Mikuni HSR Series Carburetor Tuning Service Manual
Contents
1. portion of the needle affects tuning from idle to approximately 1 4 throttle opening www ClassicCycles org 14 15 Accelerator Pump Nozzles Leaner TM42 11 50 Std for Sportster TM42 11 60 Standard TM42 11 70 Mikuni Jet Kit Tuning Kit HSR42 KHS 025 Jet Kit Contains 18 Main Jets 2 each 150 thru 170 10 Pilot Jets 2 each 20 thru 32 5 3 Needles 96 97 98 42 only 2 Pump Nozzle 60 4 Needle E Ring Clips amp Washers 2 O ring Pump Nozzle N124 063 1 Plastic Box HSR42 45 Carb Rebuild Kit Carb Rebuild Kit KHS 016 See exploded view drawing for con tents HSR48 Carb Rebuild Kit Carb Rebuild Kit KHS 031 See exploded view drawing for con tents Short Idle Screw Idle Adj Screw Short 990 605 065 See item 59a in exploded view NOTE Do not modify the idle screw or any of its component parts If you remove the screw be sure to re install it with the components in place as illustrated in the exploded view NOTES _________________ _________________ __________________ __________________ _________________ _________________ ___________________________ ____________________________ _________________ _________________ ___________________________ ____________________________ www ClassicCycles org 16 www ClassicCycles org 17 www ClassicCycles org 18 PART NO DESCRIPTION 1 C5 0410 B Screw Top Cover 2 CW2 0414 B Screw Top Cov2. org 8 9 JET NEEDLE E RING 1 Place the open end of the E ring against a hard surface 2 Cover the E ring area with your hand and press the needle down to snap the E ring off the needle Be sure that you apply pressure near the E ring to avoid bending the needle 4 Place the E ring in the desired groove 5 Place the E ring open end up against the hard surface 6 Again cover the E ring area with your hand and press down on the needle to snap the clip into place NOTE The procedures described below require that some gasoline be spilled There are obvious dangers when doing this MAIN JET 1 Turn the fuel petcock off 2 Place a container under the carburetor to catch the fuel in the carburetor and fuel line 3 Use an 11 16 or 17mm wrench to remove the drain plug 4 Remove the jet 5 Replace jet Tighten gently 6 Replace and tighten drain plug PILOT JET 1 Turn the fuel petcock off 2 Remove the drain plug and drain the float bowl 3 Remove the float bowl 4 Remove the pilot jet see exploded view for location 5 Install new jet Start the jet by hand Do not over tighten 6 Reassemble ACCELERATOR PUMP NOZZLE 1 Turn the fuel petcock off 2 Remove the drain plug and drain the float bowl 3 Remove float bowl 4 Use your finger to push the nozzle out of the throat of the carburetor see exploded view 5 Push in new nozzle O ring assembly Aim the nozzle toward the jet needle Note that t
3. some detonation during part throttle acceleration in the 2500 3000 rpm range although this may have other causes such as an ignition that advances too quickly A lean needle also results in an abnormally slow warm up If any of these conditions exist install a one size richer needle and compare the performance www ClassicCycles org 4 5 RICH CONDITION While a black sooty spark plug is a sure sign of richness others are more subtle If your engine responds crisply at low throttle openings when 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 The tapered portion of the jet needle rises out of the needle jet at about 1 4 throttle From that point until the main jet takes over near 3 4 throttle the jet needle taper is the main control of mixture strength LEAN CONDITION If acceleration seems soft or flat and the engine is slow to respond when the throttle is quickly opened from the 1 2 throttle position the mixture is too lean Raise
4. the throttle cables Modern cables are very reliable and generally do not need much attention However they do require lubrication and can be damaged if not carefully routed A light oil such as WD40 applied into the gap between the inner cable and its sheath does a good job of keeping the cables free moving and protected against binding and wear 5 Clean and lubricate the junction of the accelerator pump pushrod and carb body at the beginning of each riding season Although unlikely it is possible that dirt can cause binding and improper operation of the accelerator pump www ClassicCycles org 8 9 Figure T5 Float Level FLOAT LEVEL ADJUSTMENT 1 Hold the carburetor so that the float pin is horizontal as shown in Fig T5 above and the float is hanging away from the needle 2 Now tilt the carb until the float tab touches the spring loaded pin in the needle Do not depress the pin 3 The distance from the carb s bottom surface to the top of the float should be 18mm plus or minus two 4 Bend the actuator tab to adjust float level MIKUNI K amp N AIR FILTER K amp N air filters Mikuni Screamin Eagle and many others do not need frequent cleaning A cleaning interval of once a year or 5 000 miles is often enough However if you ride in very dusty conditions clean the filter whenever it is dirty 1 Tap the element to dislodge embedded dirt then gently brush with a soft bristle brush 2 Roll the f
