'78 XS400 - power/carb/electrical issues

[aaronbeekay]

Hi everyone! This is my first post here- I've been reading threads for a while, and I've been really impressed with the helpfulness of the answers. Hopefully my problems aren't too terrible... prepare yourselves.

About a month ago, I bought a '78 XS400 in okay condition. When I got it, it would run if it was choked all the way, and after it warmed up you could let the choke back in to about halfway. It had a pretty rough idle, and it would stall out if you left it for too long. I didn't know it (it's my first bike), but it's also pretty sluggish - I have to really let in the choke slowly when I'm starting out in first gear to avoid bogging the engine down, and it takes about 20 seconds to get up to 35mph.

So. I scrubbed it all down, checked the air filters, and pulled the carbs. I boiled them in some lemon juice, sprayed carb cleaner all over, and set the float height (though now I'm worried I may not have done that properly - I'll be checking soon). I replaced the cracked inlet stubs with some less-cracked used ones I bought, put it back together, and it starts OK without needing choke, although the performance isn't much better. I think my floats are still too high, because the garage smells like gas whenever I leave it in there for too long.

Yesterday morning, I forgot to turn off the petcock overnight, and I think I flooded the engine - it wouldn't start for anything. I checked the battery, and it was way low (more on that later), so I put it on the trickle charger, turned off the petcock, and went to work. When I came home that night, it started great after a few kicks, idled perfectly, and MAN did it have power! I took it for a test ride around the neighborhood, and it was just ZIPPY like I just realized it's supposed to be. A total joy to ride. It idled perfectly, the throttle response was lightning-fast, just perfect. It was only when I got home and put it in the garage that I realized my left pipe was belching white smoke... crap.

So I left it overnight, expecting to ride it in to work this morning- stupid me, I forgot to turn off the petcock again. This time, I saw a couple drops of gas coming out of the air filter onto the chainguard, realized what was going on, and killed the petcock. Of course, it wouldn't start, so I left it until lunchtime, then tried to start it with the throttle wide open and the petcock closed. After a few cranks, it was happy, and it's idling fine now, but... the power is gone again. It's just like when I got it, except with the idle problem gone.

I'm kind of stumped about what's going on here. I'm not sure if I've set my mix properly, and I haven't synced my carbs yet, so those are two things I could do, but why would it have that one ride where it was PERFECT? And where was that white smoke coming from? If my alternator isn't putting out good power, could that contribute to any of these problems? (I notice the battery doesn't charge well.)

Thanks for reading through all that- and thanks in advance for any help. Hopefully I'm missing something obvious here...

[aaronbeekay]

So, an update: I'm even more confused than before. I've been riding the bike around, and it's been "normal"- sluggish, as before, but still running and not leaking gas everywhere anymore. It starts up without much trouble.

Today, though, it rained for about 15 minutes while the bike was outside. I didn't have anywhere to put it, so it got rained on. About 45 minutes later, after the rain had stopped, I rode it home. It was sluggish at first, but the surging it had been doing before was getting more frequent and more insistent. It seemed like it was happening when I was opening the throttle sometimes, so I just let 'er rip to see what happened. It gurgled a few times, but the surge turned into POWER! And it stayed! The bike was zippy the whole way home. When I got back, though, I could see a little wisp of white smoke coming out of the left tailpipe... again.

So- I think the power is correlated with the white smoke, and maybe it has something to do with the left cylinder? And maybe something to do with the rain? It was surging before this, though, so I can't say that for sure. I'm pretty confused...

[HoughMade]

What are the idle mixture screws set at? Mine are at 3.5 turns out which is way richer than the manual says, but it starts first kick every time on half choke and runs great with good acceleration when warm.

Also, set the valve clearance then balance the carbs.

How old is the battery and what makes you think it is not charging well?

[johnsnownw]

Welcome. Also, post some pics of the bike.

I'll await your next reply, that hopefully has a bit more info about the issues you're facing, before offering any advice--as HoughMade has already asked and indicated the things you need to look at.

[KirriePete]

Random thoughts on a Thursday morning ....

Gas overflowing - recheck the float heights and take a good look at the float needle valve(s) - the tips may be worn allowing gas to seep past even if the heights are OK.

