XR600 big bore jetting -- silly rich?

I just installed a 102.4mm piston (and bored XR650L cylinder) on an otherwise stock XR600. It makes for about 660cc, so about a 10% big bore which is not outrageously big. I'm running a stock muffler, stock cam, stock air box (with foam filter), etc. I made no other changes than the piston/cylinder and redoing the valves.

I was running a #65 pilot with the standard bore, which was slightly rich but ran well. With the new big bore it would barely idle and I could bottom the idle mixture screw with the engine still running. I tried a #62, a #60, and a #58, all of which would still idle with the mixture screw bottomed. I verified that the choke is off.

Big bores generally require leaner jetting, but 4 steps leaner on the pilot jet seems a lot. Does anyone have a suggestion?

leaks? make sure everythings is sealed man...

try the classic carb fluid, or use a lighter around the boots etc...

Wouldn't air leaks always make the engine run lean? It seems to be way rich. But I did go back and smear RTV at every joint in the intake system.

What problems would make it run rich, other than the choke?

the reason I say that is it will run if air is entering...lean yes

I had a leak and fuel screw adjustments were nill

thats why I was saying that...


bad comp, leaking valves...plugged up filter or intake, plugged up exhaust

did you clean the headers internally by chance when reinstalling?

my old headers had a few OUNCES of carbon simply fall out of them



remove your sparkplug, clean it/ or install new one[preferable] reinstall, run bike abit only using small throttle openings, check plug to see if you are getting a colour to read on insulator, thats presuming you have already cleaned it thoroughly[ the carb], see how that goes,[ ive seen a few of these carbs still run with the idle screw fully shut, ]

Edited by temporarykiwi

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