Hi Everybody
I have a 1990 NC30 which strangely will not run when the fuel level gets below around half a tank. It is completely standard. When I bought the bike it had a performance end can on and a dynojet kit and I have jetted it back to original spec and fitted a standard exhaust. I have had the carbs off and fully cleaned, I have put new plugs in, new air filter, checked the fuel tap diaphragm and actually removed it temporarily to eliminate that, checked the fuel tap for blockage and I am now at a loss as to what is causing the problem. Has anyone else come across this problem?? I will be really grateful for any ideas that anyone would have as to what the problem could be.
Regards
Ash

Perhaps the tank breather hose is partially blocked [pinched]
If fully blocked, I would expect the fuel supply to stop flowing before half a tank is used.

Will it run with the cap open with a low fuel level?

as above is what id check first.

I had a tank that had a rag in it! as the fuel dropped the rag stuck aroudn the filter blocking it up, would only run on reserve then. (have you tried reserve?) same issue?

Mjne was doing similar, was the vacuum hose from the fuel tap collapsing and not allowing fuel to flow. An easy fix

My NC35 started running poorly on lower levels of fuel and it turned out the fuel hose had somehow managed to get a kink in it.

Yip, I agree with everyone else. If you say you've bypassed the vac, then essentially you've made it gravity fed. If you have a blocked breather, It would allow your fuel to expand wen full causing pressure in the tank which would explain why it would run with a full tank as the pressure forces the fuel down, but once that pressure deminishes, then it's like holding up a straw with the top blocked and full of liquid. the liquid stays in but once you remove the blockage the liquid flows out. Check out your breather hose or nipple for blockage, or maybe you got a "wet panty" stuck in there as "Neo" had and it's blocking your filter. Either way, I dont think it too big a prob, just keep the chicks off the tank

Cap

owen_captain wrote:just keep the chicks off the tank

But it's an NC30, they can't help themselves