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Priming SU carbs?


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:lol: SUs don't have those!! (sorry)

 

SUs have a variable venturi which adjusts to the demand of the motor. The Hitachi has to transition from idle to a huge primary barrel and it's difficult to get the air moving when the throttle is suddenly jammed open. The engine sucks air and runs lean before the vacuum builds and draws in gas. The accelerator pump fills this lean spot with a squirt of raw fuel.

 

Think of the SU as a carburetor that becomes bigger as you need it. There is a slide or piston that is drawn up to enlarge the barrel in direct relation the manifold vacuum. Thus the venturi has a constant (or nearly so) vacuum and delivers gas at any speed and sudden throttle opening.

 

 

Josh, there is a choke, of sorts, but not like other carbs. The slide is raised slightly by a linkage. Raising the slide pulls the tapered needle valve out of the jet allowing more gas to enter the air flow and make it richer. Does the same job as a choke but in a different manner.

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:lol: SUs don't have those!!

 

Good call!!:lol: I would suggest starting fluid, as a last resort. SUs rely upon vapor at the needle, and using starting fluid only disrupts the fine mixture at the needle. Kinda like starting a lawnmower with starting fluid, a good VROOOM, then you're starved for fuel until the float bowls fill up.

 

jovial_cynic, are your choke cables in place? If so, you'll have one option: pull the coil wire, apply choke, crank without spark until your float bowls fill. These carbs are hard to flood because of their construction. Once you have cranked sufficiently, you'll be able to re-connect the wire and start the car. This method allows for the carbs to do what they were designed to do, draw in a mixture into the combustion chamber, without the detonation.

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:lol: SUs don't have those!! (sorry)

 

SUs have a variable venturi which adjusts to the demand of the motor. The Hitachi has to transition from idle to a huge primary barrel and it's difficult to get the air moving when the throttle is suddenly jammed open. The engine sucks air and runs lean before the vacuum builds and draws in gas. The accelerator pump fills this lean spot with a squirt of raw fuel.

 

Think of the SU as a carburetor that becomes bigger as you need it. There is a slide or piston that is drawn up to enlarge the barrel in direct relation the manifold vacuum. Thus the venturi has a constant (or nearly so) vacuum and delivers gas at any speed and sudden throttle opening.

 

 

Josh, there is a choke, of sorts, but not like other carbs. The slide is raised slightly by a linkage. Raising the slide pulls the tapered needle valve out of the jet allowing more gas to enter the air flow and make it richer. Does the same job as a choke but in a different manner.

 

actingretarded84405ln4.jpg

By phlebmaster at 2008-03-11

 

Silly me. lol

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Twin choke cables, eh? It seems like having a single cable with a bracket would be easier to cobble together...

 

It is, and I had the same problem, but my fear was not drawing both chokes at the same time. If for some reason one carb had a bind at the choke, it could cause the one carb to run richer than the other, and performance issues as you might think two cylinders were running poorly. You search ignition while this thing makes an unhappy face O / O, instead of O - O. Perhaps mounting the choke cable center **could** alleviate the problem, but it is only a guess. I have an idea to run a 240Z choke cable set and fabricate a knob assembly under the dash. No twisted cable, as the Z cable is solid wire, and the bracket comes with the assembly to pull two cables simultaneously.

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Face the carb. On its left, the front of the car, below the casting you will feel a spring loaded pin. It's on the front, the choke cable is on the cabin side [unless you have a much later left hand / right hand pair.] That's the prime which is built into the carb. When you let the car sit for a long time fuel evaporates. There is still some in the bowl. Poke the protruding tip about twice on each carb and it will start up as though you had just shut the engine off 30 minutes ago.

 

By the way, the 2 choke cables come originally with a double choke cable assembly attached to the choke knob inside the car. Roadsters and RL411s use this setup. Might still be available?

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Face the carb. On its left, the front of the car, below the casting you will feel a spring loaded pin. It's on the front, the choke cable is on the cabin side [unless you have a much later left hand / right hand pair.] That's the prime which is built into the carb. When you let the car sit for a long time fuel evaporates. There is still some in the bowl. Poke the protruding tip about twice on each carb and it will start up as though you had just shut the engine off 30 minutes ago.

 

By the way, the 2 choke cables come originally with a double choke cable assembly attached to the choke knob inside the car. Roadsters and RL411s use this setup. Might still be available?

 

MikeRL411, I'm not so sure about that. The small spring-loaded pins (lifter pin) on the sides are for verifying the proper operation of the suction chamber and piston movement. It's effectively the method to check for the infamous 'metallic clunk' for the suction piston. Both carbs have them, and if no clunk is heard, then the relationship between the piston and the needle is not in proper alignment, and needs removed and repaired or replaced. Maybe it gives the carb enough movement to provide a small amount of fuel vapor for starting, but then again, I have choke cables.

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So to summarize:

 

1. Make sure your chokes are working. Choke it to start a cold engine

2. Maybe the fuel is leaking out of the fuel bowl overnight. This is really common with regular Datsun carbs. Solution: Fix the leak. Installing electric fuel pump will mask the problem yes, but doesn't really fix the leak.

 

Fuel should stay in the bowl 3 days at least. After a week or two maybe not.

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So to summarize:

 

1. Make sure your chokes are working. Choke it to start a cold engine

2. Maybe the fuel is leaking out of the fuel bowl overnight. This is really common with regular Datsun carbs. Solution: Fix the leak. Installing electric fuel pump will mask the problem yes, but doesn't really fix the leak.

 

Fuel should stay in the bowl 3 days at least. After a week or two maybe not.

 

I would use a short length of coat hanger wire to check for fuel in the bowls before I would rule out a leak. Run it down the overflow tube(s). I have the small overflow tubes at the top, easier to check. If you are running the long length of steel tubing for overflows, then you will obviously have to remove them to check. Do you have the brass floats? Be extremely careful not to punture them, you're just checking for fuel. Better yet, use a Q-Tip, It will confirm fuel w/o the worry of puncture.

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