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greg737 · April 15th, 2012, 11:36 AM

Finding out whether or not the petcock vacuum actuator operates properly is a start. It'll be good to know that information because your description of the details of your problem certainly sounds like a fuel delivery issue.

If the petcock checks out, i.e. flows fuel normally under vacuum pressure (you sucking on the petcock's vacuum line, sounds bad doesn't it?), and doesn't resist vacuum pull (due to a clogged atmosphere port, it's the little hole at the bottom of the petcock), and doesn't have a leak (slow, pin-hole type of leak) through the rubber petcock membrane that reacts to the engine vacuum pressure or doesn't have a leak around the edges of the membrane were it is supposed to be firmly sealed by the pressure of the screws that hold the petcock assembly together....

.... then you'll have to move on to evaluating the vacuum line from the carbs. During engine operation the carbs will experience varying levels of engine-produced vacuum. Across the engine's operating envelope a closed throttle situation generates a high-vacuum condition while a open to wide-open throttle postions produce less vacuum pull in the intake tract. If there's something interfering with the transfer of vacuum pull through the tube from the carb to the petcock, that is slowing the fuel filling of the carb's bowls, then it would be more obvious at larger throttle openings. If this is your problem then it might be true that you could ride the bike forever around the neighborhood at low throttle positions because the higher intake tract vacuum pull would be getting the most fuel through the petcock, plus you're using less fuel at the lower engine speeds.

It could be that your fuel starvation description confirms this senario. If the bowls were only just barely filling fast enough to keep the bike fed at the 50 mph criusing speed then the sustained forward "slosh" caused by deceleration might be able to cause the bike to stall.

Also, you mentioned that the bike had been stored outside by the prior owner. Another factor in the flow of fuel from tank to carbs on the EX-250 is the negative-pressure relief valving in the gas cap (the EX-250's gas cap is actually a pretty complicated item if you take the time to look at it). If something is preventing the gas cap from venting properly against the negative pressure that normally develops inside a gas tank during engine operation then it could be part of or the entire source of your problem.

April 15th, 2012, 11:36 AM	#12
greg737 - Name: - Location: - Join Date: May 2009 Motorcycle(s): - Posts: A lot.	Finding out whether or not the petcock vacuum actuator operates properly is a start. It'll be good to know that information because your description of the details of your problem certainly sounds like a fuel delivery issue. If the petcock checks out, i.e. flows fuel normally under vacuum pressure (you sucking on the petcock's vacuum line, sounds bad doesn't it?), and doesn't resist vacuum pull (due to a clogged atmosphere port, it's the little hole at the bottom of the petcock), and doesn't have a leak (slow, pin-hole type of leak) through the rubber petcock membrane that reacts to the engine vacuum pressure or doesn't have a leak around the edges of the membrane were it is supposed to be firmly sealed by the pressure of the screws that hold the petcock assembly together.... .... then you'll have to move on to evaluating the vacuum line from the carbs. During engine operation the carbs will experience varying levels of engine-produced vacuum. Across the engine's operating envelope a closed throttle situation generates a high-vacuum condition while a open to wide-open throttle postions produce less vacuum pull in the intake tract. If there's something interfering with the transfer of vacuum pull through the tube from the carb to the petcock, that is slowing the fuel filling of the carb's bowls, then it would be more obvious at larger throttle openings. If this is your problem then it might be true that you could ride the bike forever around the neighborhood at low throttle positions because the higher intake tract vacuum pull would be getting the most fuel through the petcock, plus you're using less fuel at the lower engine speeds. It could be that your fuel starvation description confirms this senario. If the bowls were only just barely filling fast enough to keep the bike fed at the 50 mph criusing speed then the sustained forward "slosh" caused by deceleration might be able to cause the bike to stall. Also, you mentioned that the bike had been stored outside by the prior owner. Another factor in the flow of fuel from tank to carbs on the EX-250 is the negative-pressure relief valving in the gas cap (the EX-250's gas cap is actually a pretty complicated item if you take the time to look at it). If something is preventing the gas cap from venting properly against the negative pressure that normally develops inside a gas tank during engine operation then it could be part of or the entire source of your problem.