Custom computers and motorcycles

†††††††††††††††† First I have to say that if you are mechanically inclined then carbís and carb tuning is not a hard thing to accomplish with the right tools and done in the right sequence. If you are not, then it can be a daunting task to accomplish with any sense of doing a good job. I am going to try and walk you through a complete carb rebuild and tuning process from start to finish. The carbís I will be using are Hitachi's that came off of my 81-750. This bike has not been run in over 6 years do to a divorce, new wife, two kidís and life in general. They have been under my house for quite a few years so a complete rebuild is in order.
†††††††††††††††† Hitachiís are a variable ventura (VV) carburetor (for you auto mechís) or most commonly known as constant velocity (CV) carburetor's. The CV carb in my opinion is one of the easiest and most well designed carbís in the industry. It is designed to give the engine, only, the amount of air/fuel it needs and no more regardless of what RPM or throttle position you are at. The engine runs the carb instead of the other way around. In my opinion when this style carb is set up correctly it does just that and can not be surpassed except by Multi Point Fuel Injection (MPFI).
†††††††††††††††† The beauty of these carbís is that unless someone has really gimped one up, there is no need to buy any parts for a rebuild, as nothing wares out under normal riding. OOOOooooo, Iím going to burn in mechanicís hadeís for that statement. Yes, I know, that there are a lot of ďExpertsĒ that will give, in detail, why I am wrong and full of the proverbial ďPOOPĒ. I will not entertain argument for the sake of arguing. This is what I have learned over the years with hands on experience. Not just what the book sayís but what I know to be fact. The only part that will need to be replaced is the slide diaphragm. It is made of rubber and may develop pinholes or cracks which must be dealt with. This goes for virtually any motorcycle CV carburetor. All other parts under ďNORMAL USEĒ will last virtually forever. These carbís are very forgiving and can be tuned for virtually any condition. If you take into consideration that each carb has a 40mm throat, times that by two, then you have enough carb for a mild small block v-8! This carb set can be split and easily be used as a one carb design for a 750 all the way up to 1100.
†††††††††††††††† From the factory, although a little lean on the idle circuit, is right on the money for a stock unchanged setup. Letís look at parts of the standard Hitachi.

Carb Tuning

This is a document in progress please check back for updates.

†††††††††††††††† 1. Diaphragm
†††††††††††††††† †††††††††††††††† By far the most important part of this carb. The diaphragm gives a flexible air chamber to let the ventura slide go up and down. The air above the diaphragm is sealed to the main ventura and does not change by atmospheric pressure. The air pressure drops as air speed in the ventura† increases. It acts like a pump trying to suck air out of the space above the ventura through the pilot air and main air jet. Below the diaphragm is ported to atmosphere. As the air pressure drops above, atmospheric pressure pushes up and raises the slide in itís boar allowing more air and fuel into the engine. As you can see if your engine only pulls at 68% the carb will only give 68%. It is imperative that there are no holes or cracks in the diaphragm to insure a proper signal to the slide. A pin hole or crack in the diaphragm will cause a lean condition off idle.

†††††††††††††††† 2. Pilot/Main Air Jet
†††††††††††††††† †††††††††††††††† These should never be messed with. I have NEVER needed to do anything to these short of cleaning. These control the signal to the diaphragm and are very sensitive to tampering.
Clean and leave alone!!

†††††††††††††††† 3. Vacuum Piston
†††††††††††††††† †††††††††††††††† The vacuum piston (also known as the ventura slide) holds the pilot/main air jets and the diaphragm. Only thing needed is a good cleaning and insure it slides easily up and down the boar.


†††††††††††††††† 4. Jet Needle and Main Nozzle

†††††††††††††††† †††††††††††††††† These are a matched set. IF you were to need to replace these, ďby the bookĒ you should replace both at the same time. Donít modify the needle or the seat. You will create more problems than you fix. Insure they are both clean and free of obstruction.


†††††††††††††††† 5. Throttle Valve

†††††††††††††††† †††††††††††††††† This should be a no brainer. If you have to ask then have someone else do the rest.


†††††††††††††††† 6. Bypass Hole

†††††††††††††††† †††††††††††††††† This is for initial acceleration. Since there is not an accelerator pump, as the throttle valve is opened, it creates sudden rich condition to get the engine to speed up. In short it is the accelerator pump without the mechanical parts.


†††††††††††††††† 7. Pilot Screw

†††††††††††††††† †††††††††††††††† This to most would be known as the main idle screw. It is the rough idle adjustment for the carb. It will get you in the ballpark and you use the external idle adjustment on the side of the carb for the fine adjust. As the rest, clean, clear and leave alone. When adjusting always do a soft seat. That means don't torque the screw in tight, turn in till you just feel it seat then back out 1 and 1/2 turn for an initial set. If you over tighten you run the risk of damaging the needle or the seat.


†††††††††††††††† 8. Float Valve

†††††††††††††††† †††††††††††††††† This lets fuel in and stops it when the fuel gets to the proper depth. If the needle is solid brass then clean and leave alone. Some have a rubber tip, if you do and it is distorted then you will need to replace it.


†††††††††††††††† 9. Pilot Jet

†††††††††††††††† †††††††††††††††† This feeds the idle mix, the pypass hole and a small port after the main needle. Some say you need to open this up to a larger size. I have never needed to and don't recommend you to. If you open it up to much you will greatly richen the affected circuits and may end up having to replace the jet for the stock one.


†††††††††††††††† 10. Main Jet

†††††††††††††††† †††††††††††††††† This jet is what makes or breaks your bike. To small and you run lean, to big and you run rich. Unless you are changing your intake or pipes, clean, clear and leave alone.


†††††††††††††††† The easiest way to do these carbís is to remove them, take them apart (donít split them), give them a good cleaning and put them back together.† Turn in the idle jet to soft seat and out 1 and 1/2 turn, put them back on and fire it up. Adjust the rough idle using a mercury stick (it doesnít have mercury in it but they use to) to sink the carbís on idle and unless you have change the air cleaner to other than stock or you have other than stock pipes everything else should be good to go.


†††††††††††††††† There are only three things you should have to change or adjust even if you run no air cleaner and straight pipes. Here is a fairly good guide to go buy.†






















†††††††††††††††† This is what I go by as a rough in the ballpark guide. This will get you to the point to where you can adjust to the final settings and sinc.

Stock air cleaner and pipes

No change to needle or main jet

Modified air cleaner stock pipes

Open main jet with a 1.2 mm drill bit may need 1 flat washer under needle

Opened exhaust stock air cleaner

Shim needle 1 to 2 flat washers leave main jet alone

Modified air cleaner and open exhaust

Open main jet with 1.2 mm bit, add 2 to 3 washers under needle