Fuel/air screw questions

[ben300]

So, with the foxburg GNCC coming up this weekend, i wanted to make sure my jetting was spot on.


My current set up was a 160 main, 42 pilot.


I decided to go up big for my first try and try and get it to cut out at WOT. so i put in a 175. it ran better, but would not cut out. So i went bigger yet to a 180. It still would not cut out and seemed to rev higher. brought it back into the garage cause i was about out of light. As i was looking over it idle in the garage, i decided to play with the air/fuel screw. This is were i noticed the screw was all the way seated.

Could this be why it will not cut out at WOT. i know i need to take it back out to 2 1/4 turns (stock setting) and adjust from there. I just feel that 180 main is a huge jet for an ex with an hmf slip on, Uni, and air box lid removed.


any ideas/suggestions?

thanks

[JOHNDOE83]

the af screw only effects idle, starting and decel, It has nothing to do with main jet.

if you went bigg and noticed improvement keep going!

[Thumpin440ex]

As in cut out you mean hit the rev limit ? 160-165 would seem to be about all you need with just a slip on and no lid. The A/F screw in almost all cases should be about the stock setting, has nothing to do with the WOT jet circuit.


John

[JOHNDOE83]

no the cut out is when you get to much fuel from being to rich and the topend rpms stutter like a rev limiter

[Thumpin440ex]

Hmmm, I can't see it running better if it is over fueling it. I have actually never done it that way.. a 180 just seems way to big for that amount of small mods.

John

[JOHNDOE83]

Hmmm, I can't see it running better if it is over fueling it. I have actually never done it that way.. a 180 just seems way to big for that amount of small mods.

John

it will never be over fueled, once its over fueled it wont run which is the sputter at WOT, it can do that at any point in the jetting scale not just the main jet.

jetting the needle and pilot like that arent needed, theres no high performance with needle jets or pilots, trust me go big on jets until it sputters out, then back down till it fully revs out and your 400ex will run better then ever.

www.jetsrus.com << in the faq section under main jet tunning, its #4 halfway down the page.

Theres stock bores on here running 210 main jets smoking 450s at the drag strip.

[Thumpin440ex]

I have to disagree to the ( over fueling). Your bike will absolutely run worse if you have to much main jet in it. Same with any engine. A leaner set up while not the safest runs better 99% of the time, unless you are in a super hot environment. The richer set up will aid in keeping the chamber cool. Not wanting to start a pissing match here but yes you can surely over jet a bike and loose out in power. I would imagine the 450s that are getting smoked by a stock 400 bore is not set up for the drag race in particular. To the OP I would start buy getting the A/F screw set correctly, along with the idle circuits.

John

[hondaREX]

I am having this same issue with my build. I am at a 215 main. 50 pilot right now in a 450r carb and i cant get it to stumble at WOT like OP states about his. I am used to a 2 stroke that when i get to the biggest main jet tested, it starts to stumble. Then i drop one size main and its usually good. Which is normal practice on 4 strokes too but I am having no luck with getting the 400ex to stumble. I cant tell if it is revving out at WOT or not. It keeps pulling but how much im not sure. I have heard of other members getting up to a 230 in a 450r with no stumble but i want to find that sweet spot with a little more on the lean side for the power.

[ben300]

it will never be over fueled, once its over fueled it wont run which is the sputter at WOT, it can do that at any point in the jetting scale not just the main jet.

jetting the needle and pilot like that arent needed, theres no high performance with needle jets or pilots, trust me go big on jets until it sputters out, then back down till it fully revs out and your 400ex will run better then ever.

www.jetsrus.com << in the faq section under main jet tunning, its #4 halfway down the page.

