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Most people that have a difficult time tuning the idle on the Nikki actually have other problems, such as a vacuum leak or a timing issue. But on a nice tight set-up with no leaks and no other engine problems, tuning the idle on a Nikki should be as simple as following a few easy steps, especially if you understand what the components and adjustments on the carburetor do.
Ten Easy Steps to Idle Tuning:
Step 1, Adjust the Deceleration Dashpot and the AC Idle Compensation Valve so that they are not interfering with the primary throttle operation.
Commonly overlooked idle tuning pitfalls include not paying attention to extraneous components attached to the carburetor. Things like the Deceleration Dashpot and Air Conditioning Idle Compensation Valve have linkage that can interfere with the primary throttle shaft, and keep it from closing fully. Other pitfalls include not having enough slack in either the throttle cable or the fast idle cable. Still another possible trouble spot is the complex automatic choke assembly. Since all of these components will need to be re-adjusted once the proper idle is established anyway, it is best to first adjust them to ensure they are not keeping the primary throttle shaft from closing fully.
Step 2, Open the primary valves to get the engine running on the main circuit by turning the Idle Speed screw in.
The "Idle Speed Screw" is located on the lower right side of the throttle body, looking at the primary venturi side of the carburetor. It's simply a screw that goes through the casting and serves as a stop for the throttle shaft in the closed position. Screwing it in opens up the primary valves just a bit, letting in more air, which raises the idle. Backing it out has the opposite effect. On the earlier Nikkis, this screw is at about a 45* angle to the top of the throttle body, and it has a locking nut. It's a terrible design for anyone who needs to reach it, particularly because that nut needs to be loosened before the screw can be turned. On the later Nikkis they replaced that design with a similar one where the screw is parallel with the throttle body top, and the screw has a spring to keep it immobile from vibration, instead of a locking nut. It also has about a one inch long head to make it more accessible.
Step 3, Set the Fuel Mixture screw to 2½ turns out from closed. (Never bear down on this screw to tighten!)
The "Idle Mixture" control is a small screw located front and center on the throttle body. The initial setting for this should be about 2½ turns out from closed. Never over tighten this screw! NEVER!!! This is a needle valve with a delicate, sharp point. It is made of soft brass on earlier Nikkis, and soft "junk steel" on the later ones, and the tip meets a hole drilled into the cast iron throttle body that has a very sharp, hard edge. Scoring the tip of this delicate needle valve is a sure-fire way to screw up your idle forever! (no pun intended.)
Step 4, Set the Air Screw to 2 turns out from closed. (if you have one on your carburetor.)
On later Nikkis, there is just one small screw that controls the fuel flow at the very end of the idle circuit. The fuel is introduced to the engine from a slit just beneath the edge of each partially closed primary throttle valve. Air flowing past the small gap in the closed primary bores creates a strong vacuum that draws the fuel out. However, on the early Nikki carbs, there is an additional, larger screw above the idle fuel adjustment. This screw is used to dampen that vacuum. It does nothing more than limit the amount of fuel that can be regulated by the Idle Mixture screw. In 1983½, the large air regulating screw was replaced by a pressed-in internal jet inside the throttle body due to a slightly redesigned emissions control setup. Without a limiter of some sort, fuel would be siphoned into the intake via the idle circuit throughout the range of the throttle body.
Step 5, Start the engine, reduce the engine RPM using the Idle Speed screw, and let it warm up.
Obviously you can't tune a carburetor without the engine running, and to tune the idle properly, it needs to be warmed up, too. If the idle is tuned cold, when the engine warms up, the temperature will effect the fuel quite a bit, and the idle will lean out and raise the engine RPM. If you guess at the settings, either by trial and error, or by using someone else's settings, you chance not getting the engine to start. If the engine cranks but doesn't start, it's likely to get flooded. Now you're in to wasting at least 10 minutes.
