Fuel Starvation? Fuel cock? 1981 GS250T

[solarae]

Hi Folks, been riding my little GS250T Cafe bike around without a hitch for a bit. Staying under 50 mph for the most part on country roads. Had to run an errand and got out on the state highway and decided to open it up. Hit 70 and it felt like I was running out of gas. Pulled off, switched to reserve, bike ran OK, filled up and went back. Felt balky again at high speed and rev but recovered again. Rode some more the next day and hit a steep hill. Again felt like fuel starvation. I've run this hill before with no issue but had been riding longer when I hit it today. After almost pulling over and switching the fuel cock again was able to continue. Went back home and had intermittent problems. Finally actually quit on me a block from home but after pushing for a few feet, I decided to try starting it. It started right up and I got on home.
The bikes history is that it sat for awhile with only 4400 miles on it. I took it apart. Cleaned and repainted it, removed the airbox, installed shorty mufflers, K&N pods, raised the needles and took the 115 mains up to 120.
I almost replaced the tank with one from a 1981 450E but changed my mind and will keep the original tank. I have a new aftermarket, made in Japan fuel cock for the 1981 450E tank. Thinking of trying the new cock on my tank but at first glance I notice the fittings face to the rear instead of forward like the 250T. Also figure I better check for plugged gas tank vent. Then of course, junk in the carbs. I have a couple more size up mains I can try as well.
Any thoughts?

[solarae]

So I'm answering my own questions here. Got some time and decent weather so I went about troubleshooting.
Pulled the gas cap apart and it seemed pretty clean. Reinstalled.
Pulled the vacuum line off the carbs and the fuel line off the petcock. Replaced fuel line with some clear vinyl into a can.
Created a vacuum on the vacuum line. Good gas flow and sucking noises from the cap so I figured that was all good.
Popped the carbs off and removed the bowls. All clean.
Pulled the sparked plugs. Right side looked just fine. BUT the left had a crumbling insulator and not much electrode.
These are the original plugs. (This bike had sat for several years in the damp coastal climate of the extreme north coast of CA.) I had ordered new ones but got D8EAs instead of the stock D9EAs somehow and since the old ones looked good at last check I just left them. I went ahead and installed the D8s and while the bowls were off I went up one more jet size just for fun. Now at 122.5, I know you're only supposed to do one thing at a time but the spark plug failure made sense. I don't think my power loss had anything to do with the jets.
Ran the little bike up the steep hill near my house and hit over 50 with ease. Really ran good. Will try the highway tomorrow.
Small bikes at WOT are a lot more fun than big bikes at barely over idle if you ask me.
Any problem continuing to run the D8EAs do you think?

TIA, Edy

[Grimly]

A difference of one heat grade won't be noticeable, unless you're running on the ragged edge of disaster already.

[difylos]

Hello i have the same bike and the same problem with you.
Have you fixed this ?
Thank you

[LAB3]

Hello i have the same bike and the same problem with you.
Have you fixed this ?
Thank you

He checked his spark plugs and found a bad one then replaced it.

[Gorminrider]

Rode some more the next day and hit a steep hill. Again felt like fuel starvation.

I'm glad you found the problem,
but just as an "aside" in case it helps someone where new plugs do not, I've had a vacuum petcock fail to deliver enough fuel running 120kmh up a steep hill....turning the tap to prime fixed it...the hill was very steep and altitude was several thousand feet. Both of these conditions can decrease vacuum and apparently this particular petcock was not getting enough vacuum for full flow....so if it seems to be happening, try Prime position which allows full flow to easily test if you have the same problem.