XR 400 breathing problems

Hi all , new here , often read posts on this great site and thought it was time to join. I am hoping someone can give me some advice about my XR please. The bike is set up as follows;

2003 Honda XR400 running on standard bore

Gordons mods done

mikuni pumper tm 36...jetting unknown but came off another XR400

FMF powercore 4 tail pipe muffler,( not full system)

Gearing ,Front sprocket 15 rear 49

riding about 1000 feet altitude

I first ran the bike with standard carb for a while ,then fitted the pumper carb which improved its performance and starting . I kept the standard tail pipe (the Honda original) but removed the baffle and rode it like that for a year on 14 45 gearing .it started ok 2 to 3 kicks even in minus 15 celsius in the snow !! although when riding on trails if I shut off or revs dropped it would die....I assumed this was lack of 'back pressure'

I then fitted an FMF powercore 4 pipe last october and did not ride it through the winter much but found it has become very difficult to start from cold requiring many kicks, when warm it starts first kick , when I ride technical stuff at low revs it splutters and seems to be gasping for air or fuel It tugs and makes trials type riding unpleasant, on the other hand if I ring its neck it goes like stink, plenty of power at high revs but wants to be ridden fast everywhere fine for flat out stuff but not good for trials stuff I love, if I shut of the throttle or 'coast down a hill it pops and pops badly ...like its running low on fuel.

Could this be a jetting problem ? or is the carb blocked with dirt from the tank. I have fresh fuel in it .

the problems started after the fmf pipe was fitted .cant find any information from Mikuni or fmf on jetting set ups...I notice that there is also a sooty black residue, not oil, around the muffler tail pipe hole too...

Any advice would be greatly appreciated .the bikes still rideable just annoying me really..I still have the original Honda pipe and I am wondering if I should put it back on......

Thanks very much

Edited by oakframer

Sounds like a partially blocked pilot jet to me. I'd say start with seeing where your fuel screw is set at (should be approx. 2 turns out from all the way in) and make sure the pump comes on as late as possible (adjustment screw all the way in) and put a new spark plug in. If that makes no improvement then clean the carb out real good, get new jets just in case the crud is stuck in there beyond cleaning. They don't cost a lot. Get with XR's Only if you have questions on what size to get. Sounds like you should be fine with the standard 15 pilot and 130 main IMO.

Add an inline fuel filter too

when were your valves done last? I would check and adjust to spec, do a leak down test and compresion test after valve adjustment strip clean and retune the carby

i have had my xr do similar things, but they are bullet proof... change my oil every 3 rides or 500km's cheap insurance...

Thanks for the good advice folks. Just the tuning of the fuel screw to 2 turns out worked wonders.....although it still pops and fizzes a bit when the weather is cooler..

popping on decel is usually jetting too lean, or an air leak resulting in a lean mix, or an plugged jet.

if the pumper was put on/set up for a stock intake/exhaust it's going to be lean flowing more air with that exhaust.

BUT the running very well full bore says it's not lean everywhere.

Post this on the jetting sight, running well on top but not on bottom points to that carb not being in good tune for your bikes setup.

Edited by MLB007

I seem to be experiencing a similar issue, though adjusting the pilot screw makes no difference. I expect it is supposed to kill the engine if it is screwed in all the way, but I have to have the idle screw up so high in order for the engine not to die on idle that I worry the pilot circuit is not even engaged at that point. no problems when I am about 1/4 throttle, so I figure there is something wrong in the slow jet or idle screw.

Here is a video showing the strange suction sounds, warmed up and choke off.

Could it be the slow jet needs to come out a bit more? I replaced the jets two weeks ago and cleaned the carb bowl out.

Could it be the slow jet needs to come out a bit more? I replaced the jets two weeks ago and cleaned the carb bowl out.

You're saying slow jet but I'm sure you mean fuel screw. So many people neglect to remove the fuel screw and clean out that passage when they clean the carb. It is as important as cleaning the jets. You must also make sure the o-ring is in there, in good shape and installed properly.

You're saying slow jet but I'm sure you mean fuel screw. So many people neglect to remove the fuel screw and clean out that passage when they clean the carb. It is as important as cleaning the jets. You must also make sure the o-ring is in there, in good shape and installed properly.

I actually was referring to what they call the slow jet in the following diagram:


OK, the slow jet (26) in your diagram is not adjustable. It needs to be screwed all the way into the carb body nice and snug.

This has come up before. Where did you get this advice?

What BR said. That's why I assumed you were talking about the fuel screw, because the pilot is not adjustable.

