Carb problems for xr650r

My brother just got a 2000 xr650r. The bike is in really good shape and has the baffle and S/A out. Whenever you pull the throttle wide open the bike lugs then revs right up.Also when you are slowing down the bike backfires and pops a lot. Thanks for any help in advance.

The air/fuel mixture in his carb is probably not setup correctly for his riding environment. What temperature & altitude does he primarily ride at?

sounds like you may be way lean, try a bigger main jet and move the fuel needle clip up. Does the bike run hot? the popping really sounds like lean to me. i would pull out the jet and see what it is, probably the stock 125. depending on elevation go to a 155 or 160ish main, maybe smaller if really high. or if you have the hrc manifold boot go to a 170 or 175. Maybe start by just blowing a couple cans of ether through the carb (great carb cleaner), see what happens, might as well check the main size while your at it just so you have somewhere to start from. let us know how it works out.

The altitude we ride at is about 450ft above sea level. The climate is hot and humid. I don't think it is running hot. I'll look in the carb and check the jets and etc. and see if that helps. The bike came from Minnesota with a high elevation. So should we change the jets?

I would recommend the following:

Make sure the airbox restrictors are removed as pictured in step 1 at the following link.

Make sure your intake manifold is not the resctrive one pictured in step 2 of the link above. It should be completely round and wide open without the rectangular slot in it.

Make sure your exhaust is either an afermarket silencer, the Honda competition tip or at least a drilled out stock tip as mentioned in the above link.

As far as jetting goes for 450ft, I'd suggest starting with a 172 or 175 main jet, a 68s pilot and either the B53E needle or the stock needle in either the 3rd or 4th clip position. There's not a one size fits all 100% perfect setup for all bikes, so you'll have to try out a few combo's to decide what you think is best, but these settings will get you pretty close to where you want to be and you should notice a very positive improvement in performance.

In terms of adjusting your pilot/fuel screw, make sure your engine is fully warmed up and set the pilot screw to

1.5 turns out. Then with the 68s installed, turn the pilot screw clockwise until the idle slows. Then turn the pilot screw counterclockwise until the idle slows again. Make a mental note of how many turns you made in between the low idle points. Then turn the pilot screw half way between the low idle points to finish the adjustment. If you turn the adjuster counterclockwise and the idle doesn't drop down, then you need a larger pilot jet. If you turn the pilot screw clockwise and the idle doesn't drop down, then you need a smaller pilot jet. After adjusting your pilot circuit, re-adjust your idle speed if necessary. Don't crank down on the pilot screw when it closes or you'll damage the seat, so be gentle when it stops turning.

Another couple things you may want to consider doing if this bike is new to you would be to adjust the valves (.006" intake / .008" exhaust), then make sure the decomp is within spec. Make sure your engine is stone cold when adjusting your valves. There's also a two oil strainer screens in the engine. The one inside the clutch housing never seems to get dirty, but the thimble like strainer screen inside the frame downtube can sometimes be contaminated with debris and should be checked every couple oil changes.

The jets on my brothers bike are 160 main jet and a 65s pilot jet. The spark plug is a tan color and the bike really doesn't act like it is running hot(judging from spark plug color). We have ordered a 172 main jet and a 68s pilot jet to see if we can stop the popping when you let of the throttle and the lug when you open the throttle quickly. If anyone has any advice, it would be appreciated!

The 160 main is too lean for good performance at 450ft. You’ll be pleasantly surprised when you re-test with the 172 main and 68’s pilot.

Also, spark plug color doesn't tell enough about the mixture to know if you're running too rich or too lean unless you're way too rich or way too lean. The spark plug will only accurately reflect your bikes air/fuel mixture immediately before the engine is shut off. For instance, to accurately read your spark plug for too lean or too rich of mixture at WOT, you must run your bike WOT for a good 8 seconds or so under a load and then immediately shut it off by pulling in the clutch and pressing the kill switch. You also want to make sure you’re not trying to read a fouled spark plug or a brand new spark plug. If you install a brand new spark plug, make sure and get at least 20 – 30 minutes of good hard riding on it before trying to read it. At this point, you can pull the spark plug to determine if your mixture is too rich, too lean or about right at WOT. If you don't immediately shut your bike off from WOT and ride back to your pit and then shut off your bike, then the spark plug will only reflect how the bike was running immediately before you shut it off and won't reflect what's going on at WOT. If you want to see what's going on at half or quarter throttle, then you must immediately shut your bike off at the desired throttle position to properly read the plug, but it gets tricky trying to figure out which of the overlapping circuits are contributing too much or too little to the mixture. There’s a good deal of trial and error if you really want to get the most from your jetting or you can pay an expert to do it for you, but most people go with known good setups that are generally ‘good enough’ as opposed to optimizing their jetting.

