what jetting for 39 fcr on 452X big bore

[joe_f7]

what pilot and main jets have you guys been using on the 350x big bores? i have a 452 big bore and just bought a brand new 39 fcr carb for it.

[joe_f7]

anyone have anything

[The Goat]

ported, exhaust, so on?

it won't use as bi of jets as you might think, same principle as when you run the same carb on a 250r and a 500r.

it's going to be pulling more through less.

guessing I'd say 55, 160

but that's just a guess.

[Mikeproc]

i would think the main would be more around 175 or more depending on exhaust intake and cam

[joe_f7]

it does have the ports cleaned up and looking into a high flow single or possibly dual exhaust. it will be a aftermarket exhaust. and i'm keeping the airbox.

[Mikeproc]

Well you probably already know but its better to start rich and work lean. Id rather see a big main jet in there and be too rich then run a new bike lean. jets are cheap. good luck with it!

[oscarmayer]

is that a much larger than stock carb? if not, i'd look at getting a larger carb and go from there. (not familuar with the 350x as much "yet") you may have to do some porting to the intake to match the flow, but it woudl make a big difference for sure. then you can look at jetting sizes. also, there's a device called a "dial-a-jet" that's an aweosme little device that woudl definatly help you out especially since you are heavly moded.

[Jason Hall]

A 350x head doesn't have high velocity Intake ports like the newer 4 stroke's. From my experiance with the 350X using a 400ex carb, It likes a bigger pilot jet, and a big early shot from the accelerator pump. I also put a 37mm FCR on an old NX250 engine, It acted the same. It wanted a huge pilot jet before It would Idle without the choke on. So, I have learned that the higher the velocity of the Intake port, the smaller the pilot jet. The NX with the 37mm FCR has a 65 pilot, with a 40 leak jet to up the pressure of the accelerator pump. It has a 160 main jet. On your 350x, I would start around a 180 main and watch for black smoke at wide open throttle. If you get smoke at WOT drop the main.

I would think you'll have to make a custom Intake manifold to fit the 39mm carb to the head. You will also need to open the beginning of the Intake port up alot so the air/ fuel won't have a big lip as It enters the engine. Boring the first Inch or so of the Intake port will help create more velocity also. Opening the Intake port crazy big might not help as much as you think with the FCR carb.

[oscarmayer]

^ yup, if you open it more than that, you will probelby hurt flow and volocity and cause performance loss. only featehr about 1" into the port to match to the larger opening on the intake or exhaust side.

[The Goat]

contact sblt500r for a custom billet intake.

well worth the money.

also...I believe the 350x stock carb is in the nature of 32mm

anymore than a 39mm and you're asking for a tuning nightmare.

[fire1]

A 350x head doesn't have high velocity Intake ports like the newer 4 stroke's. From my experiance with the 350X using a 400ex carb, It likes a bigger pilot jet, and a big early shot from the accelerator pump. I also put a 37mm FCR on an old NX250 engine, It acted the same. It wanted a huge pilot jet before It would Idle without the choke on. So, I have learned that the higher the velocity of the Intake port, the smaller the pilot jet. The NX with the 37mm FCR has a 65 pilot, with a 40 leak jet to up the pressure of the accelerator pump. It has a 160 main jet. On your 350x, I would start around a 180 main and watch for black smoke at wide open throttle. If you get smoke at WOT drop the main.

I would think you'll have to make a custom Intake manifold to fit the 39mm carb to the head. You will also need to open the beginning of the Intake port up alot so the air/ fuel won't have a big lip as It enters the engine. Boring the first Inch or so of the Intake port will help create more velocity also. Opening the Intake port crazy big might not help as much as you think with the FCR carb.


Jason do you remember what size pilot you ended up with on the 400ex carb

[The Goat]

sell that 400ex carb on ebay...get rid of it...NOW.

sorry...I hate that mod

[joe_f7]

thanks for the insight. like i said the ports are cleaned up i will check to see if the intake has been smoothed out to about 1". what does sblt500r get for one of those billit intakes?

[Jason Hall]

I don't remember the size, It's been about 6 years ago. I would start at a 55 like Goat suggested. The fuel screw will tell you what way to go. If you have to turn the fuel screw way out, you need a bigger pilot jet, If you have to turn It way In, you need a smaller pilot. Finding the sweet spot for your engine Is part of the jetting process, and needs to be figured out. I don't think anyone can tell you the exact size, you just need to start some place, and figure out what the engine wants :beer

[joe_f7]

right. just looking for a starting point. thanks guys

[Dirtcrasher]

sell that 400ex carb on ebay...get rid of it...NOW.

sorry...I hate that mod

You really had no luck with that 400EX carb?? I only ask because I have had one on a shelf I paid about 75$ for but never installed. Jetting can be such a snatch that until I have one machine I can ride, I avoid exhaust, carb and AB changes until I have the time and the jets to mess with it. My 350X runs, starts and idles great with the stock carb I jetted to run without an AB lid, I'm was hesitant to mess with until it got warmer than 40 degrees outside :lol:
Did you try and run it and tune it on your 3fiddyX??

I can't buy an FCR carb right now, but I won't put in the 400EX carb if it's that useless. I'll start a post on that mod soon and see whats up because I'm ready to ditch the stocker and the 450R can and use the 400EX carb with the Supertrapp I have for it..............

Also : Thank you JASON for your thoughts on the velocity :beer I love reading posts that make sense and help us all :D

[The Goat]

Steve the problem with the carb is size. it's a whopping 3mm bigger. now I think that would be a huge jump in most situations, but the 350x has a tiny carb stock.

it is decent. the end. I personally would just sell the 400ex carb to a 300ex guy and sell the 350x carb to the a 250sx guy. bam! you have an 37mm fcr.

in the next few months I'll be unfortunately trying to tune a lightly built ported 350x engine to a 37mm fcr carb... I honestly do NOT want to do it, but kinda have to.

[SWIGIN]

unless someone can post pics of them measureing a 400ex carb i ont beleive they are bigger then a stock 350x carb.

i know the older 400ex carbs are like 34 or 35mm and then honda changed the way they measure carbs...or so i'v bin told.

i'd like to see a caliper in a stock 400ex carb and read 38mm

[Jason Hall]

A 400ex carb Is 35.5mm If my memory Is correct, and I think a 350x had a 32mm stock. SWIGIN, I don't think you can make a calipar set read 38mm In a 400ex carb unless you bore It :beer

[SWIGIN]

see...thats what i thought as far as it being a 38mm.

i have gone rounds with members on this site and on .org before about that...thanks

but acording to the stats on this site stock was 34mm in 86