Honda 400EX weird problems

[Justice Owens]

Hey guys, any ideas would be greatly appreciated on this. 

So it started a few months ago, riding with some buddies and my 400ex all of a sudden lost power. Wouldn't fire, wouldn't sputter. Towed it home, and then it fired right up. It continued this pattern for a week or two, ran perfectly for varying amounts of time, suddenly died, and would start right back up if I waited 20 mins or so.

Now we get to the next phase... I was dinking with it one day, I think I pulled the carb off and cleaned it out, (not really dirty but just to be sure) and I was trying different fuel screw adjustments. At any rate, I didn't notice much change, except obviously running a little poorer when I had the fuel screw in the wrong place. I took it on a quick ride around the block, and it was just fine. Ran awesome, had lots of power, etc. Then went around the block again and it died on me this time right as I revved it. Ever since then I haven't been able to get it to start. 

Now when I turn it over, it sputters, fires a touch, but won't start. Ever. I replaced the plug, CDI, and coil, with no change. I do know that it has spark, but I don't know for certain that its big enough. After replacing these with no change I popped the carb off and tried firing it up spraying some either right into the intake. Upon doing so, it revved right up, but obviously there was no carb so it didn't stay running for long. While spraying the ether it seemed to run just like other engines I've tried spraying ether into no popping or backfiring etc. When it fired right up on the ether, I figured I must have a carb issue, so I just bought a whole new carb instead of taking the old one apart for the 800th time. When I put it in, there was no change whatsoever. Still turns over and sputters, but won't fire up.

I am completely befused, and any input would be graciously accepted. If you read through all that your a trooper, so thanks very much :) 

[Frank Angerano]

Welcome to Quadcrazy!!!

Have you checked the fuel flow from the tank?? All this could be caused by fuel flow.  Make sure your petcock is clear and flowing and its not dirty or bad gas.  We have seen and heard this many times before.   

Second Is if you have properly cleaned the carburetor did you set the air/fuel screw correctly?

If all of this is right my guess would be a spark issue. 
If your handy with a tester we can help you along to test the stator, trigger/pick ip coil and all other electrical parts and maybe determine whats going on.  
Its not hard to do and all you need is an electrical meter/tester to get started.   

[RedBaron357]

If it were to arrive at my shop with the listed statements I would go as follows:

Check Compression/Leakdown - Not that I think the complaint leads me to believe that there is a mechanical issue, it's just good practice. Something so simple is tight valves can cause all sorts of fun when diagnosing running issues. If you haven't checked those valves lately it wouldn't hurt, it is a normal maintenance item.

Verify spark - Typically this Japanese stuff is pretty good about a healthy spark. Most of the time it works or it doesn't. That's not saying crazier things don't happen, but generally it's good if it sparks.

Passing those early checks, your description really seems to sway my opinion towards a fuel system issue. I'd have probably asked you a few more questions on write-up, mainly trying to dig for information about what way it was running poorly when it was running, but a unit that doesn't start at all is telling in itself. The air cleaner should be clean, oiled, and well sealed to the box. The airboot (between the carburetor and airbox) should be free of cracks and fully installed on the carburetor and airbox. Your issue probably isn't in these areas but it's always good practice to check. Verify that your fuel tank cap vents. It needs to be able to flow air into the tank for fuel to leave at the bottom. This doesn't typically create a permanent no-run condition, but can create a fuel starvation issue while riding. Grab a ratio rite cup or clear bottle and prepare to drain fuel from your petcock into it. It's hard to tell the correct fuel flow rate by describing it here, but it should flow a steady stream expected by the petcock outlet size. If the petcock is not flowing or flowing all of the time just go ahead and get a new one. Even a full time flowing one can potentially create big issues. Retain the drained fuel for inspection. If within the next hour or so you notice layers of different liquids in it or trash in the bottom go ahead and plan on draining your tank. It's a good time to mention that non-ethanol fuel is what you should run if at all possible all of the time. Forgive me for suggesting to get back into the carburetor, but at this point it'd be a good time for it. No offence intended, but to a shop a customer cleaned/inspected carburetor is just a carburetor that needs to be removed and inspected again. It's easy to get it wrong unless you're very thorough.  Inside the carburetor you have a few different fuel circuits to deal with. First remove the float and its needle. If the needle has any sort of ring visible around its rubber tip when dry it should be replaced, a fuel leak is not far behind. The passageway that needle goes in should be free of obstruction between that orifice and the fuel hose inlet. If you've messed with the float height (Bending the tab on the plastic float) since the unit has last ran well it'd be good to check the installed height versus the service manual settings. The pilot circuit will consist of an air inlet in the air side carburetor throat typically, a pilot jet inside the bowl area, and a pilot screw externally. Lightly thread the screw in to verify its position from seated and then remove it, its spring, and the washer and o-ring that go on it. Remove the pilot jet. When dry a little speck of light should just be visible through the jet. If not, do not clean it with a drill bit. A wire pulled from a wire brush and some contact cleaner typically works wonders. Spray contact cleaner through the air inlet to verify it reaches the jet orifice, then spray through the jet orifice looking for fuel to exit the pilot screw orifice and perhaps its hole and bypass holes in the carburetor throat (engine side). If I were a betting man, I'd bet you've already found your problem by this point. Still if you're this far in make sure that everything else is spotless and correctly installed. While you're at it pull the intake manifold off the engine and check its seal and rubber condition. That never hurts. If all of that is good, then you're into the strange stuff category. A little light through that pilot jet, remember that. Carb spotless and choke, slide, needle all working normally.

I hope this helps. If it doesn't I'll check back soon to see what you've found.

[MarkinAR]

It's simple, but make sure the fuel cap is venting correctly.  My oldest had a 400ex and he cut the fuel cap vent hose off and put a neat looking little grenade cap on the vent line.  But it didn't move enough air and vacuum locked the tank.  So it would run a few minutes, sputter and die.  One day I was in the garage working on another bike and heard the tank slowly sucking air.  Replaced the cap vent line with a normal hose and it ran perfectly.

[JacobSlabach]

i would start simple- if it just cut out one day when you were riding, i would say it was an electrical connection issue.  i would have tested spark immediately but you cant go back now lol.  a good thing to remember is the basic components the engine needs to run:

Spark, Air, Fuel, Compression.

too much or too little of any of these can restrict the engines performance.  i always check the basic stuff first.  check spark and compression, check that gas is getting into the carb, change the spark plug, check air filter, check air ducts from air duct to carb and from carb to engine.  check the exhaust too- loose studs can cause a loss of back-pressure on the valves but i think you would hear it rattling if it were loose.

best thing is to keep common sense in your vocabulary and take a break every once in a while and dont get too focused on just one aspect of what you think is wrong.  ive been in situations like this and in the end there is always a reasonable explanation.  best to step back and consider and rule out all possibilities in order of easiness to prevent hair loss

[Frank Angerano]

@Justice Owens any luck ? 

[mrfartsalot]

Check the o-ring or gasket on the intake where it connects to the carb and the engine i have seen a case where the o=-ring on the engine side went bad and the machine just shut off and didn't start again.

[quadnut20]

Any updates on ur 400ex ??