86 VF500F Strange Fuel Issue

Hi folks!
I took a 300 mile highway trip on cool day, ran main fuel dry, switched to reserve and drove to gas station. Filled up, switched back to main fuel and hit highway again. Engine was running rough while going through gears at low rpm and seemed fine when at highway speed.

I figured fuel filter clogged but found no fuel pump or filter (normal on this model). Filter is in tank. Pulled tank and filter. It was spotless. Ditto with fuel trap etc. Didn't see water in fuel.

Fuel supply seems obvious problem - and am suspicious of diaphragm in fuel cut-off valve. It might be sticking. Vacuum does operate it.

Strangeness is that it seems like a cylinder is now missing. I took compression and found 115 to 95 psi, but will redo because I need better tools - bike designed to thwart hands my size. I think normal should be around 150 psi - but bike has run strong.

I'm just trolling for ideas. Anyone have a similar experience?
Thanks,
AB

 

check your plugs and run some seafoam. that should help out.

 

sounds like your tank isn't breathing through it's vent possibly... mine did similar one day out of the blue...breather hole in gas cap had become plugged.



Regards,


Will R. Seattle, WA.

 

Isnt it pretty easy to check if the fuel cut off is working? You know disconect the main line out the tank and suck on the vaccumm line. Gas should poor freely out the main line with vaccumm applied to the other line right? Thats how it was on my cb750...

 

Water in the tank from your last fill up?

Drain tank/carbs and use fresh gas. Check your plugs.

 

This is from an old post, but I wanted to let anyone interested know that the problem was cleared by degunking and blowing out the carbs. I'm going to put an after market low pressure fuel filter inline where I can see it, to stop this happening in the future.
I'm so glad to be on the road again, and that it wasn't anything major.
Best,
!A

 

good job.....