The S. Unfortunately, following discussions with many people, the operational principles of this carburettor appear to be poorly understood. SU carburetors have a minimal number of moving parts and are easy to tune, providing of course you understand the operational principles! It is the intention of this article to help clarify those principles in the hopes that many owners will be able to tune and maintain their SU carburtetors.
Unfortunately, this later model carburettor was not as reliable as the earlier model. I found this out after purchasing my GSR The later model carburettor is difficult to tune.
Instead of adjusting the air fuel mixture by adjusting the jet height, similar to the HS6 S. Additional air bleeds into the side of the carbie just behind the piston via an adjusting screw causing the air fuel mixture to run lean or extra lean. Most automotive combustion engines are effectively a reciprocating pump. The volume of air they draw over a given duration in time is proportional to their speed.
Example: Assume we have a four-stroke engine with a volume of 1 litre 61 cubic inches. If the idle speed is rpm, the volume of air flowing through the engine will be litres At rpm, the volume of air will be 5 times that at rpm or litres Assume the engine to which the carburettor is supplying fuel is running at idle speed.
The jet, fuel needle and piston spring are sorted to match the performance requirements of the engine. The dashpot assembly is filled with oil of appropriate viscosity. The butterfly throttle is partially open under idle conditions no air, no fuel — no combustion. There will be restricted airflow into the engine and a large pressure drop across the butterfly when compared with that of the surrounding atmospheric pressure. The lower pressure in the inlet manifold is a vacuum when compared with atmospheric pressure.
The vacuum on the engine side of the carbie throat connects to the top of the piston chamber via an orifice located in the underside of the piston. The underside of the piston connects to atmospheric pressure via an orifice located near the throat on the air inlet side of the carburettor. This in turn will draw air out of the bell chamber above the piston, causing a partial vacuum within this chamber.
There is now a pressure differential across the piston Low pressure above the piston, high pressure beneath it. This pressure differential will cause the piston to rise, pulling the tapered needle out of the jet, allowing more air and fuel to flow through the carburettor throat and into the engine causing the engine speed to increase. In both instances the engine under load and is running at a constant speed. The air velocity over the jet in both instances is the same. How can that be?
If the engine is revving at a higher speed then the air velocity over the bridge in figure 2b will be higher as well, correct? Not So! The high velocity air draws the fuel out of the jet. The opening between the piston and bridge is now much larger, but the air velocity over the jet is the same as it was at the lower speed the air velocity is proportional to the throat area.
There is now more fuel drawn out of the jet due to the reduced diameter of the fuel needle. Where the air velocity does change is in the fixed pipe diameters of the induction system, ie. The spring within the suction chamber loads the piston in a downward closed position.
I found that the red springs were inadequate, that is the engine speed would never reach redline, and the bottom end power was inadequate. At around rpm, it would miss-fire excessively.In order for the carbs to work properly the ignition timing and valve clearance must be correct. On the R16 engine two different distributors were used. The point gap for both is. The dwell angle for the Hitachi distributor is 50 - 54 degrees. The dwell angle for the Mitsubishi distributor is 56 - 51 degrees.
On the U20 engine the dwell is 49 - 55 degrees. The R16 was fitted with the early mechanical advance curve of 15 crank degrees and a late smog 30 degree curve. The early distributor requires an initial timing setting of 16 degrees before top dead center. The late distributor is set to zero degrees before top dead center.
Distributors were often swapped so it is best to check to see which on is in your car. A way to do that is, after disconnecting the vacuum advance, set the timing to TDC. Rev up the engine watching the timing mark. The early distributor will only advance 15 degrees. The later will advance 30 degrees. For a final check after setting the timing hook up the vacuum advance, rev up the engine and check the total advance. It should be about 36 degrees maximum. Valve adjustment is very important.
For the R16 all valves ate set to. The U20 intake valves are set to. Again with the engine warmed up. Insure that there is some slack in the throttle cable. The cable housing grows longer as it heats up.
If there is not enough slack the idle speed will get higher as the engine warms up. Service screw? In normal operation it is backed off till it does not touch. The service screw found on the R This service screw design was introduced with the This engine did not have the thermo fuel valve and the design worked well.