Power - the symptoms sound like a major misfire to me, probably on the left pot. It is running on both cylinders isn't it? The safe test is to splash water on the exhaust downpipes when it's running (badly) and see if both steam off - the unsafe test is to touch the downpipes to see which one doesn't take the skin off your hand. How fresh are the spark plugs? When it's running like a dog, try pulling the left plug cap off (wear rubber glovesjust to be safe) and see if it makes any difference, if it doesn't then you've got a 200cc single!

White smoke - I'd normally associate the white stuff with water-cooled motors, you sure it's not just steam from exhaust pipe condensation as the left pot chimes in? Overfuelling usually gives black smoke, burning oil is blue but you can occasionally get white smoke from bad valve seals/worn stems where the oil isn't burnt so much as vapourised and spat straight out the exhaust. Just pray it isn't that!

HTH

[aaronbeekay]

Thanks for the advice! I had to ride it to work this week, but after I get off tonight I'll be taking a look at everything you guys mentioned.

I'm pretty sure my floats are set wrong- if I leave the petcock on while the bike's off, it'll leak fuel, smell gassy, and it won't start for a while - presumably because I'm flooding the engine! So at the very least, I need to redo my floats- after that, I figure I'll set the mixture and balance the carbs. I stopped by the local motorcycle junkyard/mechanic's hangout spot, and the folks there immediately told me that my left cylinder was probably flooding. They told me that when it starts running again, it smokes white (and this is really just a little bit of white smoke, not clouds or anything) because I'm burning out the crud I put into the cylinder in the first place. Oops... maybe I've been riding a single!

I pulled the plugs last night- one of them smelled like gas and it was black, the other was black with a tan tip, like I'm pretty sure it's supposed to look, but the center electrode was very small compared to the other one. Oddly enough, the side electrode (the part that bends up and over) didn't look worn... I wonder if it's a different model of plug? I'll post pictures later tonight, and maybe cleverer minds can guide me.

What will setting the valve clearances help with?

[aaronbeekay]

Man, this thing is going to wear me out.

Worked on it for three hours or so this afternoon. I rode it home from work and tried splashing a bit of water on the exhaust headers- sure enough, the left cylinder was cool and the right steamed it right off. So I pulled the carbs and reset the float height to 26mm, then double-checked- when I push up on the floats until I feel the valve seat, the floats are about 5-6mm from bottoming out on the carb body. I put them back on the bike, and... it doesn't start. It doesn't overflow, either, so that's good, but not starting doesn't help either.

I pulled both of the plugs and held them to the engine- the left plug didn't have spark, so I sprayed some cleaner on it and scrubbed it off, and tried it again- strong spark. Hooray. But the bike still doesn't start. Thinking that perhaps I had set the floats too low (they cut off the flow of gas too early), I took the carbs off again and reset the floats higher. I'm almost certain that the gas level in the bowls is high enough- what all needs to be submerged in the gas? There's the main jet, a screw next to it, and also a brass (?) tube near the edge of the bowl. Does that need to be submerged in gas all the time, too? Still no starting.

I got pretty worn out and hungry, so I'm going to take a break and eat dinner and come back to the bike. Not sure what I'll try next. Pictures soon.

[HoughMade]

What are the idle mixture screws set at? As for he float, 26mm to almost 30mm should at least start. Are you measuring the float to where it just touches the needle, but does not depress it?

When you set the floats greater than 26mm, you decrease the amount of gas in the bowl because you set the floats with the carb upside down.

[aaronbeekay]

Okay. Progress.

I finally got the bike started that same night- I thought maybe my makeshift fuel-IV (a fuel tube attached to a Erlenmeyer filter flask) wasn't actually getting enough fuel to the engine, so I put the gas tank back on. I thought that was what solved the problem until I came back later that night and measured the battery. Turns out, the battery charger I've been using to trickle charge the battery isn't putting out any power at all... it's just "trickle-draining" the battery. *headdesk* So I jumped the bike with my friend's battery, and it fired right up. Success! I'm running on both cylinders!

Unfortunately, it's running very, very poorly. It was backfiring frequently on one of the cylinders, and still smoking out of the left exhaust for a while. I tried to adjust the pilot mixture screws, but when I looked inside the carbs, it looked like the tip of one of the needles (the needles that are moved by the pilot mix screws, not the main jet needle) may have been damaged or broken. Is there any replacement for those needles?

