Stalling Issue, Please Help

[Yusiro]

Hello All,

I've been having an ongoing issue with my 1982 xs400 heritage special. Sometimes when riding, the rpms will start stuttering - like going from 5500, dropping suddenly to 4500, then back up to 5000, then up to 5500, ect. It does this without any movement on the throttle (and freaked out the girlfriend the one time she was on the back while it did this). Eventually, the bike will just start losing power, slowing down (even with full throttle) and will stall. I stop, turn of the bike, then go to restart and it won't restart until I let it sit for a few minutes - and even then it takes some luck.

The bike used to do this more often, but I figured out that if I let it warm up for a good 5 minutes before riding, it would almost never do it. It seems that it is mostly doing it on rides that are longer than 50km (it's only done it on a short ride once, but I think that may have been due to a lack of warming up).

On saturday I was riding on my way to visit my mother, and it started to stutter again and died. This time, when I went to start it again, the battery died. The battery wasn't that old, but I guess these things happen. I got a new battery into the bike and it started up no problem. After getting home I checked the charging system with my multi-meter - it read ~12.5-12.9 volts at idle, and 14.5 volts at 5k rpm - so the charging system is working. I figured that the battery was the issue of the whole thing, so was thankful it was fixed: I was wrong. On the ride back from my mothers house last night, after about 50 km, the bike started stuttering a little bit. My girlfriend wanted to stop anyway, so we took a rest. 10 minutes later we're back on the road and it's stuttering and stalls again. I had a lot of trouble getting it started up again and it stalled 4 more times on the rest of the ride home (about 40 km).

The only way I could keep it from stalling at lights was to keep the throttle up - if not it would just slow down to a stop.

If anyone has any idea what could be wrong with her, I would appreciate the help. I sprayed out the carbs with carb cleaner, but havn't taken them apart to clean them (if that might be the problem).

Thanks very much in advance, sorry about the essay length post!

 

[OliverB]

One possibility is fuel starvation. Is your gas tank clean, is the vacuum fuel tap working properly allowing sufficient fuel to flow to the carbs? Is the gas cap vent blocked and building up a vacuum as gas is pulled from the tank? Pop the cap and see what happens.

You can't clean the carbs by spraying the outside or their open throat - but I am sure you already know that! LOL

Could also be an electrical breakdown but I would examine fuel delivery first.

 

[Yusiro]

Hey Oliver, thanks for the reply. The gas tank is clean and sealed - it was just done a few months ago. I'm thinking (hoping) it MIGHT be the gas cap vent but am unsure. I did pop open the tank last night to make sure i wasn't out of fuel, that would have allowed air back in so would have fixed the problem though right? It still didnt run well after I did that.

I have noticed though - when I take the tank right off the bike, no gas leaks out of the petcock. I'm not sure if this is an issue, or if it is working how it is supposed to be. Could you tell me what the gas cap vent looks like? I replaced the grommet on the gas cap because the old one was rotted almost all the way through, so it's possible I put something back together wrong.

Yes I know about cleaning the carbs, LOL, it was more of a maintenance thing.

 

[OliverB]

If you open the gas cap and leave it open but problem still persists, no it probably isn't the cap vent. I replaced my cap gasket also. There is a small hole in the metal place, IIRC, that allows air in to equalize pressures as gas is used. As long as this vent is operating, it's ok.

If you have the original type fuel tap on the bottom left of the tank, it is vacuum operated in the norman run position. When the engine is not running, there isn't a vacuum pull on the diaphragm in the valve so it is closed and gas should not run out when the hose is pulled off the carb or tank. When turned to the prime position, vacuum does not come into play. Gas should run freely from the tap.

 

[bentwrench]

If you have good spark, then it's likely fuel. You're doing the right thing starting at the tank, but you can also check to see if you have fuel coming into the carbs by trying to drain the carbs (with something to catch the gas.) If no fuel comes out, you have no fuel going in. If there's fuel, then likely your carbs need to be at least adjusted, and possibly cleaned. When was the last tune up and cleaning?

 

[Yusiro]

Last tune up and cleaning was before I got the bike. I'm thinking that's most likely what the issue is, but am not sure I trust myself to properly dissemble and reassemble the carbs... I might look into it though

 

[bentwrench]

If you haven't done it yet, download the manual and parts guide for the 82 XS400 - I think it's a seca, but it works for all of the 82 bikes.

And for the carbs specifically, you can get by with the XS650 carb guide. Your carbs are BS34s.

http://www.amckayltd.com/carbguide.pdf

Just ignore the jet sizes - the cleaning and adjusting process is dead on.

 

[Yusiro]

Thanks all - I'll try cleaning the carbs out. Wish me luck!

Sorry - quick question - Should I get two rebuild kits? If so, could someone recommend a place/which ones to get? I looked on a few sites, but was unsure which rebuild kit to get.

Thanks

 

[bentwrench]

Don't worry about a rebuild kit until you've done a good cleaning. Boil the jets and any brass bits in boiling lemon juice (really). Clean out the passages with carb cleaner and compressed air. Check your floats, balance the carbs, and if you're still having trouble, then look to spending money.