trx352x can it go bigger

[h352]

i have a powroll 352 kit in my 87 250x. is it possible to go bigger and how would that be accomplished if possible could i use some existing componets in the powroll 352 kit

[honda300EXtreme]

four stroke tech sells a

380 big bore and a 390 stroker kit..

i highly doubt anything will be salvagable from your existing 352 kit...

if ya dont mind me asking why do you want to go bigger arent you happy with the power?

[dork]

416 is possible, 87x70. but you start to get into diminishing returns after awhile.

[07250ex]

Originally posted by h352
i have a powroll 352 kit in my 87 250x. is it possible to go bigger and how would that be accomplished if possible could i use some existing componets in the powroll 352 kit

yeah toss a 400ex lump in there =]

[h352]

the 352 kit has a little more power than a stock 400ex this machine is having a hard time keeping up in the air cooled class.
the 352 kit has a 70mm stroke and 80mm bore. would a 400ex sleeve fit into the jug and would a 400ex piston work. both engines have a 70mm stroke. the 400ex piston would have to have the same wrist pin diameter and have to measure the same from top of piston to wrist pin center. other parts of piston could be modified. is this posible or is it time to get a bigger machine?

[honda300EXtreme]

i Beleive a 400ex cylinder will work... but the pistons have different wrist pin diameters...



i think it would be less of a headace to just get another big bore kit...


what all is done to your engine?

[dork]

what kind of exhaust, carb, and cam are you running? whats your compression ratio? stock valves? if that motor is set up right, you should have way more than a stock 400ex. might be time to call FST or TC. what kind of machines are you up against?

[honda300EXtreme]

yea, thats what im trying to find out...

if you dont have a fully built head,,, meaning oversized valves, port and polish Hd valve springs and a agressive cam

also a good carb


if you dont have any of these things done... theres still alot of potential left in the 352 kit...

[h352]

powrolls 352kit with 10.5:1 piston i have a fmf megafone complete exhaust with 10 disks, 32mm mikuni carb from powroll, powrolls billet cam, mild port and polish, hd valve springs, stock valves hardend rockers, auxilary oil cooler no airbox lid and a uni fillter and 15-36 gearing it competes with any air cooled machine that is 440cc and under

i hate to get rid of the machine. it hase +3 a arms,works shocks up from, g-force axle 01 raptor rear shock nerf bars a/c bumper dg grab bar breaded ft rear brake lines a/c skid plates

[honda300EXtreme]

dam,, with those mods and that gearing that thing should fly......

to be honest... i think you need a 450... ( and i never say that)


you next option would be a 380 big bore/ 390 stroker.

and i would recomend some over sized valves (+2mm)

[dork]

where are you lacking, top end, mid range? you probably have plenty of low end, you just need to move your torque peak up in rpm.

start with a bigger carb, a keihin fcr 39 or 41 is a great carb. you can use the kit for a 400ex, it will bolt right up and usually be close on jetting. also use a 350x or 400ex airboot if you aren't already, the stock 300 one really holds the bigger motors back.

for your exhaust, is that the megamax pipe you have? i would run 16 discs or run it open (if its not too loud) and see how you like it. that exhaust has a good headpipe.

for your cam, powroll has 2 cams, i assume you have the bigger .335 lift. either way that cam is pretty conservative for a bigger motor like yours, i would recommend megacycle's 166-20, it has .372" lift and 23 degrees more duration which will move your torque peak up and let it rev out better. your springs should be fine with it. the stroker crank places a greater demand on the intake tract and with stock size intake valves you'll need a pretty radical cam to get it to breathe.

beyond that you can get into higher compression, bigger valves, or a bigger piston but i would feed what you have first. for throttle response you can have the flywheel lightened, and do you still have a rev limiter?

if your up ther looking for 380's or 416 your better off buying a 400ex or putting a 400ex motor into it

[honda300EXtreme]

you could always put a 350x engine in it and mod that.....


but honestly with the mods you have any your still not happy with the power,,, its time to think about a 450. or a modded 400ex

i hate to say that because i hate people who say just get a 450 its better, although its true, i dont think those people know what its like to be attached to your quad....

i mean i am always working on my 300 trying to make it better or faster,,, because i like my quad better than any other....

sorry to get off topic.....

[h352]

the rev limiter is removed from the cdi box. would it be worth while to remove the 32mm carb and replace it with a 400ex carb or 38mm carb. then would i have to match the intake diameter of the cylinder head and boot with the carb diameter. right now the intake on the cylinder head is 32mm to match the cab that is on it now. what kind of power gains would be expected

[honda300EXtreme]

yes i would consider a 400 carb..... it will pull harder and boost the overall power


dont expect a night and day differencee but the power gains are well worth it...

both intake boots will work with the 400 carb...



if you have the money i would go with oversized valves (+2mm)
and a more aggressive cam..

[dork]

i don't think the 400ex carb would make much difference. its 36mm but also a round slide, and yours is a flatslide which would probably flow pretty close. a 400ex carb will fit in the stock 300ex manifold with no grinding, and no overhang into the air path. i don't know what you had to grind for that mikuni, but if you had to you could get another manifold off ebay for cheap. i would look for a used fcr 39 here or on ebay and try that, if you don't like it or decide to sell the quad, you can always re-sell it. but i really think you'd like it. then try a different cam, these would be the easiest bolt on mods without having to tear into the motor.

as far as keeping the quad, thats up to you. but i like the smaller quads once the suspension is upgraded they really can fly with the right motor.

[2muchquad]

like the smaller quads once the suspension is upgraded they really can fly with the right motor.

Agreed! I agree with Dork,the fcr carb made a HUGE difference on my 300ex.I have the 39mm fcr on it.I picked up a 37mmfcr(from a 250f) that will fit into a stock 300ex manifold,after i finish beat'en on it for a while,i'll let you know how it works but im sure it'll be better especially up top compared to the 400ex carb.It does run good now though for a 250x with just a exhaust system.:D

[h352]

how good is a pe 38mm keihin carb. i saw one in dennis kirk catalog for $245.95. it is listed for the 250x. does anyone have a 37mm 38mm or 39mm fcr carb for sale

[dork]

that carb is a 2 stroke carb adapted for the 250x/300ex by keihin. its a roundslide with no accel. pump. it would help out, but you wouldn't have the response of the fcr or edelbrock. i've bought fcr's off this site for 175-250 used, and they've been in good shape.

[2muchquad]

I remember a few yrs back before the fcr cars,the mikuni "tmx" was a hot set up for the xr400 dirt bike.Its a 2 stroke carb but i guess it worked good for the nonda thumper,im sure it can be made to work for a 300ex.I would try and get a fcr though;)

[elementryder]

ok most of the power will come from the head and cam...send the head out to tc and have him port and polish it and bigger valves and a new cam the powroll billet cam is very mild cam compard to the tc

[250X_project]

452/492 is the biggest possible. Though, this is a 350X modded engine. A 250X300EX engine can go 416 for sure. :D

[honda300EXtreme]

yes 416 is a maximum bore for a 300ex,

also a 400ex carb is 38mm, someone said it was 36mm above..

[bwamos]

and i would recomend some over sized valves

x2 w/ additional headwork.

That and an FCR Carb would be the only other things I would do to the 300ex. After that it would be better to cut the losses and upgrade to a 400ex, imho. I'm not familiar with the FMF Megaphone exhaust, but "most" most of the FMF exhausts are not large enough for your displacement.

I'd reccomend a Sparks X-5 or similar.