300ex 85mm bore kit

[2muchquad]

i will be getting my cylinder back on monday from lukesracing after having the sleeve put in for the 85mm piston.i bought the kit off of profab atv recently.I also have a wb "all around" cam to go with it.anybody have any experience with this 385cc kit?what kind of power increase will it have?im going to try a heihin pwk 38mm carb at first,if i cant get that dialed i'll try a 400ex carb.i was told to stick with the stock rev box as these motors do not like rpm.this is not a racer just something to play on.:)

[bwamos]

Never been on a 300ex bored that big, lol. The 350's are as big as I've been around. Sorry no help.

I'd honestly reccomend a 330 or 350. It just seems like the 380 would be a slug without some MAJOR headwork to compensate for the massive piston. I would think studs pulling would become a problem at that point too, there wouldnt be much material left after putting in a sleeve that big.

It just sounds like a bad deal to me.

I'll reccomend the time proven 330/350

[bradley300]

in the early 90's there were all soprts of massive 300's and 250x's. they really arent anything new. just kind of rare now. i know my 350 was really fast, dont know how to desribe it, but it was way faster than stock. gthunder built mine

[h400exrydr]

in dirt wheels they had a 330ex that kept up wit a stock z-400, wit a 380 or 385 or w/e it was it should b really fast

[Cody_300ex]

Originally posted by h400exrydr
in dirt wheels they had a 330ex that kept up wit a stock z-400, wit a 380 or 385 or w/e it was it should b really fast

But the 330ex had major headwork and a ton of R&D.

[slamdak8782]

why not just buy a 400ex man

[prepracing]

there was a local guy around where I live running one of these (his was a 250x) racing flattracks and TT's. On the shorter tracks he was beating built 400's no problem and was running right with 450's. Now this guy did have awesome setup for flattrack and great rider ability. BUT still to be keeping up with those 450's was something to see. He calaimed it was a bike that was built for Tim Farr back in the day when 250x/300ex were all there was. Now whether thats the truth or not I have no idea. but yes they are a running machine if done right

[2muchquad]

why not just buy a 400ex man

i already have a 300ex thats why.the 385 kit uses a 400ex sleeve,yes it will not leave much room in the cylinder.i do plan on getting my head done too.when i do ride i spend %99 of my time on my ktm 250 mx bike,quads is something i "play" around on from time to time.if i ever get serious ill just drop my yz426 motor into a 400ex frame.im just hoping the tranny will hold up behind the bigger motor.:(

[dork]

a 85mm piston on stock stroke will give you 372cc, 86mm will give you 380 and 87mm is 390. i put together a 380 kit for my buddy and there have been no problems with durability so far (2 years). it has larger intakes with a stock chamber, it works fine and gives it a nice fat squish band. i believe on the profab kit he uses the stock connecting rod which has a smallish 17mm wrist pin(stock). that will be your weak spot on the motor. the one i put in had a 19mm wrist pin and a different rod(400ex has a 20mm). the sleeve in it has plenty of meat and there is no overheating or stud problems(studs go into the cases on 300ex). once you upgrade the rod the 300ex motor is very reliable as long as you don't overrev it.

as far as buying a 400ex, you would need at least a 416 kit and cam to run with the 380 motor(when built right)

[slamdak8782]

it would be a lot cheaper to just build a 416 ex then to do all of that work.

[2muchquad]

i have $115 in the piston /sleeve kit complete with gaskets.$125 for sleeve installation and bore.i already had the 300ex ,so 400ex is out the question.plus i like reverse:D

[bwamos]

We're jstu saying that the 385 is going to be worthless without major headwork. You'll get more out of a 297.

You'll have to spend about an extra 1200+ to get that 385 to work right. The head is setup for a cylinder 100cc's smaller.

[bradley300]

i agree, if you dont plan on porting, bigger valves, bigger carb and good exhaust, then your blowing your moey on the piston kit

[2muchquad]

We're jstu saying that the 385 is going to be worthless without major headwork

this isnt quite true.why wouldnt 100cc extra not be beneficial?if anything the intake velocity will be increased.i think too many people believe what they read in mags and not what theyve SEEN for themselves.all kinds of falicies out there.i could put a 14mm carb off a crf 50 and this 385cc motor will still pull to the rev limiter...:rolleyes:

[dork]

i wouldn't worry too much about others who don't have experience with a 380 kit say. i do have experience, and although you're limiting yourself without headwork and a bigger cam, the engine will still pull to the rev limiter, plus you'll have a bunch of low and mid. its a good base to add to when you have the money later on. and you're right about intake velocity, look at all the interest in epoxying the intake ports, they're doing the same thing. and its not so much the port size that limits that head, its the valve size. and don't worry, as long as your tranny's in good shape it will be fine, and you won't have any problems with studs pulling. actually, the 300ex bottom end is beefier than the 350x. just don't overrev it with that stock rod.

[rneal]

velocity porting works well on high performance street bikes due to their ram air intakes. Not to mention most are also fuel injected.

A 300ex will not function in the same manner.

You should be able to have someone local clean up the intake ports fairly cheap.

You *will* be dissapointed without a cam and free flowing exhaust.

I think the motor is going to rev slower than it would in stock form.

[bwamos]

Originally posted by 2muchquad
this isnt quite true.why wouldnt 100cc extra not be beneficial?if anything the intake velocity will be increased.i think too many people believe what they read in mags and not what theyve SEEN for themselves.all kinds of falicies out there.i could put a 14mm carb off a crf 50 and this 385cc motor will still pull to the rev limiter...:rolleyes:

Well, let us know how it goes then, lol.

