400ex engine into 250x frame - has it or can it be done?

[stang623]

I'm looking into this swap for my tired old 250x engine. What is involved as far as electronics, what does the electrical system consist of? I know I will need a battery, wiring harness, and cdi box, but what else?

Thanks,
Anthony

[300exOH]

Originally posted by stang623
I'm looking into this swap for my tired old 250x engine. What is involved as far as electronics, what does the electrical system consist of? I know I will need a battery, wiring harness, and cdi box, but what else?

Thanks,
Anthony

It may be easier to use a 350x motor. I believe all you need is motor mounts and an exhaust. Also you won't have to deal with the electrics so much. The best suggestion is to just rebuild the old 250x. It would be cheaper and a lot less difficult.

[Smokin 440]

i always wanted to put a xr 650r engine in mine...... it would rip like a mofo lol, but it would be like $3000 so wasnt really an option. i still wanna though

[stang623]

That would be nasty, 650 in a 250x chassis, can you say wheelie. lol Anyways, I would guess around 1500 or so just by searching ebay so i'm seriously considering it right now?

Is it possible for someone with a 400 ex to measure that engine and give me some general dimension's?

Thanks,
Anthony Messina

[Smokin 440]

ok on my 400 its rougly 18" from the swingarm pivot bolt to the headers.... 16" on my 250 and from the frame to the top of the head on my 400 is 18" and 16" on the 250 and that extra 2 inches on the head will be in the gas tank the way it looks.....and i dont think the frame is wide enough in the rear..... and not to mention w/ the 400 engine u still need to mount the oil tank and cooler so i dont think it'd work, u could always sell ur 250 and buy the 400 less hassle.... all around a more fun machine or u could always fine a 350 x engine and bore and stroke it to like 430 and u'd have a 6 speed....... all i have to say is :devil:

[86atc250r]

I know where a person can buy a 300EX with an XR600 engine installed & 400EX plastics. Just needs a stator if I remember correctly - I believe the price was $1200.

If you're seriously interested, I can probably hook you up.

[bradley300]

yes the swap is possible, i saw one of these in the readers rides section of ditwheels a while back, it was a 300ex frame but its the same thing.

[wilkin250r]

I don't think the exhaust will just bolt right up, you would need to cut and weld the exhaust pipe.

I assume you (or a friend) are doing all the cutting and welding yourself? I hope you have lots of experience...

If you DON'T have experience with motor conversions, I HIGHLY suggest the 350X conversion instead of the 400EX motor. With just a little grinding, everything bolts right up, and you can buy a pre-made exhaust kit, and be ready to roll within a weekend.

And yes, the 350X motor will run with the 400EX easily. With a few mods, it will outrun the 400EX. In stock form, you'll leave any 250X eating your dust...

[86atc250r]

I'll have to disagree with that last statement.

It's a common misconception that the 350X outputs as much or more HP than a 400EX - below are the reasons it does not (it's about 5hp down).

Smaller intake and exhaust valves
Smaller ports in the head
Smaller carb
Heavier componentry (crank, rod, cam chain, etc, etc)
Milder cam
Smaller bore and stroke
less sophisticated combustion chamber design

I will agree that it will leave a 250X eating dust though :)

[wilkin250r]

Originally posted by 86atc250r
I'll have to disagree with that last statement.

It's a common misconception that the 350X outputs as much or more HP than a 400EX - below are the reasons it does not (it's about 5hp down).



True, it doesn't put out as much HP as a 400EX, for all the reasons you mentioned. But with higher compression and six gears, the difference isn't very big.

We all know that in stock form, an LTZ400 has slightly more HP than a 400EX, but the 400EX can keep up in most terrains, where as a 250X just doesn't have the power to keep up if both riders are aggressive.

The 350X motor doesn't have as much HP as a 400EX, but it does have at least enough power to keep up in most terrain. This is what I meant by "the 350X motor will run with the 400EX".

[86atc250r]

It's got a touch more compression (10:1 vs 9.2:1), but it's still down 5 or more hp. Six gears just make for closer gear spacing - in all actuality if you look at the ratios the 350X is much lower geared than a 400EX, even in 6th. There's a reason for this.

I've owned an '86 350X for about 14 years, we've had something like 6 or 7 of them in the family over the years & we're also getting ready to build one or two 350X conversion quads to run in the "sport class" at some local events.

We've even got one that's still 100% stock all the way down to the spark arrestor.

Needless to say, it's one of my favorite bikes.

The 350X was a good, reliable engine & was the biggest baddest 4stroke 3wheeler of it's day, but doesn't develop the power a 400EX does, especially on top - especially noticible if you ride the two side by side, even with the weight differential. The 350X did put out great bottom end though - more than a 400.

I think where a lot of people get confused is if they run a 350X against a 400EX that hasn't had it's throttle properly adjusted, then power is very similar, maybe even edging to the 350X.

Swapping a 350X engine into a 300EX chassis will make about a 5hp difference & the difference in torque will be very noticible over a 300 or 250x engine - but it will not make it into a 400EX.

For the difference in the amount of work it would take to put a 400EX engine in a 300EX chassis, the 350X is definitely the way to go though.

[300exOH]

The 350X was a good, reliable engine & was the biggest baddest 4stroke 3wheeler of it's day, but doesn't develop the power a 400EX does, especially on top - especially noticible if you ride the two side by side, even with the weight differential. The 350X did put out great bottom end though - more than a 400.

Don't forget it's torque that makes you move. With more mass(weight) in the 350x engine it probably makes a little more torque which I would think would pretty much even up the two machines. Just my thoughts.

[400exdad]

With the 400EX's getting so cheap, why would you want to mess with a 250X? Not flaming, just looking at the overall pic. Of course, if you already have aftermarket shocks, a swingarm, a-arms, and nice plastic, I may be able to understand why. I have a 300 and there is really no reason to do any real upgrades to it to make it go faster and handle better..... 400's are way too cheap and they yield better suspension, handling and power... "right-out-da-box"