i have a 1998 that iv'e owned since new...so far i have flushed the coolant and made sure there were no bubbles in the system, changed the oil to mobil 1 synthetic, checked the thermostat and changed the plugs....basically what happens is, the bike starts without a problem, usually no choke, it runs like a monster pulling very hard...the temp gauge rarely gets past halfway...BUT, after 20 minutes or so of riding, the idle gets real rough and wants to die out at stoplight, i have to rev it to keep it going.....a little extra info: the bike was out of commision for the past 4 years and this was never a problem before...stage 1 jetkit and cobra slashcuts with 8 holes drilled in the back of each pipe (sounds GREAT too)
Check spark plugs , wires & all connections especially to coils .
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If it is starting without choke then I would lean off the idle mixture screws - with the motor at operating temp turn each one back until the engine note drops the back out a bit.
It may also be worthwhile checking float heights.
"I finally got my head together...now my body is falling apart!"
Mr Max is:
A 1999, 1300cc with Mikuni 40mm TDMR Carbs, gassflowed head, Holeshot headers with custom silencers.
It is kept off the ground by Dymags coupled with Racetech springs and emulators at the front and White Power shocks at the back.
It is slowed down with an ISR radial master cylinder and six pot callipers grabbing onto Braking wave discs.
If the bike sat for four years, and was put back into use without having cleaned the carbs properly, then they need to be cleaned now. No way you can get away without doing it. If you can start that bike from cold without choke,it is because it is running way too rich, possibly from a dirty choke system which is not allowing it to close fully. The choke being partially opened would account for the poor idle when the bike is warm after 20 min., and it would also mask a poorly working , and dirty condition of the idle, and pilot circuits
Check voltage across battery with fan on, you should have around 13.8 volts. If not 13.8 your charging system has problems. A Vmax does not run well when the voltage is low.
My thoughts are you have low voltage to the battery, to prove or disprove my theory do this; If you have a car battery charger that puts out 4 to 6 amps charge rate, ride the bike untill it starts to act up and return home and connect the battery charger to the battery, on the 6 amp rate if available, while bike is running. You should see your bike start to run normal again within a minute or so. If it doen't respond then you can kick this theory to the curb and move on to other suggestions.......
Of course I talk to myself,sometimes I need expert advice.
My vote was for electrical maladies as well. Electrical components have a habit of working OK when cold, and malfunctioning when they get hot (i.e stator or rectifier). Low voltage makes the Vmax's ignition act up. Once it's acting up, check the voltage across the battery with the engine running. If it's less than about 13 something's wrong. The fan draws a good bit of current, so if the charging system is just marginal normally, once it kicks on it can pull the voltage down pretty quickly.
My bike never really needed much choke unless it was a chilly morning, and even then 1/2 choke or less, and it would idle no choke pretty much instantly, albeit slowly until it warmed up a bit.
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I also reckon that if it sat for four years, the carbs need going through. The Shotgun may help, but there's a good chance it'll start acting up again quickly and there's no alternative but to go through the carbs properly to clean up the idle circuits..