Just alittle background about this modification and the reason why I did this. The Hitachi carburator has 2 barrels, but only the primary barrel will open under acceleration. It is not until the gas peddle is fully depressed and a certain amount of vacuum is created in the intake manifold that the 2nd barrel will open up. This modification will bypass the vacuum actuator on the hitachi and force the 2nd barrel to open when the 1st barrel reaches 1/3 throttle.
Lets Get started shall we? Here are some diagrams of the part of the carburator we will need to play with. You will not have to dissasemble the carburator at all, so almost anyone should be able to do this mod.
Here in this figure we see the diaphragm that will need to be removed. It is held on by 3 screws (#2 in the diagram) and is also held on by the snap ring at the end of the diaphragm arm. You will want to plug the hole in the carburator that the diaphragm connects to.
After you have the diaphragm removed, play around with the carburator alittle and notice how there is a lever for the primary and the secondary barrel.
Here is a closeup of the moving parts that we will play with. #3 in the diagram is the actual throttle lever that connects to the throttle cable. #4 is the secondary lever that we need to make move. I've put red arrow's on the contact point where we need to atatch the primary and secondary plates. I have heard of people welding it together right there, but I dont have a welder, nore do I like that idea all too much.
I took a piece of coat hanger and cut off a piece about 4" long and bent it to the desired shape.
Here you can see how the wire should go. It might take alittle time in order to get it just right. Here are some pictures of the actual modification on my carburator:
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Throttle responce is iffy. It actually creates a huge dead spot when you step on it. Because you are forcing the secondary open, the jets arent primed and you end up sucking in alot of air, with no fuel. Only until the jet for the secondary gets fuel, then everything's a go. Power is felt past 1/3 throttle when you step on that. If done right, you can rev the engine alittle and get the secondary to open up, force the jet to prime. Then when the light turns green dump the clutch and romp on it... seems to accelerate alot better than stock. But if your just cruisin everyday driving, the dead spot can suck if your not expecting it.
I suggest doing this to a spare carburator. I did it to a junkyard carb and swapped back and forth between the 2.