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Fast Idle and Dieseling!!


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Just completed my conversion from my troublesome EFI to SU's and the car started!! It is the first time I have heard my Z run on 18 months (due to fire damage). Trouble now is that the car idles between 3500 and 4500 rpm!!!! When I go to switch it off it diesels. Also it is running very hot and warms up extremely quickly (it was a hot day 35 C). I do not think my vacuum advance is working. The fuel bowl vents are not connected to the aircleaner and the neither is the valve cover vent (I am trying to track down a stock airbox)

Help please, I really want to drive my baby.

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Have you adjusted the chokes? How about the idle screw on each SU, is either one (or both) turned in too much and causing the high idle?

With the choke lever pulled back, the fuel nozzle should be pulled down from the bottom of the carb by about 1/4 inch. The fuel nozzle is the part mounted in the bottom of the main carb body and is connected to the float bowl with a small fuel hose. When the choke lever is pushed forward, the nozzle should be pulled up against the adjustment collar on the bottom of the carb. Is either one of the nozzles sticking, and not being pulled back up against the adjustment collar?

With the engine off and the air filter box removed, you might have a helper operate the choke lever in the car while you watch the action on the carbs to insure the nozzles are moving properly.

And as 240kman said, check the linkage to make sure nothing is binding and causing the high idle.

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Kindal , Have you disassembled the butterfly to throttle shaft of the carbs ? If the linkage adjustment isnt the problem , check the butterfly to see that they close and seal when the throttle is closed . As to the heating , be watchful of this when you first start the engine . You can have a ''air lock '' when the engine if first filled with coolent. The thermostat is closed sealing the passage . The temp sensor can be dry in a air pocket the same with the thermostat , so It will open too slowly . Overheating can happen . Now that you have started the engine and allowed it to cool , recheck the coolent level . Most likely you will find the level is vary low . Conducted heat will open the thermostat allowing the air to pass , then the next time you start the engine the water pump can pass the coolent . That is if the thermostat is in good working order . By the way , to adjust the butterflys and or check to see if they are sealing. Look down the barrel with the throttle closed , holding the carb with a light in the background and see if there is any light showing around the butterfly disk . If there is , this is the cause of the high idle . Loosen he screws that hold he disk to the shaft , just so the disk can move with some resistance. Now forceably close the throttle . Do this a few times then check against the light again . This should allow the disk to shift and mate to the barrel and seal . Then tighten one screw and remove the other one and apply LOCK-TIGHT to the screw and replace it , do the same thing with the other one. You do not want these coming loose . All the best , Gary:rambo:

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Thanks guys, I disconnected the throttle linkages and the chokes this morning and the engine idled at 900, turns out one of my linkages was too long and holding the throttle open.

Tomorrow I will borrow a timing light and set the timing.

The car still diesels, could this be because it is running lean (240 needles on and L28)?

topped the radiator up with coolant and did not have any heating issues today, temp was much lower than yesterday though.

Any advice on the dieseling?


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I am running a 82ZX with cam , 9 +to 1 compression . 54 needles in the SUs and she runs strong . I am close to sea level . I have headers and 2 1/2 plumbing with a dynomax muffler . I am not having any mixture problems . I also have a '79zx ignition set at 10 degrees BTDC . Set the timing , then check the carb ballance and the mixture as well , even though you just did it. Make sure the nozzles are not sticking down a little , this will cause a rich mixture if they are . This should take care of the dieseling , if not add some premium gas and see if this helps. Has the engine been run rich for a while ? It could be carbon deposites causing the run-on. If you run a tank of Hi-test fuel through this may clear it out . If the SUs came off a L-24 with a automatic trans origionally , it could have N-58 needles . Also if they were set for high altitude. The 58s are leaner . I have 58s ,54s, 27s witch are richer stock needles for the '70 , and a set of SMs witch are vary rich especially for street use . So far the 54s have been the best for me . Both power and the mileage seems good as well. I cannot account for the mileage untill I recalabrate the speedo due to a gear change in the diff. The N-27 ,and 54s are still available from Nissan . All these needles use the same nozzle. Keep us informed on how this pans out. Gary , one more thing run some 20w or 30w oil in the carbs , this makes a big difference in throttle responce .

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