Also, I can start the bike, but often after 5-10 seconds of a high idle, the engine will start to rev up to 4-5k rpm, and hold there or continue climbing. I suspect I have a leak in the rubber manifold boots between the carbs and the engine- specifically, the left boot. I have a homemade carb balancer- one of the "long-tubing-with-ATF-inside" deals- and when I attached it to both vacuum nipples, the ATF moved up on the right side and down on the left side and kept going until I hit the kill switch. It seems to me that that's a symptom of a manifold leak, yes?

I'll be back at it today- posting here is a good way to think about what I'm doing, which I'm not always so good at.

[aaronbeekay]

What are the idle mixture screws set at?

The idle screws are currently about 1-1.5 turns out. I had them set at 2.5-3 turns out, but the bike was backfiring frequently- this is before I realized I might have a manifold leak, though, so I may have to adjust again.

[HoughMade]

Fluid moving one direction may well only mean that you need to adjust the screw between the throttle plates. In fact, I would bet on it. Turn the screw to open the throttle more on the side the ATF was moving away from, then start it again and see what happens.

[aaronbeekay]

Fluid moving one direction may well only mean that you need to adjust the screw between the throttle plates. In fact, I would bet on it. Turn the screw to open the throttle more on the side the ATF was moving away from, then start it again and see what happens.

Even with the engine jumping up to very high RPMs symptom? If you're right, I'll be very very relieved- I'll try it tomorrow.

[HoughMade]

I think it's too lean. If it were me, I'd turn the idle mixture screws out to 4 turns and see how it runs. If you still suspect and intake leak, spray wd-40 or carb cleaner where you think it may be and see if the rpm changes. If so- leak- if not, probably not. If not, like I said, take the screws out to 4 turn- popping can be too rich, but usually it's a symptom of too lean. If you can get it to idle, then balance the carbs. At least try to get a consistent, if not smooth idle before trying to balance.

Good luck!

[aaronbeekay]

Well, HoughMade, I have no idea how you intuited my problem so well, but you did. I came back to it last night and took the mixture screws out to 4 turns, and I managed to get it to start (albeit with some difficulty). It was idling OK, so I started to balance the carbs, which went well. It smelled like it was running rich, though, and when I took the plugs out they were dark with soot, so I assumed that 4 turns was a bit much and took it down to 3.5. I had to rebalance them, and when I tried edging the pilot screws a little bit leaner, the bike started to do the "runaway RPM" thing again. When I bring it back to 4 turns, it will still idle OK most of the time, but the left pipe still smokes a bit, it seems rough to me, and now there's a sort of snapping noise coming from the exhaust.

What made you suspect that making the mix richer would solve the runaway RPM issue? When I roll on the throttle, sometimes it will "stick" at whatever speed I had left it (i.e., if I rev up to 4k RPMs and then let the throttle back to idle, the engine will stay at 4K for a while). I know that my throttle cable isn't sticking - is this the same issue as the runaway RPMs?

I took a short video of the bike idling when I got it to idle. I had to upload it from my phone, so it's not the best quality - I'll try and replace it with a better version tonight. You can hear the snapping noise near the end of the video.

[aaronbeekay]

Oh, also- I know my carb holders are cracked on the outside, and it looks like the consensus around here is that they'll need to be replaced. My local shop is quoting me $65 for a pair- is that reasonable? I found some on this page on MikesXS (part 48-7479)- would those work?

Sorry about all the questions- thanks so much for any help.

[drewpy]

thats backfiring through the carbs.

valve and ignition timing spot on? (ignition sounds retarded to me)

don't carb balance, its the last thing you do. You need to get the rest spot on first.

[aaronbeekay]

thats backfiring through the carbs.

Is that different, or more dangerous, than backfiring through the exhaust?

valve and ignition timing spot on? (ignition sounds retarded to me)

I bet they're not. I guess that's what I'll do next- I've been avoiding them.

[aaronbeekay]

Here's the high-quality version of the video I posted earlier:

[HoughMade]

The answer to your previous question is: because mine was doing something similar. I can't tell from the vid, are the air boxes in place? if not, put them on and see how it runs. Is it now running on both cylinders?

[aaronbeekay]

The answer to your previous question is: because mine was doing something similar. I can't tell from the vid, are the air boxes in place? if not, put them on and see how it runs. Is it now running on both cylinders?

Yes, the airboxes are in place, and yes, it's running on both cylinders.