Originally Posted by c_hasbeen
. . . I put a regular pipe cap on it.
If it is supposed to be vented cap will I notice a vacuum if it is running and I hold my hand over the tube?
maybe a piece of plastic bag?
If it is supposed to be vented but is not, I don't think the fuel draw will be fast enough to feel suction on your hand, but I may be wrong. But once the engine pumps enough fuel out, there will usually be a vacuum that will prevent drawing any more and seem like you ran out of fuel.
As a result, you open the cap and see there is indeed fuel, put the cap back on and voila!, the engine starts and runs again until it doesn't. My snowblower lost a vent spacer in its cap this winter, and with the cap on tight it was pulling this trick until I figured it out.
On the sheet metal passenger car tanks, I have heard that with a strong enough fuel pump the vacuum can collapse the tank instead of starving the engine. But the heavy tank on a bus should be strong enough to resist this.
If the engine will run for an hour with the un-vented pipe cap on the tank, you should not need to add a vent and be good to go.
If you have any doubts, a pinhole in the pipe cap would relieve the vacuum. I'm guessing that 1/8 inch or even smaller might do the trick. I would maybe even start with 1/16 inch. If the bus idles and cruises fine, but seems to starve going up long hills unless you take your foot off the throttle, you might gradually enlarge the vent until you have continuous power at all loads.