Hesitation / Surging on Throttle

I wanted to ride my new XR650r this weekend, but it seems to be slipping away as a possibility.

I just bought a used 2000 xr650r up around 8000ft and it was running great. Seller told me it was jetted for the desert and when he took it to the mountains it ran fine so he did not rejet. Bought it, put it on trailer and brought it to Tucson at 2500 ft.

A few weeks later now and I change the coolant and oil and want to run it for the first time in Tucson. I struggle to start it (had the process wrong). Get it running fine at idle, but when I start to give it a little throttle it starts to hesitate. If I really open it up (to clear it out?) it starts surging (not fast like the rev limiter I have a trx450r also) but about 2 Hz at high RPMs. I tried to ride it thinking it needed some load, but it just seemed to want to rip my arms off and buck me off.

The gas was questionable so I dumped it a saw some crap in the tank. Cleaned the tank, dried over night, rinsed with clean gas. Put new 91 gas in it that I had for the quad. Check the main jet and it was 175 which seemed reasonable (has an Uni filter and a white Bros slip-on) and with all my quads I have never seen jetting do this. I tried to remove the carb, but could not get the screw holding the throttle wire mount out, so I just rotated it to look in the bowl, and everything looked ok {I intended to pull it apart after this weekend} ... I did not open up the top side. Decided to check the plug and it was BLACK (probably too rich up in the mountains). Went and bought a new plug. Put everything back together this morning and it seemed to start much easier. Throttle response seemed better up to about half throttle after which the surging was still there ... GGRRRR!!! I want to ride this thing.

It has a Baja Designs dusl sport kit on it (but the turn signals and brake light switch appear not to work).

So, any thoughts on what I am dealing with? Main jet seems ok, air filter a little dirty but ok, gas is new, plug is new. I can not figure out if just the altitude change is causing this or if something happened in transit. Does it sound like a carb issue? Could it be electrical?

Any help is greatly appreciated.

Not enough info to trouble shoot. We need more info to digest the problem. Describe engine starting technique. Does the carb piss fuel after shutting the bike off or stalling the bike? Any changes warn the bike is warm or cold?

What is 2 HZ? (but about 2 Hz at high RPMs. I tried to ride it thinking it needed some load, but it just seemed to want to rip my arms off and buck me off.)
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WHAT DO YOU HAVE FOR THE PILOT JET AND NEEDLE?
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air filter? CLEAN OR DIRTY?

What does the petcock fuel filter look like? Clean? Dirt? Is it collapsed?

What are you doing with the choke? Is th choke off half or full position (note: there is a XRR choke issue where the choke becomes loose and falls off)?

float position?

Thermostat stuck closed?

Check the valve lash. (probably not out of adjustment but goo to check on a newly purchased bike)

Engine is now starting just fine. I am having a bit of a leak of fuel out of the float bolw drain. I have to play with the screw to get it to stop ... but it is not leaking when I am running it or stopping the engine.

2Hz (hertz) = 2 pulses per second

I just disassembled the carb, cleaned, and put back on the bike last night. This morning same issue (although seems more of a popping)

Pilot = not sure, could not remove ... but idle and low throttle seem fine
Needle = B53E ? (forgot the paper I wrote it on) clip in the middle
Main = 175

Airfilter = slightly dirty ... this morning I took it completly off briefly while running and while the low in response seemed a little better. Mid-throttle hesitation / pop was still there. It also seemed to hole high RPMs a little better, but not right

Just cleaned the tank and fuel filter looked fine (although I did not remove it)

Choke is off and intact.

Not 100% sure if the float is set correctly. I did not change it from when it ran fine.

Thermostat was found to be failed open. Now replaced with Suzuki Swift with hole drilled.

I have not checked the valves, but was thinking of doing that tonight.

I made a video this morning, but I do not have the software with me to convert so I can post it.

A couple friends of mine think it sounds like a rich popping in the exhaust, but we are not sure how that is possible given that it used to work fine at a much higher elevation.

Current thought (since the carb seemed fine) is that maybe the electronics (CDI?) are messed up and not initiating spark. Which would explain how the problem seemed to be sudden and not a gradual appearance.

