400EX Won't Start

[MoparManiac117]

So I'm trying to help my brother in law out with his '05 400EX. It died while riding it and now won't start. I thought a spark plug would be a quick and cheap idea so I changed that and still nothing. The starter is spinning and the starter clutch is catching because I could see the piston moving while I had the spark plug out. I'm thinking it's something in the electrical system but I'm not great with electrical stuff. What should I check next? Any help is greatly appreciated.

[LxMxL97]

I'm sure you've already checked but does it have gas? Lol but it sounds like electrical to me Check the fuse back by the battery

[MoparManiac117]

Sadly, it has gas. When he told me it wouldn't start, the first thing I checked was the gas.

[Drifterboy3]

check to see that the plug is sparking. put it in the wire and hold the "finger " on the header. thwn turn it over and watch for a spark at the tip. if it has spark check compression, incase you dont have a compression tester that will fit i reccomend harbor freight. they have a 3 piece compression tester with interchangeable sizes. they are 30-40$

[DragonGunner]

check to see that the plug is sparking. put it in the wire and hold the "finger " on the header. thwn turn it over and watch for a spark at the tip. if it has spark check compression, incase you dont have a compression tester that will fit i reccomend harbor freight. they have a 3 piece compression tester with interchangeable sizes. they are 30-40$


Yes...check if it has spark. If it does, put plug back in and throttle it a bunch like your trying to flood it while hitting the start button....now take out the plug, it should be wet with gas....if the plug is dry your not getting gas. You cab also shoot a little gas down the plug hole and if it has spark it will ignite and run a split second. Let us know if you have spark and gas.

[MoparManiac117]

Will check it this afternoon when I get in from work. Good place to start. Thanks guys. I'll let you know what I figure out.

[MoparManiac117]

It has spark. I put the plug in the boot and touched the electrode to the header. Hit the starter and it was sparking. I put the plug back in and tried to turn it over quite a few times while punching the throttle. Pulled the plug back out. It was hard to tell if it was wet or not where I had anti seized the threads. Should it have been noticeably wet with gas? Either way, I went ahead and took the carb off. Just out of curiosity I left the throttle cable attached and took it to full throttle a couple of times. It squirted gas out of the engine side. So that should mean it's getting fuel, right? What should I check now?

[Drifterboy3]

Yes it is getting gas. That's the accelerator pump. I would check compression now.

[Drifterboy3]

As the motor turns does it sound like it "pumps" or does it just sound like it's turning?

[MoparManiac117]

It doesn't sound like normal. I at first thought maybe it was the starter clutch because the starter sounded like it was spinning at a much higher RPM than normal. But once I saw the piston was moving, I started looking at other stuff. Couple of questions: can I rent a compression tester? And what would cause it to just lose good cylinder pressure? It started just fine and was running, then just died and wouldn't refire. Head gasket? It's not got any oil or anything leaking out between the head and cylinder.

[KKiowaTJ]

Advance auto or autozone rents compression test kits orielys sp? does as well I think. Under the 'loaner tool' program.

[KKiowaTJ]

You could also pull the valve cover and check the cam chain and tensioner. But that's all ballpark from my end, I don't know too much to help ya other than the above rent a tool help lol. Try and see if you can get it to start on brake cleaner and idle for a couple seconds. It will burn clean and wont harm anything. Other chemicals have oily residue and will make it stumble and or not pop off. Good luck, Wish I could offer more help

[Drifterboy3]

You may be able to rent a tester from an auto store. BUT it's not the same thread size for a quad and car so that is worthless. Of it's low on compression my bet would be either valves are slightly out of spec. Or your cam could have skipped a tooth on the chain. (I don't know how it happens bit it happened to my cousin on his yfz)

[KKiowaTJ]

They come with different adapters and the fact NGK is one of the worlds top oem spark plug maker auto and bike/quad/sled/boat/anything that takes a plug, NGK makes it run flawless. Plus Honda cars would take a similar plug so the thread pitch adapter would be included. Can always call and ask, Beat buying one just to 'test'.

[DragonGunner]

It has spark. I put the plug in the boot and touched the electrode to the header. Hit the starter and it was sparking. I put the plug back in and tried to turn it over quite a few times while punching the throttle. Pulled the plug back out. It was hard to tell if it was wet or not where I had anti seized the threads. Should it have been noticeably wet with gas? Either way, I went ahead and took the carb off. Just out of curiosity I left the throttle cable attached and took it to full throttle a couple of times. It squirted gas out of the engine side. So that should mean it's getting fuel, right? What should I check now?


