Any time a change is made to the air intake/flow, fuel delivery, and or exhaust rejetting and carb tuning may be in order, usually these are changed with aftermarket or modified exhaust and intake. The exhuast may only require a simple fuel mixture screw adjustment on the carb, but an aftermarket intake or a modified stock intake requires more fuel to match the increased airflow which can only be corrected with a carb rejet.
Pulling the carbs apart for a rejet, cleaning, or rebuild is something that gets easier the more you do it, and may seem scary or difficult at first but is a necessary task for keeping your VT750 tuned properly. Below are a few pictures for reference to help you when working with the particular carburetor setup on your VT750.
All CV carbs follow the same theory to operate. There is a pilot jet, a main jet, a needle, a slide, and a mixture screw. See the picture below to understand the parts and components of all CV Carbs including how fuel is delivered at idle versus at throttle.
Based on throttle position fuel id delivered from different circuits within the carb. See the Chart Below for reference to see how each throttle position leads to fuel delivery overlap.
Also see the Jet Needle diagrram which explains the needle clip position effect on fuel delivery, as well as the position of the needle at various throttle positions:
Below are exploded diagrams of both the DC Spirit / CD Ace and the C2 Spirit / C Aero carb(s) included for reference pulled from the factory service manual. This will help you understand the parts of your carburetors as well as your intake manifold and air box.
If you have a single carburetor model VT750 pulling the carb for cleaning or a rejet is a fairly easy task requiring you to meerly disassemble and remove the intake, then disconnect the vacuum hoses and fuel line, and finally loosening the clams so you can remove the carb. From there the build will be similar to the pictures below of a VT750DC carb except there is only one of them.
Carburetor Diagram (right click images and choose to open in a new tab or window to see a larger version) for reference:
Intake Manifold and Airbox Assembly diagrams for reference:
If you have a dual carb setup model VT750 pulling the carbs is a little bit more difficult and requires a little finesse as it is a tight fit with limited space to operate in between the frame and the engine where the carbs sit.
Diagram of the dual carb setup of the DC Spirt and ACE (right click images and choose to open in a new tab or window to see a larger version) for reference:
The stock airboxes will differ between the ACE and the Spirit. Below is the DC Spirt Airbox for reference:
The theory and similarities in steps between the VT750DC and other VT750 bikes are incredibly similar. The biggest differences are accessing and removing the carbs between models. The single carb models have a more accessible carburetor while the dual carb models require a bit more effort to rejet or clean.
1. Remove the seat.
2. Remove the Gas Tank by unbolting and removing the speedometer, unbolting the underseat gas tank bolt, disconnecting the petcock fuel line (make sure petcock is set to off), disconnecting the right side rear air/overflow line, then sliding the tank back so it becomes free to be removed and placed somewhere safe.
3. Remove the stock airbox or your aftermarket air intake by unbolting it, and then unclamping any clamps. On the other side of the bike remove the carb cover.
4.Remove the upper intake manifold by unscrewing the clamp screws and disconnecting the airbox vacuum line.
Once the screws are loosened and the vacuum hose disconnected you will need to pop the intake boots off of the tops of the carbs and then slowly remove the airbox assembly turning and twisting it slightly when necessary to get it completely off of the bike.
5. To make more room remove some of the decorative engine covers. Also removing the front cylinder coil from the frame also makes more room to work with. Remove any other items you feel may be in the way.
6. Detach/unbolt the choke cable from the engine.
7. Unclamp and disconnect the two breather tubes from each carb and disconnect the two throttle cables. The pull cable should be removed first and then the 2nd push cable can be removed to make it easier.
8. Disconnect the fuel line from in between the carbs. This is tricky and may require some needle nose pliers to unclamp and disconnect the hose.
9. Loosen the clamps on the carb boots connecting the carburetors to the engine. On the rear carb just loosen the top clamp that connects the boot to the carb. On the front carb loosen both the top and bottom clamps because this boot will need to be removed in order to get the carb assembly out. This is also a good time to remove the carb fuel heater wires form the terminals on the bottom of each carb.