I forgot to mention that there looked like puffs of smoke with the popping when the throttle is held at mid-throttle. This could also indicate a rich condition right? But I am just not sure how that is possible.

I would look at the float adjustment...should be 16 mm. Bend the tab to adjust the height.

Is the float good?

I assume all the jets and the needle valve look good.

Clip position counted down from top of needle. As a guide your should be at the second position. Though all bikes are different.

Is the AJCV intact? Leaking air? Blocked off?

Knowing what the AJCV (Air Jet Cut Off Valve) does might help in knowing what will happen when you disable it. The pilot and needle jets have small holes (emulsion holes) in them that adds air to the fuel that is drawn through them. This atomizes the fuel. Makes the mixer right is a nice way of saying it. This air comes through the air jet....at about 19~20psi of vacuum the diaphragm is pulled back (vacuum comes through that small hole that we are plugging) against the 18+pound spring, in the AJCV and the little plastic door that the diaphragm held open; closes cutting off the needed air. This is designed to soften deceleration...The AJCV makes the idle on deceleration richer, except that there is no extra fuel added, the AJCV does nothing more then turn off the air NEEDED, to make the pilot jet mixture atomized right. There might be more fuel pulled through the pilot, do to the loss of air but, I don't know how much that would be. The un-burnt fuel will pop as it hits hot fuel still burning in the exhaust. When this happens, the rich mixture makes a dead cylinder, less vacuum on the intake stroke and still burning on the compression stroke. Somehow this makes less engine braking compression. . Why you would want to soften the engine braking on a race bike is beyond me. Mikuni, Edelbrock, and the FCR carburetors do not have this...maybe because they were made for race bikes. If you are going to ride the street then this would be good but, you might have gotten a XR650L. As in what you use to plug the hole is up to you RTV works ....I got lucky and found these real small rubber plugs that were to be used for one of my kids toys.....he lost them...they look like little top hats and fit perfect in the hole.......hole plugged no vacuum diaphragm doesn't move bam disabled. Leave the spring in or this won't work. There is fuel that leaks into this small vacuum hole when the bike is laid over (crash/bike falls over) so, whatever you use has to be fuel proof. You should readjust the idle mixture screw once you do this modification.

There are a lot of carb's that have the mixture screw being an air screw but, this is mostly on the small 4 strokes and two strokes. If the idle screw is in the front it's a fuel mixture screw like the XR650R is as you know, an Idle fuel mixture screw. If it is in the back it's an air mixture screw. The big thing that is needed to make the AJCV disable trick work is to jet richer pilot (because you can) and adjust the idle mixture screw to work right. Turns out from full in....

2~2.5 for the 68s, 1~1.5 for the 70 and .5 to almost 1 turn for the 70s. depending on the bike.

The other reason is to load the carburetor up for smoother idle while coming to a sudden stop and still being able to run the bike lean to pass AQME standards (smog). The bike does not pass smog if you run it rich enough to run right. Having this AJCV lets Honda run the pilot lean but, not have the bike stall coming to a stop as often.
The big confusion comes from some of the first guys on some groups, getting my modification backwards and cutting off the tit to the diaphragm, cutting the air from the air jet off all the time making the bike run bad. There would be no air going to the pilot emulsion holes at anytime. Some did this and had to buy new diaphragms to fix it and are sour on any talk about the AJCV. Make sure it will resist fuel, fuel will get in that hole if the bike is stalled and on it's side. The diaphragm when pulled out by the vacuum, closes a plastic block cutting off the air from the air jet. AJCV is not needed or wanted for most racing. Having the air to fuel mixture right at all times is the goal for most.

Air leak? Not as pronounced at 8000 feet but pronounced at 2000 feet. Did you start the bike. Then spray starting fluid or carb cleaner around the carb and or the carb/intake/cylinder head? When you do this does the engine increase in RPM?



Any leaks around the exhaust manifold (exhaust manifold gasket) and cylinder head?

I still would like to know what you are running for your pilot jet. 65? 68? 68s?