XS400 idle problems

[James in Atlanta]

Just bought a 1980 XS400, and it runs...sometimes. It cranked today relatively easily with the choke all the way out. I let it warm up for a few minutes, then pushed in the choke. It seemed to be idling around 1400. I ran it around the block for about 30 minutes and it was fine. Went home and turned it off.

About 20 minutes later I started it up again, but it was having a hard time idling. RPMs dropped several times before it just died. I kick-started it , and revved the throttle. When it dropped to idle, it would die again. Sometimes when it was trying to die, I revved the throttle, but that just made it die quicker. I adjusted the idle stop screw between the carbs so it would idle a little higher, but I had to drop it back down because after revving the throttle it would stay at 3000RPM.

The air filter boxes are very poorly connected to the carbs. I tried to make a secure connection with the clamp on the rubber hose to the box, but the metal shaft sticking out of the box kept getting pushed inside the box. So, the boxes aren't really connected to the carbs. Air can enter the hoses directly to the carbs and bypass the air filter boxes. Is this the reason the bike dies while idling, or do I have a more serious problem? Also, any suggestions on the air filter boxes? Are they supposed to do this, or are mine screwed? Thanks for any suggestions!

[FZR AL]

Just bought a 1980 XS400, and it runs...sometimes. It cranked today relatively easily with the choke all the way out. I let it warm up for a few minutes, then pushed in the choke. It seemed to be idling around 1400. I ran it around the block for about 30 minutes and it was fine. Went home and turned it off.

About 20 minutes later I started it up again, but it was having a hard time idling. RPMs dropped several times before it just died. I kick-started it , and revved the throttle. When it dropped to idle, it would die again. Sometimes when it was trying to die, I revved the throttle, but that just made it die quicker. I adjusted the idle stop screw between the carbs so it would idle a little higher, but I had to drop it back down because after revving the throttle it would stay at 3000RPM.

The air filter boxes are very poorly connected to the carbs. I tried to make a secure connection with the clamp on the rubber hose to the box, but the metal shaft sticking out of the box kept getting pushed inside the box. So, the boxes aren't really connected to the carbs. Air can enter the hoses directly to the carbs and bypass the air filter boxes. Is this the reason the bike dies while idling, or do I have a more serious problem? Also, any suggestions on the air filter boxes? Are they supposed to do this, or are mine screwed? Thanks for any suggestions!

seems strange that the first time you started it it was fine

it could be your throttle cable not been routed properly, if it is staying at 3k revs, but if it was mine i would do a carb strip down , has you seem to have more than one problem there,is it stock ? check your mains and pilots for correct size? carbs could be sucking crap through from the tank?, float heights set wrongly so not enough fuel getting through when you rev it, it will be trial and error

[OllieB]

Since you are new to this bike, the first thing you should do, if you haven't already done so, is to take some time to search back through old posts about carb issues on this bike. I did so when I bought mine in June this year and the info I learned was tremendously helpful.

First, before a carb tear down, I would go through all the connections to make certain they are proper and not leaking air/vacuum. You described an issue with the intakes. These systems are set up from the factory to run properly and any deviation can cause problems. Yes, leaking air at the intake side of the carbs can cause problems. If you can't fix the filter boxes, get used replacement ones. Open the boxes to see if the air filter elements are intact. You can find info here about how to resurface the old filters. Next, look at the rubber (and metal) intake manifolds between the carbs and the engine. If they have never been replaced, do so with new manifolds and gaskets. These are notorious for leaking as they age and develop cracks. Also, on top of each carb is the idle mixture screw. If your carbs have never been disturbed, these screws will be covered with a metal plug from the factory to prevent us peons from messing with the adjustments. Usually these screws are set to about 1.5 turns out as per EPA emissions requirements. If the plugs are still there, remove them by carefully drilling them out. Under the idle mix screw is a very tiny o-ring that deteriorates and can cause issues such as you describe. I bought a new set of screws and o-rings. Start with the screws about 3 turns out (counter-clockwise). Run the bike then set idle speed again. See how it behaves. If you just turn the original screws out to 3 turns and don't replace the o-rings, you will still have issues.

Let us know how it goes. You can buy the manifolds and jets/o-rings from mikesxs.com. Those from the XS650 also fit our XS400 bikes.

