YZ400F valve cover removal? need tech help

does anybody know how to do this without taking the coolant pipe off? and also what size socket is the spark plug supposed to be? I can't fit anything larger then a 5/8 down there the the size is like 18mm or 3/4.


you take the two alen head bolts out and it comes off. the two tubes are breathers. simple as that.


you should also wash it off before removing.

I know that but the coolant feed to the engine is blocking the cover from lifting up more then a few centimeters, the previous owner said that the owner b4 him had to replace a radiator, idk which one is was. one is black and the other is silver. if no one else is having my issue then I suppose that pipe was bent or changed.

The spark plug is 5/8". On the steel framed bikes you also have to remove the top engine mount to gain enough clearance for valve cover removal. Unless that coolant pipe is bent back you shouldn't need to remove it to get the cover off.

Normally, the pipe isn't in the way, but it can be tricky, anyway.


The spark plug hex is 5/8".  On the steel bikes, it's also dead center to the frame top tube, so use a swivel joint of some kind to avoid snapping the insulator. 

looks like I'm going to have to unbolt the one bolt holding the pipe in to get the cover off and and I have had the top motor mount off so thats not a problem. the plug the is in there isn't 5/8 which worries me a bit. the plug pictured is a NGK C7HSA (5/8 socket) out of my motoped with a piranha 140 and shot from the same angle the plug in the bike looks a lot bigger then 5/8. looks like I'm going to have to get the valve cover off to pull the plug.. I can get a 5/8 plug socket down there with ease but I'm going to need a bigger socket that won't fit past the valve cover




The cover comes off without removing the plug.  You'll be able to see what's going on better when it's off. 

ik the cover comes off without having to remove the plug and the plug should come out without having to remove the cover but I can't fit a larger socket then 5/8 down there and it is definitely bigger then 5/8. it's like the previous owner stuck a car spark plug down there. this is all the farther the lead will plug in.


And when you get the valve cover off, you can figure out what it was that he did and get the right plug for it (NGK CR8E, or better, a CR8EIK)

what's better about the cr8eik?

It's an iridium electrode plug.  More reliable starting and longer life.

I got the plug out, it was a cr9e, without looking at it I set a friend to go pickup a new plug while I was at work. so a new cr9e is in it for the time being. my plug socket was jacked which was why I couldn't get it off, so I got a new one. I checked the valves and all but one exhaust valve were pretty spot on. I cycled the engine a few times and then gave it a kick, started first kick. I brought it outside and started great again, let it run a few minutes and then it sputtered to a stop and now that's all it does. ik I need to get a new brake vacuum hose bc the old one is well old but I trimmed up the ends and plugged it back in. any tips for tuning it to get it running good? https://youtu.be/KTALoOMsUhk

running for a min with the choke on

The CR9E will foul rather soon after you get it running.  


There is no brake vacuum hose on the bike. 

after work I'm going to pick up a cr8ek. I'm still getting spark on the cr9e I put in there. there is but it old.




Aha.  That isn't a brake vacuum hose, and the brakes don't use vacuum.  That is a somewhat rare piece, actually; a bar mounted hot start button.  The hot start on any YZF is there to provide a controlled air bleed into the intake manifold to create a lean condition when starting the engine when hot to counter the tendency for them to "flood" slightly right after they have stalled or shut down at full temperature.


On the 426 and 450, the hot start is built into the carb, but the stock setup on 400's was a pull valve mounted to a tab on the left side of the frame near the air boot that opened a passage running through a hose from air boot to the intake port.  You should verify that the valve at the handlebar does not leak when not pressed by applying vacuum and operating it.


I think it's safe to say that you have a carb/fuel related problem.  Start by checking the flow of fuel into the carb is adequate to keep it running, then get the idle mixture and speed adjusted.

idk how many times i've tired to write this post....

ok good to know, when i get to the store later i'll pick up a new vac hose and the CR8EK. I got it started again! WOO! I plugged the manifold hotshot and it started back up after a few kicks and a little gas in the carb.


I had noticed that it was cracked at the mani and i trimmed it back and plugged it in when i first tried starting it. when it was running i noticed that is was cracked and leaking at the button and tried to slow the leak and died shortly after. 


Would it be fine if i just left it plugged untill i can get to the store later so i can get the bike dialed in(running good not everything)?


hot start plugged, choke on:



hot start plggued, idle turned up a bit, choke on:



one other the previous owners must have raced or really like the bike, take a look at these and let me know what you think. The wheels and the hot start are a given but i found some other stuff as well






You can leave it plugged and probably just leave it at that.  The hot start on the 400 wasn't terribly effective, IMO.  As far as gas in the carb to start, if you're referring to squirting some down the carb throat, know that the bike has an accelerator pump, which you can use to prime a COLD engine simply by twisting the throttle a couple of times.

gas in the carb normally how the gas tank would feed, I just don't have the tank on right now. I knew it had accelerator pump for when the throttle opens quickly and I'm assuming that's what the red factory pro (upgrade?) part on the carb is, but I did not know it functions as a primer on a cold engine.

The red item is some sort of "upgrade" accelerator pump cover.  An accelerator pump does nothing more than squirt fuel down the carb mechanically when the throttle is twisted to compensate for the sudden drop in intake vacuum.  Same as squirting gas down the carb any other way. 

I got to ride the bike tonight for the first time, idles great, loves to start(if I do it right) and idle with or without the choke on (20°F out tonight when I ride it) it didn't like getting any throttle without the choke on so I putted around in second with the choke on a little bit but just felt like it was going to die without the choke. I feel like my jets could be still dirty because it would just cough and sputter. im going to cut a part from the recording i have where i start in second when the choke on and when i shift to fifth the choke is off and the bike sputters and then pops when i down shift. would it be the pilot causing the bike to run to lean?

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