Aw that would make a big impact. This is my first bike and the guy who I bought it from did the rejetting with me and told me the slow jet also had to be unscrewed a bit. I just checked the manual and indeed no mention of it. Thanks for noticing this, I am going to go tighten down the jet now and see where that leave the bike status.


Either the guy was misinformed himself or he just mis-spoke, saying slow jet and meant fuel screw(sometimes referred to as pilot screw).

So I tightened down the slow jet, but still the pilot screw seems to not be having an effect on the bike's idling issues. I took the bike out this weekend with it running a bit rough, and once it got up to 92 degrees I was able to turn down the idle screw and it ran smooth at 1500 RPM. The bike cooled down as I ate lunch, then when I started it back up it was running pretty rough again, the bike was at 82 degrees.

One thing I noticed is that when I need to slow down going down a hill to a stop light, I pull the clutch and the brake in and the engine bogs way down and the engine stops. I can prevent this by flicking the throttle a bit, but it becomes quite the technical challenge to keep the bike running while coming to a stop from a higher gear (3-5)

Don't ride with the fuel screw out more than 3 turns. It can fall out at 3.5 to 4 turns. I know.

The pilot jet controls low rpm fuel metering. The fuel screw is a fine adjustment of the pilot jet. Think of the fuel screw as the 0.1-0.9 pilot jet adjuster. If the fuel screw has no effect on idleing from 1/2 to 3-1/2 turns out, the wrong pilot jet is installed. Try one size larger or smaller. Hint, you can fake testing a larger pilot jet by adding some choke, which will richen the mixture. See FAQ and dreaded bog for fuel screw adjustment tips.

Do they put ethanol in the gas? I use to put away my bikes for the winter, then start them in the spring. Not anymore. The alcohol pulls moisture out of the air, then evaporates, leaving water to corrode the carb.

Possible fixes

New gas. Old gas goes in the lawn mower.

Add some SeaFoam cleaner to the gas.

Adjust fuel screw.

Clean or replace pilot jet.

New spark plug.

Adjust valves.

Check manifold for air leaks.

Clean carb.

Adjust float level.

Edited by Kev_XR

Problem Resolved!!!

So I checked the slow jet size and it was #52, but the guy had put an aftermarket exhaust on, so I swapped out for a #62 and that fixed the problem instantly. It runs smooth and the pilot screw now works as a fine tuning for the idle circuit. Thanks everyone for the input :)

A quick update ....so I took the Mikuni TM36 pumper carb apart to see if I could fix the problem, cleaned everything , passed thin wire through all the jets I could see and adjusted pumper nozzle to spray on needle. cleaned out but did not touch the float height. Fitted carb back on bike checked intake manifold connection and airbox connection also cleaned out all breather pipes . Wound out fuel screw 2 turns laid bike on its side until fuel leaked from overflow pipe, pulled choke and tried to start....still required many kicks to fire but once warm started ok, however it still popped on decel and also flames were coming from exhaust...!!! So I let it cool down and pulled the plug to see the colour it looked ok not white but tan colour with a bit of carbon....so I wound it back in just a bit tighter than I normally do adjusted fuel screw to 3.5 turns .....bike started 2nd kick ....slight popping on decel and now some bogging off idle !! There doesnt seem to be much info available on jetting the TM 36 mikuni pumper for running with the FMF powercore . I tried FMF they came back quite quick with a reply but, unfortunately the jetting specs were for the stock keihin carb no luck there. Tried Mikuni still heard nothing from them. The carb has a 130 main jet and a 15 pilot jet at 3 to 4 turns out the bog starts to disappear and also the popping reduces...question is should I go up on a pilot size ? I am wondering if the spark plug was loose and that was the main problem all along, the bike starts from cold one to two kicks so a great improvement. Thanks for the advice folks.

Edited by oakframer

Has no one given you this link yet? This is what we use to reference the TM36.


Yes, I think I would try a 17.5 pilot jet.(next size larger).

Did you at least check the float height to see what it was?

You shouldn't need to lean the bike on it's side before starting. If you do, then your float is screwed up.

Best cold starting drill for the Mikuni I've found is:

Fuel On, Choke on

Pull and hold the decomp lever, kick the engine over several times

Release the decomp, bring engine to TDC, kick hard, wicking the throttle open off idle at the same time.

Thanks Trailryder42, have the tuning TM 36 manual (your link) also l only laid bike on its side to make sure fuel was getting to carb, dont do it regularly. rode the bike today and bogging awful....getting fed up with it now.....maybe valves need adjusting...:-(

Edited by oakframer

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