Oh boy, not fun because you're thinking, "I've bought a pig in a poke, and I'm holding the bag." Sounds like you need to begin with the basics.
A fully-charged battery.
Fresh fuel, that you know the carburetor bowls are filled, with fresh gas, without crap from a rusty tank.
A good, powerful spark, at the right time.
The battery is easy, charge it. Try load-testing it, to ensure it's not nearly-dead, a low battery can boggle your starter and its drive mechanism! Ensure that the battery contacts are shiny-bright clean metal, and that the thick ground wire by the oil filler, where the wire connects to an engine case cover screw, is also clean and tight.
Under the seat, (remove it) you will find a tube from the pipe off the top of the gas tank, going to the gas filter. Remove the tube where it connects to the gas filter. Place it into a clear receptacle, and turn on the key. The fuel pump should immediately tick-over, and fuel should flow into the receptacle as-long as the key is 'on.' Be sure to do this in the open, and preferably not in your garage. You should have a hose and a fire extinguisher easily-retrieved for use, "just in-case." Fumes from spilled gasoline can travel to an ignition source, like a gas-powered hot water heater, a home heating unit, etc. and can ignite, even if it's 20+ feet-away! Safety.
Let's assume that you got plenty of gas, and that it's clean, with no debris, nor any water. Water in gasoline makes a 'lens' of different viscosity, very easy to see. Water contaminated gas needs replacement.
The next in-line component is the fuel filter. Remove it, and try to blow through the side closest-to the gas tank. You should easily have no-problem doing this. Any impediment, or obstruction to blowing a good volume of air through it, and you may have one culprit to the bike not-running. You can replace it with a generic filter sized similarly, take it to your local automotive parts house for match-up.
If I have a stopped-up or poorly-flowing gas (petrol) filter, I slice it open, to see what crud is causing the obstruction. At this point, I would also be shining a flashlight into the fuel filler neck, to check on the condition of the gas tank interior. If you do-
not see shiny metal on the bottom of the gas tank, either you have rust or gummed-up crappy gas contaminant there, or you may have a coated gas tank, they are not coated from the factory.
A rusty gas tank needs to be removed from the bike to restore it. Cleaning the tank can be done a number of ways. Electrolysis, pressure-washing, throwing a bunch of small nuts/bolts into it, and shaking it for enough time to break-loose the rust (some guys wrap it in carpet and throw it into a clothes dryer when the missus isn't looking), 6% or stronger vinegar (remove the fuel low-level sending unit and block-off the holes for mounting it, or your liquid may eat it alive), Evaporust liquid, are some methods for doing this. You can use the forum 'advanced search' function to read-up on all these.
Say you got lucky, the tank is shiny inside. You got petrol to the gas filter, found a dirty filter, replaced it, now see what comes out of the carburetor overflow hoses, those small black rubber hoses sticking out the sides of the bike's carburetor brackets (the attachment shows how a member used pop rivets to attach his black rubber carb drain hoses on the inside of the carb mount brackets, normally the hoses stick-through the brackets-no pop-rivets. He also removed the individual caps from the carburetor nipples used for carburetor synchronization, and ran a short length of hose from one carb to the other, this is not how the factory did it), below the constant-velocity carburetor round caps. You'll see a small phillips screw to one side of the base of each carb, loosen that a few turns, and gasoline should flow-out, capture the gasoline to see what crap it may have in it. Do 1 carb drain at a time, and empty the gas from your receptacle in-between. You want to inspect what comes-out from each.
As to spark, the bike already ran, yes? You can remove one spark plug boot, install a spare spark plug to it, and ground it out to bare metal, with insulated pliers as the starter button is engaged. You should bet a fat blue spark, a weak yellow spark, and that indicates an issue, likely a poorly-charged battery.
There is a brass crimp inside the wire harness, you can solder that to get better conductivity of the charging system, it costs literally-nothing, and you can often gain more-than 1+ volt in the system when you do this. You remove the insulation (see attachment) do the soldering (a 25 watt iron may not be sufficient) and re-insulate things
well.
Well, that's a start.
Cleaning the carburetors requires removal from the bike, partial disassembly, at least into two pairs of carbs, and float bowl removal and then jet block removal, the removing the two small brass jets in each jet block, ensuring they are
not plugged-up, carb cleaner into passages, and compressed air, at a minimum. There are on-the-bike attempts you can try (search '
peashooter,' search '
shotgun') but I don't bother with them if I suspect an obstructed carburetor circuit, off they come and apart, and into an ultrasonic bath. The pilot jet seems to be the bane of VMaxes left-sitting with gas in the float bowls, it's a terribly-small passage, any crap and it can plug-solid, resulting in a poor idle. The only effective way to solve an obstructed pilot jet is to remove the carbs, split them into two-pairs of carbs, and then open the float bowl, and remove the jet block, to remove the tiny brass jets inside (carburetor diagram: #15 pilot jet; #17 main bleed pipe), hiding behind those black rubber plugs, and
yes, you need those plugs in-place and intact. I've bought used bikes where some well-intentioned idiot neglected to tighten the jets, they screw-in/out, it doesn't require much torque. Then the rubber plugs need to be replaced upon assembly, you cannot neglect to re-install them! On one used bike, besides missing carburetor jet block rubber plugs/caps, on one of the jet block rubber plugs, when I removed it, the jet fell-out, it wasn't even screwed-in! No-wonder it 'ran like crap!' That lack of mechanical aptitude just drove-down the price. Gee, thanks, fellah! Fortunately he left hands-off the heavy reciprocating parts of the engine.
A word about the pilot jet, that's the one with a single strand of stainless steel wire through the hole (attachment). If you don't have the
exact size of slotted screwdriver to fit the jet block hole, and the slot in the pilot jet or the other carburetor jets, buy ones which do fit, well. You do-not want to bugger-up the slot to remove the jets each has for installation/removal. In the event that you do break-off one side of the brass slot, you can probably use a reverse-thread drill bit (left-hand twist) to remove the jet safely. However like COVID, it's better to take precautionary steps to avoid it in the first place.
Service manual:
vmoa.net/VMX12-Service-Manual01.pdf
Parts:
PartNumbers.pdf (vmoa.net)View attachment 75697View attachment 75698View attachment 75699View attachment 75700View attachment 75701View attachment 75702View attachment 75703View attachment 75704
