Take the air filters off and while idling and opening the throttle, spray some starting fluid or WD-40 in there. If it revs up beyond what it did before, I'm afraid your carbs aren't as clean as you think or there is otherwise some sort of fuel restriction. If that still doesn't cause it to rev, check the timing and the only way to do that is with a strobe light on electronic ignition models.
Do your carbs have plugs over the idle mixture screws? If so, time to remove them and adjust the screws to 3 turns out (from a very, very light seat). The first 15% of throttle is almost all the idle mixture circuit and it contributes up to about 35%. If that is not all clear and working correctly, you'll never get past 2000 or so rpm to get the main jet on line.
from trial and much error, I have discovered that getting this circuit completely clean is difficult. I had my carbs apart and together at least 5 times before I got it clean enough to work right. Here is what I finally discovered to clean and verify that the circuit is clean:
1. With the carb off and the idle mixture screw in 3 to 3.5 turns and the enrichener (choke) in, use aerosol carb cleaner (I like Berrymans...and it will burn on your skin and melt any plastic in the area) and with the straw on, spray cleaner through the air jet at the carb inlet. While doing this look through the engine side of the carb. You should be able to see s strong stream coming out where the idle mixture needle is. If you don't, keep spraying until you do.
2. Once that is clear, turn the screw in and seat it lightly. Now spray the carb cleaner in the air jet again. It should not come out the needle hole anymore, but there are 3 tiny holes right above the throttle butterfly. You should see 3 distinct streams coming out the holes. Not trickle, not dripping- thre strong distinct streams. You may have to hold the butterfly open a bit to get a good view. If you can't, spray until you do.
3. With that done, turn the carb over and if the float bowl is not off, remove it now. Remove the plug on the idle jet tube. It is a smaller tube built into the side of the main jet. Then, with a small flat screwdriver, remove the idle jet. Make sure it is clean. the hole through it as well as the holes on the side should be clean. Use carb cleaner if you have to, but do not stick anything harder than the brass it is made of in any of the holes.
4. With the idle jet out, repeat the same procedure that you did spraying through the air jet, but this time spraying up from the bottom where you just took the idle jet out. With the screw in at 3 to 3.5 turns, you should see the strong stream coming out from the hole at the idle screw. With the screw lightly seated, again, you should see the 3 strong streams from the holes above the butterfly.
5. Re-install the idle jet and plug. Set the idle screws to 3 to 3.5 turns- reassemble everything else.
- if you do all this and actually see the streams of carb cleaner each time you should from each location, your idle circuit is guaranteed to be clean. If you do not actually see the streams where they should be when they should be, you can not be sure.
Finally, as to the main jet, you can look through it with the slide out and see if it is clean, but unless you take the emulsion tube out, you will not know if the holes in the emulsion tube sides are clean. With a broad screwdriver, take the main jet our, the firmly, with a flat punch or something wide enough to span the center hole, push the emulsion tube up from the bottom into the carb throat- remove, inspect and clean if necessary. Heres the kicker- I boiled my carbs in vinegar water for about 2 hours- twice, but when I finally got around to taking the emulsion tubes out, the outside, the part you cannot see when they are installed, were caked with varnish.
Good luck!