Datsun SU Carbs
In the thermo fuel valve was introduced and there is an interference problem between the screw and the valve. Some of the screws were made shorter to address this problem.
The shorter screws are too short to work properly. A small folded up piece of paper between the screw and the manifold will allow it to work. When you are done using the screw it needs to be screwed in as far as it will go without touching the manifold and the screw head to thermo valve clearance checked.
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Hitachi Carburetor Kit
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Twin Carburetors 1. Condition see all Condition. New Used Please provide a valid price range. Item Location see all Item Location.By donhaywoodNovember 28, in General Discussion. Hi guys I went through some of your posts from about a year ago that dealt with the and they were pretty extensive.
Right now I'm having a lot of trouble with my carb not atomizing the idle fuel very well and the result is poor idling. Today I re-built the and did all kinds of cleaning of the jets, orifices etc.
I can see beads of fuel laying on the butterfly itself too. I know the float level is good and that the vacumn lines are all hooked up correctlyaccording to the Haynes LUV manual and the Chevy LUV series 6 manual.
I don't think I have any vacumn leaks either. I really think the problem is because the fuel spray is not well patterned or fanned well. There seems little chance of complete atomization when this happens. I allso had a Datsun and three Bugeye Sprites. I also think it is neat that some Datsuns and the Isuzu have the same carb. Thanks for all your help I also use the DCH and like them, but like anything else you can get the odd one with probs deeper than what's readily apparent.
I too had one like yours that seemed to wanna keep the top side of the butterfly moist with fuel Don - I don't have an answer for you, but thought I'd post in hope keeping this thread alive. I noticed the other day my recently brought back to life which has a rough idle seemed to be doing something similar to what you describe. At the time I didn't look too closely, but thought something didn't look quite right. Seems to smell like it is running rich at idle although it smooths out and runs fine at higher speed.
I intend to take a peek down the carb on my SX today and see how it looks at idle. Did you try spraying carb cleaner around the intake manifold and carb base when at idle? That is supposed to sometimes detect hidden vacuum leaks from a bad gasket.
SU Carburetors Explained
I didn't realize LUVs used the same carb as Datsuns.Rebuilding a stock Hitachi is not too difficult if you are careful and patientso why pay somebody to do it? Because it is so easy to make the carburetor worse, the last thing you want to do is disassemble the carburetor.
Rebuilding doesn't usually cost much. You may not even need new gaskets, even year old ones are often resuable. The most you'd need is the top float chamber gasket, and that only if it tears. Because it is so easy to make the carburetor worse, the last thing you want to do is dissasemble the carburetor. So first See Carburetor Adjustment.
Only proceed to this dissasembly if the carburetor is known bad. For example: rusted, sitting in mud bucket, or all other engine factors are known good but if it still runs poorly :. If the carburetor is missing parts or shows signs of being poorly assembled stripped screws, etc it may need a partial disassembly for inspection. A repair manual will be useful here. Album click to view. You don't even necessarily need to take the carb off the manifold which makes it much easier, there are no throttle linkage and wires to disconnect, and removing the carb is harder than fitting a new kit.
The keys are to success are:. Put the top back on see Carburetor Top Removal. Just tighten the bolts enough to compress the lock washers.
No more. Hitachi Power Valve Plunger the suction hole is just to the lfe of the valve, in the top casting Album click to view. It includes all the items in the Quick Rebuildplus:.
Use exploded view as a guide. The numerical sequence may generally be followed to disassemble unit far enough to permit cleaning and inspection. Reassemble in reverse order of disassembly. Note special instructions and follow numerical outline in making adjustments necessary for carburetor being serviced. DCG manual choke Album click to view. DGH automatic choke Album click to view.
Diagram is of manual choke model circa Other models are slightly different. This page has been accessed 45, times. This page was last modified1 August Content is available under Datsun Club. Main navigation. Main Menu. Username: Password: Lost Password? Register now! Advanced Search.
Dieseling - Engine run-on. Carburetor Inspection. Diagram - Parts breakdown. Specifications - year by year, including Jet specifications.