Interesting tuning problems.

[flyboy1294]

I've got a 400ex with the engine mods in my sig.

Before I get started, let me say that I had some engine work done (over size valves installed, port and polish, hone, etc). Everything in the top end is literally brand new. So I'm still in break in, even though I've yet to really start.

Right now, I am running a 185 main and a 55 pilot on the 40mm FCR. The 185 is just a tad rich, but that isn't my problem.

When I rev the bike, it back fires and spits fire out the exhaust when I let off the throttle. I have tweaked the a/f screw. Actually, I started at a 50 pilot jet, and have worked my way up from there.

From everything I know, the symptoms above indicate that it's lean on the bottom end. However, a 55 pilot seems pretty dang big for my setup.

So, with that said, does this sound like a simple lean issue, or do you guys think there is an underlying problem that I need to look into? (Note: I've already checked for airleaks. It's good.)

Thanks guys.

[honda400ex2003]

I would say that fire out the back is too rich imo. probably rich throughout the whole carb. I would do some testing with plugs at least to see how rich it actually is or lean it is at pilot (idling), needle, and main. do each of them seperate and on new plugs AFTER warmed up as best as you can. do the chop method on each of them to at least get an idea of what you are working with. steve

[78ta]

fire out the exhaust is unburnt fuel, way to rich on the main. dial in the main first then do the rest of the circuts

[bherriman]

I would start off going down a couple on the slow jet and then mess with the main. She's way too rich if its turning into a flame thrower...how ever cool it may be lol

[flyboy1294]

Here is what I did

First, stepped down to a 180 main and a 50 pilot. It seemed to improve somewhat but still back-fired, literally.

Next went to a 175 main and a 48 pilot, similar results.

Finally I went to a 170 and 42 pilot. It still was blowing fire, but I could get it to idle with the choke on.

I don't have any smaller jets. My old carb was using a 185 main and 42 pilot, and the engine was much more close to stock then, so I would've never dreamed it'd be using smaller jets now.

Am I going the right route here? I wanted you guys' opinion on this before I took off making assumptions again.

I don't know if a video is beneficial, but I will try to take one in the morning.

[78ta]

im running a 460 stroker, ported head, +1mm valves, stage 1 hotcam, 39mm fcr, air box with k&n lid

. I don't remember exactly what jets r in mine, but if i remember right in in the high 160's low 170's on the main. factory kehin jets. can't rem the pilot somewhere around 50

I was at a 230 main with the stock carb, no lid. still too lean.

You are on the right path, keep leaning it out on the main, watch the plugs. Cut your plugs and look at the "mixture ring" Wide open through 4th pref up hill for a little while clutch in and kill it. coast back and yank the plug and cut it. Do the main first, then mess with the rest. all circuts overlap, gotta get the main close first wot b4 you can mess with the rest.

most everybody on here has their 400ex's way too rich for their mods, too rich is just as bad as too lean, too much fuel washes rings and prematurely wears them out, to lean gets hot, detonates and breaks stuff.

[flyboy1294]

Originally posted by 78ta
im running a 460 stroker, ported head, +1mm valves, stage 1 hotcam, 39mm fcr, air box with k&n lid

. I don't remember exactly what jets r in mine, but if i remember right in in the high 160's low 170's on the main. factory kehin jets. can't rem the pilot somewhere around 50

I was at a 230 main with the stock carb, no lid. still too lean.

You are on the right path, keep leaning it out on the main, watch the plugs. Cut your plugs and look at the "mixture ring" Wide open through 4th pref up hill for a little while clutch in and kill it. coast back and yank the plug and cut it. Do the main first, then mess with the rest. all circuts overlap, gotta get the main close first wot b4 you can mess with the rest.

most everybody on here has their 400ex's way too rich for their mods, too rich is just as bad as too lean, too much fuel washes rings and prematurely wears them out, to lean gets hot, detonates and breaks stuff.

Alright, this is extremely helpful.

What are safe increments to buy jets in? From what you say, I'd assume I need somewhere between a 155-168 main, and a 32-40 pilot.

I have been getting every other size, but I have 8 mains and 6 pilots that I have no use for anymore. The cost is adding up.

[honda400ex2003]

Stock pilot is a 38, i would say to buy a 40 and a 38 if you dont have one. mains i would say 155-170 would def suffice. you should def be in there somewhere. you already have a 170 so 155-168 by 2 or 3 is the jump that there is. there are only 5 jets in that group so you can get those easy enough. take the ones that you have back and exchange them for the ones you need. they should at least do that for you. as i stated before and others have also, keep doing the chops and see how they look. steve

[flyboy1294]

Originally posted by honda400ex2003
Stock pilot is a 38, i would say to buy a 40 and a 38 if you dont have one. mains i would say 155-170 would def suffice. you should def be in there somewhere. you already have a 170 so 155-168 by 2 or 3 is the jump that there is. there are only 5 jets in that group so you can get those easy enough. take the ones that you have back and exchange them for the ones you need. they should at least do that for you. as i stated before and others have also, keep doing the chops and see how they look. steve

Alright thanks, that's helpful.

I bought all my jets from RockyMountainATV for $2.25 a piece + $7 shipping :mad: so I doubt I can exchange them. I'd say I'll try to sell from for like $1 a piece of something.

Where do you guys get jets from normally?

[honda400ex2003]

i try to get them from my local dealers if i need them otherwise i have them order them for me. Yami ones and honda ones along with any other keihin running brand will have them if you are lucky. my honda dealer doesnt stock them so i get mine from the yamaha dealer in town. Jets r us is where alot of folks get them also, along with rocky mountain. steve

[78ta]

i got mine from jets r us. i do a lot of jetting other peoples quads, i have every size main they make lol

when jetting carbs that i have no clue what size they should be I jump 5 sizes at a time till i get it close but lil rich, then buy all the sizes in between that rich one you have and the next leaner one you have.


be forewarned fcr's are very picky. stock carbs are way more forgiving. try to get everything close so that it will run and atleast halfway idle, then mess with the main till you get it close wot by doing plug chops, then mess with pilot, fuel screw and needle. then you can mess with the leak jet. you will pull some hair out in the process but mine was worth it though, takes a lot of tinkering and tuning to make one work right on the 400ex