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!

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.

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.

I just got my cover from the above eBay vendor today. It looks really pretty good. I will be installing soon and I will post pics, probably in a separate topic about the install.

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?

I have now replaced the stock headlamp on the XS 400. I used a headlamp from Candlepower that is designed to replace headlamps on '70s and '80s Japanese middleweight bikes with a 162mm headlamp. It uses a standard H4 halogen lamp. Here is the product: http://store.candlepower.com/ca61quhahe.html It has attachments on the back that are supposed to work for several bikes. the top and bottom tabs bolt right in, but there was no tab for the right/left adjustment screw. I drilled out the spot welds on the tab from the original headlight and epoxied it on the Candlepower unit with hi-temp epoxy. So far, so good. I have ordered a 35w/35w H4, but I do not have that yet, so I installed a 60w/55w that can be found anywhere to see how it did. I road about 35 miles yesterday and the bike ran fine and it appears it is charging well. Candlepower claims a '79 XS 400 should be able to use this higher wattage bulb if the charging system is in decent shape. If the bike keeps running well and charging, I'll probably just keep the higher-wattage bulb. I'll have the 35w/35w just in case.

If I were you, I'd go ahead and take the caliper off and disassemble it, including removing the piston. Clean everything and then reassemble with out a rebuild kit. All the rebuild kit contains is the piston "O" ring (seal) and a cap for the bleed screw. Just try cleaning everything and then flush the brake fluid. My bet is on rust or crap around the piston itself- that's what was going on with mine and i had the same issue.

All I can say to you is "good luck". On mine, I adjusted the valves on the LT mark (and RT, respectively). To tell the truth, I did not pay that much attention to checking if it was still going up or down or not. Even with the plugs out, I have to wedge the wrench holding the alternator nut against the footpeg or else the engine would "flop" past the LT mark. Same on the RT. When exactly on the LT mark, I could feel play on both intake and exhaust which is what the manual says I should be able to do when it is at TDC. Following this procedure and setting the adjusters in the middle of the range, I can hear them, but the sound is light and not at all loud. Perhaps you are correct that someone has modified something over the years. I wish I had more info for you.

The hole in the piston seems like the tell-tale sign of running real lean, as is the fact that it would get too hot. Could be other things, but my guess is an air leak somewhere.

I'm in the U.S. where we do not have such things (can't say it's a bad idea, though), but it is my understanding that the "restrictor kit" is just a plate that chokes off the air available to the engine. It has a hole much smaller than the intake manifold inlet. If that is the case and the bike is currently running smoothly, I highly doubt that the larger pipes will need much jetting, but it is possible. The reason is that the 2 components that affect airflow are the intake tract and the exhaust tract. If the original pipes are designed to work well with the unrestricted 650, on a restricted one, it is likely that the exhaust is not providing a great deal of backpressure (there is much more to silencing an exhaust than restricting flow, so the quietness is not a good indicator of restriction). On a restricted engine, the intake tract is so choked down that I doubt that opening up the exhaust will increase airflow that much. It is possible that re-jetting will be necessary, and it would certainly be necessary on a non-restricted engine, but I kind of doubt it as is. That being said, if it were me I would leave it as is for now because I imagine that it would be a bear chasing a decent tune when changing components on a restricted engine. The turbulence caused and the fact the carb isn't in its sweet spot with a restriction will cause some unpredictable results. I would wait until I could de-restrict it and do the mods then.

Been there, done that....did this:

On U.S. bikes, the headlight only works when the engine is running- the tail, brake and turn signals should work, though.

Here are a couple of pics that hopefully explain the process and result a little better.

I agree- I took about 2 or 3 hours on Saturday and completely replaced mine, but within the original box. I also took every electrical connection apart I could reach (one at a time) and cleaned the connections. Believe it or not, I noticed the difference the moment I turned on the ignition switch- even the indicator lights were brighter.

Here you go: http://cgi.ebay.com/ebaymotors/YAMAHA-XS400-E-F-78-79-SEAT-COVER-YPAL-_W0QQcmdZViewItemQQhashZitem27ab10bdc7QQitemZ170373725639QQptZMotorcyclesQ5fPartsQ5fAccessories#ht_5855wt_952 It's on ebay, but it seems that's what this company uses as a sales website, so they always seem to be available.

