Honda 400ex Has Weird Problem. I NEED HELP! 

What do you guys think is wrong with this 400ex? I'm stumped on it!

Anytime I have an electric issue, I refer to the manual ALL Voltage and OHM specs for every part of the system. If everything is within spec, it is the CDI. Everything HAS to pass tests in spec to determine if it is the CDI. The trouble is, sometimes aftermarket parts test out with the quad off or at low RPM (idle), but fail at higher RPM or hen they get warm - especially coils. Coils tend to start failing at hot temp 1st before total failure. I bet $1000 it's that aftermarket coil. I've had this same exact symptom on a trx450r, a YFZ450, aaaaaannd my 02 Chevy Trailblazer. I would (if this 400ex was at my house) order and OEM coil from Rocky Mtn ATV.

It was never an electrical problem to begin with,it’s running out of fuel under wot/full load in higher gears & choke won’t help in that situation either because there’s no fuel left in the carb. The issue is fuel flow & or float level in the carb float bowl. But 1st check to ensure the fuel tank & cap are properly vented because if not a slight vacuum with firm in tsnk reducing fuel flow. If that checks out ok then check fuel lvl in the carb. When check fuel lvl in carb do it the proper way by pulling the carb off motor & then hooking up the fuel line & opening petcock to fill the carb float bowl. Then carefully remove float bowl to see if fuel lvl is at upper end of oem spec for fuel lvl in the bowl in Honda fact srv’c man. If fuel lvl isn’t high enough adjust/bend tan on float to adjust & reattach float bowl & refill & check again till you it right. Again, this has. Keats been a fuel delivery issue from the start & is why none of the electrical ign & charging fixes you threw at it didn’t work. This is coming from over 5 decades of 1st hand experience wrenching/rebuilding/restoring classic motorcycles & American muscle cars too. Good luck & Happy Motoring!

I had this same problem mine would run great till I hit 4th or 5th gear and would turn off but start right back up. Turned out I had a Chinese carburetor so I ordered a OEM carb and haven’t had that issue Since.

I've had a fuel cap vent plug up on me before. The only reason I found it, was because I had a clear fuel filter between the fuel tank and the pump and it was dry. I would try a clear fuel line between the petcock and carb and see what happens when it does it again. Right now that "parts cannon" is getting pricey. Good luck.

i have a carb gsxr 750. I had a high rpm cutout in higher gears. I changed the stator, reg, plugs, swapped coils, adjusted my timing forward and back a tooth since my base timing was always a half tooth off, changed fuel pumps, petcock. cleaned my carbs like 3 times. What it ended up being was my air filters were causing a restriction. Sounds simple, but a restricted air filter or air box will cause a condition like that at higher rpms.

Great video sir. The only common issue you typically will face is, *_all_* the work, large and small repairs, brakes etc., will almost *never* be appreciated or felt by the owner as you going the extra mile and then some. They somehow focus on maybe the engine performance, but not give a second thought as to the numerous extras that it needed, yet the owner didn't realize or notice.

Put the bike on a stand n run thru the gears n see what happens..it will help w diagnosing the problem...not gonna just guess n throw new parts at it !!!! 😃👍👍👍👍👍👍👍👍

After lapping valves we always took a black sharpie and marked up the valve seating area and put back in the head and spun it around just like lapping a few times and took back out to see where the valve was sitting and how wide the seat area was. It always gave me peace of mind knowing the valve was seated good all the way around. If a wide area of the sharpie was gone we need the seat needed to be cut to change the angle.

Check for tight valve tolerances. I had an XR 250 that had tight valves and would cut out under heavy throttle just like that.

If you have an old school inductive timing light, you might connect it to confirm spark or no spark at your high rpm condition.

Thanks for all your Informative videos the fact that the engine cranks over at full speed after cutting out leads me to believe charging system is adequate I would look at the wire from the Kill switch to see if it’s shorting out on something usually it cuts out when grounded

I think that stock CDI has a rev limiter. I've heard that some Honda quads have a low RPM cutout point. I would check the specs on that and compare it to an aftermarket unit, which probably would have a higher RPM limit.

thank you for your concistent uploading. i m learning so many things from your vids. keep up the good work!

Had a similar thing on my Sportster. Drove me nuts for a while. After rebuilding the carb, new gas cock, new fuel filter, new plugs, new cdi, new plug wires, it turned out to be a blocked vent in the gas cap, and t he tank was getting too much vacuum. However, yours to me sounds more electrical. It doesn't do that at high rpm in lower gears, just higher gears? That "may" be time based (takes x amount of time to go through gears and get into 4th and 5th where it happens), or intermittent wire connection where higher speed air flow aggravates a wire. To see if it is speed/air flow related, I think I'd run it at high rpm in 2nd a long time and see if it still cuts out. However, you are one of the best at finding these black magic anomalies on these things. It will be interesting to finally find out what it is. Please keep progress posted!!

