View Full Version : Rejetting a GPX250

25th January 2006, 00:12
I'm having some trouble with my GPX, symptoms of fuel supply problem, just cleaned and refitted carbs, problem not solved. Going to tear them down again next sunny day.

In the meantime, I'd like to look into performance options as well, starting with a higher flow air filter, which will need a rejet. I know kits for this are available overseas, but any local suppliers? Not looking for much power, only a street bike, ideally looking for a little more pep in motorway speeds, she's fast enough for the suburbs...

Toying around with the idea as I'm looking into tossing the irritating stock airbox for something a bit easier to work with...

Any advice appreciated, I'm new to carbs, my major mechanical experience has been with EFI vehicles.

25th January 2006, 11:56
if you toss the airbox then it will take some hard work to make it run smooth again. well so i'm told.:baby:

F5 Dave
25th January 2006, 16:16
To be frank you won’t be getting much mondo improvement out of such a beast. Best get to run as well as std & be happy until it is time for another bike. You will have to sort your original problem out regardless. Don't throw that airbox, trust me.

It is not a new bike so there are a few options: Your floats could be leaking. The float jets could be leaking. The float height could be way out.
The fuel line could be kinked which is hard to see & catches a few out esp if you have a fuel filter.

The airfilter has to be clean (if foam oiled only with proper oil, if paper can vacuum a bit -replace if real dirty), the tank vent open, fuel tap filter clean.
The diaphragms could be perished & finally the needle valve can wear oval, but that’s another story & would make bike rich.

& when that is all cool the front forks are often incredibly soft. I’ve heard of two that didn’t have the right amount of oil from new.

25th January 2006, 16:48
The float height could be way out.
Do floats lose their buoyancy as they get older?
If not, how else would the float height go out?

F5 Dave
25th January 2006, 16:53
Not really unless they fracture or start leaking. Try shaking & listening. [can't think of a funny maracas song]

Vibration possibly may move, but more likely someone misadjusting it, perhaps to cure the float jets leaking or just error of measurement. Sometimes experiment is the way.

My RF didnít run up real steep hills (ran out of gas) & surged apparently the bike had been tuned up before I bought it. Spec seemed incorrect. Found a decent posi & worked well ever since.

26th January 2006, 01:14
Thanks Dave...yeah, wasn't expecting anything much could be done with performance...no replacement for displacement and all. Still, that airbox is such a bastard to get in there...

When I had the floats out I didn't notice any leaks, will check again on disassembly...would be looking for too low a float level though?

Checked all the lines, they're clear and I see fresh fuel in the sight glass in the fuel line from tank, so up to that point at least its fine. Nothing appeared perished when I had it apart...but just in case, wheres a good place to get parts like the needle, diaphragm, floats etc? I tried asking the bike shop I usually go to, after 3 times of "haven't got round to checking for it" I gave up.

Yep, I have noticed the front forks are soft...not leaking or anything but its a bit annoying under heavy braking...

F5 Dave
26th January 2006, 08:40
Can’t suggest shops up in Auckers, but there’s a few to try from.

With the airbox, sometimes it is easier if you undo the battery or whatever so you can shift the airbox back & slide the carbs out. If you have drama with the boots on the back of the carbs try put the backs in first do the clamps up then forward, or make a small $2 shop Phillips screwdriver with a right angle bend to help run around the edges to ease them over the carb mouths.

So what is the real symptom?

Does the bike surge like it is running out of gas? Does it run fine then get worse? Even stop? These are fuel starvation probs. Tank vent is often the petrol cap, can get blocked esp if tank has had a rust sealer put on it.

Only at certain revs?

Have you changed the plugs & checked the plug caps? This will cause a stumbling misfire.

For the forks you could loosen the caps then take the forks off, drain the oil, flush with kero, drain again, (drain some more overnight) & then fill them with the correct amount of oil + maybe 10cc per side & you should be amazed at the diff.

26th January 2006, 13:29
Hmm, hadn't actually though of putting in the airbox that way...cheers, that oughta be a lot easier, should have thought of that... battery's already out, I've been operating her from heavy duty jumper leads to save the time of changing the battery out every time, airbox and battery mount are integrated.