If it is a little deeper than Scratch X will handle (have used it, works for light stuff), I have had success wet sanding with 2000 grit, the going to rubbing compound, then polishing, finished with a wax with cleaners. Word to the wise- go easy with the sanding and if it is clear coated, be careful not to go through the clear. Scratch X is like a polishing compound, so pretty much any polishing compound will do what it does.

Yep- lean. Turn the screws in to a light seat, then turn them out 3.5 turns and adjust from there. Yes, I know the factory manual says 1.5 turns....I don't know what they were smoking- 3.5 to start.

That is exactly what I was referring to...though the innards of that carb are a whole lot more modern than what I am used to.

On the XS400...which is all I personally know, you hold the carb upside down, but hold the float to where it just touched the needle, but does not start to press it, and you measure to the bottom of the float- which is now on top....clear? I'm confused, but thats the process I am familiar with.

I road the XS400 to work Friday. I experienced a couple of electrical problems. First, I lost all power- nothing. No lights, no starter...no start, not even with kick. Eventually the power came back...but I did not know why. Later in the day, the bike started, but no lights. Hmmmm. Saturday, I went through and cleaned every electrical connection, one at a time. All inside the headlight, under the seat, everywhere I could find a connection, I took it apart, used solvent on a q-tip, scrubbed it clean, then dried it with another q-tip. Guess what- still had the intermittent problems (though all lights were brighter when they came on). I even disassembled the ignition lock and cleaned it inside. Logic told me that when all the power was out, the problem was very close to the battery, either a bad main ground or a a bad positive connection before the power is distributed. I knew that at some time in the bike's past, someone had replaced the main fuse with an add-on, so I started there...and that was it, in part at least. The junk aftermarket, AutoZone add on fuse holder was not holding the fuse firmly. that add-on had to go. The other 3 circuits still used the original fuse box. When the bike ran, but with no lights, there was another problem. Turns out the brass clips that held the glass fuses were breaking down and I was getting intermittent contact with the lights fuse too. Knowing that the rest of the fuse holders would follow in losing integrity, I decided to replace the holder for all 4 fuses and try to get the main fuse back into the fuse box. I went to Radio Shack (sorry UK riders, I do not know your equivalent). The Shack had this 4 place fuse holder available: http://www.radioshack.com/product/index.jsp?productId=2062258 This holds 4- 1/4" x 1 1/4" automotive type glass fuses. The original fuses are closer to 1". I removed the fuse box and removed all of the original fuse holders. I then used a Dremel to remove the mounting tabs for the holders so the bottom of the fuse box is flat so I could mount the fuse holder in it. On the holder above, you will see 8- 1/4" male tabs for terminals. I bent these down so that now the wires will enter the fuse box from beneath instead of from the side as is standard. The fuse holder is too wide to fit in the original box with the wires coming from the side. I then cut slots in the bottom of the fuse box to provide a place for the wires to connect to the tabs from underneath. I used insulated female 1/4" terminals on the wires as below: http://www.radioshack.com/product/index.jsp?productId=2103499 I used very small bolts to bolt the fuse block inside the original fuse box, then plugged the terminals into the appropriate places so that the fuses were in the original order. I then glued the original sticker that had been in the fuse box (carefully removed earlier) onto the new fuse block, installed the new fuses (20A main, and 3-10A), and tested it. Everything works- no more power loss. Only down side is that because the fuses are longer, only 2 spares store under the cover instead of 3. I will post pics of the finished product later, but if you have had fuse block issues and want a clean, mostly original look, this is an inexpensive way to do it- soldering skills a plus. For a couple of hours (including all the cleaning of connections mentioned) and less that $20, I have a pretty nice result.

An engine that will idle, but not rev is kind of a classic sign of a lean idle/low speed condition. Another vote here for new fuel and cleaning the carb. I don't think new fuel alone will do it.

No, as long as the tank is above the carb, fuel should flow by gravity. Sounds like your float is not letting fuel in the bowl.

They sound a little loud to me, but microphones can be deceiving. Sounds louder on left than right. I always hear mine, but it sounds lighter than that and is almost gone at speed. What weight of oil are you using?

Check your float. if the gas doesn't have anywhere to go, it won't even clear an air bubble in the line.

My question would whether it runs well. On my bike, at times the fuel filter will get low when I am riding, but there is always gas flowing through it. I say if it runs fine, don't worry about it.