gas cap and petcock?

[badbrown]

I was having a problem with my 78 550 where I could not cruise at highway speed because the bike would surge and not maintain a steady pace. I've continuously read that the problem is likely caused by the gas cap not venting. It turns out that the petcock was clogged, I cleaned it, and the problem went away. So theres the answer. Opening the gas cap while riding is DANGEROUS. I 'm glad I did'nt try it because that was'nt the problem. Just clean the damn petcock. Also now the on position is working properly and there are NO vacuum issues.

[tconroy]

I was having a problem with my 78 550 where I could not cruise at highway speed because the bike would surge and not maintain a steady pace. I've continuously read that the problem is likely caused by the gas cap not venting. It turns out that the petcock was clogged, I cleaned it, and the problem went away. So theres the answer. Opening the gas cap while riding is DANGEROUS. I 'm glad I did'nt try it because that was'nt the problem. Just clean the damn petcock. Also now the on position is working properly and there are NO vacuum issues.

That's what I like to hear! Someone who has a problem, fixed it, and is telling other's how it was done!Good going man!!

[denydog]

I was having a problem with my 78 550 where I could not cruise at highway speed because the bike would surge and not maintain a steady pace. I've continuously read that the problem is likely caused by the gas cap not venting. It turns out that the petcock was clogged, I cleaned it, and the problem went away.

Now that you mention it, I've been having a little problem with an occasional bogging when I open the throttle wide in top gear at about 4,000 rpm. Only happens once in a while, but is very pronounced. It sort of comes on for a few miles and then clears up some, or more likely is fine when I start my next ride. Kind of feels like not getting enough fuel when it's doing it. I've been dealing with this for several months. I recently cleaned my carbs for the second time, mostly because of this. My petcock flowed good when I sucked on the vac hose, and the screen is clear. I took my gas cap apart (still hoping I got it reassembled correctly) and the vent is clear.

For a while I thought it was related to my carb screw adjustments, because the problem seemed to occur right after I adjusted them out, and would go away after I readjusted them. But that doesn't make sense because it's not a problem associated with idle or low throttle opening.

I'm still thinking inadequate fuel flow, so I tried turning the petcock to prime when it bogged during a ride. But I'm not sure if that had any direct effect, because the problem occurs on such a hit or miss basis anyway.

My current thinking is that my petcock vacuum line is a little too long and kind of pinches where it doubles back on itself, partially shutting off vacuum and the petcock. Might make some sense with my experience with the carb screw adjustments, because when I lift the tank to do the adjustments, the vacuum hose sometimes settles back into a slightly different position. So when I lifted my tank the other day to sync the carbs, I tried to place the vacumm hose into more gentle bend. I know, I could just shorten it. Anyway, I rode the last couple of days without any bogging. So maybe the hose was pinching off slightly and slowing fuel flow out of the petcock. I've got my fingers crossed, hoping that I'm on the right track.

Don't you just love a good mystery? :-D

[badbrown]

True. You had the same effect happening. Your bike was starving for fuel and leaning out at higher rpm as did my550. Getting fuel then starving for fuel and on and on. Glad to see you solved the issue. I went for months on end tracing the problem down. Ignition? Timing? Vaccuum? Badly Tuned carbs? not enough air? Gas cap? as I 've read a bunch of times. Not enough fuel causes the bike to do this. I guess that's part of owning an old bike.

[denydog]

Ya, I HOPE I've identified the problem. Like I said, I've got my fingers crossed, and I might just shorten the vacuum hose to make sure it's not kinking. If you hear a loud screaming noise in the next few days, it might mean that my problem has returned.

I really know what you went through with months of trying to think of anything that could be the cause. I almost was hoping my bike would just quit completely, so I could get to the bottom of the problem. I was just about ready to order new coils and spark wires last week. Those problems that come and go are really difficult to diagnose sometimes.

[badbrown]

Remove the vacuum line from the petcock, and block the line with your thumb. You should feel the vaccum in the line while the bike is idling. If you feel the vacuum then it's not your carbs. Try removing the petcock. If yours still has the filter attached it should be ok. Also look for the pinch. Mine doesn't have the filter and the ON hole was fully blocked making my bike only run on RES. Probably happed the last time I cleaned it. Some gas mixed with some carb spray and gelled up in the hole. No that I found this the bike revs and rides normally, even though the carbs are NOT tuned the BEST. And the ignition is NOT timed the BEST.....Also I run an inline fuel filter before the carbs.