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SUs adjustment


beandip

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Adjusting SUs ,round tops only, is realy not difficult. There is a must in the way of tools to do it correctly. To ballance the ammount of air being drawn into the two units you must have a method of measureing the air flow. The tool comes with different names and brands. Mine is a UNI-SUN made by Edelbrock. Available at the local speed shop and some cycle shops. I paid $25.00 for mind. To start with remove the air cleaner cover and filter. Start the engine and warm it to driving temp. This may take 10 + min. to really warm the engine and not just the thermostat and sending unit. These engines do take a while to get up to temp. When sufficently warmed up and the engine at Idle , should be about 7 or 800 RPM , insert your finger in the inlet of one of the carbs. Lets start with the back one. Lift the piston of the rear carb up an inch or so. This will dissable the rear carb , and the engine will now be running on the front one only , and this front one is the one we are testing. If the engine dies when the piston is lifted the mixture on the front carb is to lean. If the engine continues to run and fairly smooth manner the front carb is too rich. If the engine continues to run but roughly the mixture is correct ,for now. If the setting is too lean , reach under the front carb and feel for a small fuel line , about 1/4'' in dia. this line is connected to a adjustment knob on one end. Turn the adjustment CLOCK WISE , one half turn. This will richen the mixture . Restart the engine and do the test once again. Now if at first the test determines the setting is too rich because the enging continued to run smoothly . Reach down and adjust the adjustment COUNTER CLOCKWISE . This will lean the mixture. Repeat the test on the rear carb by lifting the piston on the front unit. When reading the manuals I find it confusing to try and follow there instructions.What we are doing is by lifting the piston and disableing one of the carbs we are seeing how the other unit adjustment is set. Disableing one while adjusting the other. SIMPLE. Now after all has been adjusted and the ballance has been rechecked and adjusted. With the cover still off the air cleaner and the engine at idle, BLIP the throttle to wide open and let it go back to idle . Just wide open and shut. If there is a backfire through one of the carbs, this is known as a lean pop , and adjust that carb richer slightly about 1/8th of a turn and repeat the final test. No Pop and you are done. take her out and drive her. I have found that the oil that workes the vary best for me in the dampiners of the carbs is 20wt . Auto trans oil and Mistery oil is too thin andit causes a lean condition when the throttle is first opened. This can cause a stumble . 20wt is what was called for when the cars were made and I have found it works for me. Another thing check the small fuel line that attaches the float bowl to the adjustment under the carb for stiffness. they get hard from being close to the manifold and will cause the choke to stick open . Do NOT use vacume hose for this or water hose , ZTherapy has replacement hose . Nissan dealer should and or look to a cycle shop. Hobby shops will carry fuel line also but be sure it is for gasoline not nitromethane fuel. The wrong line will fail and dump fuel on the exhaust manifold !! Not good. If you have trouble send me a PM or email .Gary

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  • 4 years later...

Adjusting SUs ,round tops only, is realy not difficult. There is a must in the way of tools to do it correctly. --snip--If you have trouble send me a PM or email .Gary

Thank you, thank you, thank you! I just got the car love of my life back on the road after nearly 15 years, and after 200 miles and almost 2 weeks, it seems fine. Except for the fumes, and....I'll stop there! (No jinx here) So I really didn't want to be beholden to a carb voodoo doctor (and the attendant $$$) so your primer here is **exactly** what I needed...enough to make these adjustments and hopefully deal with a major contributor to my gas fume/richness (?) problem. I'm also lining the whole tub from stem to stern with a variety of butyl skin products for air seal(CO!)/vibration/noise abatement. I just want to exclude potential carb issues and this seems like the best way to start. Thanks again!

2/71 240Z, 920 Gold-Black, and all original except for the 5-speed. Bought from the original Japanese owners from California. Under 70k miles. I'll be documenting more soon.

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