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MikeZ

XR650R - low rpm surging - help?

6 posts in this topic

OK, so my street legal R is not as smooth as it should be.

BTW, we are just getting into riding season here in Oregon. Wet, but warming. So I am just starting to commute on the pig again.

Anyway, the bike is uncorked and jetted properly. 68s, 175. Run great through the top and mid and while giving it any gas.

But while putting along at low rpm at pretty much any gear I get what feels like a lean surge. Meaning it is hunting for rpm and just generally not happy.

Does that make sense to anyone? Any great ideas from the world?

Thanks in advance.

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Air screw. You can start at 2 1/4 turns out then fine tune from there. Idle probably is a little hit and miss as well I'd bet. Also keep in mine, the air screw can move after riding a while so it's a good idea to put a drop of RTV on it once you get it right and remember how to readjust in case it moves anyway. I know on the L model they have a little tab so that you can only move it 180 degrees in stock form. Most guys that do any mods grind off the tab so it can be properly adjusted, then it can and does move out of range. Your R model may not have the tab.

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I had a similar deal where I would ride and then stop and the bike's idle would take quite some time to settle down. It took a little bit of lube on the throttle cable and double checking the routing to make it go away. I have the identical jetting as you and ride mostly at sea level to 2000.

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Air screw. You can start at 2 1/4 turns out then fine tune from there. Idle probably is a little hit and miss as well I'd bet. Also keep in mine, the air screw can move after riding a while so it's a good idea to put a drop of RTV on it once you get it right and remember how to readjust in case it moves anyway. I know on the L model they have a little tab so that you can only move it 180 degrees in stock form. Most guys that do any mods grind off the tab so it can be properly adjusted, then it can and does move out of range. Your R model may not have the tab.

While I can agree with the above in a general way, let me make note of a couple things. The screw is a [fuel] screw. 2stroke carbs have air screws and a rare few small 4 stroke bike carbs. For all intensive purposes all 4 strokes have fuel screws. Also, 2 1/4 turns out as a starting point is too far out. With the proper pilot jet size, the fuel screw setting should be from 1.5 - 2 turns out. Only if your pilot jet was too small would the fuel screw setting need to be past 2 turns out. If you go through the proper fuel screw tuning procedure, you can bet that it will most likely fall somewhere around 1 5/8 or 1 3/4 turns out. Bikes will differ slightly depending on a few things. The 'R's pilot screw has a good amount of tension. It was designed with such, without any need for external tab or holder to keep it in place. It most likely will not ever move. I would leave it alone as far as trying to add RTV or anything to the screw.

Another factor might be your needle and it's position. What needle are you using? What position is it in?

Also, possibly a small air leak at the carb isolator boot. Check the clamps. If there is an air leak, it can cause an irratic idle.

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My R is doing this same thing(I have the same mods and comp needle in middle position) except mostly going down hills, it wants to surge a bit, maybe a little on level too, but less and only if I keep it at a steady low RPM. I played with the fuel screw some, and also no leaks that I can see, so could it be the needle? It's not really bad enough for me to tear into it yet, because it is running good otherwise but wanted some thoughts?

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