. Got the plug sitting back down nicely and tried to start it with no luck. After say 10-15 minutes on the phone with another friend I gave it another try and it started and got it home but not without stalling 8-10 times.

Yesterday I pulled the plug off and gave it a proper inspection in the light. It had some blackened areas on it so I gave it a good cleanup and took all the carbon off. This made me think that the issue could have been the carbon tracking causing it to short out. During this time I gave the chain a good clean and lubed it up. After this I let it sit for about an hour then decided I'd try ride it. I started it up with the choke on but not fully and it came to life straight away. I had about 20 minutes of riding before it went rough again and stalled. Gave it a few minutes but it didn't want to start. Because of where I was I couldn't ride/walk home so I got a friend to bring his trailer to get me home.

I've spoken to a few people about this and there could be about few things causing this. One thing I think is causing problems is the shortened exhaust. It is very loud and backfires a bit too much for my liking. I plan on welding a longer one on this weekend.

Another suggestion I've had is change out the CDI because that could be causing issues. Is there a way to test my current CDI so I know if it's faulty or not? I don't want to spend $50 on something that might not even be broken.

The last thing that I think could be it is that the carby has been played with and is putting too much fuel into it even at low rpm. Hence all the backfiring and and roughness. I don't know much about adjusting the carb so I have left it as it is for now.

Sorry for the big wall of text. I just wanted to make sure I had put everything I have done since it started running rough and what it was doing at the time. I've made sure there is always adequate fuel in the tank and that the fuel is set to off when I'm not riding to stop fuel getting out. Any suggestions would be amazing as I would love to try fix this myself. Thanks in advance for your help.

[Akzle]

new plugs are cheap. So are ht leads. Do that to fix the observed problems.

Shorter pipe will be providing less backpressure, which will need to flow more fuel and air, =carb tune.

A compression test wouldnt go a miss, either, as if it's blowing by it wont be blowing out.

Also valve clearances.

But it is fucken old. And only a gn. So you may be better putting it on a bonfire and drinking beer while thinking about less shit bikes.

[p.dath]

1983. 32 odd years old. I would get someone to look at the carburettors first. Perhaps the engine is getting starved.

Perhaps the fuel filter has crap in it. Perhaps the air filter is passed its day.

[sugilite]

Due to the inconsistent nature, it is unlikely the pipe. It "may" be some crap floating around in the carbs, possibly even water in the carbs float bowl. While not completely ruling it out, I think it is unlikely related to your CDI.

[Voltaire]

new plugs are cheap. So are ht leads. Do that to fix the observed problems.



Also valve clearances.
..... and drinking beer .........


wot he said mostly :lol:

[Zerollax]

The air filter had changed when I got it. When I test rode it, it had the oem intake on the carb. It's now got a little pod filter.

I will clean the carb up and give the bike a service + I little wiring I'm not happy with. Whilst that is done I will trickle charge the battery overnight. Change out the plug and the ht lead for a new set. Might also try test the conductivity of the lead etc.


Also could changing the exhaust to a aftermarket one be better than the straight pipe with tiny baffles? Haha!


I like this style of bike because my grand father had one since it was brand new. I remember getting to go on it you know.

[ducatilover]

Back firing is because it's a little lean with your hectic exhaust.
Raise the carb needle with little washer by 0.5mm, if one or two washer won't sort that, go up a jet size
However, this will not cause it to cut out

Do the usual checks:
Earths all clean and not breaking down
Valve clearances in spec or even a little looser is okay
Carb set up correctly and bowl vent tube/passage clear
No air leak between carb and manifold
Decent oil (GNs will heat seize when low on oil, don't ask how I know)
Fuel tank vent not blocked
New plug

Neutral switch working and sidestand switch (or bypass both)


Has it got spark when it fucks out?
If no, test coil/lead resistance when it does
Test pick up coil resistance when it does
Then test the CDI last as there is no real definitive way to do so without a machine that emulates your crank trigger and having the plug under normal load

Here's a GN manual https://www.dropbox.com/s/ry84u8swvektnhj/GN250%20Manual.pdf?dl=0

[jellywrestler]

cheuck half a cup of meths in the fuel tank, if there is water in it the meths will absorb it and will burn much like petrol would, that may help but all the rest is relavant too.