Mikuni HSR Tuning Manual

Tuning Manual. Revised 02/10/03. 2. 3. Tuning the HSR42/45/48. Carburetors … We have found that Harley-pattern. engines tend to develop minor air …

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HSR HSR Tuning Manual Revised 02/10/03 2 3 Tuning the HSR42/45/48 Carburetors Eagle parts as a  reality check. When re-tuning is required, it usually involves small alterations to the idle and/or main system. The following pages supply enough information to make such alterations relatively simple. Please note that there is no point in attempting to tune any carburetor unless the engine is sound and in a good state of tune. If you have any doubts about the general condition of your engine, have it checked by your dealer or an experienced mechanic before attempting to fine-tune your Mikuni. AIR LEAKS: We have found that Harley-pattern engines tend to develop minor air leaks between the manifold and heads. These leaks affect air/fuel mixtures at low throttle settings and can be troublesome at idle. For best performance, it is important that you test for and eliminate any such leaks. The test is simple : With the engine warm and idling, spray WD-40 or similar paint-safe liquid around the junctions of the manifold, carb and heads. If the engine changes from its steady idle, if it surges or misses, then there is an air leak that should be corrected. Be sure to keep the spray away from the air cleaner to avoid a false indication. Your Mikuni HSR comes from the factory with the tuning parts we found to work with the great majority of engine performance modifications. However, the large number of differing after market exhaust and air cleaner systems makes it virtually impossible to accommodate all possible combinations with one carburetor set-up. Your HSR will almost certainly run correctly on your engine with the installed parts. But, if it doesn’t, you may alter its tuning to suit your engine’s needs by following this guide. Some exhaust system designs strongly interfere with carburetor tuning. For instance, it is very difficult to get smooth and responsive carburetion through the entire rpm range with open straight pipes and other unbaffled exhausts. In addition, very small volume, small diameter mufflers are often ‘seen by the engine as straight pipes and present similar tuning difficulties. Very long duration cams often cause relatively poor running below 3,000 rpm, depending upon the individual cam’s intake valve closing point. Such cams cause reverse airflow out the mouth of the carburetor (often called  reversion or ‘stand-off ) that can be mistaken for a carburetor tuning problem. If you have any doubts about a particular exhaust system, air cleaner or ignition, you may substitute the Harley Screamin 2 3 For a quick and accurate analysis, when fine-tuning your HSR, we recom- mend using  witness marks on the throttle grip and throttle housing. Use masking tape on the grip and an indica- tor mark on the throttle housing. Mark the tape in 1 4 throttle increments from idle to full throttle. You can then accurately identify the throttle opening and adjust the proper tuning components. Figure: T1 HSR TUNING SYSTEMS: The HSR carburetor is divided into four interdependent systems: 1. Choke system 2. Idle system 3. Main system 4. Accelerator pump system Each of these systems has its major effects in a different throttle range. While there may be some overlap, each system can generally be treated as though it is completely responsible for air/fuel mixtures within its range of throttle settings. Three of the systems have replaceable components that allow fine-tuning should the need arise. CHOKE SYSTEM: The choke system’s purpose is to provide the rich air/fuel mixture an engine needs to start and run reliably when cold. There are no replaceable tuning parts in the HSR choke mechanism. The choke is designed to work correctly with the throttle closed. Opening the throttle greatly reduces the action of the choke. Make sure that the stock Harley- Davidson choke cable is fully seated in the metal elbow at the carburetor end of the cable assembly (see Installation 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

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