GN250 running lean

**autobatch** · 8th September 2007, 16:28

I think my GN250 is running lean. I had a bad engine noise so I tore the head off and my friend Victor said the piston had got too hot due to a lean mixture and distorted. I replaced the piston and rings and the engine noise seems to have gone away. Now I want to fix the fuel mixture problem so it doesn't happen again. This bike is nearly new with only 4,000 k on it. It has allways taken forever to warm up enough to push in the choke before taking off. For this reason I suspect it has always been running lean. Does anyone have any ideas on the best way to richen things up a bit? I could bend the tab on the float so the float level is a bit higher. It all sounds a bit unscientific. The picture in my workshop manual shows a round float and mine is a bit square so I don't have any scientific way to adjust the float. Any ideas on the float or any other way to adjust the mixture for more fuel?

**manwithav8** · 8th September 2007, 19:16

offer advice for tuning? Could be as simple as setting the air/fuel mixture screw to manufacturers specs. Also check things like your fuel filter isn't partially blocked, and check your not getting extra air past your filter or airbox. To much air will lean it out as the carb will not deliver enough fuel to compensate if it is excessive. Also if you can't set your float level, take the carb off and take it into the bike shop and have them do it.

**merv** · 8th September 2007, 19:26

If the bike is nearly new how come you "tore the head off" and didn't talk to the dealer or is it old enough to be out of warranty?

Don't touch the float level. If it really is too lean you may have to think about rejetting it, but I some how doubt this is the problem.

If its anything like the Suzuki singles we've owned they all run pretty lean in the sense of needing a good warm up before the choke can be fully in as you say, but they've never buggered a piston.

You can richen the mixture in the mid range by lifting the needle one notch - you have to take it out of the slide and move the circlip, if its got one that you can alter, and you can alter the mixture in the low range by adjusting the pilot screw. For the upper range you've got to change the jet.

**cynna** · 8th September 2007, 20:03

Originally Posted by merv

You can richen the mixture in the mid range by lifting the needle one notch - you have to take it out of the slide and move the circlip, if its got one that you can alter, and you can alter the mixture in the low range by adjusting the pilot screw. For the upper range you've got to change the jet.

what does it mean when they run rich. in my dr the top of the needle broke off and the bike shop must have lifted the needle a notch (i never even knew u could alter them). they said it might run rich - seems to be running ok but i have no idea .... what will it be doing if its runing rich

**twinkle** · 8th September 2007, 20:31

Originally Posted by autobatch

I think my GN250 is running lean.

Is there a ngk DR8EA spark plug in it?(thats the plug it comes with). Is there enough oil on the air filter? Is the exhaust still stock?

the reason i ask about the spark plug is that i tried the bosch equivalent, and it didn't seem to be equivalent

**MikeyG** · 8th September 2007, 20:55

Where did you get the piston, rings etc and how much were they. I looked at some but they were way too expensive.

**merv** · 8th September 2007, 22:25

Originally Posted by cynna

what does it mean when they run rich. in my dr the top of the needle broke off and the bike shop must have lifted the needle a notch (i never even knew u could alter them). they said it might run rich - seems to be running ok but i have no idea .... what will it be doing if its runing rich

If they are slightly rich they'll kind of blubber a bit on and off throttle and your exhaust will look blacker than usual and you'll use more gas per km. If they are really rich it just won't run well at all.

If the "top of the needle broke off" why didn't you get a new needle?

**autobatch** · 9th September 2007, 09:54

Wow guys! Thanks for all the ideas. The bike actually isn't new even though it has only 4,000 k on it. I bought it from an American Baptist preacher who never rode it. He was a George Bush fan so we didn't hit it off all that well. Anyway, I would never take my bike to a bike shop as a matter of principle. I bought the piston and rings from Haldane Motorcycles in Auckland for around $350. I will check out all your ideas. I rode the bike around the East Cape and pushed it pretty hard trying to keep up with my son-in-law so that it got pretty hot. I still think it is running lean and will try all your suggestions to get it a bit more rich. The workshop manual doesn't say anything about adjusting the mixture. The exhaust is stock standard. I will check out the air filter to make sure it doesn't have any leaks and put oil in it (if it has oil). I will try moving the needle up one notch if it has any notches.
Thanks again,
JC

**autobatch** · 9th September 2007, 12:40

Thanks again to all those who supplied ideas for my running lean problem. I pulled the carb. apart. The needle does indeed have notches and it was on the middle one. I raised it two notches to place it on the most rich setting. Perhaps I should have only gone 1 notch but I can pull it apart again without too much difficulty. I started it up and it is now running much richer. I judge this by the fact that I only need to pull the choke to get it started and can push it back in right away and it runs fine. Hopefully all is well now.
JC

**merv** · 9th September 2007, 12:57

Raised, meaning you put the circlip down two notches eh, which lifts the needle up? Also remember what I said though, at the top end, i.e. throttle wide open, its still the jetting that counts so wound full on around East Cape if it was lean before it will still be lean now. However, hopefully it will be sweet for general riding now.

**cynna** · 9th September 2007, 21:52

Originally Posted by merv

If the "top of the needle broke off" why didn't you get a new needle?

thanks for the info

im ordering a new needle. they just put the carb back together for me so i can use the bike in the meantime

**autobatch** · 10th September 2007, 08:24

Merve,
Yes, I moved the circlip down two notches to raise the needle. I would have thought that would make the mixture more rich over the entire range. The needle is tapered over its entire length is it not so raising it should make it more rich even when flat out.
JC

**merv** · 10th September 2007, 08:41

By the time its wound wide open though it is still limited by the jet size itself i.e. you can't open the slide any more and it makes no difference to the amount that can get through the jet - if you want more then you need a bigger jet.

**klingon** · 10th September 2007, 12:48

Did you check the air filter issue as well? My Volty (TU250 - same engine as GN250) was running lean... it turned out that the foam air filter sleeve had totally disintegrated! Replaced it with a new filter sleeve (properly oiled up) and a new spark plug and the problem was instantly solved.

Gotta love these simple little engines where everything's visible and accessible.