Lacking power in 3/4/5 gear
 

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!

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

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.

Stock sprockets/gearing g.

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

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.

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

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

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.

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

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