XR600R sputterring at cruise speed

Hi all,


- really can't figure it out... I have a 1998 XR600R that runs OK at most rpm ranges, except at very low turns, when cruising. Say, I hit the throttle, reach 50km/h, then decide to cruise at that speed (I know it is slow :)). The bike will sputter, like it's too rich, and the ride will be rough. If I stop, let the engine run at idle, it also runs awkwardly, seems rich and smells gas. If left long enough at idle and I pull the throttle then some black smoke comes out. Sounds like it's rich right? The problem is here are my settings:

- FMF Powerbomb headers

- FMF Powercore silencer with screen removed

- Air box snorkel removed

- Pilot jet 62 (back from 68 as recommended by FMF on their website (http://www.fmfracing.com/RiderSupport/JettingCenter))

- Needle 4th (as recommended by FMF)

- Main jet 158

- Altitude 300m asl


The air/fuel mixture screw is a bit loose (must be the O ring that's old) but let's say it's 1 turn out. However, turning it in or out does not make much of a difference... quid?


Changing from 68 pilot jet to 62 did not seem to make much of a difference - if at all, the bike now is a bit slower to react to throttle pulls.


I feel I've tried everything! Anyone would have an idea of what it happening? Is the carb the only culprit here? The engine runs fine in high turns, it pulls really well, which is great, but I can't go anywhere without either accelerating or using engine brake!! Cruising is just too rough!


Any help greatly appreciated!



Don't know..Is the airfilter clean of excess oil and dirt..If not soak it in some fuel allow it to dry then spread a bit of oil on it,,not bucket loads..a small amount sprayed on or spread on with your finger is all it needs so dirt sticks to it ...Reinsert in box..test..


You're right 50k an hour is crazy slow..You don't run it at that speed all the time do you..Engines need a good hammering from time to time,,Granny driving is no good for them whatsoever..


That air/fuel mixture screw seems a bit odd..Turns in or out are done in full turns,,360 degrees,,or twice around is one turn..Mark one side of the screwdriver,,go around twice till you come to that mark again..that's one full turn


How does the bike start..ok , or difficult..Maybe you're just running the thing in too higher gear for 50k an hour..I'd be in 1st-2nd running my bike at that speed..,,You should maybe be in 2nd-3rd..

Edited by Horri

Hi Horri, thanks for this! 


I just tried to ride without an air filter at all on a clean street and that showed no betterment...


The sputtering happens at all speed in fact. 50k or 100k, as soon as I'm cruising it goes all hesitant. I guess I must be between 5% and 15% throttle when that happens, say on flats. Not there that often, I'm no granny  :lol: but from home to the tracks I do sometimes have to cruise. And on long rides out country, cruising at times is pleasant enough.


Anyway, no luck so far. What am I going to try next?? 


I've ordered a new fuel screw kit, and a new insulator, as I read the one from 1998 is made smaller due to emission control. Let's see how this goes.


RE: starting, the bike starts VERY well, either cold or hot, no problem there at all - which makes it even weirder. I feel I am looking at the wrong thing (carb?).

Two things come to mind.  The spring, washer and o-ring on the fuel screw must be in good condition and installed in the proper order for the bike to run right. Check the float level, might be too high.

My XR is doing the same thing but i mostly trial my beast and don't cruse along at steady speed very often, so havent tried to fix the spluttering but i would really like to fix it.

I would like to get a new carb but if i can tune it out i would be happy. If you fix yours i would love to know how you did it.

Edited by madxr600

I think creeky outlined the problem

Two things come to mind.  The spring, washer and o-ring on the fuel screw must be in good condition and installed in the proper order for the bike to run right. Check the float level, might be too high.


Hi guys - was offline during the weekend - celebrating my son's first birthday :rolleyes: .


Did not get much done on the carb BUT many thanks for your help here. Looking at how illogical my problem was I decided to overhaul the carb, give it a real good clean (I may have to get a XR Only choke plate now that the O-ring has been soaked in thinner... :banghead: ). Ordered a few new o-rings, gaskets and screw sets. They should be here next week.


In the meantime I discovered the air cut off valve system was in a pretty bad shape ie o-ring broken, gasket maker all over the place, bits of dust on the inside, which resulted in no air coming in. I suspect now that this must have been the culprit > the ac valve letting no air in, I was always going to be too rich in low turns, whatever pilot jet was in. Correct?


Am I on the right track? No proof yet as carb is still on the shelves and I'm out of town for a week - but I will let you know how things go as I put the bike back together.

Make sure you have no exhaust leaks. My XR250 had the same exact condition as yours. I fixed an exhaust leak where the muffler and header and connect and it solved the problem

I have just pulled my carb off and I have a 65 pilot 170 main jet. I have a very open airbox and wmp muffler. The float measured at 14mm my manual says 14.5mm. My bike is the same as it idles starts and runs fine except miss as cruse.