[James in Atlanta]

Since you are new to this bike, the first thing you should do, if you haven't already done so, is to take some time to search back through old posts about carb issues on this bike. I did so when I bought mine in June this year and the info I learned was tremendously helpful.

First, before a carb tear down, I would go through all the connections to make certain they are proper and not leaking air/vacuum. You described an issue with the intakes. These systems are set up from the factory to run properly and any deviation can cause problems. Yes, leaking air at the intake side of the carbs can cause problems. If you can't fix the filter boxes, get used replacement ones. Open the boxes to see if the air filter elements are intact. You can find info here about how to resurface the old filters. Next, look at the rubber (and metal) intake manifolds between the carbs and the engine. If they have never been replaced, do so with new manifolds and gaskets. These are notorious for leaking as they age and develop cracks. Also, on top of each carb is the idle mixture screw. If your carbs have never been disturbed, these screws will be covered with a metal plug from the factory to prevent us peons from messing with the adjustments. Usually these screws are set to about 1.5 turns out as per EPA emissions requirements. If the plugs are still there, remove them by carefully drilling them out. Under the idle mix screw is a very tiny o-ring that deteriorates and can cause issues such as you describe. I bought a new set of screws and o-rings. Start with the screws about 3 turns out (counter-clockwise). Run the bike then set idle speed again. See how it behaves. If you just turn the original screws out to 3 turns and don't replace the o-rings, you will still have issues.

Let us know how it goes. You can buy the manifolds and jets/o-rings from mikesxs.com. Those from the XS650 also fit our XS400 bikes.

It seems the air filters have a mesh cage connected to a metal cylinder. The metal cylinder part sticks out of a hole in the filter box, and this is the part that connects to the hose leading into the carbs. The rubber between the carbs and the engine and between the carbs and the air filters looks brand new, so I assume they were recently replaced. The air filters look home made, but with the original cage. The metal cylinder on the cage was wrapped with black electrical tape, so there was no way in hell they were going to fit inside the rubber. I removed the tape, but I still have a hard time pushing the metal into the rubber. It seems to be an extremely tight fit, and the metal part of the air filter retreats into the filter box when pushed making it even harder to connect. I think the rubber intakes may be a size too small? Did the intakes and air filters I described sound normal, or has something been 'ghetto rigged'?

I unscrewed the battery box between the two air filter boxes to make this task easier, but that was rather annoying. I got the left air box connected solidly, and the right one still lets air in at the connection so I will work on it tomorrow. It ran great today after I fixed this. It idled around 1600 which is high (should be 1200 I read). I ran it around the block for about 30 minutes and I had no loss of power all the way up to 8000RPM. The idle didn't stick when I made it to a stop sign, so its some improvement I think.

There 'was' an inline fuel filter, but I managed to break it yesterday :-/ I will get another one, it looks the K&N one I saw on a previous thread. I am trying to avoid removing the carbs because I live in an apartment and management gives me dirty looks when I work on the bike, but if I have to I have to.

The idle screws still have the metal plug from the factory, so I will have to drill those out to access the screws. If the bike is running, what are the downsides of having it idle a little high?

Thanks for your suggestions, I am not only new to the XS400, but this is my first motorcycle.

[OllieB]

It seems the air filters have a mesh cage connected to a metal cylinder. The metal cylinder part sticks out of a hole in the filter box, and this is the part that connects to the hose leading into the carbs. The rubber between the carbs and the engine and between the carbs and the air filters looks brand new, so I assume they were recently replaced. The air filters look home made, but with the original cage. The metal cylinder on the cage was wrapped with black electrical tape, so there was no way in hell they were going to fit inside the rubber. I removed the tape, but I still have a hard time pushing the metal into the rubber. It seems to be an extremely tight fit, and the metal part of the air filter retreats into the filter box when pushed making it even harder to connect. I think the rubber intakes may be a size too small? Did the intakes and air filters I described sound normal, or has something been 'ghetto rigged'?

I unscrewed the battery box between the two air filter boxes to make this task easier, but that was rather annoying. I got the left air box connected solidly, and the right one still lets air in at the connection so I will work on it tomorrow. It ran great today after I fixed this. It idled around 1600 which is high (should be 1200 I read). I ran it around the block for about 30 minutes and I had no loss of power all the way up to 8000RPM. The idle didn't stick when I made it to a stop sign, so its some improvement I think.

