Jetting

Hello, ive read most of the jetting comments.   My new to me Xr400 has gordons mods with a FMF Q4.   Its jetted at 155/55 according to my mechanic who i just got the back back from.   My question on rejetting is if i go to a 160/60 for sea level where i do most of my on street riding 1) would that solve some popping i get on decel 2)  help when i go to 7000 feet elevation 2 times a year to ride?   Would i be able to mess with fuel screw when going to higher elevation to get it to run better without changing the jets?

 

JK

The 160/60 will probably run better at sea level. The popping on decel indicates a lean pilot circuit.

Where is the fuel screw set with that 55?

No, the bigger jetting wouldn't help when going up to 7k in elevation. It would be too rich. You can figure a need of 1 jetting size change per 3k in elevation change. Your current 155/55 would run ok at 7k.

The engine would run with the bigger jetting at 7K but it would blubber and be down on power. Tuning the fuel screw leaner would help but only a little past 1/8 throttle. You're better off to rejet when making an elevation change of that much.

I've just put my 96 model back together with the 160/60 jetting, Unifilter/no snorkle/exhaust baffle out and all seems really good.

 

Idle mixture screw best at 2 1/4 turns out ad seems to rev readily from idle, but I haven't ridden it around yet as it's unregistered, just sorting a trailer out.

 

I'm at sea level using Australian PULP fuel.

The fuel screw is set at 2.5 turns.  It seems to be popping quit a bit.  The mechanic told me to turn a 1/2 turn either way and see how performance is effected.  Other things to note -  the little spring on the internal part of the carb is broken so the little piece attached by spring flops around.  He said that might let in a  little more air at times.   Lastly,  throttle response is a little slow i think.  If i just hit the throttle itll almost die.  If i turn the throttle a little until i feel resistance and then hit it its fine and revs up.  Kind of quirky.  Not sure what thats about.  

If you're at 2.5 turns out already and it's still popping on decel, going another 1/2 turn out puts you at the recommended limit of 3 turns out, so go ahead and swap to a 60 pilot.

 

The internal part with the broken spring you speak of is the backfire flap on the choke plate. Either replace the choke plate with a new OEM part, the billet part from XRs only, or remove the screws holding the flap to the plate and remove the flap all together. Don't leave it as is.

 

Even with the carb tuned well, whacking the throttle open quickly off idle is not going to get you instant response like what you would expect from a 2 stroke. The hesitation is normal, don't get hung up on it.

I have a 99 xr400 with about 15k miles of baja riding (sea level to about 8,000 ft, tight trails to high speed highway) and the best jetting I have found is #62, #160, 2-1/4 out and 3rd position on needle.

This is with stock engine, stock carb, uni air filter, no snorkle, and exhaust baffle removed, 15/42 gearing. Best of luck.

thanks for a dive

 

There isnt any difference between 15/42 gearing and 16/45?    I had 16/45 and liked the mid range and higher end but it really wanted to sputter at slow speed when hanging a uturn or something.  I have 15/45 now and its great at slow speeds but i missed the higher end speed.   

You are correct that gearing is the same except the 15/42 uses less chain length. You are also correct that the gearing is a little tall for ultra slow riding, but  I just got used to it. I have never have much trouble

in the slow challenging stuff but the reason I use tall gears is to conserve gas. I have a 4 gallon tank and  I seem to get 60-62 mpg no matter how easy or hard i ride. In Baja I need the 240 mile range.

Also I have never had to mess with fuel screw, but if it is really cold (say 30 F or less) I do increase to idle screw a little for cold starting.

Edited by bajanutt

I am late on this subject. Have had an xr4 for a week. Today I took the carb apart (was told that it was jetted) it had the 52/152 jets in it. The float bowl was clean as were the jets. The previous owner had remove everything on the muffler but the chrome cap. I think he punched it out. I installed a 1.4" insert into it.

 

It starts good on full choke.  But pops for nearly a minute, The next step is toward going for some jetting.

I am late on this subject. Have had an xr4 for a week. Today I took the carb apart (was told that it was jetted) it had the 52/152 jets in it. The float bowl was clean as were the jets. The previous owner had remove everything on the muffler but the chrome cap. I think he punched it out. I installed a 1.4" insert into it.

 

It starts good on full choke.  But pops for nearly a minute, The next step is toward going for some jetting.

 

Still stock jets in there. Bump them up in size to 60/160 range and tune from there. Your bike should run much cooler also after jetting. When I opened my exhaust up with stock jets in, the heat difference off the motor was very noticeable.

Edited by michigan400

Obviously I don't know how hot it should get but the oil cooler gets hot within a mile.

Still stock jets in there. Bump them up in size to 60/160 range and tune from there. Your bike should run much cooler also after jetting. When I opened my exhaust up with stock jets in, the heat difference off the motor was very noticeable.

Today I tore into the carb on the 2001 XR400R. The needle was a A16A. It didn't correspond to anything in the Sudco catalog as an OEM and didn't have enough digits to convert to Option. Let me back up and say when I got the bike the exhaust didn't have anything but the empty chrome cap. I welded on a a 1.4" tip in it.

 

When I took off the seat, the airbox lid was open, no rubber intake on the airbox. The kicker was that it didn't have an o-ring between the rubber intake and head.... OOPS the prev owner must have lost it. Called the local honda dealer, didn't have it would have to order, I am going to look for a 1.75x1/16" as the part is a 45x2.5mm. Part # 91303-397-000.

 

I raised the needle a notch, getting bigger jets.

The A16A needle is the stock 49 state needle and the one most preferred. So you're good there. Stock clip position on the needle is 3rd from top

I don't know how the bike ran at all with no oring between the intake manifold and head.

Removing the rubber airbox snorkel under the seat is one of the normal Gordon mods. But someone went overboard with removing the entire exhaust baffle/spark arrestor. You should not be running an open exhuast. Either find a stock baffle/spark arrestor on ebay or buy an aftermarket unit to replace the whole stock muffler/spark arrestor.

Thanks, I do have a homemade insert welded into the chrome cap so it isn't an open type anymore. I raised the needle to position 4, second from the bottom. Put in a 60 primary and a 160 main. My local Kymco dealer got me set up with an O-ring for the manifold base. Idle mixture screw is about 1 turn out instead of 2.75. Starts one kick. Idles nice. The sound has lost that hollow sound. Engine seems to run much cooler, I could barely touch the oil cooler after a mile. Now after 2-3 miles it is noticeably hot but touchable. Sorry no oil temp gauge. Main thing is that it revs 2x as fast. Front wheel air has become commonplace. Yeah baby it runs. I rode a Honda Motocrosser 450 a few years ago. Every time I opened the throttle in a pasture it was instant wheelee. This is much nicer and more affordable.

 

Time for a another pic.

 

Thanks to all, Michigan400 and trailrider42

 

It is great.

I finally got the UNI filter yesterday.

When I got the bike the previous stock foam filter had disintegrated leaving the metal screen about half clogged. I had burned the foam out, more or less with a propane torch then built a foam cover from some open cell foam. In truth the screen although better than when I got the bike, was clogged about 30%. It ran good with the 60 primary and 160 main jets.

 

I started the bike after installing the UNI and got intake popping, just as it had done when i got the bike, before jetting it up.

I richened the idle mixture screw. Didn't ride it as we just had two inches of rain and the ground was not suitable for testing the power.

I see further jetting changes coming up with this new more free flowing intake.

Edited by lump_tha_thump

Finally I got a chance to ride my XR400 with the UNI filter and the 60/160 jetting with my 1 1/8" exhaust tip. All I can say OMG.

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