The plug where you gap it should be wet with gas.....sounds like its getting from carb, but we don't know if its getting to the cylinder.....you said it quit while it was running....try a little gas down the plug hole, put plug back in and see if it fires. If doesn't fire a low compression reading could be several things. Also how good is your battery....a weak battery well spark a plug but may not be enough, if you got a charger try and boost it while starting.....however a weak battery would not of made it quit running while he was riding.

[DragonGunner]

You may be able to rent a tester from an auto store. BUT it's not the same thread size for a quad and car so that is worthless. Of it's low on compression my bet would be either valves are slightly out of spec. Or your cam could have skipped a tooth on the chain. (I don't know how it happens bit it happened to my cousin on his yfz)

If gas and spark are good the timing is the next thing I would check too.

[Drifterboy3]

If you can't or don't want to buy or rent a compression tester you can go straight to pulling the valve cover. It's pretty straight forward and can be done with basic hand tools and common sense

[MoparManiac117]

The compression tester from autozone wasn't the right thread size and didn't have an adaptor to go down to the needed 12mm. I'll try putting a little gas in the cylinder and see if it fires. I'll pull the valve cover and check timing too. Battery was just taken off the charger because they killed it trying to refire it when it died. They had to push it up the hill I get it back in the barn anyway.

[MtnEX]

Go to advance auto... they have one.

Could just be a plugged pilot jet though. Might be what killed it and why it won't start back if it is trying to start at all.

If not, cam chain or compression.

[Drifterboy3]

One thing before you pull the cover. Pull the plug and put one finger over the hole and turn it over. Of you have decent compression it will push your finger (from the air preasure)

[MoparManiac117]

My advance auto said they didn't loan compression testers. So I'll try the trick with the gas or starter fluid in the spark plug hole first. If I get combustion, I can assume it's still got decent compression and dive into checking the timing. I'll keep everybody posted.

[DragonGunner]

My advance auto said they didn't loan compression testers. So I'll try the trick with the gas or starter fluid in the spark plug hole first. If I get combustion, I can assume it's still got decent compression and dive into checking the timing. I'll keep everybody posted.


If you have good spark and it wants to fire, but won't run then these problems could be the reason..

improper choke operation

incorrect adjusted carb

leaking carb insulator

improper ignition timing, faulty ICM or ignition pulse generator

contaminated fuel.......like water, moisture in fuel.

Also if you have spark, but it won't start and your plug is wet with gas.....

flooded carb

throttle valve open

dirty air cleaner (you need air so if you got a mouse nest in there from the barn.....)

improper adjusted pilot screw

[Drifterboy3]

If you have good spark and it wants to fire, but won't run then these problems could be the reason..

improper choke operation

incorrect adjusted carb

leaking carb insulator

improper ignition timing, faulty ICM or ignition pulse generator

contaminated fuel.......like water, moisture in fuel.

Also if you have spark, but it won't start and your plug is wet with gas.....

flooded carb

throttle valve open

dirty air cleaner (you need air so if you got a mouse nest in there from the barn.....)

improper adjusted pilot screw

Those are all a possibility. But it doesn't make sense that those would just happen in the middle of riding. Given the problem with cam chains on 400ex that would be the next thing to check.

[MoparManiac117]

I think we're getting closer to the timing being the culprit. On the chance that maybe the battery wasn't making full spark, I hooked a jumper box up to it. No differene. Then I took the plug out, sprayed some starter fluid in the cylinder, put the plug back in and left the jumper box hooked up while I tried to start it. Still nothing. Didn't even make an attempt at starting. The starter just sounds like it's free spinning. I have an '07 400 I keep parked right behind his. Just for a comparison I fired mine up. The starter sounds like it's trying much harder to turn the engine over. I guess checking the timing is the next thing in the list

[Drifterboy3]

Given how it sounds I would be willing to be it is a snapped timing chain

[87TRX310R]

It could be the cam decompression, If it sounds like it wants to start when you keep pumping the throttle then that's your problem. I had that happen to my 400EX.

[MoparManiac117]

Given how it sounds I would be willing to be it is a snapped timing chain

If the chain was snapped, I would think the valves wouldn't move. They're moving in the head, so it could be just out of time. I'm curious about the valve clearance though. Once I dig my feeler gauges out, I'm going to check that before I pull the valve cover off.

