ATV Florida Forum

what would you do to a 400ex engine.  i was thinking of what engine mods i could do to it.  already have k&n filter, jet ket, white brothers pipe, and dyna FS ignition

send the bike to ATP they send it back dialed in, they will do all the engine work!! horsepower machines !!

how much power do u wnat out of it
i would get a hot cams stage 2 and make it a 416 or u can go bigger like a 426,440, also they have a 460 kit and they also have 500 kit
                                                                                        ^  im geting one this summer

why do so many people have the 416?  it seems to be the most popular

More reliable.

When you have a bike with more power that it was intended for, it takes a while to get it started and warmed up. A  416 isnt that much larger than stock, therefore it will be noticable, but not a pain.

Just a recomendation, dont get the 500 kit..

y do u say that have u had experinces with them befor ???

why get a 500cc kit? Unless you first of all, like to buy 45dollars worth of race gas, and when you turn it off, you have to wonder if that will be the last time before you have to rebuild it.

as much power as i can get without putting to much stress and pressure on the engine.  what about getting an edlebrook carb for it

why when you get the engine work ::)

I've only got a 406 in mine, but everyone who rides it thinks it's much better.

ATP is the Honda horsepower king.

If you ever get the chance to meet me out on a ride, I'll let you ride mine and see what you think.

Id@

I would suggest tuning the power that you have and use what you have. once you stroke and poke the internals you may run into reliablity problems. maybe the sleeve coming out of the cylinder and locking the motor while riding,....HINT HINT
SERIOUSLY you might want to consider sprockets to better use the power you have you learned alittle on jetting this weekend. tuning is a key. just bolting things on and hoping to go fast is not beneficial. porting a cylinder head ,maybe an option
larger head pipe for increased top end.

you dont have to have a huge unreliable bore to get power, there are other ways such as porting, cams, carb.I would just go with a 426, and get cams, porting, maybe a carb...

never thought to ask but what year is your EX?

reason I ask is that if it's within the 4-6 year old range, don't bore it out unless you are prepared for some reliability issues.

This was something that was found out after my 400EX went through three headgaskets in the span on a month.  When those engines get to that age range, the metal of the middle of the engine where the sleeve is(don't know the name) get's brittle.  This causes the head bolts to come loose when it gets hot and whammo, there goes your head gasket.

The only REAL fix for the situation is to get a new piece and start over.

There are kits out there that claim with stronger bolts you can get by, but to make absolutely sure that it will hold together you need a new metal piece.

and it aint cheap.

Just ask ATP about it and they will steer you in the right direction.

and follow the advice of Mr. Norris above.

Id@

it's a 2001.  i was thinking about going from a 15 tooth to a 16 tooth front sprocket to get more power.  i've had previous experince with a 300ex that had a wiesco piston in it,  but after a couple rides the piston sleeve chipped and the pieces got all cought up in the cylinder or somethin like that.  so i'm not real sure if a big bore kit or hot cams is the way to go

Listen to what Ida and Billl are saying they are both knowledgeable on the subject. The 416 or 406 can make plenty of power and still be reliable. Stay away from the 500cc bore/stroke combo's there are a lot of problems that go along with them. Only way to build a reliable big bore in a 400ex is to fork out the big bucks for a Duncan PC4000 billet cylinder. It elimates all the problems w/ stock of pulling the head studs from the constant heat and most of the overheating issues since you have a lot of extra cylinder wall.

Also, to add: J&E pistons don't have the brakage problem that you described from the Wiseco. We use J&E in everything we can. I have only used other brands when a JE is not available. 

Whether you stay with a stock bore or a little bigger don't go too high on the compression if you want reliability. The high compression tends to stretch out the studs and you will have to replace them. Not to mention the added expense of race fuel.

If you have your heart set on more than just portwork I would recommend a 416 with a 10.8:1 JE piston. It will have more power, still be reliable and you can use pump gas.

Are there any problems with really modded 400's overheating when ridden hard?  It can get hot here in the summer, and if you're in the mud, forget about it.

The 16 tooth sproket with not give you more power it will however use the power you have at a different level. in a nut shell you  will increase you top end speed. and you will not shift as often or has fast. you will find the you will use more of the gear you are in.

Not sure how modded you mean but I have never had a problem with mine overheating. in the bigger bores though yes it will produce some heat problems due to the very thin cylinder walls.

As far as the sprocket goes the 16 tooth will give you more top end with a little smoother acceleration while a 14 tooth will give you a little harder accl. but lose some top end. Also with the 16 you can't run a case guard which isn't a great idea. You can also look into a rev box that get's the limiter up around 10K rpm....like a TC or Dyna but you will need some mods to get the motor to pull those kinda rpm's in the higher gears Like p&p, cam and pipe should well do it. I run the 14 tooth with 18" tires and can still hit 70+ mph in 5th just to give you an idea.

i got a 14 t and i hate it lol yea i can get threw the gears faster and much quicker but i hate it lol

ok so just to get an idea, how much would port work cost?  and how much more power would i get?

Porting usually runs a couple 100 bucks...by itself won't make a heck of a lot of difference but along with a good cam, exhaust and maybe a piston.....then your talking a good bit of power...maybe around 10hp increase over stock. Give or take.