No Acceleration at 40MPH
 

I did a search and couldn't find anything so I figured I'd start a new topic. I've been getting to know my 2003 gz250 in the past month and from what I've read about top speeds and acceleration I think something is wrong. On flat roads the bike does not get above 45 even with wide open throttle in fourth gear.

It wasn't running when I bought it but all I did was put a new battery, new spark plug, and dismantled/cleaned/reassembled the carburetor. I did go an extra half turn on the pilot screw and shimmed the needle (not a true shim, just switched the position of the washer per one of the topics on this forum). I also changed the oil to synthetic (bike had about 6000 miles when I bought it). The engine sounds good but power doesn't translate to the wheels.

I weigh ~170 lbs so I don't think its a weight problem. I did take the windshield off but I don't think wind resistance should keep me from hitting 60-65.

At a loss for what it could be - any suggestions?

I wonder if the air filter is plugged up a fair amount. To check the filter it must be removed and inspected. Air flow is from the inside to the outside( reverse to what we normally see in filter systems). The inside gets dirtier than the outside.

The airbox is a sealed system. You can't drill holes in it to increase airflow, all you will accomplish is feed dirty air directly into the engine.

Changed the air filter and still have the same issue. Went from 2.5 turns on pilot screw to 1.75 turns and no change either.

From what I read it seems that this could be a symptom of a slipping clutch? I don't really know how to gauge whether or not it is slipping. I don't have problem shifting gears but I do notice that between halfway and full throttle the engine revs up but doesn't actually move any faster. Any thoughts?

OK, find a long uphill stretch of road, get into 4th gear at about 40mph or 5th gear at about 50mph, then try and accelerate using full throttle, if your revs increase, but your speed doesn't, your clutch is slipping.

Ah, more info to work with. Likely as suggested..........clutch adjustment needed.
Secondly...........did you do an oil change and exactly what kind of oil did you use? You must use moto specific oil or the clutch will likely slip. Must be a JASO certified oil for wet clutches.

When adjusting the clutch cable, keep in mind that the clutch handle (grip) needs to be moving freely about 1 cm at its tip.

Should be about 1/2" (12mm) at outer tip of clutch lever.About 2 to 3 mm between inner end & perch. ;) :)

Can the bike rev up, strong and loud, if it's in neutral?

Changed the oil two months ago and used synthetic royal purple 10w-40. Also changed spark plug back then - guy at Napa told me it was NGK DCPR8E

I thought it was the oil so I changed back today to Castrol GTX conventional 10w-40. Only had about 70 miles with the synthetic - what a waste!

Anyway the bike did not perform any better so I thought maybe the spark plug. Looked up the spark plug and found the internet says it's NGK DR8EA, not what I have in there. Took the old one out and it's black - covered in soot. Not wet which is what I thought I might find. Took her for a quick spin after changing the plug. I think I notice a slight uptick but it got dark so I'll have to wait until tmrw

Also I did check the clutch and it's correct. And yes it revs up strong and loud. Too loud actually, I changed the idle screw to the bare minimum. Do you think that may have something to do with it?