5. 8 Hose Overflow 76 VM28 254 O Ring Drain Plug 77 TM32 41 Drain plug 42 45 77a TM32 41 1D Drain plug 48 78 C2 0412 B Screw Flt Bowl short 79 TM36 44 1A Rod A P 80 TM36 64 Boot A P Rod 81 TM36 60 Plunger A P 82 VM14SC13 89 Spring A P 83 N198 063 Rubber Cap Purge Port Alternate Parts Jet Needles HSR42 HSR45 48 J8 8DDY01 95 J8 8CFY02 95 Richer J8 8DDY01 96 J8 8CFY02 96 Richer J8 8DDY01 97 J8 8CFY02 97 Std J8 8DDY01 98 J8 8CFY02 98 Leaner Accelerator Pump Nozzles TM42 11 70 Std TM42 11 60 Leaner TM42 11 50 Leaner Needle Valve Assemblies 786 27002 1A 4 5 Std HSR48 786 27001 4 2 Std HSR42 45 786 27001 3 5 Smaller 786 27001 2 3 Pressure feed only Rebuild Kit HSR42 45 KHS 016 HSR48 KHS 031 NOTE 1 Lined through part num bers are not available 2 Parts in bold are included in rebuild kits www ClassicCycles org 18 19 www ClassicCycles org U5FB15E0 book Page 3 Tuesday May 11 2004 3 55 AM PARTS amp ACCESSORIES Click on links below Save Up to 45 on Harley Davidson Motorcycle Tires Shop at CruiserCustomizing com Shop Revzilla for the Latest in Motorcycle Accessories Harley Davidson Motorcycle Parts amp Accessories Vintage Harley Davidson Motorcycle Parts Free Shipping No Minimum JC Whitney Harley Davidson High Performance Exhaust Systems Save up to 75 on Clearance Bike Gear
6. HSR Tuning Manual Revised 02 10 03 www ClassicCycles org 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
7. ation is correct Harley s choke knob has the word CHOKE in white The Mikuni knob has a small brass bump in its center The Mikuni spring is longer and stiffer than the Harley part www ClassicCycles org 10 11 The Mikuni nut has a smaller hole where the cable fits and must not be used with the Harley choke cable The Harley nut is larger and is knurled on its outer diameter The Mikuni Starter Plunger and the Harley plunger are different and must not be interchanged While they are very similar and both slide into the HSR carburetor the Harley plunger does not work in the HSR The Harley plunger does not seal and causes a severe rich condition The Mikuni choke cable has an adjuster the Harley cable does not The Mikuni threaded section is steel while Harley s is plastic The Mi kuni cable has a hold open detent Harley s cable has a friction adjuster The Mikuni knob has a small brass center while Harley s has the word CHOKE in white The Harley cable end may not completely bottom in the socket formed in the metal elbow If the cable is not bottomed the starter plunger does not seal Poor mileage and a rich idle results The fix is to re route the cable so that it can bottom A simple wiggle may be enough to get the cable completely seated www ClassicCycles org 12 13 TROUBLE SHOOTING OVERFLOW Fuel runs from the tube on bottom of float bowl or from the vent f
8. be enough that small differences in performance can be detected Figure T4 ACCELERATOR PUMP The accelerator pump has two adjustments and one replaceable tuning part It injects a metered amount of fuel at a controlled rate into the engine when the throttle is opened from or near its closed position The size of the accelerator pump nozzle determines the fuel flow rate The two screws adjust when the pump action starts and when is stops Screw 1 in Figure T4 adjusts the starting position and screw 2 the stop position The total amount of fuel injected is determined by the positions of the two screws www ClassicCycles org 6 7 Screw 1 see Figure T4 on the throttle lever adjusts the starting point of the pump stroke To start the pump sooner smaller throttle opening back the screw out To start it later turn the screw in Screw 2 adjusts the pump s end point Best performance is generally achieved when the pump stroke ends between 2 3 and 3 4 throttle NOTE The accelerator pump nozzle size 50 60 or 70 determines the rate at which fuel is delivered to the throat of the carburetor A larger nozzle delivers fuel at a higher rate richer over a shorter time than a smaller one SETTINGS amp ADJUSTMENTS The standard nozzle size is 70 If the engine seems to run too rich when the throttle is first opened the nozzle may be too large and the fuel delivery rate too high In this case fit a smaller no
9. 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 www ClassicCycles org 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 increment
