Jerky at Mid RPMs

[AvalancheCCM]

Ever since I've owned this GZ250, the bike tends to bog down at middle RPM's. It runs and idles fine, but when I accelerate, it gets about half way through the RPM band and starts to jerk. If I am in a lower gear (1-3), I've found that if I keep the throttle at about 75%, it will eventually "push" past the jerkiness and accelerate rather quickly to higher RPM's. If I use the higher gears, sometimes it will do the same thing, but sometimes there isn't enough power to push past the jerkiness.

This is my first bike and I'm still new to carbs, etc. I've run Berryman B-12 through a couple tanks but it's not done a lot. It does get better about 15 minutes of riding but the jerkiness is always there. If I leave the choke half way out while I ride, it seems to help.

Before I start taking things apart, maybe I could get pointed in the right direction?

[blaine]

Sounds like the pilot jet (Idle mixture jet) is plugged. The carb will need to come off for a good cleaning/rebuild.Anytime the choke helps the problem it is a plugged idle jet.

[AvalancheCCM]

So I manned up and took off the carb. I could see daylight through the main jet and the idle jet. I didn't use any cleaner this time around. Could it still be the idle jet if I can see through it?

[jonathan180iq]

ATTABOY!!

Being able to see daylight through the mainjet and idle jet means they are flowing but it doesn't mean there isn't a tiny obstruction. When you have passages that small, even a slight accumulation of particulate will make a big difference.

You can take a soft brass bristle or just pluck some boar's hair from your shaving brush and shimmy shimmy shine the inside of your jets.

My first gut response while reading your introduction was that there was a slight air leak. I used to tell people to pull their "S" tube that runs between the bottom of the air box and the crankcase to feel what a vacuum leak feels like. And that bucking feeling is a little similar to what you'd experience by doing so. To check for this, once you get it all put back together, spray some WD-40 over the boots, connections, and seals that you are associated with the carb and vacuum tubes. If the idle changes at all while spraying a certain area, then there is a leak. (Plus, you can later smear the excess WD-40 over everything and make it shiny pretty lookin')

That being said, the mid range bucking would also be best isolated by what controls the mid range fuel flow, and that's the needle jet.

Pull the black top of the carb, being cognizant of the spring that will try and shoot the lit off. Keep your hand over it so you don't lose anything. Once removed, be gentle with the little rubber diaphragm and pull it out. You'll see the needle way down in the bottom of the recessed cavity. Take it out. Move the circlip down one notch on the needle and put it all back together. This will slightly enriched the fuel flow through the mid range and hopefully up whatever your little issue has been.

Even if it's not the problem, lots of riders over the years have really enjoyed the slight difference and increased mid throttle response that this mod delivers.

[AvalancheCCM]

Thanks for the replies. I figured something must be wrong with the needle since it's happening at mid rpm. I took apart the jet needle and noticed that I don't have little notches like the other photos/guides listed in the forum. I do have a plastic washer that has some friction so I tried moving it down further to create the same effect.

When I tested the bike, it was 90 percent better. I actually went for a 90 mile ride the next day because I was excited to have the bike working again. The bike performed pretty well, but over time the problem came back. I haven't taken apart the carb again but my guess is that the washer has moved back to its original location.

I think I found the problem and need to fashion a permanent fix. Any one ever seen a needle without the notches before?

[blaine]

You must have a aftermarket needle.The OEM one has no notches & must be shimmed to raise it.

[jonathan180iq]

There were SOME factory needles that didn't have notches. In this case, you can use a nylon or brass washer form any large home improvement store as your spacer.

Take the needle with you to make sure it fits properly. You will place the new nylon washer between the top of the needle and the old washer, essentially permanently raising your needle jet height. This enriching of the mid-range is very common on this bike and solves a lot of the issues that you're talking about.

You can even experiment if you like, try one washer first and then try two or three after that to see what works best for you.