bogging down on the highway

[techyguru]

I have been lurking around here for a bit and now I need some help. I have a 1980 Yamaha xs400 special II. I have been learning to ride it for the last month. I was coming back from visiting the parents and was doing a steady 60mph on the highway, I started to come up a hill and the bike bogged down. After that, even back down the hill, it would not go more than 55. You could tell it wasn't running right, felt heavy and slow/no acceleration. I moved off the road and looked the bike over for a bit and when I fired it back up and moved on it ran fine until I got back, about 10 miles after that.

It happened again on the freeway this week. Was going around town and jumped on the freeway. I was going 70 for just a few minutes and it started again. Luckily traffic was slowing down so the bike was able to keep up, but it would struggle to do 55 after that. After exiting I was slowing down for a red light and the bike stalled. It did this a few more times but was able to keep it running while sitting at the other lights with a little gas to keep the idle a little higher.

It had half a tank of gas the first time, and full the second. It was about 60 degrees both days. I can drive around town for hours without a problem. Is it overheating on the highway? Is the bike too small for these speeds? Any help or suggestions would be greatly appreciated. Let me know if you need anymore info.

Thanks in advance.

[BuzZz]

Next time it happens, pop the gas cap and see if the problem stops.
Can you hear/feel air rushing in when you open the cap?
If you can, you have a clogged gascap vent.
You can try to take it apart and and clean/fix the one you got, or just buy a new cap.

[techyguru]

I'll give it a try in the morning, any other idea's?

[BuzZz]

I'll give it a try in the morning, any other idea's?

'Fraid not. Just a lot of confidence.

Seriously, I think that's your problem. It is common. Common enough, I should make this thread a sticky......

[techyguru]

Well I went out riding today and hit the highway for a bit. It started bogging down again after about 10 min. I pulled over and took another look. The fuel line was half empty! I switched it over to prime and the fuel went back in. Put it back 'ON' and got back on the road. After another 5-10 miles it started again, flipped the petcock to prime an the engine picked right back up.

I figure it's either a clog in the petcock or something like that. Anytips for clearing it up or should I just rely on the prime trick while I'm on the highway? Also anyone know if having it one prime while riding will cause any harm to the engine?

Once again thanks for the help

[Damian]

Well, it depends on how the petcock on your bike works (I'm not familiar with bikes other than mine), but if your floats are working right, you can probably ride with it on prime until you get it fixed (assuming that's soon).

If it flows in prime, but not in On (again assuming yours works similar to mine), then it's not likely a matter of needing to clean it out. When it's switched to ON. it uses vacuum provided by the engine to open a valve and lets the fuel flow. On Prime, this valve is bypassed and fuel flows even without the vacuum.

So, I'm guessing that you might have a vacuum leak somewhere (which will also effect performance) or the valve is malfunctioning.

Did you try slowly removing the fuel cap and listening for pressure like Buzzzz suggested?

If you're sure that's not it, I'd start checking connections on hoses coming from the carbs to the petcock (I'm guessing there will be a black one that doesn't contain gas), this is the vacuum line. Check it for cracks and that it is seated on its nipples.

If that is good then start from your air intake and check all the connections where air goes to make sure there is no other leak. If you can't fix it there, then take off the petcock, break it down, and see if there is anything obviously out of place/cracked/missing.

I hope someone will chime in to confirm my suggestions.

[storysunfolding]

My old 81 xs 400 wasn't a vaccum operated petcock. If there's just one line coming out of it then yours isn't either. I'd listen to buzz and fix the gas cap vent- a vaccum building up in the tank (from fuel flowing out and no air getting in) would do exactly what you're describing.

It buzzes like hell at 60 mph doesn't it? If you haven't already gotten new handlebars (heavier) think about it and get some nice foam grips. Make the ride nicer. Kick starts also rock out.

[techyguru]

I tried the gas cap thing a few times and didn't hear any noise. Also opening the cap didn't improve the performance.

It is a vacumed opperated petcock, so I will check on the tubes, ect for leaks. Could this be caused by bad airfilters. The previous owner did mention that he had some issues when they got wet once and he didn't change them out.

Off to try some more things, Thanks

[storysunfolding]

I tried the gas cap thing a few times and didn't hear any noise. Also opening the cap didn't improve the performance.

It is a vacumed opperated petcock, so I will check on the tubes, ect for leaks. Could this be caused by bad airfilters. The previous owner did mention that he had some issues when they got wet once and he didn't change them out.

Off to try some more things, Thanks

Might as well replace all your fuel and vaccum hoses. It's $10 worth of peace of mind.

[techyguru]

took a quick look and there is a hose that has a nice crack in it and it goes all the way through. according to yamaha's web site its a PIPE, BREATHER 2L0-11166-00-00. and goes from the crankcase to the tubes comming off the air filters. Anyone know of a yamaha dealer in the south metro of minnesota?