I EASILY Whooped A Raptor..

I went riding yesterday with my buddy that has the 660 Raptor. We traded off and rode each others machines for a while. He seemed to think that his Raptor was faster than my bike. Honestly, I was scared to ride the Raptor very quickly as it scared me. When you hit the gas, it would take off in a different direction everytime. It "power-steered" under accelleration and it was never predicatable as to which way it would go.

Anyway, we finally lined them up and raced. Each time he would get the jump on me as I had to control the wheelie action.. But I would quickly catch him and then blow past him with ease. No real comparison as it just was not much of a challenge.

We rode about 60 miles in total. Towards the end of the day my bike started mis-firing under hard accelleration. It would not do it all of the time, but occassionally when running it pretty hard it would lose power, then pick right back up. No pop out the exhaust or anything like that, just an instantaneous loss of power momentarily.

This was the first outing with the new 68S pilot, and the 158 main jet. I also swapped to a UNI filter which I purchased from XR's only. In case anyone is thinking about the UNI filter, it does come complete with the cage, ready to be oiled & installed for $28 bucks. I had a K & N but everyone said they flowed better at the expense of letting more dirt in. That is the reason that I changed them out. It seemed to perform pretty well.

At the end of the day the bike was running like crap. It would not idle and kept stalling out unless you kept on the gas. Under acceleration it seemed to run okay but if you let off the gas you could feel that the engine was wanting to die as the compression braking felt like I was riding the brake.. If you pulled in the clutch-it would die instantly.

Any ideas as to what I should check first? I was thinking perhaps the valves might be out of adjustment after all of the hard running, high speed, killer RPM runs I was doing. Tomorrow I will adjust the valves, change the engine oil, pull & inspect the carb to see if something is blocking one of the jets. The problem only seems to affect the first 1/4 of the throttle.

I forgot to mention that I also turned the adjustment screw (wha'ts it called again?) in 1/2 turn at the beginning of the day and the bike ran well that way until it developed the problem it has now. I was thinking about turning that screw back out, but I don't think that is the problem as it ran all day that way and was not a problem until the end of the day. The bike will still rev up, but if you try to let it idle it dies, and under very light acceleration (1/4) throttle or less) it spits & sputters and wants to stall out. HELP!

I have another riding trip planned for this weekend out in Highlands Texas and would like to have my baby running properly again.

On a side note, I just purchased a complete aftermarket exhaust system with stainless steel header.. I will be installing it along with the Hot Cam as soon as it gets here.

Sounds like a little spec of something clogging the pilot to me. That'd be my first check.

i would definately say you are a little lean.. maybe even two sizes off... i would check the rest of your jetting specs just incase. but as far as the problem goes i think you are on the right track... check the valves and clean the carb and see what happens.

you also might want ot just start it up after it has sat for a day and got completely cold again. see if it gives you the same problem now before a day of riding

definately get back to us if you find out what the problem was.

If you are riding an uncorked XR650R, then I think you need a 170-172 jet (or somewhere in that range). The 158 is closer for a corked up bike. If you are riding a XR650L, then that is a different story.

How is your filter. Did you over-oil it? When you oil a filter, you want to spray it on then squish up the filter in your hand by folding it over then compressing it to distribute the oil evenly. Be careful with the UNI filters as the tend to come apart if you are too rough with them. If its over-oiled then the bike runs richer. Your filter may have been too oily thus choking off the air inlet near the beginning of the day while letting more air in later in the day.

If you have a XR650R and it is uncorked, then read on. Read the whole thing before you start working on your bike so you can tell if it is lean or rich and identify what is going on.

Carburetor Air Screw (recessed brass standard/slot head screw on right side of carb) - This should be at 1.5-2 turns out from screwed in.

Carb Idle adjustment - This is the knob on a flex line at the rear of the carb under the gas tank - I screw it in a bit when the bike is cold to raise the idle and then screw it out to keep the idle low when it warms up.

Valves - If they are loose, they make alot of noise. If they are tight, then they are still quiet and its very difficult to start your bike. Your bike should be easy to start with a 68s pilot jet and the air screw at 1.5-2 turns out.

