HSR Tuning Manual Revised 02/10/03 2 3 Tuning the HSR42/45/48 Carburetors Your Mikuni HSR comes from Eagle parts as a “reality check.” the factory with the tuning parts we When re-tuning is required, it found to work with the great majority usually involves small alterations of engine performance modifications. to the idle and/or main system. The However, the large number of following pages supply enough differing after market exhaust and air information to make such alterations cleaner systems makes it virtually relatively simple. impossible to accommodate all Please note that there is no point possible combinations with one in attempting to tune any carburetor carburetor set-up. unless the engine is sound and in a Your HSR will almost certainly good state of tune. If you have any run correctly on your engine with doubts about the general condition of the installed parts. But, if it doesn’t, your engine, have it checked by your you may alter its tuning to suit your dealer or an experienced mechanic engine’s needs by following this before attempting to fine-tune your guide. Mikuni. Some exhaust system designs strongly interfere with carburetor AIR LEAKS: tuning. For instance, it is very difficult We have found that Harley-pattern to get smooth and responsive engines tend to develop minor air carburetion through the entire rpm leaks between the manifold and range with open straight pipes and heads. These leaks affect air/fuel other unbaffled exhausts. In addition, mixtures at low throttle settings and very small volume, small diameter can be troublesome at idle. For best mufflers are often ‘seen’ by the performance, it is important that you engine as straight pipes and present test for and eliminate any such leaks. similar tuning difficulties. The test is simple : With the engine Very long duration cams often warm and idling, spray WD-40 or cause relatively poor running similar paint-safe liquid around the below 3,000 rpm, depending upon junctions of the manifold, carb and the individual cam’s intake valve heads. If the engine changes from closing point. Such cams cause its steady idle, if it surges or misses, reverse airflow out the mouth of the then there is an air leak that should carburetor (often called “reversion” or be corrected. Be sure to keep the “stand-off”) that can be mistaken for spray away from the air cleaner to a carburetor tuning problem. avoid a false indication. If you have any doubts about a particular exhaust system, air cleaner or ignition, you may substitute the Harley Screamin’ 2 3 NOTE: The choke is designed to work correctly with the throttle closed. For a quick and accurate analysis, Opening the throttle greatly reduces when fine-tuning your HSR, we recom- the action of the choke. mend using “witness” marks on the throttle grip and throttle housing. Use NOTE: masking tape on the grip and an indica- tor mark on the throttle housing. Mark the Make sure that the stock Harley- tape in 1⁄4 throttle increments from idle Davidson choke cable is fully seated in to full throttle. You can then accurately the metal elbow at the carburetor end identify the throttle opening and adjust of the cable assembly (see Installation the proper tuning components. Instructions). Harley ‘s cable is stiff and can fail to fully seat in the elbow. This condition results in poor mileage and a poor idle. The Mikuni choke cable is more flexible and less likely to jam. Still, it is best to check to be sure the cable is installed correctly. Figure: T1 IDLE SYSTEM: HSR TUNING SYSTEMS: (Idle - 1/8 throttle) The HSR carburetor is divided into The HSR idle system has two four interdependent systems: tunable components: the Pilot Air 1. Choke system Screw and the Pilot Jet. The air 2. Idle system screw’s purpose is to fine-tune the 3. Main system mixture at idle. The pilot jet controls 4. Accelerator pump system the total amount of fuel passing Each of these systems has through the idle system. The pilot its major effects in a different jet can be exchanged for a richer or throttle range. While there may be leaner one if needed (see the note some overlap, each system can on the following page). generally be treated as though it is The pilot air screw is set at two completely responsible for air/fuel turns open from the factory. This is mixtures within its range of throttle the position we have found to be settings. Three of the systems have correct most of the time. If the screw replaceable components that allow position has been altered, gently fine-tuning should the need arise. bottom it and re-open it two full turns. Next, run the engine until it CHOKE SYSTEM: reaches normal running temperature. The choke system’s purpose is to With the engine idling smoothly, provide the rich air/fuel mixture an adjust the pilot air screw in slowly engine needs to start and run reliably until the idle either slows or becomes when cold. There are no replaceable irregular, then turn the screw out until tuning parts in the HSR choke the engine again slows or begins to mechanism. idle irregularly. 4 5 Count the number of turns between the two positions. Set the air screw mid-way between these too-rich and too-lean positions. Use the Idle Adjuster to re-set the idle speed. If the engine becomes too hot during the adjustment procedure, the resulting idle mixture will prob ably be on the lean side of correct. If you have a large fan, use it while adjusting the mixture. If you do not have one, you may need to take time out for a short ride to cool the engine back to normal temperature. Figure: T2 NOTE: JET NEEDLE (off-idle - 1/4 throttle) 1. If the best idle is achieved with the air (see Fig. T2) screw less than one turn out, the pilot jet The straight diameter portion of is too small and should be exchanged for a larger one. the jet needle controls the mixture 2. If the engine speed does not slow after from just above idle to approximately two-and-a-half turns out, the pilot jet is 1/4 throttle. If the mixture is too rich too large and should be exchanged for a or too lean in this throttle range, the smaller one. needle will need to be exchanged for one with a larger or smaller diameter. MAIN SYSTEM HSR jet needles are available in four sizes. Only the diameter of the NOTE: straight part of the four jet needles Cam design effects differ. The richest is the “-95” and When testing, consider the rpm “-98” is leanest. effects of any accessory cam you may have installed. Long duration LEAN CONDITION: cams with late closing intake timing If the needle is too lean, part tend to perform poorly below throttle acceleration will be flat some critical minimum rpm. If you (remember the possible effects of attempt to test below this rpm, the cams & exhausts). There may also engine may seem soft, flat and be some detonation during part unresponsive. No carburetor can throttle acceleration in the 2500 compensate for the engine being - 3000 rpm range, (although this “off the cam.” may have other causes such as an All jet needle and main jet testing ignition that advances too quickly). should be done with the engine A lean needle also results in an near the middle of its rpm range, abnormally slow warm-up. If any of but high enough to be “on the these conditions exist, install a one cam.” All testing should be done size richer needle and compare the with the engine at normal operating performance. temperature. 4 5 RICH CONDITION: While a black, sooty spark plug is NOTE: a sure sign of richness, others are See the “How To” section at the back more subtle. If your engine responds of this manual for jet needle adjustment crisply at low throttle openings when and removal procedures. cold, chances are the needle is one size smaller (richer) than it needs to be. This assumes, of course, that the idle mixture is correctly tuned. Poor fuel mileage is another sign of an over-rich condition. Fuel mileage is usually measured at cruising speeds (65 mph on a flat, windless road) where the jet needle is the main mixture control. Poor milege at these speeds can be corrected by fitting a leaner jet needle. Raising or lowering the jet needle has no effect on fuel mileage below about 70 mph. JET NEEDLE (1/4 - 3/4 THROTTLE) Figure: T3 The tapered portion of the jet MAIN JET SELECTION: 3/4 - full needle rises out of the needle jet at throttle about 1/4 throttle. From that point until the main jet takes over near 3/4 DYNAMOMETER METHOD: throttle, the jet needle taper is the Dynamometer testing is a quick main control of mixture strength. and safe way to determine the cor- rect main jet size. The main jet that LEAN CONDITION: delivers the most power is the best If acceleration seems soft or flat one. If two jets produce the same and the engine is slow to respond power, use the smaller jet. when the throttle is quickly opened One possible issue with dyno test- from the 1/2 throttle position, the ing is temperature. Most dynamom- mixture is too lean. Raise the needle eters do not have enough cooling one notch and repeat the test.