2000 XR650R carb question

i'm trying to help my buddy out and finish putting his XR650R in supermoto trim.

Here's the carb question

Is it a fuel screw or an air screw on the Keihin PE carb? It's got pop on decel suggesting lean but it'll kick over and start with no choke in 40 degree weather. It could easily be lean....I haven't pulled the pilot jet out to look and see what's in it or pulled th eplug yet.

but I wanted to know if I am adding air or fuel when I back out the screw.

Thumpers have a fuel screw. Unscrewing, lefty loosy, = more fuel. If the bike will start with no choke in 40 degree weather then the pilot jet is RICH. Cold air has ALOT of oxygen, and his pilot jet has PLENTY of fuel flowing to match the extra O2 in the cold air.

Here is a test, close the fuel screw slowly while the bike is at temp and idling. The bike should start to idle bad at around 1.5 turns out. If the bike is still running at .5 turns out, or when the fuel screw is closed, that means the pilot jet is to big (rich). With the usual airbox/exhaust mods and full uncorking of the intake boot, the pilot should be ~65s.

One of the reasons for the pop on decel is can be due to the air cutoff circuit on the stock PE carb. The mechanism behind this un-marvel of technology , and how to correct it is best described here:


I did this mod when I still had the stock PE carb on my 650 and it works very well. :mad:

Or you can just drop kick you carb to the curb and upgrade to an accelerator pump carb such as an Edelbrock. I have a Keihin FCR41. It was a pain to get fitted correctly, but I absolutely love it.

snaggle, the pilot was a 65S. could not get over the pop on decel with any setting. The fuel screw setting was about 2 turns out for smoothe idle. So I picked up a 65 (nonS), 68 and 70 to test out and no getting rid of the pop. at any setting. tried the air hole blockoff as sugested no change. There is no smog qweep on the bike and I'm getting frustrated.

I can't believe I need to go leaner than stock. but that is next I guess.

You might have an air leak somewhere between the carb and the engine or an exhaust leak. Either could cause popping on decel regardless of pilot circuit settings.

BTW, a quick way to distinguish an air screw from a fuel screw is to note its location on the carb. Fuel screws are on the engine side of the carb while air screws are on the air filter side (in other words--in front of or behind the slide). And like already mentioned, nearly all four stroke carbs have fuel screws while two strokes generally have air screws.

could not get over the pop on decel with any setting. I can't believe I need to go leaner than stock. but that is next I guess.

I have a Edelbrock pumper and have dialed it very close to optimum. I still get a mild popping on decel. My understanding is some mild popping is normal in an R that's dialed in. (Somebody correct me if I'm wrong here).

To check whether or not you've got your mixture dialed in correctly, I'd put more emphasis on having proper plug color than I would on anything else. Check for a good ash-gray color after at least a 30-min ride.

Take heart, friend. I know you're frustrsted, the good news here is the R's Kehin is not a bad carb. Once you get that baby dialed in its done and you will start grinning again

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