Hey guys I'm new to these forums here and thought i'd drop a note to say hi and ask a few questions. I'm not new to forums so I know how things around here work so to speak.

Now for my question.....

I just recently purchased a used 400EX Quad from a friend of mine for only 1G and I thought I got a steal on it considering the shape its in and everything that has been done to it. It has a bad head gasket at the moment which im aware of these 400cc motors being plagued by but I plan to properly rectify that problem next fall. I previously owned a 99 300EX for about 5 years and always loved it, had plenty of power for trail riding which I do mostly. This Quad ive just purchased however is definitely not stock and im aware of the stock HP ratings being about 26-28 hp and for a trail machine thats more than enough I think. My machine however has the mods listed in my signature and im not sure how much power I might have now. If I had to guess im thinking about 35 peak HP, if im lucky nearly 40 but I know im not that lucky [img]i/expressions/face-icon-small-happy.gif[/img]. What do you guys think about those HP figures? Thanks in advance for any input you might have

 

<div class="FTQUOTE"><begin quote>Originally posted by: Marty1985It has a bad head gasket at the moment which im aware of these 400cc motors being plagued by </end quote></div> Not sure where you got that idea??? I have a very high mileage 00 and have a dozen friends wtih EX's, and not one of us has ever had any "head gasget" problems. You don't see any posts in here about it either!

These engines will run forever, if you just leave them alone! Once you start messing around the reliability is gone. Trouble is, yours has been "messed with", so you are going to have to spend $$$ to keep it running.....

As for the HP, only way to tell is to dyno it.

 

I'm with Recon on this one. Never heard of the 400's having head problems.

Now, looking at the mods it has had done to it, and the first one stands out. 12.5:1 comp piston. Seems he's had some work done on the head already. Not to say this isn't a reliable mod, but any time you do a mod, you will take away from the reliability the stock from had. Could be something as simple as an improperly installed head gasket.

Have fun with the new ride, and welcome to the forums Marty1985. [img]i/expressions/face-icon-small-smile.gif[/img]

 

Dude i had a 02 426ex and all the mods that i had r listed below. We have a very good mechanic here in elreno, oklahoma his name is phil he has i shop and dyno and with all the mods i have i would have still had 2 put a hot cam and high compression pistol to get up to 40hp! And even with all the mods i had mine ran awesome tell the day i sold it.

 

I know there are lots of highly moded older 400EX's around. That made sense 10 years ago when it was THE high preformance machine, but now it is at the bottom of the heap in terms of engine performance (but still a sweet sweet trail quad!).

So, those of you buying an EX today, just leave it alone and it will last forever! If you need more engine, then step up to one of the 450-700 class machines.........

 

Yea I know all hondas are very reiable but this one used to be raced somewhat then it was completely redone. I talked to the guy who rebuilt it and he said its possible it wasnt broken in properly which could have caused the blown head gasket. Especially since the only thing this quad has ever seen from the previous owner was water and mud because thats all he done so he got rid of it to get a 4x4 utility more suited for his needs. Im going to replace the head gasket and be really tedious about how I torque the head bolts down as to be sure not to pull the studs out. Oh well it runs great and doesnt smoke at all so im going to drive it like it is for now. Once the engine is completely warmed up the head gasket doesnt leak anymore and even cold it only does at low rpm throttle bursts. Plus I figure if I have to sink some money into the engine to get it reliable then ill just go ahead and do it right the first time instead of letting it nickel and dime me to death. Oh and in case your wondering that is the machine in my avatar lol.

 

You did good for a grand. I wanted to find a cheap one, and ended up spending quite a bit more than that.
Seems to me, there is an issue with 400ex head gaskets and high compression. Stock motors usually dont leak, but modified motors do. Usually the 8 mm bolts, get way over tightened, and that strips them out, and then they dont seal. Is it on the right side of the motor? Usually, where the oil stain is, the leak is.
Make sure you use a top shelf head gasket when you change it out. Theres alot of differece in quality out there.
Most guys move up to heavy duty head studs, when the compression gets over 11.5. You might want to look into that, for a more lasting fix. http://www.gtthunder.com/TRX400EX.htm

PM sent

 

Welcome to the forum. As hondabuster said, I have read on here that once you start doing stuff like yours has that the stock headbolts need to be replaced by heavy duty ones.

 

Yea I think thats going to be the first thing I ever do as far as engine work is get upgraded studs from GTThunder. After that I might look into fitting a larger carb or going with a stage 1 cam. I was told the only thing not done to this quad was a cam and I think a 440 kit. Up until I looked at JE Pistons I couldnt find a standard bore 12:1 piston unless it was a 440 kit but JE does offer a 12:1 standard bore and im pretty sure my friend who built before I owned it said he put in it along with a thinner head gasket to boot.