There 'was' an inline fuel filter, but I managed to break it yesterday :-/ I will get another one, it looks the K&N one I saw on a previous thread. I am trying to avoid removing the carbs because I live in an apartment and management gives me dirty looks when I work on the bike, but if I have to I have to.

The idle screws still have the metal plug from the factory, so I will have to drill those out to access the screws. If the bike is running, what are the downsides of having it idle a little high?

Thanks for your suggestions, I am not only new to the XS400, but this is my first motorcycle.

The connection problem between the air filter and the filter intake sounds weird. They should be just a little tight but not impossible to get on to the metal ring. Sounds like you may have some damage to that side filter box.

An idle in the range you describe is not going to hurt anything. While the specs call for 1200 rpm, that will not always be consistent. We are spoiled by fuel injection and computers on our modern cars and think older carbureted motor vehicles should perform the same way. Even on a bike of our vintage in perfect condition, we may see some slight variation in the warm idle speed from one run to the next or even with climate changes. Idle speed rpms are also somewhat a matter of preference. Some folks like to have the idle so low that they hear the individual cylinders fire one at a time. Others prefer idle speed closer to 1500 rpms. You shouldn't need to turn the idle screw under the carbs all the way out to achieve a near spec idle speed though. Sounds like you are getting closer.

Be careful when you drill out the plugs. Use a bit just slightly smaller than the opening and go slowly with light pressure. Be certain the bit is centered so you don't drill into the metal of the carb. Once you have drilled out the plugs and removed debris, note where the screws are set then slowly turn them in (clock-wise) to see how many turns out they were set to. Do this gently and don't screw them down hard when you feel them seat. As I said, probably about 1.5 turns. You can also remove the screws to inspect the VERY SMALL o-rings at the bottom of the screws. Even ones seem to look ok need to be replaced as they become distorted and hardened. Inspect the tips of the idle mix screws. Sometimes the very pointed tips break off and remain in the hole, causing problems. Most likely this will not be the case with yours as no one has been able to monkey with them. Even if you re-use the original screws, install the new o-rings and set to 3 full turns out to start. Some have found 2.5 works on their bikes while others need up to 4 turns out. If you play with the settings, only move them 1/8 to 1/4 at a time to see how idle & performance is affected. A lean mix caused by leaking o-rings, intake manifolds and/or air filters can mess with the idle speed as well as low rpm performance. BTW, is the vacuum hose from one of the intake manifolds to the top of the vacuum tap (fuel selector under left side of tank) in good shape? If not, replace it as well as the gas hose.

A gas filter is not original equipment on our bikes and not necessary if the inside of your tank is clean and the filter on top of the vacuum tap is ok. The connection problem between the air filter and the filter intake sounds weird.

An idle in the range you describe is not going to hurt anything. While the specs call for 1200 rpm, that will not always be consistent. We are spoiled by fuel injection and computers on our modern cars and think older carbureted motor vehicles should perform the same way. Even on a bike of our vintage in perfect condition, we may see some slight variation in the warm idle speed from one run to the next or even with climate changes in ok.

If you don't know when or if your spark plugs, plug caps and ignition wires have been replaced, do so. The plug caps should be 5 ohm type. Take one to your Yammy dealer and get new ones. Replacement wires can be bought cheap from MikesXS. Spark plugs: NGK BP7ES are about the best and pretty cheap at Advance Auto. If your plugs have an "R" in the number, they are "Resistor" type plugs and not proper for our engines. On a used vehicle of unknown service recirds, I prefer to start with new stuff so I have a base line.

Engine oil: If you live in Hotlanta, GA an excellent oil would be Valvoline Blue Diesel grade oil 15W40. Why Diesel oil? It is better suited to our older engines and has MUCH MORE anti-wear additives than modern gas engine oils. This oil is ok with cycle wet clutches, too. No need for Synthetic oil. NAPA stores sell this oil if you can't find it at other places. They also sell NAPA brand oil filters made by "WIX", one of the older filter companies and they make excellent oil filters.

I think you have selected an excellent bike for your first machine. Personally, I prefer smaller engines bikes as opposed to the massive two-wheeled recliner/cruisers we see here in the U.S. Be careful!! Car/truck drivers are not looking for motor cycles and will look right through you, not even seeing you.

