twin flat top su problems

[Max_Pain]

i have a L18 datsun 720 ute with twin flat top su carbs(Hitachi) but i cannot get the idle low+balanced yet everything is loose(undone) 1 su is very unbalanced

i have found how ever there are 2 other screws that no body mentions at all these are outlined in yellow in this pic i have added (note)not of my ute but exactly the same as what i have

the carbs are from a 180b sss from what i have been told)

Yes i have a carb balancer and all the stuff needed

also they do not leak any air fuel(only brake booster leaks air but i have blocked the very tiny leak off)

does anybody know what these are set to or how to set them???

any info is much appreciated

[SteveJ]

The location of the back screw isn't quite right, but look on page EF-19 of the 1974 260Z FSM. You can find it on the XenonS30 website. IIRC, the American flattops are not quite the same as the flattops in other markets, but the manual may help you anyway.

[Bruce Palmer]

Any time we send out a set of those SSS1800 flat tops we remove the pieces on the bodies just behind the domes and block off those holes so that takes whatever's there out of the mix. What the circled item on the balance tube/manifold is is anybody's guess. Just make sure it doesn't leak.

And for any who are ready to jump in here about flat tops, these are 38mm SSS carbs not 46mm Z carbs. I absolutely wore out a set of these in the 90's.

While I'm thinking about it, That preformed hose that vents the float bowl..... where does it go (to the rear) and does it T into the hose off the back float bowl lid. That's an unusual arrangement, at least to me.

Spray carb cleaner around those throttle shafts with the engine running. If the idle speed doesn't change then start scouring the area for vacuum leaks.

About all I can offer at 5 something on a Sunday morning..... COFFEE more COFFEE!!

[Captain Obvious]

The circled screw on the rear carb is your idle mixture screw. It meters air around the suction pistons to lean out the mixture coming through the venturi.

The circled screw between the two carbs on the balance tube is your idle speed screw. It meters the idle mixture into the balance tube which is then split for front bank and rear bank.

Not having messed with a set of those carbs personally, I would expect the procedure to be... a) Get it to run well and have it be synchronized above idle. For that you would use the nozzle drop adjustment for mixture and the throttle plate opening adjustment for speed. Then Once you have it working above idle, go back and work on the idle performance using the idle mixture and idle speed screws.

I suspect all of this sounds a lot easier than it is in reality.

One thing to note is that you should be able to get it to idle with the throttle plates virtually closed. Darn near all the fuel/air needed for idle should be coming through the idle circuit and into the balance tube, not past the throttle plates. If you need your throttle plates to be open to get it to idle, then you're not really idling.

[Max_Pain]

ok thanks

so what i have to do is screw in the idle speed adjusting screw(to get low idle) and also screw in the idle mixture adjusting screw(to have air balanced between the carbs), balance the carbs using there idle screws and she be done. i hope it doesnt play havoc with the anti dieseling solenoid

[Captain Obvious]

In theory, any changes that you make to the idle mixture and idle speed screws should have the same impact on each carb. The air that is metered by the mixture screw is supposed to be split evenly into the two carbs, and the idle air out from each carb is supposed to be the same as well. In theory at least.

That said, if you crank both of those screws fully down, then your carbs should operate just like all the standard Z round top carbs. Use the high speed idle screw (I assume there is one of those) to simulate an idle circuit and sync and tune just like you would any round top. Then once that is done, you should be able to release the high speed idle screw and open up the idle speed screw instead. You will probably have to mess with the idle mixture to get it right at that point, but if everything else is working right, it should fall into place. The theory would be that you run a little rich at idle using the nozzle drop, and then lean it out again using with the idle mixture screw.

What else? The anti-dieseling solenoid should have no effect as long as the key is in the "ON" position. If everything is working as intended, then you should be able to completely ignore the anti-dieseling stuff completely.

Also, I'm traveling right now, and my internet access is spotty, but hopefully I'll have some access.

[Captain Obvious]

That preformed hose that vents the float bowl..... where does it go (to the rear) and does it T into the hose off the back float bowl lid. That's an unusual arrangement, at least to me.

The hoses on the float bowls are part of the "anti-dieseling" system. When the engine is running, there's a solenoid valve that vents the float bowl chamber to atmosphere through the air cleaner. This means that when the engine is running, everything works the way you're used to seeing with the standard round tops.

The trick is, however, when you turn the key off, that solenoid valve de-energizes and the float bowl vent is no longer connected to the air cleaner anymore, but is instead connected to the decaying manifold vacuum as the engine grinds to a halt. This negative pressure above the fuel in the float bowl prevents fuel from being pulled into the engine through the venturi and renders the carbs ineffective. This prevents dieseling after the key has been shut off.

I absolutely wore out a set of these in the 90's

I'm not surprised... They look like they've got a good mix of features from both round and flat tops. They've got the round top nozzle adjustability and the independently controllable mixture on the idle circuit of the flat tops. Best of both worlds?