JAW's 150Y: Twin Hitachi Constant Velocity Carbs

These pretty looking things are 38mm (1 3/4") Hitachi carbs, came out on some A15 engines. They are a constant velocity carby, that shiny 'pot' looking thing houses the piston that varies the venturi size... very similar operation to stromberg, SU and zenith carbies.

Hitachi Constant Velocity Side Draft Carburettor


I've spent much time getting to understand those little bastards, and I am now convinced that they are *excellent*.

Okay, I got these off a mate who had them 'lying around'. I completely dismantled them - the old petrol had completely blocked up the fuel rail, and unfortunately the runners into the fuel bowl too. Cleaned up okay with carby clean and the right size drill bit. It was pretty stuck.

The flexible pipes from the fuel bowl to the jet were perished. I replaced with normal fuel hose. Let me tell you right now if you intent to use the choke, normal fuel hose is no where near flexible enough. You can buy special silicone pipe for the job, I didn't bother.

What I should have bothered with is the fuel bowl inlet valves. I cleaned them up, and even carefully reseated the brass plungers with valve grinding paste. After the 10th time they got stuck open while driving and pissed fuel everywhere I did 2 things: bought a fire extingusher and 2 new valves ;) Fortunately the valves have not got stuck since.


I got my mate Tom to make me up some spacers to go between the cars and the manifold, basically because I didn't like the heatshield. I used 12mm polycarbonate. It last 3 months before i refabbed out of some 15mm fibreglass resin insulating material used in transformers etc. Polycarb might withstand 170 degreesC, but it is not suitable.

Some dodgy fuel hose setup and a just as dodgy throttle cable got the whole show running. I made those airclea

The first month was full of annoyances. I was ready to go back. It coughed and spluttered upder load, the throttle cable was incredibly sensitive, there were vacuum leaks, bad adjustments and the occasional fuel leak. Here's some good pointers that I have learned the hard way about these little beauties.

  1. Okay yep you need to balance them. And you can do it by ear. Wanna know why I say that? Cos I balanced them by ear but they broke down under load. I bought the vacuum measurer because I thought I had it wrong, but I was dead right - the problem was something else. So to balance them do this:
    • grab a bit of hose, 'bout a meter long.
    • rip off the aircleaners, and stick one end of the hose in your ear.
    • put the other end near the piston in the carb. Listen to the shshshsh
    • whip it out of carb one and onto carb 2. Listen to the shshshsh on that one
    • the one that is making the louder shshshsh is the one that you need to back off.
    • the throttle mechanism should work like this:
      • 1 and 2 are the carbs, with the throttle shafts coming out.
      • 3 is the balance screw. you only need to adjust one side, the other side just has a tab.
      • 4 is the idle screw. lets both carbs rest against a fixed point on the manifold.
      • 5 is where the throttle pulls from.
  2. Ya with me? The carb linkages are actually little ball joints, but I think you get the jist of it - the throttle pulls both carbs but is balanced by the one screw, and the ilde just sets the low point of the throttle action. That's all you need to do to ensure the airfolw through both carbs is correct.
  3. Mixture - not too hard either. There are little lifting pins under the pistons. Push one up. Watch what the carbs do. Give it a few revs.

    Push the other one up. again watch what they do. What you should see is when you push the piston up (quickly to the full extent of the lifting pin) the revs rise a little then drop.

    Both carbs should do the same. If the revs drop and the engine is dying, wind the bottom screw down a bit. That will richen it. Keep lifting the pin. The revs won't drop as much, then eventually it will get rich again. You've gone from too lean to too rich. Of couse, this assumes you started too lean - ie the bottom screw was wound up high.

    You think it is right? Go for a blatt. So long as it runs and lifting both the pins seems to have the same affect you'll be okay, well, kinda. Feel the response. sniff the exhaust. Pull out the spark plugs. If you are putting twin carbs on then you know what a rich carb and a lean carb is like.

    Okay, I have assumed that there are no vacuum leaks and that the needle is the right size... maybe some time I'll do a "JAWs Simple Engineering Guide to CV Carbies". Later.
  4. Emission control. Hey man, it's a 1974, emission control wasn't invented yet! :) Okay so I'm a bit out of character, un-green, but hey it's just a 1.5 litre, how much harm can one car do? apologies...

    There were two big air bleed pipes on the top of mine. Once I decided that they served no purpose (an easy way to spot emission control) I blanked them off. Throttle response went right up, economy too.

    What they did was to firstly balance the amount of air bled from both carbs (with one big screw, can ya see it?) and scavenged extra air from within the aircleaner.

    However I should install pipes from the fuel bowl floats into the air cleaners. That is emission control that does not affect performance, just saves the environment and also stops dirt from getting into the fuel bowl.

Original JAW, April 1998