[78caferacer]

It seems the air filters have a mesh cage connected to a metal cylinder. The metal cylinder part sticks out of a hole in the filter box, and this is the part that connects to the hose leading into the carbs. The rubber between the carbs and the engine and between the carbs and the air filters looks brand new, so I assume they were recently replaced. The air filters look home made, but with the original cage. The metal cylinder on the cage was wrapped with black electrical tape, so there was no way in hell they were going to fit inside the rubber. I removed the tape, but I still have a hard time pushing the metal into the rubber. It seems to be an extremely tight fit, and the metal part of the air filter retreats into the filter box when pushed making it even harder to connect. I think the rubber intakes may be a size too small? Did the intakes and air filters I described sound normal, or has something been 'ghetto rigged'?

I unscrewed the battery box between the two air filter boxes to make this task easier, but that was rather annoying. I got the left air box connected solidly, and the right one still lets air in at the connection so I will work on it tomorrow. It ran great today after I fixed this. It idled around 1600 which is high (should be 1200 I read). I ran it around the block for about 30 minutes and I had no loss of power all the way up to 8000RPM. The idle didn't stick when I made it to a stop sign, so its some improvement I think.

There 'was' an inline fuel filter, but I managed to break it yesterday :-/ I will get another one, it looks the K&N one I saw on a previous thread. I am trying to avoid removing the carbs because I live in an apartment and management gives me dirty looks when I work on the bike, but if I have to I have to.

The idle screws still have the metal plug from the factory, so I will have to drill those out to access the screws. If the bike is running, what are the downsides of having it idle a little high?

Thanks for your suggestions, I am not only new to the XS400, but this is my first motorcycle.

Hi guys are just bought the same bike xs400 1978 and is weird im having the same problem bike starts but wont idle,as we speak i'm looking for a carburator repair kit,my friend saids we should repair it, the bike was sitting for a while.

[HoughMade]

In every idling problem with an XS400, my suggestion is that if the carbs have not been cleaned in a while, take them off and clean, clean, clean them. With them back on, turn the idle mixtures in to a light seat, then turn them out 3 turns to start. Then start the bike screw, let it warm up and adjust the throttle stop screw for 1250 rpm (1200 is stock, but halfway between 1000 and 1500 is good enough). Try riding it. If it runs good, after riding a while, the idle may creep to 1500. That's not a problem, but you don't want it idling faster than that. If the engine will rev quickly and fall back to the original idle, does not pop at idle or during deceleration, you have it right. If it does any of those things, back the idle mixture screws out another 1/2 turn, try again and so on. If it runs perfectly the first time, you can try screwing the screws in a 1/2 turn until those issues arise, then go back to the last setting where there were no issues.

[James in Atlanta]

I found pictures of what the air filters are supposed to look like, and I think the ones I have are almost unusable. The metal on the sides of the air filter are not attached so the whole things falls apart if you touch it. I think this is why its hard for me to attach them to the hoses. I saw some pics where others have used a different air filter in lieu of the stock filters and boxes, but it sounds like the carbs have to be rejetted for this to work. This may be too much work for me at the moment, so I might try and find some original replacement air filters.

One problem at a time though. Once I get the air filters properly connected, I'll get back to you if it is still having problems idling. The previous owner said he cleaned the carbs out well, so I'm going to avoid doing it myself for a few months if possible. I should have more time for such a task in December. Thanks all!

[OllieB]

I found pictures of what the air filters are supposed to look like, and I think the ones I have are almost unusable. The metal on the sides of the air filter are not attached so the whole things falls apart if you touch it. I think this is why its hard for me to attach them to the hoses. I saw some pics where others have used a different air filter in lieu of the stock filters and boxes, but it sounds like the carbs have to be rejetted for this to work. This may be too much work for me at the moment, so I might try and find some original replacement air filters.

One problem at a time though. Once I get the air filters properly connected, I'll get back to you if it is still having problems idling. The previous owner said he cleaned the carbs out well, so I'm going to avoid doing it myself for a few months if possible. I should have more time for such a task in December. Thanks all!

New replacement O.E.M. filters are available but they are about $70 a pair, IIRC. My preference in these older bikes is to leave them the way they left the factory and not mess around with filter, exhaust and carb jet changes. You get yourself into a good bit of work and expense. I think your plan to find replacement O.E. filters is a good one. If you find a set of decent used ones and the rest of the filter is OK, you can always wrap it with new mesh as some have done and described here on the forum. Get your filters and go from there. Good luck.

