2009 trx400x jetting - Honda TRX Forum
 
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post #1 of 5 Old 08-08-2018, 08:02 PM Thread Starter
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2009 trx400x jetting

Hi All,

I'm new here so please go easy on me. I'm also from the UK, there isn't a massive quad scene over here so it's hard to get info.

I've recently brought a 2009 trx400x which has a standard muffler fitted, with the spark arrestor removed and a K&N air filter fitted, still in the fully sealed air box. Everything else is standard.

Question time.

Should I re jet the carb? If so, which jets and to what sizes?

Should I leave the spark arrestor out of the muffler?

Should I get more air into the air box?

Any advice would be gratefully received.

Cheers!

Lee
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post #2 of 5 Old 08-08-2018, 09:09 PM
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Welcome to the forum!

Changes to the exhaust affect jetting only minimally. Normally no jetting changes need to made when changing the exhaust unless installing a zero restriction exhaust which will negatively affect performance while making exhaust noise obnoxious. Only super-charged or turbo-charged engines benefit from zero restriction exhaust.

Changes to the intake affect jetting immensely - as long as the changes do not affect idle, no changes other than adjustment of the fuel screw is required - if the engine will not idle smoothly, then a change of one size larger on the pilot jet and a fuel screw adjustment is usually adequate. Just having a K&N air filter pretty much requires changing the main jet from a 142 to a 150 or 155. Taking the top off the air box or other restriction modifications may require an even larger main jet in the 160 to 180 range. Just have to operate it at full throttle to determine how far to go. The mid range (1/4 to 3/4 throttle) is metered by the slide needle. If it runs smooth at a steady throttle, no change is needed - if it surges (like it wants to do more but can't and choking it a bit smooths it out, then the needle needs to be raised a notch or two (move the clip down on the needle) - if it blubbers like the choke is on and chocking it just makes it worse, then the needle needs to be lowered a notch or two (move the clip up on the needle). Getting it right is up to the builder making the modifications. There is no one size that is right. It's all dependent on the combination of parts used and modifications made.
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post #3 of 5 Old 08-08-2018, 09:16 PM Thread Starter
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Hi,

Really appreciate your reply, that is very helpful.

In your experience, is there much to be gained by getting more air in and then re jetting? Or is it pointless on a standard setup?

Thanks
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post #4 of 5 Old 08-09-2018, 02:22 PM
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You will gain a little mid range and top end, but lose a little low end. If you got a pal you want to beat in a drag race or just out run in a flat all out high speed run, it might be worth it, but for all around fun, the stock set-up is best. In my opinion anyway.

I just spent $2500 on a 2005 400EX to get it right. Took off an HMF slip on and put on a stock muffler. Jetted the carb back to stock, removed the performance air box and installed a stock air box. Plus fixed all the brakes (including a new rear brake caliper and caliper mount), installed a new axle carrier, new front wheel bearings and fixed the reverse lockout. Now it's a blast to ride. Before it was hard to start, didn't want to idle, had terrible throttle lag, was obnoxiously loud (hurt my ears even with a helmet on) and would not out accelerate a TRX250X.
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post #5 of 5 Old 08-09-2018, 04:44 PM Thread Starter
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Wow! Good advice, thank you very much.

Lee
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