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What has changed with my bike????

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Greetings all. I've been battling this issue for a month now and cannot figure this out. Wondering if you guys can shine a light for this blind fellow to follow.

Bike is a 2003 Yamaha YZF250. It used to run perfect through the RPM range and always lit first or second kick hot or cold. if I cartwheeled the bike it would take about two or three kicks to fire up. It was my first four stroke and i was glad I was not having the starting difficulties other people were ladened with. Lately however things have changed for the worse. At first it started having a huge bog and running a bit rough. I checked the AP settings, check valves in the bowl, AP squirt timing etc. The cure ended up being a one step richer pilot (42) and one clip richer on the needle. This did not make sense as it was running fine before with the stock jetting:banghead:.

I could not argue with success but was clueless on what was changing in my carb. I assumed an airleak so took the carb apart cleaned everything out, jets, orifices, etc. and installed a new carb to cylinder manifold just to be safe and put the jetting back to stock. Reinstalled the carb making sure everything was sealed and aligned properly. This did work and so put the 42 back in and went back to the one step richer on the needle, It runs fine once i'm rolling above idle. No bog, just pulls cleanly all the way through the RPM range. However the bike now takes dozens of kicks to light up both hot and cold and has me absolutely frustrated.

Things I have also checked for a source of the problem:

* Valves adjustment in spec

* Timing marks aligned correctly

* TPS voltage checked out, both throttle open and closed

* recent piston and rings

* new spark plug cap and plug

I've even tried going back to the stock 41 pilot but the difficulty of starting is still there.

I've even taken a buddys WRF250 carb and bolted it on to no avail. It still starts reluctantly hot and cold.

So gents where else should I be looking for solutions to my issue? Where else should I be checking for possible air leaks as I'm still confused why the richer jetting fixed a bog problem when I never had an issue prior with the stock 41 pilot.

Thanks in advance.

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Thank you for the reply William1.

Any idea how I can check the stator to confirm it is the issue?

Can the malfunctioning stator cause the bike to need the richer pilot jetting that fixed the bog that was not there in the past?

Again thank you for your insight.

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Pilot and fuel screw are for idle only.

The only thing that affected jetting on an untouched bike is air density (air temp/humidity/air pressure-altitude) and wear (typicallt the needle jet emulsion tube). If the carb wears, it runs richer, not leaner. If it is only a bog, then it may just be the AP. Confirm you have a good squirt.

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Just gonna throw this out there.. Whats your air filter look like? I assume you do normal maintenence, but you didnt say, so I figured I would ask. A dirty air filter will almost exactly what you describe.

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Pilot and fuel screw are for idle only.

The only thing that affected jetting on an untouched bike is air density (air temp/humidity/air pressure-altitude) and wear (typicallt the needle jet emulsion tube). If the carb wears, it runs richer, not leaner. If it is only a bog, then it may just be the AP. Confirm you have a good squirt.

William1 - The bog began to appear and I checked the AP. It was squirting fine and missing the slide. Since it worked fine with the stock pilot I never thought about rejetting. Out of frustration I put in a one step richer pilot and it cured the bog. Head scratching time as I could not figure out why the jetting needed changing since the weather is consistent where I am. I also went richer one step on the needle clip and that improved performance though when initially dialing in the bike I tried that position and it was not ideal.

Is it possible for the carb to have an air leak from somewhere other than an unsealed joint between the carb and rubber boot or a hole in the boot? One place I thought might be the air screw hole if the oring is missing, however the o-ring is in the place. Any other places to check?

Dirty907 - Yes air filter cleaned and oiled.

Fluffdad - Piston replaced 3 rides ago.

Thanks all for the advice in searching for the cause of my hardstarting YZF.

One symptom I have that may be a clue is that I get a hanging idle I cannot get rid of if i set the idle correctly. If I back off the idle screw to where it barely ticks over and stalls there is no hanging idle however the bike will often stall when I come to a stop since the idle is set so low.

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