Lacking power in 3/4/5 gear

[new_bee]

New gz250 owner (2003 15k miles).. seemed to run great on a brief, back-road test drive.. Luckily it was cheap, and factored in some $ on maintenance, etc..

Honestly the bike seems to run great... it revs up well in neutral.

In first gear the bike accelerates well and buzzes right upto ~25mph, and upto 35mph+ in 2nd gear... in 3rd/4th/5th, I can get the bike upto ~45 (barely maybe 50mph downhill)... in 4th and 5th the bike just won't speed up much at all.

There are a few similar helpful posts..but nothing conclusive!

I have gone through and read a lot, downloaded the service & owners manuals, and done searches on this forum & google, and I think I have covered most of the basics..

A couple quick notes:
-previous owner had the oil about 1/4-1/2 quart overfilled. not sure type of oil or anything, or for how long, but oil didn't look bad when drained.
-in neutral I can hear a slight ticking sound that is not there when in gear, seems like it's possibly from the clutch/ transmission area... not bad or obvious (not sure if this is normal) but noticeable
-old spark plug maybe seemed a little whitish or dry..

So far I have
-Emptied fuel tank, new fuel filter, checked petcock for good flow
-Fresh gas with strong dose of seafoam.
-Drained carb and filled with straight seafoam, then let sit overnight. The gas that drained out of the carb didn't seem visually bad, gelled or anything at all..
-Fresh motorcycle rated oil
-New air filter
-New plug
-Adjusted clutch cable, set to dime gap, then tried going for extra slack/play before it engages to be sure.
-Bike coasts well so it doesn't seem like any extra resistance anywhere.
-Tested for air leaks around intake boots and rubber hoses by spraying starter fluid around those areas while running, no change in idle speed. Tank vent is working.


I haven't yet pulled the carb for a clean/rebuild, or checked the valve clearances, or checked the compression yet.

I have put about 50 miles on it after doing the things listed above and it hasn't changed much at all..so I don't think it really feels like a carb issue that is going to clear up with carb cleaner in the gas.


I am hoping, at this point that a full carb cleaning will be all it needs! Really crossing my fingers because beyond that, I might be hiring a mechanic.

If that doesn't do it, next I will try to figure out if adjusting/checking the valve clearances is something I can do myself.

Finally, I wonder if anyone can describe or link me to any references for checking or evaluating if this might be an issue caused by the clutch? (clutch slippage)

IS THERE ANYTHING I AM MISSING?

thoughts, advice or encouragement greatly appreciated! thanks!

[Fawlty]

If the bike has a 16 tooth rear sprocket, you can expect some loss of power in 4th and 5th. Worth checking.

[Vegas Street Rider]

I have a 16 tooth front sprocket and although it is not as peppy in 4th and 5th, I still have plenty of power to take it up to 70/75 mph on the flat with no head wind. I don't think that is a factor.

[new_bee]

Stock sprockets/gearing g.

Anything else I should check before I pull the carb as the next step?

[new_bee]

Carb rebuild did it. I think the carb was perfectly clean, but possibly the diaphragm was pinched on the upper portion around the lip where it seals.

[5th_bike]

Good. To finish things off, I'd check the valve clearances.

[new_bee]

So after around 20 miles of running, the problem was back. It seems like the same exact symptoms, lacking power, and the symptoms seems to have come on fairly quickly.

I believe the diaphragm might not be sealing correctly in the grove, I took it apart twice, the second time I put it back together with a small amount yamabond adhesive around the o ring, but still runs the same. I've ordered a new diaphragm but not sure if I should have ordered the whole diaphragm slide assembly.

I checked the valve clearances and they are within special. But I noticed a minor oil leak behind the left top engine access cover.
I wasn't really able to identify where it was coming from.

Any thoughts on either issue appreciated

[Vegas Street Rider]

Check the head bolts. Left top oil leak seems to be pretty common with the GZ. I had a minor leak and after torquing the head bolts, all is well.

[derbydave]

Is there junk in the tank that is clogging a jet? I would clean the carb again and install an inline filter in the fuel line. A problem with the diaphragm sounds unlikely. It doesn't take very much at all to plug up a jet

[new_bee]

There's a new inline filter (old wasn't bad), and tank is clean.. No junk in carb at all both times I cleaned it.

Thanks for heads up on head bolts