Going lean/Fuel Starvation?

[Thompo]

Had my 2011 250 out last night for an extended run since swapping needle.
Jetting is
40 pilot
NECW /3
178
Sea level - 300m @ 20?C (UK Summer time lol )

Riding a turn tracks for few laps was fine.
Moved over to MotoX track and after a couple of laps it felt like it was running out of fuel/ Going lean, revs climbing, erratic throttle and making tons of unexpected power approaching lip of jump etc

Pulled in flipped the tap onto reserve and rode on for another few mins until it done the same then I stopped to investigate.

Checked my tank and still had a good 1/2 - 1/4 left.
Couldn't find any air leaks, fuel tap is flowing well on both positions, idle is stable, im thinking float needs raised a little?

What is throwing me is that I had the carb slightly out of alignment vertically, maybe 5-10? and when on the side stand it was over flowing. I realigned the carb and the overflow stopped.
This change was made inbetween jetting and running last night. So I thought the floats were about right?

Cheers

 

[Jakobi]

You've checked the right things off. If the fuel is flowing freely and the engine is still leaning out on sustained throttle, then perhaps the bowl is running dry. Or is it a sticky needle valve that's not opening? Sticky check valve in the cap?

 

[Zman]

Double check the floats are aligned with each other. It is common that they get bent and one float sits higher than the other. Take the carb off and make sure there are no obstructions between the needle seat and the fuel inlet. Occasionally we get junk in there. Make sure the petcock filter is not plugged up with junk in the tank.

The carb is on a complex angle so it needs to be aligned (carb horizontal) when the bike is upright (not on the side stand). If you can not eye it you can use a small level. If you are jetted rich it can increase this problem.

 

[kdxkid]

Check the hose form the gas tank lid (to the frame) is not clogged. If that is clogged it could great a vaccum effect and prevent fuel from flowing into the carb. Seems crazy, but it can happen.

 

[swazi_matt]

Check the hose form the gas tank lid (to the frame) is not clogged. If that is clogged it could great a vaccum effect and prevent fuel from flowing into the carb. Seems crazy, but it can happen.

especially common if you have those shorty tank cap hoses

 

[Zman]

Excellent points! and some of those checkvalves on the breather restrict flow too.

 

[Thompo]

Cheers guys for the suggestions.
Breather pipe is clear and venting ok into the frame.
Bit of crud in the tank but nothing significant pulled and cleaned anyway.

Not sure if I've found the source but the feed pipe from tank to carb had a small kink just before the inlet barb. Would have been worse when fitted.
Only noticed it when I took the cover/spring off the pipe.
Trimmed the pipe back a bit just to test it.

Going to pull the carb now any for a clean and check over.
Out tomorrow hopefully for more testing

 

[Jakobi]

Kink would restrict flow for sure!!

 

[Jacob 'Berg]

My main jet backed itself out a few months ago and I had similar symptoms..

 

[Thompo]

Actually found some Go-Pro footage of the bike doing this for 1st time.

https://www.youtube.com/watch?v=wcvpqCUmMJQ

No comments on riding ability (or lack there of..) please lol

Didn't get around to pulling the carb instead trial fitting stuff waiting for my SSS to arrive.

Will give it a once over this evening and see what I find, as riding again tomorrow.

 

[Thompo]

Carb checks out ok although the floats may have been a little low.

Tested by blowing gentle through the feed pipe and seeing where it stopped flowing. Set it to just off level which measured around 6.5mm.

As an aside it's unseasonably warm here in the UK at the minute with temps around 25. That would lean it out a little more too wouldn't it? So a 1/2 turn in on the air screw would be worth a try too?

Hopefully get out for some testing tomorrow evening.

 

[(F5)]

Colder will lean.