[Drifterboy3]

Your correct they would not move. I didn't know you ever pulled the valve cover plugs

[MoparManiac117]

Your correct they would not move. I didn't know you ever pulled the valve cover plugs

I just pulled the lash inspection covers to make sure I was getting valve movement. Once I find my feeler gauges, I'll check the clearance and then pull the valve cover and check the timing. I think it's in the valve train.

[Drifterboy3]

We'll make sure you set the engine to TDC before you pull the valve cover. Otherwise you can strip bolts in the head.

[Stickman400]

I have the same problem with a friends 300ex that he brought me to fix. It's getting everything it needs, spark, air, fuel is getting in and out of the carb., valves are in spec, has good compression, new plug, fresh gas, wiring is good, kill switch isn't on. Damn thing still wont start for ****. Some how the carb is getting fuel into it and out of it, the valves are opening and closing yet fuel isn't getting into the cylinder. I took it to a friends to see if he can find out what's wrong with it. I ran out of ideas, other than ether, but that wouldn't help the main problem.

[DragonGunner]

I have the same problem with a friends 300ex that he brought me to fix. It's getting everything it needs, spark, air, fuel is getting in and out of the carb., valves are in spec, has good compression, new plug, fresh gas, wiring is good, kill switch isn't on. Damn thing still wont start for ****. Some how the carb is getting fuel into it and out of it, the valves are opening and closing yet fuel isn't getting into the cylinder. I took it to a friends to see if he can find out what's wrong with it. I ran out of ideas, other than ether, but that wouldn't help the main problem.


Maybe really low compression....takes compression to suck the air and fuel in.....

[Stickman400]

Yea, I didn't check it with a tool but it seemed like it had decent compression when I stuck my finger in the spark plug hole, but I guess I wouldn't really have anything to compare it to. My friends going to see if it will fire or run with ether, then go from there. My friend that brought it to me said the guy he bought it from hit a water hole and it died and wouldn't get started again. They managed to pull start it in 3rd gear and it ran for a couple seconds and died, hasn't started since. That makes me think a wet connector or something electrical. Anyway I don't wanna thread jack, but thanks for commenting Dragon!

[MoparManiac117]

So I finally got time to check the valve clearance. There was none on any of the valves. So I adjusted them to the proper clearance. Tried again and still nothing. Guess it's time for the head to come off. Would the fact that there was zero valve clearance give any clue as to what's wrong? I'm getting low on patience and time (I have race season starting in about a month). Again, advice is appreciated.

[Drifterboy3]

Check the timing before you pull the head.

Set engine to TDC
Remove valve cover
Remove upper engine plug.
There is a "T" marked on the flywheel and should line up with the notch in the case.
There check on the cam. There are two - - marks that should line up parallel with the side of the head.

If it still doesn't fire after that I'm out of idea. It wasn't smoking before it quit was it?

[MoparManiac117]

Yeah. After I check the timing, if it's right and it still doesn't fire, I taking it to a shop.

[Drifterboy3]

Get back to us!

[KKiowaTJ]

Did you ever try and swap another CDI box on it and try to fire it? Shot in the dark and free if you have a friend with a 400 too.

[DragonGunner]

So I finally got time to check the valve clearance. There was none on any of the valves. So I adjusted them to the proper clearance. Tried again and still nothing. Guess it's time for the head to come off. Would the fact that there was zero valve clearance give any clue as to what's wrong? I'm getting low on patience and time (I have race season starting in about a month). Again, advice is appreciated.

You made sure it was TDC when you checked the valves right?

[MoparManiac117]

Yeah. The 'T' was centered up in the small inspection hole on the left crankcase cover. Engine rotated counterclockwise to get there.

[2001400exrida]

definitely double check your timing and make sure. you should almost always have play in one of the rockers if it's at TDC. i can't imagine they are all that tight that there is no play when you're TDC.

[MoparManiac117]

So I checked the valve clearance again to be sure...still right where it needed to be. Rotated it around to the 'T' again (top of exhaust stroke) and the exhaust valves had no clearance (as they should...they're supposed to be open) and still had clearance on both intakes. Rotated it around to 'T' again (TDC compression stroke), all valves still had correct clearance. Pulled the valve cover off, everything is in time--marks on the cam sprocket lined up with the head. I have no idea what could be wrong with this thing. Can someone enlighten me on this cam decompressor mechanism? I'm not familiar with it.

[Drifterboy3]

all i know is there is a tiny plunger in the head with a spring underneath...