GZ250 losing power on the motorway after downshifting - scary stuff

[rok-the-boat]

Sounds like a carb issue to me. Can't see it being anything to do with transmission. Pull the carb, inspect it, clean it etc. Do you have an air filter? If it has been removed, dust may cause the slide to stick.

[FROSTY:Me]

Possible causes I can see.
1)crap in the tank -clean out the tank
2)crap in the carb -clean out the carb.
3)carb icing.-I dunno how to fix that

[maraudingkiwi]

Just an update on this, thanks very much for the informative suggestions btw guys. I'm learning a lot about engines thanks to this at the very least. I bought a can of carb cleaner and took the carb out, prepared to clean a lot of gunk out but it turned out to be spotless like a mirror inside (even in the float chamber). I can only assume the previous owner had given it a good clean shortly before I bought it because it's not a new bike by any means. Regardless, I took out the slide, wiped it down and applied a small amount of light sewing machine oil to it. However it seemed to travel smoothly even before applying oil so I would imagine it'd take some serious icing to cause it to stick?

Unfortunately the problem reared its head on the motorway again the other morning (ie since cleaning the slide), although not as badly. This time I got some warning in the form of a drop in power - suddenly the bike could not accelerate over 90 even at full throttle. I managed to get to a petrol station and it turned out the fuel level was a lot lower than I had thought (but still far from empty). So I guess this would fit with the theory of the fuel tank pulling a vacuum because of a blocked breather hole? Ill look into this, and may end up just taking it to a reputable mechanic as suggested.

[Drew]

Still sounds like fuel not getting to the carb to me.

[kiwi cowboy]

Just an update on this, thanks very much for the informative suggestions btw guys. I'm learning a lot about engines thanks to this at the very least. I bought a can of carb cleaner and took the carb out, prepared to clean a lot of gunk out but it turned out to be spotless like a mirror inside (even in the float chamber). I can only assume the previous owner had given it a good clean shortly before I bought it because it's not a new bike by any means. Regardless, I took out the slide, wiped it down and applied a small amount of light sewing machine oil to it. However it seemed to travel smoothly even before applying oil so I would imagine it'd take some serious icing to cause it to stick?

Unfortunately the problem reared its head on the motorway again the other morning (ie since cleaning the slide), although not as badly. This time I got some warning in the form of a drop in power - suddenly the bike could not accelerate over 90 even at full throttle. I managed to get to a petrol station and it turned out the fuel level was a lot lower than I had thought (but still far from empty). So I guess this would fit with the theory of the fuel tank pulling a vacuum because of a blocked breather hole? Ill look into this, and may end up just taking it to a reputable mechanic as suggested.

Does sound like a vacuum problem unless you have actualy fixed it and you were running out of fuel on main tank and the fuel you heard was reserve???.

[Drew]

Does sound like a vacuum problem unless you have actualy fixed it and you were running out of fuel on main tank and the fuel you heard was reserve???.

Doesn't have to be a vacuum issue. Taps get blocked up from time to time.

Dunno if his bike even has a vacuum operated tap.

[kiwi cowboy]

Doesn't have to be a vacuum issue. Taps get blocked up from time to time.

Dunno if his bike even has a vacuum operated tap.

Ah yer vacuum I meant was possibly the breather to the tank but could be the tap oh mighty one

[Drew]

Ah yer vacuum I meant was possibly the breather to the tank but could be the tap oh mighty one

Makes more sense now.

[Marmoot]

If you are using 95/96/98 octane fuel, try switching to 91 for a bit and see if that helps with the problem.

Had similar problem with '99 fireblade a few years back. I ended up having to use 91 for winters and 96 for summers. Worked well in the end with no power loss.

[ducatilover]

Dunno if his bike even has a vacuum operated tap.

Nup. But it'll have a little bowl and filter on the bottom of the tap.

Dirty shit in the carbs is not always visible to the eye.

[haydes55]

Next time it happens open the fuel cap. If you regain power it's a blocked breather. If not it's the tap.

[Drew]

Like anything mechanical though. There are several scenarios that fit the symptoms described.

Low float level can present this way too.

But I am picking it's a blocked tap filter, (which is easy as fuck to fix if it's the system Ducatilover said), blocked float needle or gallery (I've had this happen and spent a long time trying to figure it out), and lastly the cap thing.

Cap thing is cheap and easy to check. Go to supercheap or repco and grab a cheap temporary cap, (don't have a full tank of gas when you do this, half would be best), and go for a burn on some back roads to emulate the conditions it happens under.

If it doesn't happen, then you pull your cap apart and find out why it's not breathing. If it does, go back and pull the tap apart.

One thing at a time though. Don't fix two things and test, because you will never know what it was.