[James in Atlanta]

New replacement O.E.M. filters are available but they are about $70 a pair, IIRC. My preference in these older bikes is to leave them the way they left the factory and not mess around with filter, exhaust and carb jet changes. You get yourself into a good bit of work and expense. I think your plan to find replacement O.E. filters is a good one. If you find a set of decent used ones and the rest of the filter is OK, you can always wrap it with new mesh as some have done and described here on the forum. Get your filters and go from there. Good luck.

I find some knock off air filters for $14 HERE. I was thinking about picking up a pair.

I put in new spark plugs and the new fuel filter today, and the bike ran beautifully for a few hours I also noticed I have a 2 into 1 exhaust, and after reading the forums here I don't think thats standard. Am I correct in that assumption? Mine is a 1980 XS400 (special I think?). Would the bike run well with new exhaust without rejetting the carbs? Again, the idle screws still have caps on them, so nobody has touched those. I won't know for sure about the jets until I take the carbs off in December. I guess its more for my curiosity and to make future plans if I ever decide to change the air filters.

Thanks for the suggestion OllieB, I'm trying not to add any aftermarket stuff if I can help it, at least not while I'm on a poor college kid budget.

[OllieB]

I find some knock off air filters for $14 HERE. I was thinking about picking up a pair.

I put in new spark plugs and the new fuel filter today, and the bike ran beautifully for a few hours I also noticed I have a 2 into 1 exhaust, and after reading the forums here I don't think thats standard. Am I correct in that assumption? Mine is a 1980 XS400 (special I think?). Would the bike run well with new exhaust without rejetting the carbs? Again, the idle screws still have caps on them, so nobody has touched those. I won't know for sure about the jets until I take the carbs off in December. I guess its more for my curiosity and to make future plans if I ever decide to change the air filters.

Thanks for the suggestion OllieB, I'm trying not to add any aftermarket stuff if I can help it, at least not while I'm on a poor college kid budget.

Yep, you're correct about the exhaust. That may be a plus, though. If the bike runs well after the usual tweaks (idle mix screw adjustment, new plugs & wires), nothing to worry about. The next step is most definitely the new o-rings for the idle mix screws. Sounds like no reason to open up the carbs. Carb removal isn't really that difficult and it will be much easier to drill out the plugs with the carbs removed. They are connected by a metal bar. Leave them that way. After doing the o-rings, put it back together and see how it runs and idles. Tweak the idle mix screw adjustments, if necessary. Set the idle speed. Your next step will be to check valve adjustment unless you know this was already done by the P.O. Last, but not least in the tune up is to balance the carbs. I haven't done that yet to mine but will be doing so soon. You can make your own manometer as described here or elsewhere (http://www.obairlann.net/reaper/motorcycle/manometer.html) or buy a balancing kit (http://shop.ebay.com/items/carburetor%20synchronizer?_dmd=1&_sop=12&rvr_id=135762117158&MT_ID=69&crlp=5619449165_1&tt_encode=raw&geo_id=1&keyword=carburetor+synchronizer&adgroup_id=1617145025). You want one with 2 gauges.

If you will be riding at night and your headlight is still the O.E. type that is a sealed beam unit, buy a new reflector (http://store.candlepower.com/ca631quheleu.html) without bulb and buy a Sylvania SilverStar H4 bulb. Replace the standard filament type stop/tail bulbs )2) with these: http://stores.ebay.com/warden-jp2008/_i.html?_nkw=1157&submit=Search&_sid=1006980729 Select the ones that are described as : 2 pcs "1157 13W High Power LED (Red) Stop/Tail Bulbs". The LED bulbs will reduce the amp draw on your bike's charging system and compensate fot the added draw by going up to the brighter headlight bulb.

I get the "poor student" problem. Been there years ago myself. I have given you lots of things to think about but all will improve your bike in the most important ways - reliability, proper performance and safety. Keep us informed and PM me if you have any other special questions. I don't have experience with engine tear-down like some here but have picked up some knowledge along the way about the more general stuff.. Don't forget to check your chain adjustment!

BTW, haven't seen that site for Yammy parts before. Thanks for the link. If the filters fit, they should be fine. That's a good source for all of us. Let us know what you think of them.