I personally dont think you'll have enough air flow to create any extra HP above 6000rpm. You'll have lots of torque in the 2000-3500 range. But, you are going to flat line beyond 6000 rpm.

You can always try it, then do the head work if it's not to your liking. You came asking for advice, so we are giving you our honest opinion on the matter. I know the 330 is a slug w/o head work. I would think the 385 would only make it worse.

I've also owned my 330 for 5 years. (2 years stock bore prior). I do all my own work (except for head work, I let the pro's do that), so I am very familiar with the 300ex. Nothing I am telling you here comes from some ATV Mag.

[slamdak8782]

300ex has the same head as the older 250x so it is a stretch just for it to work on the 300 much more a 385

[2muchquad]

i wouldn't worry too much about others who don't have experience with a 380 kit say
my thoughts exactly dork.as i stated in my intial post,i have a wb "all around"cam for it already and i also said i was getting the head worked as well.i was just wondering about the performance and reliability issues of this kit.:)

[rneal]

The larger the big bore kit, the less reliable it will be. Just out of curiosity, why the 385 and not the 330?

[bwamos]

Originally posted by 2muchquad
my thoughts exactly dork.as i stated in my intial post,i have a wb "all around"cam for it already and i also said i was getting the head worked as well.i was just wondering about the performance and reliability issues of this kit.:)

As long as you do get head work done you'll be fine. Origionally there was no mention of headwork. Then it was argued that you didn't need it. I just wanted to say that you wouldn't see top end benefit without it. If you are getting headwork the argument is a moot point anyway. Besides as astated above, you wanted to know the performance and reliablility issues. And that's exactly what we pointed out.

My only reservation at this point would be the reliability on a "play quad". On a "play quad" I'd reccomend a 350 or 330. But that's just my personal opinion. I have not owned a 385, so take that for the little it's worth.

Other than that, you'll be spending as much money on the motor work for the 385cc as you would buying a used 400ex. Unless you have a lot of cash into the suspension, you may want to do some weighing of the options. It's going to be $1500 - $2000 total (including pipe/cam/etc that you may already have) for a proper job.

Even if not practical, the 385 would be cool though. ;) It's not always about being practical, lol.

You'll definatly want an oil cooler or 2 since you're loosing a lot of metal.

We're jsut here trying to give you the best advice we can. We're not trying to be nay-sayers, but we also dont want people going into these projects with false hopes. You're never going to beat a modded 400 or stock 450r more that 20% (rider error) of the time on any "reasonable" level of modded 300ex (in a drag race).

[wilkin250r]

From what I've seen on other threads, bwamos knows his stuff about the 300EX.

It's been said already, but I'll say it again. Headwork is a must. A 380 would out-run a stock 400EX, but not without headwork. The ports on the 250X and 300EX were really small, almost too small for the stock displacement, let alone a 380cc. I bet you would pick up 10HP just by porting the head and getting oversized valves, and a cam to go with it.

As for reliability, I think dork nailed it. I think the crank and rod is going to be your weak point. I'd keep the screaming revs to a minimum.

[2muchquad]

Other than that, you'll be spending as much money on the motor work for the 385cc as you would buying a used 400ex. Unless

yeah normally that would be a lot of money in this kit BUT i got everything for $115 off ebay.thats why i ended up with a 385 and not the 330kit.:)

[2muchquad]

Almost a year later and this motor is stilling kicking major arzz!I did go with a heavy duty rod kit and a curtis sparks cam.I have done some mild proting on the head but otherwise its stock.It does have gobs of lowend torque though,this is pretty much a short shift motor,there is no need to rev the piss out of it,its deceptively fast.With the fcr carb it revs faster and hits like a mx bike in the midrange,almost like a different motor,who would have thought a fcr would make such a difference over the 400ex carb.I will put my 350x head back on it later on once i build me a custom twin port exhaust,then it should really run.;)

[00tn300ex]

why does everybody say go for the 400.I like building a 300 where it can either stay with or outrun a 400.Really pisses ppl off.

[2muchquad]

Agreed.Its not like we build 300s to compete at the pro level.Its just a hobby i guess.I get people all the time ask me if i have a 330 kit because they are the most popular,i go "something like that" or i lie and just say its just bored 80 over..;) My exhaust note with the cam sounds nowhere near stock,so only the trained ear will know its got a lumpy cam.

[bradley300]

i miss my 350, ive been thinking of building something similar for the sport class

[2muchquad]

Why go back to a 350?Shoot for the stars,go with a 406ex.I heard rumors of 500cc based 300ex's but i couldnt tell how it was done.My cases are already bored for my 86mm piston.I would like to know how its done though.:confused:

[bwamos]

Or a 430cc stroked 350x motor. ;)

I wonder if the 350x conversion pipe from powroll would fit up on your motor? I'm sure their header would work.

[honda250xrider]

i've seen as high as 426cc on a 300ex motor right now im up to 406.... some of the most common are the 416cc 250x's but anymore there pretty much unheard of

[2muchquad]

I wonder if the 350x conversion pipe from powroll would fit up on your motor? I'm sure their header would work

Probably but those clowns want $350 for just the headpipe!I think they must be smoking.A case of mountain dew and sunflower seeds and a few hours of my time and can fab my own. ;)

[bwamos]

Originally posted by 2muchquad
Probably but those clowns want $350 for just the headpipe!I think they must be smoking.A case of mountain dew and sunflower seeds and a few hours of my time and can fab my own. ;)

Holy cow.. lol.

Ok.. change of plan. Get a 400ex header and take it to a muffler shop to get it extended.. lol.

[00tn300ex]

bwamos, what brand cdi do you run and wat is its cutoff.

[bwamos]

Stock CDI w/ the rev limit increased 2000rpm.