The carburetor does not need the idle circuit for the engine to run. It needs the idle circuit for the engine to idle. It's only there because the main circuit can't deliver any consistent mixture with so little air running through the venturis. But the main circuit will run at 1800 RPM. Take a look at what that Idle Speed screw does again. It opens the primary throttle, so if you crank the screw in a bit, the carburetor will basically run on the main circuit. That's why I send all my carbs out with the idle speed screw tightened in so that the engine will initially start. Set the Idle Mixture screw to an initial adjustment of 2½ turns out from closed. If you have a Nikki carburetor with an air adjust screw as well, set that to about 2 turns out from closed. If you cannot get the idle to drop down to a reasonable RPM, the air adjustment may need to be closed a bit. Use ¼ turn increments, and repeat the tuning steps.
Before starting the engine, it's a good idea to practice locating the speed screw with a long screw driver so you can turn it down quickly after starting the engine. After turning the idle speed screw in, when you initially start the engine it may rev right on up to 3500 RPM, and it's never a good idea to rev a cold rotary up high for very long. Fire up the engine and get out and turn down the idle speed screw so that the engine is running at a comfortable warming-up RPM (around 1800 RPM). Let the engine get up to temperature because idle mixture changes while it's cold won't stay that way.
Step 6, Turn the Idle Speed screw out until the engine almost shuts off. (This will lower the engine RPM.)
Once it's warmed up, back out the idle speed, lowering the RPM. Keep going until the engine "hunts", or sounds like it will shut off. Turn the screw back in at that point, just about an eighth turn, or enough to be sure the engine won't stall out.
Step 7, Turn the Idle Mixture screw in, in ¼ turn increments, waiting 2 seconds after each change, until the engine starts to skip. (This will raise the engine RPM.)
Now direct your attention to the fuel mixture screw. Turning it in is going to let less fuel in, which is going to lean the mixture, which is going to raise the idle. Too lean a mixture, and the engine will die. To rich a mixture, and your plugs will get fouled. Turn the screw clockwise until the engine begins to stumble. It's important to keep in mind that this adjustment tends to take a second to have a full effect, so make your turns in small increments, and wait 2 seconds for the result before continuing. When the idle begins to stumble, back the mixture screw out just a bit (about an eighth turn), and back the speed screw out again until the engine begins to hunt.
Step 8, Repeat steps 6 & 7 until the desired idle speed is achieved. (Usually this is between 750 & 850 RPM.)
Simply repeat this process until the engine is at the lowest RPM it can go and give a consistently purring idle. The RPM goal is 700 - 750 RPM, but I consistently get 650 on my stockport. (I do have an aluminum flywheel and direct fire ignition, however.)
Step 9, Back the Fuel Mixture screw out less than ¼ turn to ensure that the idle is not too lean. (This may require readjusting the Idle Speed screw first.)
When you've gotten the idle to where you like it, back out the Idle Mixture screw so that the idle is a little richer. This will cause the engine to bog and stall if you've already gotten the idle down to around 700 or 750 because the stockport rotary really doesn't want to idle at anything lower than 750 without an extremely lean mixture, so you should probably adjust the Idle Speed screw in a bit first. The reason we want a slightly rich mixture is because giving the engine a good workout is going to heat the fuel in the carburetor up. Because the fuel is expanded, the mixture becomes leaner, and coming to a stop can cause the engine to stall. If you can't get below 1000 RPM, there is a problem. Either there is a vacuum leak somewhere, or there is an idle jetting issue, such as the incorrect jets, or a fuel blockage inside the idle circuit.
Step 10, Readjust the Deceleration Dashpot and the AC Idle Compensation Valve.
Once the idle is all set, adjust the other components on the carburetor, like the Deceleration Dashpot and the AC Idle Compensation Valve, so that they do what they are supposed to do without effecting the initial idle. The deceleration dashpot simply "cushions" the return of the primary throttle shaft, and should never interfere with the idle setting. The AC Idle Compensation valve, however, is designed to increase the idle via a vacuum signal to compensate for the drop in idle RPM due to the engine load from engaging the AC Compressor. Be sure that when the AC is switched off, the idle returns down to where you set it. Keeping these components on the carburetor lubricated with white lithium grease is the best way to ensure they do not bind the linkage.
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Copyright © 2009 Dennis Williams, Sterling Metal Works. All rights reserved. gorealfast@sterlingmetalworks.com |
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