45mph top speed on 250?

[wrzesinski]

Ok I bought this bike off craigslist and I got it home, I flushed the tank [there was rust in it] and cleaned the carb [jets were clogged] I got it running and I took it out and when I get to 45mph it starts to choke and die. I thought it was the rev limiter but at lower speeds it limits at 7-8k but in 4th or 5th gear it starts choking at 5-6k.

If I baby the throttle and the stars line up I can get it passed 55 and ONCE I got it to hit 65 going downhill with the wind on my side. What could be wrong?

I was also thinking that the original owner didnt change the oil on schedule, there is only 1700 miles on it and I'm probably going to change it to have peace of mind about when the next change is due, could this have anything to do with it?

I would really like to be able to get on the highway without being a hazard to drivers.

one note is that I can consistently get to 40 in 4th gear by babying the throttle, then when I shift to 5th I gain maybe 5mph then choke out.

what can I do?

[atrayu8nc]

Could just be the jetting of the carb. I don't know a lot about bikes, but owning a 72 honda cl 350 gave me a crash course. I used to have the same problem and the more and more I messed with the float adjustment the better or worse it got. I almost had it dialed in when I sold it. Then like you I found a zongshen on craigslist and it was to cheap to pass up. Been a fun little city commuter. Feel like the skinny kid in gym class whenever I stop at lights with the big bikes, but they aren't getting 95 miles to the gallon sooo. Think I filled up twice last summer. They are cheap bikes, but great for the intended purpose. Good luck getting yours true again.

[kristifer]

I have noticed that since i put an iridum plug (and gapped it correctly) as well as changing the oil over to a synthetic blend, it revs up much higher/smoother.  I get mine up to near 70 on flat roads and i weigh almost 250lbs.

[z250gs]

Just something to check that may help:
1. Whats the fuel filter like in the tank? Is it clean now?
2. Try "reading" the plug. You should get a idea of whether its running rich or lean. Or if its wet,etc.
3. Did you check your air cleaner. If theres a lot of oil in the overflow, you may have a overoiled foam filter. Maybe clean and reoil it with a FEW drops of oil.  
4. Check for overfilled oil in sump. PUll the bike up dead vertical and note the oil level is BETWEEN the high and low marks. If too high drain it out and refill. With a 2nd hand bike, best change oil anyway and definitely check the oil filter . (Some riders previosly mentioned they werent aware of the location of the main filter.) On a low mileage bike thats just run in there will be plenty of gunk on the filter medium-enough to clog it right up!.
5. Check for air leaks around carb clamps etc. (WD40 sprayed on inlet and outlet clamps to check for air leaks.)

[grumpy]

Could be the sign of wrong oil type in the case. Car oils can contain friction modifyers that work well in cars but cause clutch slip in bike engines with wet clutches. The clue to the problem is where you say you baby the engine throttle up to 40mph, then slowly get up to max speed . Does the engine sound like the clutch is slipping as you accelerate with a bit of grunt? If so, try changing the oil and make sure it reads " for wet clutch bike apps"  and does not contain friction modifiers or any reference to "energy conserving".
A lot of the fully-synthetic and semi-synthetic blend oils are designed for auto use and will contain friction modifiers so be careful to read the packaging before using those oils. The manual for the bike specifies SAE15W40SF mineral oil.