The symptom before was idle under half-choke upwards, but opening the throttle would cause revs to drop rapidly, I think leaning out the mixture. It is fuel starvation, but I don't know why as yet, I'd say blocked jets but I just cleaned them out. Tank vent on the GPX is in the fuel cap, left it open on some starts, no change. Plugs and caps are new, tested the ignitor and coils, matches manual spec resistances. Now not starting, gets a couple of turns with some engine start, further pointing to carb issues.

Waiting for a sunny day as I'm working outside and don't want a sudden drizzle wetting down the bike..
Thanks for the tip on the forks, as soon as she's running that and the new clutch are going to be my first jobs...

F5 Dave
26th January 2006, 13:35
Bike will take a while to empty the carb bowls. So I'd say that the problem isn't petrol supply to bowls.

But if it won't start then I'd want to open the float drains to see if the bowl is full after all. Assume the fuel tap is vacuum & it has the hose connected to the carbs or manifold to open it. Sucking on that hose (not the petrol one obviously) should make the tap work.

27th January 2006, 04:11
Another day of iffy weather, didn't get the bike out yet, float level's the first thing I'll check on the next sunny day.

Just a quick question while I'm at it, though...say I come across a set of carbs from a bigger bike...what would happen if I mount those onto the little ninja's intake? Would it just be like a larger bike pulling lower revs on the same carb, or would the mixture be way out?

F5 Dave
27th January 2006, 08:20
Fix the prolem first or you will forever be chasing your tail.

Jetting will be off & bigger carbs will work worse accross the range but esp low down.

Iffy weather again? We had some rain wednesday. That was novel.

1st February 2006, 22:17
Sorry for late reply, on a whim I got hold of a length of hose, connected to the fuel tap vacuum line and applied a vacuum while running...yep, even though last time around it confusingly gave the problem even with fuel visible in the sight glass, all troubles went away...further inspection revealed my dumb ass hadn't tightened the screws on the fuel enricher and I had a vacuum leak...marvellous.

New problem, though, this one a bit easier thankfully...hole in the exhaust, probably from the last drop, missed by the repair shop...knew there was a reason for it sounding unusually loud, lol.

Its corroded as well, 20yo exhaust...so I'm thinking I might need to replace the headers, if its too corroded and brittle to repair properly. I asked some questions here (http://www.trademe.co.nz/Community/MessageBoard/Messages.aspx?id=8031660&threadid=8031660) about the balance pipe, its function etc., started doing some research - if I have to replace it anyway might as well look at performance options. Are there any changes you'd make to the header section of the exhaust to get better performance out of the bike?

F5 Dave
2nd February 2006, 08:16
Ahh. Glad there was a resolution.

Anything I'd do? Well avoid spending too much money on it & just get it warrantable. Money should be aimed at keeping the tires & brakes in spec & any spare to slowly save for a bigger bike for when the time comes. My old GSX250 cira 1980 taught me a lot about riding that I would have missed if I had had a bigger/more powerful bike.

2nd February 2006, 18:49
Yeah, very glad its running again, still have to pull the carbs out again as the floats are a touch low, but should run fine after that.

I'm still interested in what I can do to the exhaust, though, as I just pulled them out and theres more corrosion than I though...going to get it quoted on tomorrow, but by the looks of it the headers will probably have to be replaced...if a performance alternative isn't too much dearer I might as well get it done in one go. You're right, though, warrant is first priority.

I replaced the tyres about 2 months before the drop and I've already budgeted for the new brake pads, discs appear to have plenty of life left. I'm probably going to be keeping this bike for a while, as I've got two more years at uni and the 250 is just unbeatable for traffic, I personally wouldn't use anything bigger for the type of commute I do. I have a project car in the works which I'm going to sell shortly after completion and may buy a bigger bike for fun with the proceeds anyway....only reason I'm interested in performance is that I really want to explore a bit for experience's sake, I know that in the long run theres no replacement for displacement.


F5 Dave
3rd February 2006, 08:57
Well if youíre at uni or commuting then Iíd agree & anything above 250 is a waste. Glad itís going better.

The only aftermarket stuff will be simple cheap replacement sot of stuff. If there is an increase or decrease in performance it will be by luck. Or the originals may be collapsing or the new exhaust louder so it seems faster.