Theres stock bores on here running 210 main jets smoking 450s at the drag strip.


see thats the problem. the needle on an 05+ carb is not adjustable. theres only one notch and thats where its at. lol

on the bottom end the thing runs good, like it doesnt sputter and has great acceleration, but its hard starting, really hard when its cold even with a 42 pilot. which to me, seems like if the AF screw was all the way in, could be the culprit for that.

so john, what do you think i should do? set the AF back ot 2.25 turns and then mess with it till it gets good idle, and then start messing with the main jet? i personally would think that 180 is way way to huge, but it didnt cut out, but hwen i was running that jet yesterday evening at about 8pm it was about 48 degrees out. im gonna try it out as soon as i get home from work today

[JOHNDOE83]

I would go to 2.5 and ride happily, if it started and ran with the screw closed, then began to run slightly better after the 2.5 turns leave it and forget it.

To Thummpin: thats just the thing, its not overfueled until it wont rev out fully then it becomes "flooded" and the fuel is not getting burnt. www.jetsrus.com isnt wrong...lol, so your not dissagreeing with me your disagreeing with the largest jet seller in america. Not saying your wrong either but Im gonna go with whats reccommended by the guys whole job is tunning motors. Most people to actually go out and try this have reported back that once they got to 200 in size the machine ran WAAAAYYYY better then it did with the 160 in it. Another rule of jetting is that if you continue going up in size and "DONT" see performance loss KEEP going bigger till you do.

Anyway, heres exactly how jetsrus describes main jet tunning: 4) Main jet: Start with the biggest numbered main jet and run at wide open throttle, the engine should stumble at wide open throttle. Install the next smaller size until the stumble is gone. A general rule is to increase the main jet as long as each jet causes the engine to run as well as or better than the smaller jet in it before. When there is no longer any additional performance gain from the larger jets the jet size is too big - go back down one size.

Note this is generalized for 2 and 4 strokes, the 4 strokes will sputter out when they get to rich and wont run. The 2 strokes will run when to rich but will foul out plugs and just not run correctly to full potential, like where it says " no longer any additional gain " would pertain mostly to a 2 stroke and the " go down till stumble is gone " is mainly 4 stroke.

To the OP: we all strive to run a perfect motor, in reality once anyone has gone in to the 200s and not seen the stumble theres really no reason to keep going, with a 200+ size main jet youll be safe and reliable enough not to worry anymore and have the performance value to help. Us drag racers tune as accuratly as possible but were going by ET and every .001 sec counts.

You can keep going up and try to tune it perfectly under the stumble, but when the weather changes the jetting will become finicky because one day it could be perfect the next day rich in the morning to lean in the afternoon.... Ive been through it....lol.

[hondaREX]

So John,in my case, is a 215 in a 450r carb a safe point equal to the 200+ in a stock carb you mentioned above? Or should I go a couple higher to be safe? Im really not feeling any performance gain but I haven't climbed anything crazy for fear of it not being right.

[KKiowaTJ]

Id run the 180 or if you think too much 170 minimum. Then adjust idle etc from there. Whats the plug looking like after the runs? To each their own, Id rather have it running a little rich and have fuel, Than lacking and the motor running hotter. Good luck either way

[Thumpin440ex]

To the OP, I would get the A/F set correctly, along with the pilot. You have basically a stock bike, yes they are jetted a little lean from the get. One circuit at a time. Maybe I am just being lazy, but I am not going to run a 210 jet in a bike that it has no reason being in there then back out. Yes set ups vary, I can understand. Not saying the jet folks are wrong but saying that a 180+ jet in a slip on stock bore stock compression bike is a little big IMO. I am not a pro by any means, but have messed with some fast stuff. People do things diff all the time, so yes there is more then one way to skin the cat. Can you run the bike untill it hits the rev limit ?? You can tell the diff in ignition stumble and fuel stumble. And for sure check your plug after good WOT runs. I run a 440 with decent compression, and head work, am about see level in most riding instances , I run a 185 main 50 pilot A/F screw ruffly 2 1/4, needle 2nd notch from the top on a 450r carb. Any thing bigger and the bike gets noticeably laggy on the top end, never jetted untill the stumble, but tell it instantly.

John

[hondaREX]

Hmmm, guess I need to put some more seat time in. I haven't had much time with a new baby and a house renovation all summer. Thumpin, what gear do you test the main in? I am also getting a slight pop on decel, should I up the pilot? I believe I'm at 2.5 turns out now.