ARGH! Hate the fight fight fight to find every issue. Love the sound when you open up the throttle going down the road

You’ve more patience than I have. Could it be cut off switch! Great to watch as always 👍👏🏻👏🏻👏🏻👏🏻

I would take the seat off, take off the airbox top and maybe air filter. Carry a flammable liquid with and when it cuts off spray the flammable liquid into the carburetor to see if it is a fuel delivery issue. You do great work and are very generous when you repair people's rides for free. Side note. I am not responsible for any injuries or equipment damage when doing this. 🤣😂🤣😂🤣😂. Good luck and I know you will have it figured out soon.

Run a clamp on spark indicator on the plug wire. Run the lamp end up to the handle bars and road test it. See if the spark fails by the indicator light flashing. This will tell you if ignition is the problem!!!

It’s good to see the progress of your knowledge you do compression and leak down tests now you are gain and passing on knowledge great videos

Check if there is an AC leaking through the reg/rec I know you replaced it but always a chance the new one could be a dud. Multimeter on AC Volts across the battery and you should see 0v so no AC power. Otherwise I'd say it's a CDI that's breaking up at high RPM but it's unusual in that it only happens in 4th 5th gear. Also you could do the wiggle test on the connections and wires coming out of components like the CDI while it's running and see does it shut off. Bit of a mystery though as to what's happening.

Check the wiring to the cdi box it may have a bad connection, that when you get to a certain speed the wind blows against it causing the cdi from sending any signals.

I had a boat motor that did the same thing and it was coil worked fine, till longer runs at high rpms.. You fixed coil wire but coil could have been damaged from it not being right. Quick easy check if you swap coils with a known good one.

I watched a youtube video where a guy had a chinese quad and it cut out at high revs. He lowered the needle in the carby slide as he thought it was getting too much gas. He fixed it. Could be what your issue is as that would explain why you could restart it as it was still getting gas compared to not getting enough gas! Love your vids mate! Cheers from Australia!

Good to finally see s bike giving you a challenge, I know you'll figure it out and make another sub happy to have a good bike again

If it has a parking brake sensor unplug it. Not sure if 400ex’s have one like banshees but if the sensor is bad it would bog at high rpm revs sometimes.

My guess is that the fuel bowl is not able to fill up fast enough when you pin it. Starved for fuel as you empty the bowl when pinned. Pull the bowl and turn the fuel valve on and see what kind of flow you have into the carb. If it pees or drips. If it pees real good, then the vent out may be restricted causing it to only dribble in when put together.. Another thing you can do is if the bowl has a drain screw, you can remove it and check for flow. Again, if the vent is restricted, you may show good fill rate but still have a problem if vent is restricted. If you have a bowl drain screw, you can replace it with a fuel line nipple and using a clear fuel line connect to it and bend the clear fuel line up and this will allow you to see where your fuel bowl level is and when test driving, take a look and see if the fuel bowl level is keeping up when under extended full throttle runs.

you sure the timing chain is right. you said the valves had gone out of wack after first starting it. maybe its off by a tooth. that seems like it'd be effecting it throughout all gears tho. i'm sure its something simple. hope you get it.

It could be the CDI or even just a loose ignition wire. I remember one time I had a TTR225 that would only spark intermediately and it was just my ignition wire was loose. Shove it back in there and it ran like a champ! Also, here’s a fun fact for ya, you can bend back valves… I did it once in a video a few months ago with my TTR90 and it worked great with some vice grips lol. Just don’t clamp down on anything too hard so you don’t scratch it.

Very nice job Joe !!

Job well done, looked comfortable to ride as well

Great video joe cdi is about the only thing left mate unless there's a starvation of fuel to the carb at high revs crackin quad though 🔧🔧🔧👍👌

Either your gas tank isn't venting right or it could be the float height in the carburetor. Try and run it on a separate full tank first

Front brakes are working great....lever goes all the way to handlebars. 👌

I had a yfz450 that would cut out in high RPM and the way to fix it was to run with the headlights on. Common issue on the old YFZ450. It sounds like it's cutting out right away. It doesn't sound like it's starving for fuel or loosing fuel in float bowl. See if you can replicate the issue in the low gears. Maybe jiggle your wire harness around. Maybe it's shorting somewhere

Best RU-vid channel regarding repairs on 2 and 4 stroke quads & bikes.You are very modest when it comes to repairs... Waiting for your next video,just know you going to find the problem.Had to sell my 97 CR 125 due to financial reasons last year.Miss my baby!

Check all of the wiring connections to the cdi, and I would try to find out ohm readings on the cdi, but check it right after it's warm.

this man is the best really is inspiring me to start my own work to fix bike in my garage and i will start doing it when im 16 but what a great inspiration and way to make money

Try a voltage drop test on the primary ignition wires after warm up.

