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Dies letting off throttle. Driving me crazy!

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Hi guys,

Not a dirt bike, but running a VM carb on my bike, so figured I'd look here for help, seeing as you guys are the jetting experts. Anyway, my xv500 v-twin 4-stroke is running a vm36 carb. I can get the bike to idle nicely, and it accelerates great, pulling hard with no hesitation through the rpms.

My problem is that the bike will die after letting off the throttle hard. I'll be cruising at 4000rpm, and slow down to make a 90' corner (or such). I clutch, engine brake, drop down to first, maybe touch the brakes, coast through the corner in first, then touch the throttle to take off again. The bike goes "blah", stumbles, idles at about 600rpm. If you try and tap the throttle to get the revs up, the engine dies. If you just let it sit, it seems to regain its legs after about 12 seconds.

Guys, this is driving me crazy!!!! I can't seem to figure out what causes this. Am I jetted too lean on the pilot circuit? The bike will idle great (no popping or stumbles), and she starts very easily. Float level is correct. I'm stumped. Any help or suggestions would be VERY much appreciated at this point.

Jetting specs:

55 pilot

0.7 air

p2 needle jet

6f9 jet needle

250 main

3.0 slide

elevation is 3500' and temperatures have been around 50'F

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After a bit of experimenting, I may have answered my own question. I don't know if this is correct however, so take from it what you will.

I richened up the pilot jet to a 60, and advanced the "idle-to-2000rpm" timing from 13' to 18'. No knocking, so I stayed with it. Seems to work alright, with no hunting. Also bumped up the jet needle clip position from a 2 to a 3, with a washer shimmed underneath to get a "3.5" position. @ 1/4 throttle and 4000rpm in 3rd gear, plug straps are a nice mocha colour, but the porcelain is dark brown. Mind you, these aren't brand new plugs either, so the porcelain colour could be left over from when the bike was jetted too rich due to too big a carb.

Mind you, I was riding at around 4 degress Celsius, and the cylinder heads at operating temp were only around 120'C. EGT temps were 600 on the rear and 715 on the front. I wonder if the plugs are the correct heat range for hot summer weather, but I should be running one range hotter for the colder weather right now.

Anyway, just figured I'd toss out all the numbers in case somebody else has the same issue as me. Like I said, I don't know the exact reason the bike dies out, but advancing the timing and richening up the idle circuit and jet needle position seems to have done the trick.

Edited by hansol

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