10. You will need to wiggle and pull the carbs to unseat them from the rubber boots. You may even need to utilize a flat head screwdriver to gently pry and separate them from the boots. Once the carbs have been unseated and separated from the front and rear cylinder boots remove the front boot from the bike completely and set it aside.
11. In order to get the carbs out you will need to lift and rotate them towards the front of the bike as you are sliding them out towards the right side of the engine. Gently maneuver the carbs around wires, hoses, etc.
12. Once removed you will need to drain the fuel from each carb float bowl using the fuel drain screws to prevent spilling gas everywhere. Also rotate the carbs and pour out any remaining fuel from where the fuel line was disconnected.
13. Find an area you can work comfortably. Outdoors, I often utilize the bottom of a 5 gallon bucket as a simple "work bench" setup.
14. To change the jets for a rejet or to access the floats to change out the float valve when performing a carb rebuild you will need to turn the carbs upside down working on one at a time to access the float bowl. Stock there are 4 phillips head screws holding each float boal on. If you buy a Factory Pro jet kit it comes with stainless allen head replacement screws that are fantastic to prevent stripping. Remove the 4 screws and then remove the bottom cover to expose the pilot and main jets as well as the floats.
This is the Main Jet:
This is the Pilot or Idle Jet:
Replace each jet with the new jet or remove the jets to clean them and then put them back in place. Note that the carb with the throttle cabe assembly is the rear cylinder carb so it will take the bigger jet due to that cylinder naturally running warmer in a dual carb setup. Reassemble when done.
15. To access the needle flip the carb over and remove 3 of the four screws from the vacuum side cover on the carb. THERE IS A SPRING INSIDE, be careful not to loose it. Hold the cover in place as you remove the last screw and then slowly lift it once removed.
Slowly lifting the cover relieving the tension of the spring:
The cover removed:
16. Pull the slide out careful not to damage the rubber diaphragm. Looking at the bottom you can see a plastic piece with a phillips screw head. Turn this to unlock it. You can then remove the plastic piece with a tiny spring attached (do not loose) and then the needle.
The factory needle is not adjustable, any jet kit will come with a replacement need that you can adjust by moving a clip. Once needles are changed out you will need to insert the plastic piece into the slide and lock it back into place so that the needle is now spring loaded. I find it is easiest to hold the needle in place with the slide upside down and then use my finger to lift the needle cover into place. Once I have made it to the top, I flip the slide over and then use a screwdriver to lock it in place. Insert the slide back into the carb and gently re-seat the diaphragm the only way it will fit. Replace the spring and then gently put the cover back in place holding it while inserting all four screws.
17. Now is a good time to adjust the mixture screws on each carb. The default setting is 2.5 turns out. You can go as high as 4 turns out to richen the mixture, or less than 2.5 to make it more lean. Once installed on the bike, you may need to pull the air box and carb side cover to adjust this further.
18. Once both carbs have been rejetted and reassembled it is time to put everything back onto the bike and then reattach all vacuum hoses, fuel line, intake manifold etc. in reverse order. You will need to insert the carbs at the same angle they were removed and then slowly rotate so they line up with the engine intake boots. Re-seat the front cylinder intake boot, then align both carbs so that are ready to be wiggled and pushed into both boots. This can be tricky to get both carbs to seat all the way ensuring the best seal.
I have found that a small piece of 2x4 to gently push each carb all the way into the boot is a great tool. Prior to being able to use the 2x4 you have to have begun the seat by standing over the bike, and holding onto both carbs and gently rocking, wiggling, and pushing them into both engine intake boots.
19. Once the carbs are mounted back to the engine, tighten all boot clamps to lock the carbs in place. Then you can begin to reassemble everything in reverse order to removing it. Be sure to get a good seal on the intake tubes to prevent air leaks which can cause problems.
Rejetting and tuning a motorcycle is a painstaking process that is often never perfect the first time, therefore, you may find yourself removing and reinstalling the carbs to change the jetting numerous times until you get it right. The more you do it the easier and quicker it becomes.