You ever think about opening up your own shop ? You should with all you know about bikes and quads man. Its cool that you help people out by fixing there machines man 🤟🙏

I wouldn't mind finding out about what happens to the bikes that you do sell. How much they sell for. Whether they made a profit or loss if so how much. I can't believe how often you buy bikes and post. You can't have much personal time lol. Not that I'm complaining keep on posting dude👍

Sounds like a fuel flow issue at maximum demand. Fuel filter, lines, petcock or petcock filter, and if not try raise the float level. Even try with fuel cap really loose. Might be mud wasps in the fuel cap breather

It’s best to use a piece of wood to tap the side covers off.

I had a bad plug wire that would arc to the frame at high speeds. The g force would bend it just enough to open the shield at the nick and ground out the coil. The part was rubber mounted so I never got the voltage through the frame to get shocked by it....until one day....then buddy...I knew! Cheers good luck...and do tell.

If you're going with the parts cannon, the cdi is next...

Havent seen a comment saying this yet although I haven't read much - too rich will make it not rev all the way out when you're lugging it and will also cause it to stall when you're pulling hard and pull the clutch in and let off the throttle, might wanna try a smaller main and turn in the mixture screw

Check the cdi where the harness plugs in and the harness plug itself, one/both may be corroded. And some cdi boxes will act like they trip a relay inside if the get hot, like under a load. Also, check a Honda manual and see if there is a inline relay for your charging/ignition system.

I'm willing to bet the CDI is cutting out at the rev limiter at high temp & high revs. Sound like a 2-part issue. Other than that, I say a solid bike considering what it went through with the previous owner.

Hey Joe. I am 100% positive that you'll figure this out!! (If you haven't already). Every other video I've ever seen where you have been stumped, you have always eventually figured it out!! I (along with most of your subscribers) have faith!! Hang in there buddy!! The answer will reveal itself sooner or later!! P.S. I think it shows amazing character that you're not afraid to show that you don't know everything!! And you're asking for help as well!! Dude, that's the shiznik!! Thanks for the great video!!

Maybe vapor locking usually a air filter causes it if the air filter is restricting air and pulling less fuel I have a atv that was bogging down about every mile and died replaced it hasn't vapor locked since also check your carb boot to the engine

If the regulator was was fried it might of messed up the CDI unit. Using jump leads the opposite way fries the charging circuit . Bypass the kill switch aswell.

I have a 04 recon that was doing that my problem was a short in a wire in the wire loom as the wires would heat up from the motor it would cut out from the wires expanding and becoming loose

Hello 2 vintage great video loved watching it but sucks that in the end that problem you were having occurred. I Myself do not know what that could be seems like a hard problem to fix. but other than that would you ever consider doing a shop tour of the tools and setup/ workbench you have to work on all of your awesome motorcycles and quads and maybe even show some of the vehicles in your collection just a thought since i love the channel and like all the bikes you work so hard on to get running. This was a dream of mine to work on motorcycles but maybe for a later date but other than that suggestion i hope you find a way to fix this cool quad have a good day!.

What about if you take of a tach wire going to the cdi module

Hey 2v my 400ex is have a bogging problem at high rpm’s but it ran fine before. I cleaned and rebuilt the carb and put a new plug but it just pops above half throttle. No jet changes or anything, what do you think it could be?

That cable from the battery to the stator lookin kinda sketch... dunno it this help. Great vids bubba

Check to see if there is throttle switch contacts. if so at wide open it may be safety cut off in case throttle cable lock or break. adjust may be required if present.

Hey I have a 2002 Honda TRX 400ex and can't get it to idle we have played the the idle screw and running 155 main and 42 pilot cleaned the carburetor and adjusted the valves and stills runs but dies could it be a exhaust leak. Sounds like a hiccup though the air filter also

Nice rebuild. Easy

Had a similar problem with a yamaha 350, changed Cdi, stator, Pickup, rectifier, coil, sparkplugwires and on/off switch, still the same problem. Then I changed the keyswitch and it all worked. The guy that owned it before me said that he had tested the keyswitch with a multimeter (He was an electrician and a close friend of my dad) and it worked fine. So i changed the keyswitch last :/ It cost me ca $550 instead of $6 to fix it.

Check the carb vent hose routing. The 400ex has a finiky way you need to vent the carb hoses.

I want say ohm everything out like others have suggested but my hesitation is that things are working at low RPM, so I think you would do all the testing and everything would check out fine. However, the testing might be worth it just to rule it out. If you have a known good CDI box off another machine, you could confirm/dismiss it as the problem pretty quickly.