Coolant - Did your bike boil off the coolant and overheat? That might explain the erratic power near the end of the day. Is there any coolant remaining on the reservoir on the lower left side of the bike? If the bike is cold, then start it, open the radiator cap and pour more in. Keep it running until it starts to get warm so you can get the air out of it. Put the cap back on, leave it running and fill the reservoir. When the bike cools, it will suck more coolant in from the reservoir to displace the water vapor when it condenses back to liquid. Watch the reservoir the next couple rides. I use prestone 50/50 coolant that is premixed or water wetter plus distilled water. The latter has 15-20% more cooling capacity but can freeze and damage your engine in the winter.

Spark plug Electrode color - White=Lean, light Brown=Good, Dark Brown/Black= Rich

____Main Jet Tuning_____

Since you already have the 68s pilot, here is what I do for tuning the XR650R main jet. I install a main that I think is a bit undersize and start it. Let it warm up. From idle, I blip the throttle. If it bogs before it revs, then the main jet is too small. I then change to the next biggest size main and try it again. As soon as the bog goes away, then you are probably good.

The main jet is accessible from the 17MM nut on the bottom of the carb. Make sure to get the main jet threads through the upside down brass bowl that works as a baffle. Tighten the main but not too tight. If you unscrew the emulsification tube (the tube the main jet screws into) you might have to remove your carb to get that back in properly. That is a whole new post.

At that point I go and ride it for about 10 minutes then roll back into the driveway and shut it off immediatiately without letting it idle too long. Next, pull out the spark plug without burning yourself. You can do this the next day if you wish. You want it to have light brown residue on the part that is in the combustion chamber. If it is white, then you are still too lean and need to go 1 size bigger on the main jet.

If the main is oversize, the first indication is that the bike will bog as you roll off of the throttle from wide open. This means that you are at wide open and as soon as you start to untwist the throttle, the bike will bog for a second or split second. You want the roll-off to be smooth.

The main jets are sized by xx0, xx2, xx5, xx8, meaning that there is a 172 and then the next size up is a 175 or the next size down is a 170. Follow the table on the http://www.xr650r.us/jetting/ link.

Billahjack, I am not 100% sure I agree with you.

The blipping of the throttle actually is more indicative of your pilot jet and needle height choice, not your main jet.

Billahjack, I am not 100% sure I agree with you.

The blipping of the throttle actually is more indicative of your pilot jet and needle height choice, not your main jet.

Thanks for the input.

The thing that gets me here is that he is using a 68s pilot and a 158 main. The 158 main is a good XR650L main, and not a good uncorked XR650R main.

That is why I was wondering which bike he was using.

Ahh ok, the only thing I considered is maybe he lives at high altitude? I live at 6100 feet and am running a 162. I was running a 168 in the winter, but as the temps came up it needed to be leaned some, it screams now. I can't wait to get this monster to sea level.

My bike is a 600 NOT THE XR650R!!!!!!!!!!!! And I live at SEA level.

Here is an update. I pulled the carb and checked the pilot jet. It was

clean/clear. Nothing inside of it. I sprayed carb cleaner through it

anyway, and then blasted it with compressed air, and then confirmed it

was clear at the end just to be sure that was not the problem. Put the

carb back on and the same symptoms. The bike will not idle, and under

1/4 throttle or less it spits and sputters badly.

I adjusted the valves today. They were all spot on. I thought at first

they were a little tight, but it is difficult to get the feeler gauge

in there. I adjusted them to .005" exhaust, and .004" intake as best I

could. I started the bike again, and now I can hear a mild tappet

noise. Not enough to be concerned about but if I put my head close to

the motor you can definitely hear them. It did not do this BEFORE I

"attempted" to adjust them. No big deal, I can remedy that later.

Started the bike up-same symptoms. It will rev up easily, and produces

great power-but does not want to idle and will not run under light


I pulled the spark plug and it was black and sooty. A sure sign it has

been running rich. This current plug condition may have happened since

the problem started so I am thinking at this point that the bike is

running rich either due to too much gas, or a plugged idle air bleed

which would give the same result. I put a new plug in-exact same

running conditions.

Tomorrow I will pull the carb and blow out all of the air bleed

circuits. If I remember correctly, you can see 3 of them if you look

into the inlet side of the carb. Perhaps something has plugged one, or

more, of them up.

Got my aftermarket exhaust in today. I purchased the Big Gun system. I

have read that people have LOT'S of bad stuff to say about them, but I

got a really good deal. I was mildly surprised when it arrived today

and the can has already had an update done to it that would prevent it

from blowing off the mid-pipe. There is a stainless steel strap that

encircles the entire can with rivets about 1" apart all the way around.

