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400ex top end rebuild..

13449 Views 56 Replies 9 Participants Last post by  SlammedRanger
Okay, so about a week ago at the dunes and noticed i had oil leaking from the head gasket. Finally got around tonight to tear down the inspect the engine and measure the bore. PO told me it had a stage 2 hotcam and a 440 kit but as i just found out, he was lying out his rear end. After measuring the bore, its an 85mm piston and looks to be a stock (very worn) camshaft. All in all, exactly what i was hoping for. Was also extremely surprised with how well the bike ran and how much power it had. When i removed the crank plug to rotate the piston, with the spark plug still in, i was able to move the piston with nearly just the weight of the ratchet.. I assume there should be considerably more compression on these..

Anyways, I want to do at the very least a top end rebuild. Thinking about doing a 416 kit and stage 1 cam. But from the looks of my head i need a new one.. center bearing has some pretty good grooves in it and i dont think there is much that can be done to save it (correct me if im wrong)

I want the best balance of power and reliability. So just off my head what i need is:

Wiseco 11:1 piston
Hotcams stage 1 cam
GT Thunder head studs
Cometic gasket set




Anything else I should worry about right now? What about valves and springs?

Anyways, few pictures of the parts..

First off the cam



The head



and the piston/cylinder
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A couple things.
About to cooling, with only having the 416 you shouldn’t have a problem with heat. I added to the scoops to my bike and didn’t notice a difference

About the caps. I have a couple off one of my parts bike that I could send you, PM me


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A couple things.
About to cooling, with only having the 416 you shouldn’t have a problem with heat. I added to the scoops to my bike and didn’t notice a difference

About the caps. I have a couple off one of my parts bike that I could send you, PM me


Sent from my iPhone using Tapatalk

I agree with the heat not being an issue to an extent. But i cant help but notice there are sand tires on this quad. And if hes riding in sand then he will 100% have over heating issues. I only ride a sand quarry sometimes and it get so hot i can feel the heat come up through the seat. Sand is a whole different animal. Its like your riding in an oven.
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A couple things.

About to cooling, with only having the 416 you shouldn’t have a problem with heat. I added to the scoops to my bike and didn’t notice a difference



About the caps. I have a couple off one of my parts bike that I could send you, PM me





Sent from my iPhone using Tapatalk




I agree with the heat not being an issue to an extent. But i cant help but notice there are sand tires on this quad. And if hes riding in sand then he will 100% have over heating issues. I only ride a sand quarry sometimes and it get so hot i can feel the heat come up through the seat. Sand is a whole different animal. Its like your riding in an oven.


I didn’t catch that, good eye slammed. I never have ridden in sand so I wouldn’t know :)


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Yeah mixed dunes and trails. Would rather it be over cooled then under haha

Xplicit youre my hero, PM sent.
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Yeah mixed dunes and trails. Would rather it be over cooled then under haha



Xplicit youre my hero, PM sent.


If only they had liquid cooled on the Honda 400ex. I think you would quite literally have a tank. And no problem bro :)


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Try the richest main jet that still runs good.....this will keep the heat down quite a bit a may help your mid range torque. It may take a few revs off the top tho.
Okay, got her all wrapped up today!! Fired right up and is mostly broken in haha

So far all i can say is WOW. What an beast. This thing has no issues pulling the front up lol

But, go figure, my starter soleniod decided to give up, so new started soleniod ordered.. right now ill stick to jumping contacts

Also went back to an OE chain tensioner. Idk, just dont trust myself to set it right and would rather just not worry about it.

Also ordered new front plastics so i can get rid of the dang race cut up front. Shes just about done!!! (for now)







Want to say THANK YOU to everyone that helped!!
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Your welcome bro! Always glad to help!
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Anyone know why my choke keeps working its way UP. Itll be running great then start bogging and trying to die and i look and the choke is back full up.. idk how that even happens.. im about to just bungee it down.
Anyone know why my choke keeps working its way UP. Itll be running great then start bogging and trying to die and i look and the choke is back full up.. idk how that even happens.. im about to just bungee it down.
Done this before^^^^^. The detent that holds the choke in the positions is worn out. There is a tiny little ball in the plastic that drops into the holes in the metal plate with the screw that holds the arm on. Either the ball is gone or the metal plate has a groove in it.
Cool thanks Ranger, i may look into it when i rejet (if needed, at 42/162 right now) otherwise ill just keep a bungee on it haha
Your welcome! The bungee trick works. Ive ran more then a few races that way. Until i got the fcr. It uses a pull out plunger style instead of a lever action choke.
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Cool thanks Ranger, i may look into it when i rejet (if needed, at 42/162 right now) otherwise ill just keep a bungee on it haha


Don’t be a hater of the zip tie choke holder. Hahaha I’ve been running one on my 400 for years


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Okay. So finally got today to actually ride the quad. Curious tho if it’s already getting too warm. My head is about 300-350, right at the spark plug about 4-450, intake side about 250-300, oil tank about 180 and cooler about the same as the tank. This sound about right? This is using a cheap IR temp gauge
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Those nunbers are high. Not insanely high. But higher then i would like to see. Although it is winter and temps will be cold so it will be running leaner then when your riding in the summer. It is very very hard to jet properly in the winter. I would suggest waiting till it warms up to your normal ambient air temp to jet it. But again if your riding sand dunes go up a few extra steps on the main and i wouldnt be shocked if you need a 45 pilot jet. Full pipe, cam, 416kit i was running a 45 pilot with mixture around 2 turns out.
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Okay. I may bump my jets to a 45/168 and try that. I really need to/want to add a fan. Any recommendations?
If you want to PM me your email or phone number i can send you my setup. I used a 4 or 5 inch spal water proof dust proof fan and i mounted it on the engine side of the oil cooler. I used a puller fan so it pulls cool air through the vents in my hood and blows right at the head cooling fins. You can feel air on your hand holding about a foot from each side of the engine. I also wired it on the stock headlight switch since i dont have a headlight anymore.
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