Carb help - Suzuki GN250 sputtering and dying at idle

[investinwaffles]

Apparently none of these were sold in the US or something, but there seems to be tons over at NZ. Been lurking on the forums for a while now and I love the down under attitude (need to come visit sometime).

Anyway, I bought this thing as a project bike to make into a really sweet bratstyle bobber (still a work in progress).
The previous owner said he had it running perfectly before he sold it to me - and it was running great actually. Started up on the first crank, every time, and the engine was running pretty well.

Then, a few days ago, it simply bogged and died on me at a light. Wouldnt restart for a few mins, but then it cranked right up and stayed running the rest of my journey. The only thing that was weird is it was kind of misfiring for a stroke at higher sustained RPM, but as soon as I modulated the throttle it would be fine.

Yesterday, the bike did the same thing as I was approaching a light, with the clutch pulled in. I tried to give it some throttle, and it would bog, then run OK for a second at 0% throttle, then it just died. Couldnt get it resetarted for SHIT, and I was about a mile from home, with a steep uphill on the way back. Finally got it to crank, and gave it the beans.
Ran perfectly at higher RPM's, but as soon as I pulled into my neighborhood and hit a stop-sign, it died and wouldnt restart. It only does this at low RPM when it is hot, mostly with 0% throttle.

I made a video - you can see it doing it towards the end.
http://www.youtube.com/watch?v=P648jnHLGro

I tore it down, cleaned it out (it was already really clean inside), and put it back on the bike but it still hasnt changed.

Anyway, when I sprayed carb cleaner through the idle jet pickup, it came out of the jet to the left of the intake (Upstream/front side).

I also found another screw that I apparently missed before - it is on the top-side of the "outtake" side of the carb, where the actual throttle-body is. The screw is marked in red, and the yellow screw someone previously stripped but I got it off just now and everything looks fine inside - needle is OK too.



And here is the front "intake side" of the carb - the jet on the left is the one that is "connected" to the idle-jet pickup in the bowl.




*So* - I assume the Upstream (front side screw) is the air-idle valve, and the downstream (throttle body side), is the fuel regulation.

I screwed both in till they were fully seated, then backed them out 2.5 turns but I am a dolt and forgot to mark the position of (what I think is) the fuel-mixture screw on the outtake side before changing it. Does a 2.5 turns sound OK for that as well? I cant figure out what is wrong
Plug looks fine too (a little black if anything, so its running rich I think).

Would appreciate some help if anyone has advice. I already ordered a new carb, but this is my only transportation and I hate taking the bus

[investinwaffles]

Heres a few pics btw - first streetbike so I wanted something stylish and safe (miles better than a Buell blast or a mid 80's lookin Ninja 250)

[haydes55]

Cool looking bike.

Another option to look at, check the timing hasn't slipped. If the timing was out it would be making all those issues as well.

Otherwise get the bike running and set the idle higher and try fine tune the fuel mix for lower rpm.

[Akzle]

that is possibly the only cool gn ive seen.
Haydes hit it tho. Good luck.

[investinwaffles]

Aww, so much hate for the GN - haha. It's super comfy, and gets like 80mpg too!

So does tightening = less fuel/air, and loosening = more?
It sounds good in the top end so I think I need less fuel since It bogs if I give it throttle.
And how do I set the timing on one of these? Should I retard or advance?

Thanks

[unstuck]

I wouldn't think its a timing issue, sounds more like a fuel issue. Float hight ok? Needle valve? Start with the basics and go from there.

[mossy1200]

Assume you have a small air filter and the black box is battery. I would say a CV carb isn't going to like that a lot. I think you should chuck the carb away and get a flat slide seeing as the bike looks so good (for a gn).

[investinwaffles]

UUgh this is driving me fucking insane. I just burned the shit out of my hand on the header trying to adjust it while it was hot so I am getting really pissed at the thing.

Adjusting both the fuel mixture screw and the idle air jet screw had literally NO effect. I leaned the fuck out of it, made it run pig rich, and everything inbetweeen and there was absolutely no difference. It still idles fine most of the time, still revs fine, but if I give it even remotely sharp throttle at idle it will just fucking bog. Revs through the top-end just fine, and comes back down just fine - it is only when I give it throttle at idle that it bogs really hard (and dies unless I close the throttle and slowly open it to rev)

I am getting to my wits end with this and fucking US postal isnt delivering my carb until wednesday at the earliest.

Could it be anything else?? Jetting shouldnt be an issue either - both are clean and it ran perfectly before - I have not changed anything to do with the engine since I got it.

I hate carburetors more than I can even express. Getting depressed and more agitated because I cant even leave unless I get a ride from a friend or my roommates.

[unstuck]

Hows that float hight?

[hayd3n]

have you checked the valve clearence??

[investinwaffles]

Hows that float hight?

Float hight looks OK. I am not sure how to get it back to "stock" tolerances (I think it is a mikuni BS34 carb), but it is more-or-less level when upside down. I adjusted it up a touch, but I cant do the hose trick to check how exact it is.
Do you think it being too high would be causing this?

I still cant tell if there is too much fuel + too little air, or vice versa.

have you checked the valve clearence??

I have not checked the valve tolerances - it doesnt seem like they should have an effect on it bogging like this? No ticking either so I think it should be OK.

You guys rock for the fast responses though.


EDIT:
I would also just like to say thank you suzuki/mikuni, for putting the fucking adjustment screws in the most inaccessible places, absolutely necessitating the removal of the carb to adjust.
What a horrible design

[unstuck]

Have a search through the forums, there are a couple of threads on GNs and their issues. http://www.kiwibiker.co.nz/forums/ar...p/t-95619.html

[investinwaffles]

I read through that and searched a ton, but most has to do with adjusting the mixture screw (my rubber plug is also missing, but I cleaned it out thoroughly and it feels fine.

My hand is pretty burned and hurts like bloody hell, so I am going to call it a night and see if I can get anything to change tomorrow. Other than that I can only wait till the new carb shows up (some knockoff chinese POS), and pray for the best.

[investinwaffles]

Worked on it some more today - I think I eliminated the issue down to the actual function of the carb.

It runs really well with 4- turns out on the A/F mixture screw and ~3 turns out on the air idle jet.

Idles perfectly now, gets hot and in neutral it still bogs under sharp throttle but it does it more-so when in gear. I can power out of it if I go 0% throttle, give it a second, and then smoothly open the throttle, but it only works for 15mins until it gets really hot. Then it just really really wants to bog and die.
Sticking my fingers into the inlet, it is really wet with gas.
I have adjusted it about 10 times now, every which way on the idle and A/F screw but its the same result every time.

Final checklist on this carb -
Float height - OK
Cleaned Jets + passages
Slide looks unworn
Float needle looks OK I think
A/F + Idle screw - tried all positions
Spark - OK
Gas level is full
Petcock was also cleaned out and flows with some force
Gas-cap is actually not on the motorcycle so vac should not be an issue
It wont run for a long time but the plug looks OK right now (a touch black but nothing extreme).

I will try to check timing sometime soon, if the new carb doesnt solve anything. I will try to drop the oil tonight and see how it looks. I only have automotive 10w40 so I am hoping that will be OK until I get the thing running.

Giving up for now becuase I feel like I have tried everything.

[fridayflash]

sounds like youve had a thorough go at it...whats the diaphragm like? anyway good luck with the new carb, gn's are fucken ace bikes man! your one looks cool, was it imported into usa you reckon?