Hi all! Well, well, well... after having put it all back together, I can't say it's worse than before. The bike runs much better, especially when hot, but it's still sputtering at cruise speed a little.  :banghead:


The bike is definitely happy with the needle settings recommended by FMF, as well as the 68 pilot and 160 main. Acceleration is really good (need to change clutch discs now!). And idle is crisper than before, so I guess fixing the air cut off circuit was a good thing to do.


I've only got to re-fine-tune the newly-replaced fuel air mixture screw but will it make a huge difference?


Whatever happens I will post some news as it goes.

Another thing to check is the choke , make sure it is sealing when closed and not letting gas by. The mixture screw should help by getting that sealed up. Have you tried lowering the needle ? Raise the snap ring up one groove to see if anything changes. The rpm your having issues with is the very end of the pilot and as the needle takes over.

Is there different needles for modding the carbs like some other bikes?

My klr is running a KFX needle as an example.

Mine was doing that rich crap at idle and the o-ring on the acv was bad :( so I plugged it off. Don't need that crap anyway. 68 pilot needle second from bottom main 165 :) fuel screw adj out to 2 3/4 turn to get rid off rough cruise. Runs perfect now.


Hi all, better late than never, a quick update. The stator died on me a couple of weeks ago. The mechanic, friend of mine, says it could be the cause for the rough cruise thing. Indeed it is better now (he put an XR250L stator in there, which is good as I now have an extra AC wire to send directly to the headlight :thumbsup: ) but I feel it's not quite there yet. Fuel mileage is very very low so it definitely is too rich somewhere (I suck more gas than a friend's 650R :lol: with plenty of mods). I will try a 65 pilot. Don't think the needle is the cause - it would be a shame if it is because the way it is now allows for a really good crispy throttle response  :rolleyes: .


As often, it looks like there were many causes to the symptoms. So far the following checks have all improved the running:

- carb clean

- ACV rebuild

- stator/electrical check


More when the 65 pilot is fitted!

Check that the spark plug isn't half fouled.  Double check that the choke is staying in the off position and that the flapper door part isn't dangling down into the air stream.


A #68 pilot is very rich.  That's good for acceleration, but will run very poorly when cruising at low throttle openings as you described.  A #65 is probably better; a #62 will probably give the best carburation when cruising.


If you're in Laos, fuel from a 55 gallon drum that's been sitting in the sun beside the road for a month may be suspect.

Edited by heart_of_darkness

Hi all - seasons greetings! 


Indeed Lao fuel is a bit dodgy... I have two filters to help prevent dirt getting in but that will not help dodgy mixing...


Anyway, tried a #62S pilot over the weekend and it was not very good - pretty much similar cruise sputtering and less interesting acceleration. Still have not ordered a #65, so am staying with the #68 for now. But I changed the #160 main for a #158 (needle 4th clip). Let's hope I just get better fuel efficiency for now...  :jawdrop: Indeed, look at this: 137 km and 10.7 liter of fuel sucked through the thumper... a friend of mine, same distance on an XR650R, with a few mods and XRs Only exhaust, only 7.2 liters - that is 7.8l/100km against 5.2/100km!!  :lame:

Big XRs like ours are not tremendously sensitive to jetting or needle position. Your bike should run without misfiring using most possible combinations of the jets you already have. The middle clip position works adequately for just about all situations. 1.5 to 3.0 turns out on the idle mixture screw is what most XR600s will want.


I imagine you don't need to use the choke with a 68 pilot. I know I don't. Make sure that it's not closing on its own.


the o-ring on the acv was bad :( so I plugged it off. Don't need that crap anyway.


Do you know what the ACV does?

So when you are cruising along, you're in top gear and the engine is spinning very slowly and the throttle is open only a very small amount?

Others mentioned the float adjustment, and I'll mention the float needle...

But I don't think you specifically mentioned checking these?

Float level # seems debatable, just make sure the float tab barely touches the float needle button when the float is parallel to the carb/bowl mating surface.

The float needle's rubber tip should be smooth, no wear or ridges.

Hi guys (and girls? :rolleyes: ),


quick update, and maybe the final word...


Still same config (pilot 68, main 158, needle 4th), and it seems all is good now. Maybe the carb has settled in after the clean and feels better now? Very little sputtering left at cruise speed. However still planning on putting a 65 pilot when I order parts next. Going on a week-long ride in a couple of weeks so I'll be able to check that awful fuel consumption.


@Headtrauma, indeed - no need for choke in the morning! Although when it gets cold and the bike hasn't turned for a week... I am tempted by easiness (but choke off as soon as engine started)

@Heartofdarkness - yes that is about it - although it seems the throttle must be little more opened than that, since putting in a 158 main helped reduce sputtering

@ignatz72, float levels are good, as you say, and float's needle is as smooth as silk :)


Cheers - and many thanks all for jumping in the chat!

Edited by GGN406

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