I was reading that this is the area to watch out for. I was prepared to

modify/strengthen this one when it arrived, but alas it was already

done for me. The stainless header is beautiful! I can't wait to install

it and get that old rusty factory system off. The factory pipe on my

bike is the only eye-sore it has. But, I won't get a head of myself. I

need to figure out this problem with my bike before I move on..

If anyone has any ideas, please reply and let me know. Specifically, if

you have any ideas about how to check/clean the idle air bleeds, or

their locations, let me know.

Could it be a float needle seeping at idle but burns up excess when throttled up? Try turning off pet cock after starting & see if it starts to clear up temporarily.

Make sure your pilot screw isn't creeping, the above poster also has a good point about checking the float level, perhaps the float has a leak and as you ride it is filling up with fuel, or just plain getting stuck.

I will check/confirm the pilot screw to see if it has backed out. Help me understand about the pilot screw-if I turn it in, it should DECREASE the amount of air/fuel mix for the idle circuit, and if I back it out it should INCREASE it? Once I correct the current condition, I am going to experiment with the screw to see what effects it will have.

I should have the carb off later this afternoon since today is "D-DAY". I have to have it running or I scrub my planned trip to Spring Creek for this weekend.

Opening the screw makes richer.

That begs another question; Does that screw only control the fuel flow for the pilot circuit, or does it control the QUANTITY of emulsified fuel for the idle circuit?

I think your pilot jet is for a XR650R and your main is for a XR600. I think you need a pilot around the 55 range to lean out the idle. The 68s is too big and that is what is making your spark plug black and sooty when it idles.

Unfortunately I don't know much about the air cooled bikes as far as tuning, but take a look in here to get the right jetting for your XR600.

On the XR650R, the screw controls the air flow but I'm not sure what rev range that is. I just know it is supposed to be 1.5-2 turns out.

Billa, are you sure you don't have the pilot screw term backwards? At least on the XR650R and most other 4 strokes the screw is a FUEL screw. Very few, 4 stroke bikes have air screws. 2 stroke carbs are the ones that have air screws.

An air screw and fuel screw are tuned in the opposite direction to provide the same result. Fuel screws you turn out to make it richer while an air screw makes the pilot circuit leaner by adding more air to the mixture when turned out.

I would have to believe that the XR600 has a fuel screw.

You just may be right on about the pilot jet though. That would be my guess also.


On the XR650R, the screw controls the air flow but I'm not sure what rev range that is. I just know it is supposed to be 1.5-2 turns out.

Everything I read says the pilot on the 650R controls fuel, otherwise known as a fuel screw.

A fuel screw regulates emusified fuel.

All four stroke carbs I'm aware of use fuel screws. All two stroke carbs that I'm aware of use air screws. You can tell which one a carb has by looking at it's location on the carb body. If it is up-stream from the slide it is an air screw. If it is down-stream from the slide it is a fuel screw. It would be more technically accurate to say up or down stream of the pilot jet, but using the slide location is easier and still accurate in every case I'm aware of.

Screwing a fuel screw in makes the pilot circuit leaner by cutting down the flow of emusified fuel. Screwing an air screw in makes the pilot circuit richer by cutting down the air entering the circuit.

Here's a couple visuals showing the differences in air screws and fuel screws.

Fuel screw carb:


5 is the fuel screw

6 is the pilot jet

blue area is air

orange area is fuel

green area is air/fuel mixture

Air screw carb:


1 is the air screw

5 is the pilot jet

blue area is air

orange area is fuel

green area is air/fuel mixture

Well, I appreciate everyone taking the time to respond. I got the floats worked out in my carb, but the problem still persists. I do not believe it is a carb problem. At this point, I am leaning towards burned valve, cracked rings, or some other type of mechanical issue. Bike runs just like before. Rev's up fine, but does not want to idle. When ever you rev the motor, and then let off the gas as the RPM's come down it wants to stall. I idled the motor up to a point where it will at least idle, but something is not right still as it wants to hesitate and sputter at low RPM.

One thing I did notice was that the headlight is dimming quite a bit at idle. I have read where the stators can be a problem on these bikes. Is it possible to test the stator for weak output while still on the bike? What about the coil and CDI?

Kawabuggy - Sorry about your woes. Good luck and I hope its is an easy one to fix.

Thanks all for the education on the carb.

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