There are many factors to inspect when reading a spark plug, but the mixture ring (also known as the fuel ring) is one of the most critical for jetting and not insulator color. If you think about it, how can the insulator color be an accurate guide for the air/fuel mixture? What is the color of unburned fuel? The byproduct from the combustion process from too lean or too rich of jetting is the abundance or absence of carbon (more fuel burned = less carbon // less fuel burned = more carbon). If there’s too little fuel in the mixture, you’ll be running lean and there won’t be much of a presence of carbon on the insulator. Your engine will also be running hotter than what it should be and you may experience pre-ignition/detonation depending on how lean the mixture is. If the mixture is too lean, then you’ll see tiny black or silver aluminum flakes (like pepper) on the insulator, which is aluminum being blasted from your piston’s crown. If you’re running too rich, then you’ll see varying amounts of carbon on the insulator depending on how rich you are.

The mixture ring is simply a carbon ring that’s formed on the insulator during the combustion process which indicates the presence of unburned fuel. The closer this ring is towards the electrode, the richer the mixture (the less fuel that’s combusted). The deeper this ring appears into the shell on the insulator, the leaner you’re mixture (more fuel is being combusted). You want the mixture ring near the junction where the insulator joins the shell, which means most of the air/fuel mixture is being combusted. You don’t want this ring to disappear; otherwise you’ll be running too lean.

The other problem is viewing the mixture ring. You can’t always see it just by looking down the spark plugs shell with your eyes, so you’ve either got to cut off the spark plug shell or use some type of illuminated magnifier at about 10X to 12X to see what’s going on. You can find spark plug viewing tools at certain performance & specialty shops if you really want to get into it, but most people never take it that far and are happy enough with a general setup that’s good enough as opposed to being optimized. There’s plenty more to reading spark plugs, but that covers some of the basics.

just thinkin, you may be too rich, that explains the hesitation. If you have the stock manifold boot and that big of a jet you may be too fat. Check which manifold boot you have before you put that 172 in, if its the stocker your bike will run like crap and probably almost blow black. when i first rejetted mine i had the exact same problem,i was told i had the HRC boot and put in the jet, didnt check it, it ran sorta like the choke was stuck on, i was right pissed off. checked and changed manifold, instantly all problems solved. I am running the standard uncorked 650R with no tip and a 175 main right now at 1600 ft and it runs great. still dont know about the popping though. Good luck, JR

I've got an uncorked '00 650R with a 172/68s set up. I ride mainly at about 1000'. I've been plagued with a nasty lull at about 1/8-1/4 throttle it bogs for a mili-second then revs right up but it kills me when I need to get the front tire up.

I've been through the whole process. I installed the HRC needle and messed around with the clip positions (between 3rd & 4th) but ended up putting in the stocker because it seemed to work better for me. I also did the fuel/air screw adjustment and I still can't get rid of this hesitation.

I pretty much have come to the resolve that the XR's have a natural hesitation like this when the throttle is whacked.

Can anybody tell me different.

Further more my bike sometimes, when I've been trail riding and I go to turn a corner or pull the clutch will die and it will be a bear to get going again. I'm not falling or dumping just dies...any info?

I've been plagued with a nasty lull at about 1/8-1/4 throttle it bogs for a mili-second then revs right up but it kills me when I need to get the front tire up

Make sure your idle is turned up enough and possibly turn it up a bit more. A low idle will contribute to more noticable off idle hesitation.

Your jetting looks pretty good, but you may want to try the stock 65s pilot and re-adjust your fuel screw to see if that helps.

If what you're feeling is a very small hesitation from idle when you whack open the throttle, then you may be feeling vacuum lag and there's not much you can do other than fine tune the heck out of the carb to minimize it, live with it as most people do or buy a pumper carb that doesn't depend on a lagging vaccum signal to supply enough fuel during at low engine RPM. All the newer MX bikes come with pumper carbs for improved low end throttle reponse, but unfortunately Honda didn't give our XR's that courtesy.

In regards to your bike dying in corners, verify that your float height is within specification. Also make sure you have the white plastic shround installed around your main jet.

Every BRP with the stock carb seems to have a wee bit of the lag. It drove me nuts.

I got the Edelbrock pumper carb, and I cant say enough about what a difference it makes. WAY better fuel economy, starts with 1-2 kicks, idles on its side, etc. etc.

But best of all- absolutely no lag-just a rim saving 4th gear 6" wheelie right before i'm gonna hit that un-expected rock....

Thanks for the info qadsan. I will mess around with it this weekend and see if I can't get a little better performance out of her.

You're right Irondude a pumper carb would solve all my problems I'm sure. But it's either a pumper or suspension work and I think I'm going with the latter right now so I will have to figure out what I can do with the stocker.

Thanks again.

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