Begins to die at high rpm under load – GT250

[Devon]

Hey guys, made an account here to ask for help with this issue I’m facing.

My bike is a 2004 gt250 that I bought at the start of this year. When I bought her she ran decent enough and didn’t have any major issues I was aware of accept a loose shift lever. Until one day after a decent trip it begun to what seems like misfire every now and then when at about 7k rpm or so. I tested a little more and it kept it up so I thought it may be a fueling issue. pulled apart the carbs and cleaned them, put it back on and ran decent.

About 3 more smaller rides later it began doing it again and would sorta just come and go, couldn’t really get it to replicate its issue. Until now…

Every time I try and get it near 7k rpm (100kph) it would begin to die and the tachometer would freak out and so would the bike, I’ve pulled the carbs to bits a few times to really make sure I cleaned them good (guy I bought it off was an idiot and had treated the bike terrible) as they had still a bit of gunk, still no good. However all times it will happily Rev past 7k prm when sitting, just when under load

So think it might be electrical, spark plugs are new, fuel lines and filters are new and I’ve cleaned the pump. It could be the coils are shot but I’m not sure, tried buying new ones but there’s absolutely no oem or after market support for these bikes in NZ and I’m not super into the idea of getting Chinese mock oem ones, it could be something else electrical but I’m not super knowledgeable in the electrical field.

Tldr. Bike dies at 7k rpm while under load, confident its not fuel issue.

Sorry for the long post but I just wanted to give a decent amount of detail.

Thanks,

and this forum has helped a ton with other info so thanks for that also 👍

Can add more detailed info if necessary about the bike

[♠️ MARCEL]

It might sound stupid, but have you taken the “Slides Off”  (carb diaphragms?)  , what if it revs past 7k, then the slides stick ? incase they are too scratched up  (then its time for a new set)  and also verify the mixture is around 2.5 full turns out each carb , and leave it there forever.

Test ride it again, to see if it helps.

And also try this free mod  (Slide Tuning Mod)

Also have a can of carb cleaner, when she is idling and revving, spray carb cleaner around the airbox and intake pipes = Do the revs change ?
If yes , your airbox is loose
Or the intake O-Rings are shot , meaning as soon as lot of air is sucked in to the bike (full throttle), the slides hang due to excess air still passing through ?

Like these little ones (if its a 2004, it is probably time to change the o-rings), like these you see here

Intake Pipe O-RING (Inlet Boot Seals) x2 – GV125 GT125 RX125 GV250 GT250 Hyosung

If we can confirm 100% the intake & carb side is all good.  Then we can progress to the electrics.

// Meditation doesn't mean you have to sit still....

[Devon]

Once I’m home from work tonight I’ll do said tests and report back.

Some other details about the bike might help. The bikes muffler has been messed with to make it louder and has very little in the way of restrictions, air filter is stock as far as I can tell, when I pulled the carbs to bits the most recent time I checked needle and slides. Needles were set to the third position (same as slide needle mod) and the slides did have some scoring, more so on one of them, can’t remember off top of head what side but will add photos when testing tonight.

I’ve set the slide needle back to stock to see if that changed anything however not alot happened. Main jets are the stock 90 and 92.5.

Also there is an extremely small oil leak in the front cylinder somewhere, not sure where the leak is coming from but after any ride, whether or not it’s a big ride or just a trip to the supermarket it will have about a drop of oil on a flat bit of casting (will add photos later) not sure if a gasket is unhappy or there’s a crack somewhere.

Thanks for the help

[Author]

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