When honing it’s just proper to have it centered and not at an angle and it’s a tad of a pain but put the piston in the cylinder then lower it and put the pin in I’ve broke rings doing it the way you did it. But bravo better then most experts

Does it have a speed limiter I had trouble with a 03 Raptor 660 that did the same thing

i cant belive you laid that brand new cam chain in the dirt

Definitely fuel problem. Like the other guy said. Plugged vent, or tank filter. Maybe something in float valve seat.

run with gas cap off. The vent might be plugged. Then carb for fuel delivery. Good luck

I work in a automotive shop I had a customer Came in head The same problem you heaven i test the wiring harness It was a bad wagon harness I replaced it He said he didn't have no problems with it

Fantastic content Brilliant vlogs

Put clear fuel lines on it to where you can tell if it isn't a fuel issue like the cap not being vented, or the cap not working like it is supposed to

Betcha it’s running out of fuel on the full throttle pulls. And once it shuts off the bowl fills back up.

Does it have a good Ground?. The harmonics of vibration May be the problem.

I enjoy these videos.

something grounding out. if it was fuel or air related it would bog then die. this thing just cuts off immediately like hitting the kill switch. check kill switch and all the wiring and cdi.

Can you run it with a spark tester installed so you can see if you are losing ignition at high rpm?

You’ve done a good job on the rebuild love your content man keep up the good work love to see you fix up a Harley Davidson sportster

Hey just wondering if you do any riding. Like atv parks or trail riding. Would be dope to see you testing the bikes to see the true abilities they hold

Yay I’m one of the first to like and comment!if I can help I will I’m not half as educated as u but I can try bud.love your videos,I’ve learned a lot from u

Just asking, is there some kind of rev linter on the unit ?

Check the screens in the fuelpedcock. Sounds like you don't have enough fuel flow at high RPM to refill the float bowl.

I'm just curious have you ever tried the carbide scrapers I heard they're amazing but I've never tried them

Did you recheck your valve Clearance’s. where they are new they may have seated a little more causing a very small valve clearance.... then when you really work the engine they lengthen from the heat resulting in zero clearance and then will suddenly loose compression. If you let it sit for a few minutes it will start right back up only to repeat the problem again. Just a thought ....( common problem with lawnmower engines).

I'd replace the CDI box it's more than likely has bad solder joints inside and it's arcing out internally at the higher RPMs because it has to fire quicker at the higher RPMs.

I would not use a manual timing chain tensioner especially on a new chain they will wear the chain out prematurely and the chain guides I would find a auto tensioner if you plan on making it last. And d.I.d makes a great timing chain also they are as close to oem as you will get but I love wiseco products.

I was knocking this design at first. Having to pull the clutch to get to the timing chain at first thought seems like a lot of work but I guess it isn’t much different than having to pull the flywheel/ stator. Interesting the difference in brands design

Any possibility that the fuel tank is not venting fast enough to keep up with fuel consumption at higher demand of 4th and 5th. I have had a bad fuel cap cause a simular problem. Peace ✌️

Fuel delivery. Try running it with the gas cap off and see if it dies.. or the vent on carb may be plugged up

Did you ever get it figured out? I'm leaning toward bowl going dry, like some others. Great vids!

If the piston killed three outta four valves , I would expect damage to the crankshaft , or connecting rod also .

Good job

Did ya get all that valve lapin compound out that’s like sandy mud not good combo in the engine

Only thing i can think is a low oil sensor getting shook from all the vibration and shutting off you can test by just shaking it back and forth while it's idling if it shuts off then there, and run it without an air filter for one ride, it might not be getting enough air at a high rpm, and check to see if there is a stuck throttle sensor because if the cable gets "stuck" it should just either cut off or just hit a limiter at anything higher than idle, and mabye just mabye something with the ignition is tripping

Warm the CDI unit. I had an old NX 250 and I had to keep the CDI warm with an electric heater to get the engine running. I eventually replaced the CDI.

Not the cdi it would just shut off and not run . I had the same issue with my 2000 400 ex after hours of tracing wire found that it was the plug att the rectifier were wire go into back of it. Was not making connection all the time the vibration from motor would cause it to shut off. I cleaned back side of plug made sure wires were all the way in and then hot glued them to keep wire pins from being able to move fixed it.

Love your videos

Great video

I can't believe you put that new cam chain in that dirt. Yikes. Those 400s can be a bit ticky.

check that the kill switch is not the issue could be once you get to 4th or 5th vibration could be causing kill switch to kill it try riding on rougher terrain see if it happens at lower speed

Only suggestions I can make are to inspect and clean e every connection on the wiring harness and deals with the ignition system and find a cdi to trade out from a know running bike. I'm thinking the cdi is the issue. I have experienced similar issues on a yamaha raptor cdi.

Whenever possible go with OEM. The Chinesium is often gar-bage out of the box.