02 250 starts and then sputters out

[Axeman88]

Originally Posted by Demon49

I don't understand how a bike went from running good to barely running with nothing happening to the bike.

When I came in, at the start of this thread your GZ wasn't running well.

This sounds to me like a fueling issue. CV carbs have a diaphragm on a piston that runs from engine vacuum. We control the butterfly, but the main slide runs from the engine vacuum. I've seen engines with CV carbs run wild when there was an intake manifold leak on the engine side of the carburetor. Extra air coming in anywhere after the jets can have the effect of making the engine runaway. This could be the vacuum line serving the petcock, the carb mount, even the carb itself if air is leaking in somewhere.

Also, engines tend to run faster as they lean out, so fuel starvation would account for both increasing rpms and imminent dying. The prime suspect, that would be consistent with all these behaviors, is what suggested in my first post, the vacuum valve in the petcock, but there are other possibilities as well. Have you tested the petcock for functioning under vacuum? I use a hand vacuum pump for this kind of test. https://www.amazon.com/Performance-T...7WG/ref=sr_1_6
If one disconnects the vacuum line from the intake manifold to the petcock, and puts vacuum on it, it should HOLD that vacuum, not bleed off.

At this point, if I was you, I'd probably remove the carb, disassemble and clean it, and note the jet sizes. I'd check and set the float position to spec. I'd inspect the intake manifold for cracks and leaks, and carefully reassemble, but leave the air box and intake conduit off for a few tests. I'd try capping the vacuum nipple for the petcock and running strictly in the petcock's "prime" position, just temporarily, and always being sure to return it to "run" position when the bike is parked. When the bike was running "on the bench", I'd be peeking down the carb throat to see if the CV piston was doing its thing correctly, not hanging up or sticking. I've heard that CV carb operation can be tested by using a shop vac to suck air through them as well.

As I tested, If the engine failed at any point, I'd drain the carb. bowl into a container, and compare the resulting fluid volume to the known correct bowl fuel volume, which I'd establish early on.

A good way to find the source of an issue, is to remove as many elements as possible, then add them back in, one at a time. For example, one thing I'll do during diagnosis/testing is to run from a hanging, gravity fed auxiliary fuel tank, thus bypassing all the fuel system components, including fuel tank, filter, pump, and all the fuel lines. https://www.amazon.com/Auxiliary-Ser...6BL/ref=sr_1_5

If components and subsystems can be individually tested, the diagnostic/repair process gets a lot less hit or miss. One is then in a position of building up from known good building blocks, rather than trying this or that change to a complicated complete system. It's not impossible that more than one thing is wrong.