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Discussion Starter · #1 ·
I have a 07 trx400ex with a HMF exhaust and a K&N filter with outerware. Stock airbox with lid. I live in Texas so temps range from 25*-100*. I just purchased it used and the guy said it was jetted, it starts fine BUT as soon as you press the throttle it bogs, sputters and then it will take off. Revving it in neutral does the same and sometimes backfires till it gets past the 1/4 throttle and its then fine. I took the carb a part and it is very clean so my first thought that it was dirty was wrong. It has a 168 and a 38 jet. Fuel screw was at 2 turns out. I cut a groove in it so it can be adjusted easier. Should I jet to a 40, 42? and leave the 168 since the top end seems ok? What kind of jets do I need, I see b series, c series and I have no clue.


 

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The carb has an accelerator pump which shoots a shot of fuel into the throat of the carb upon throttle opening. Check to be sure it is working properly. It may need serviced, replaced or adjusted.

You might replace the pilot jet with a #40 and readjust the fuel screw for a smooth idle, but the pilot jet has little to do with acceleration. (take the pilot jet to a shop with an assortment of jets and match it) The setting on the fuel screw is totally dependent on the pilot jet and the pilot air jet. Make the setting with the engine at full operating temp.

Try running with the choke partially on and see if that helps or makes it worse. Try a stock filter and see if that helps or makes it worse. You might have to raise or lower the slide needle depending on what the "try's" indicate.
 

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Discussion Starter · #3 ·
I will see if the local Honda dealer has a #40 jet and give it a try. I did happen to press the accelerator pump and it squirted good so I think that is good. What is a good setting to start the fuel screw at? Turning it out gives more fuel correct? In is less?
 

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2 turns out is a good starting point - yes,out is more fuel - in is less - are you using 93 octane fuel? The octane will make a difference too - 93 to 95 is good - anything above 100 octane will cause a loss of power.
 

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A 168 is pretty big, but if it runs at full throttle without blubbering or fouling plugs, then it is OK. Remember - more fuel is not more power - less fuel is more power, but if you change the velocity of air flow through the carb with changes to the intake and exhaust, then you have to compensate to correct the mixture. A bit of experimentation is required unless you have a flow bench. With a dyno you can see the results of changes. Without either, it's a seat-of-the-pants try till you get it right process. Too lean will cause spark knock and (over time) engine damage. The sensation of too lean is surging at top end (kinda like it want's to rev higher, but can't). Too rich will cause poor high end performance, fouled plugs and excessive carbon build-up in the exhaust. The sensation at top end speed is like the choke is on, engine kinda falls on it's face suddenly, blubber's and slows down. Backing off the throttle slowly will find a spot where it suddenly pick's up speed again and as it accelerates it starts to blubber again.
 

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Discussion Starter · #7 ·
What is the difference between a Pro X - N424-26 series vs a Keihin N424-74c series? I cant seem to be able to find a 43 pilot jet locally and there are so many different series of jets.
 

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First off - there is no #43 manufactured - of 2 or 3 (35,38,40,42,45,48,50....) with 35 being the smallest that I could find commercially available.

The N424/21 and the N424/25 are the two most common. Send me a pic of your jet and I can probably tell you what it is. Check your PM
 

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What you have is a Keihin N424/21-42

Standard jet in a #38 - Usually the pilot jet does not need to be changed and if it does, almost never more than one size increase. I would recommend getting a 38 and a 40. Install the 38 and set the idle (hot motor - fuel screw about 4 full turns out and adjust from there for smoothest idle - then ride it to see how it performs. Best if air temp is over 50 degrees. Under 50 degrees - ride it about 20 minutes and then make the settings.
 

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Nowhere yet, my wife and I have about 235 acres up in northeast texas about 2hrs. away. I've met some guys from Okla. that ride on the Ok. side of the Red River on a
sandbar, they have TRX's, Banshee's, etc. sounds like fun up there, I want to try that,
Sounds like you and I have been on the same job repairing our quads, my 400 is an 05' all stock, I installed a new sparkplug, set valves, changed oil & filter, serviced air filter, installed a Dynojet carb kit, and replaced the rear tires. Still lack servicing the chain, I'm also adding a tach/hourmeter. It cranks up real easy with no choking and
runs great.
 

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Discussion Starter · #15 · (Edited)
Same here, I heard the red river area is nice but have never been. Wish there was somewhere around here to ride. My wife wants a automatic but we don't have the room for one right now. Let me know if you take a trip up north, I can meet you up there or something. The only thing I am lacking now is new rear plastics. P.O. cracked the left side, its not bad but annoys me.


As for my carb, I put in a 42 and a 170. Starts with no choke, a little rough the first min. but once warm it runs like a top. I think the 42 may be a bit rich but for now its fine.
 

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Discussion Starter · #16 ·
Recent update: with the warmer temps here in the 80-90's I went riding some trails this past week and had some bogging. It would start fine even when warm and 1st-2nd gear was strong and then it fell off on power. IF I gassed in in 3rd or 4th she would fall on her face unless I feathered the throttle then she would take off. Right now I have a 42 and a 170 in there as HMF suggested. I have the slip on pipe, kn filter and outerwear and stock lid.


I plan to put in a 40 since the 42 is a tad too rich I think and drop to a 168 or lower. What do you think?
 
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