10. e assembly 2 Remove the spring and plunger from the stock cable 3 Remove the Mikuni Starter Nut from the HSR 4 Remove the Mikuni spring and plunger from the HSR 5 Install the Mikuni spring and plunger on the Harley choke cable 6 Install the Harley choke cable with the fitted Mikuni spring and plunger into the HSR carburetor DO NOT use the Mikuni Starter Nut discard it DO NOT use the Harley spring or plunger discard them Note If you do not have installation instructions you may download them from the www mikuni com website in the manuals section If the Mikuni Starter Nut is fitted to the Harley cable the choke plunger is held off its seat and the choke is open all the time If the Harley spring and plunger are used the plunger does not seal and the choke is open all the time The result in both cases is very poor fuel mileage 30 mpg or less Another possible cause of poor mileage rough idle and fouled spark plugs is incorrect cable routing The stock Harley choke cable is very stiff and may not be fully seated in the metal elbow at the carburetor end On occasion one of these errors may have been made by someone else at another location The unfor tunate mechanic who inherits the task of correcting the rich condition poor idle and poor fuel mileage may have no clue as to the mismatch of parts For this reason we include the following photos and text to help you discover if your install
11. em has two tunable components the Pilot Air Screw and the Pilot Jet The air screw s purpose is to fine tune the mixture at idle The pilot jet controls the total amount of fuel passing through the idle system The pilot jet can be exchanged for a richer or leaner one if needed see the note on the following page The pilot air screw is set at two turns open from the factory This is the position we have found to be correct most of the time If the screw position has been altered gently bottom it and re open it two full turns Next run the engine until it reaches normal running temperature With the engine idling smoothly adjust the pilot air screw in slowly until the idle either slows or becomes irregular then turn the screw out until the engine again slows or begins to idle irregularly NOTE NOTE www ClassicCycles org 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 NOTE 1 If the best idle is achieved with the air screw less than one turn out the pilot jet is too s
12. er 3 776 39005 Top Cover 42 45 3a HS42 081 Top Cover HSR48 4 TM42 04 Gasket Top Cover 5 BS32 126 E Ring Jet Needle 6 826 03002 Washer Jet Needle 7 J8 8DDY01 97 Jet Needle 42 7a J8 8CFY02 97 Jet Needle 45 48 8 TM42 03 Lever T V 42 45 8a TM42 08 1A Lever T V HSR48 9 B40I 56 E Ring Link Lever 10 B40I 10 Packing Link Lever 11 834 23041 Pin Link Lever 12 TM42 08 3 0 Throttle Valve Slide 13 739 13002 Screw Needle Retainer 14 TM42 16 Clip Needle Retainer 15 TM42 13 Sealing Ring T V 42 45 15a TM48 02 Sealing Ring T V 48 16 TM42 10 Seal Throttle Valve 17 925 98006 Pulley Cable Bracket 18 53974 E Ring Cable Bracket 19 TM42 51 Bracket Ass y Cable 19a TM42 53 Bracket Ass y Sportster 20 B3 0520 B Bolt Bracket 21 VM28 204 Spacer Bracket 22 TM42 38 Plate Lock Tab 23 C2 0514 B Screw 24 640 12001 Starter Nut Choke 25 VM14 241 Spring Starter Plunger 26 N189 192 Starter Plunger 27 TM42 06 Body HSR42 45 27a TM48 02 Body HSR48 28 616 94029 Seal Spigot Body 29 925 19011 Ring Steel 30 TM42 43 Lever A P 31 N138 019 Pin Throttle Lever 32 TM42 48 Lever Throttle 33 M12F 46 BB Spring A P 34 MC 0316 B Screw A P 35 TM42 47 Spring A P 36 TM42SS1 01 0 Mixing Body 42 45 36a TM48SS1 01 Mixing Body 48 37 B36 95 Packing Shaft Plastic 38 TM42 36 Ad
13. he nozzle orifice is located at 90 degrees to the screwdriver slot 6 Re install the float bowl 7 Turn the fuel petcock on and work the throttle several times to prime the accelerator pump 8 Use long nose pliers to adjust the fuel stream to the middle of the carburetor throat NOTE It is important that you use a tight fitting screwdriver Mikuni float bowl and top cover screws are slightly soft This is a deliberate choice We would rather strip screw heads than the threads of an expensive aluminum casting For this same reason we recommend against installing hardened Allen screws NOTES _____________________________ _____________________________ _____________________________ _____________________________ _____________________________ _____________________________ _____________________________ _____________________________ www ClassicCycles org 10 11 CHOKE CABLE INSTALLATION The HSR series carburetors use either of two choke cables the stock Harley Davidson cable 1990 and later or the Mikuni cable Mikuni designed the HSR to accept the Harley cable to save you money and because the stock cable works well The Mikuni cable is furnished for those installations that do not start with an installed stock CV carburetor The majority of HSR42 45 installa tions use the Harley cable Installa tion is simple when the instructions are followed step by step Those steps are 1 Remove the stock Harley choke cabl
14. ilter element in a large shallow pan of K amp N air filter cleaner Simple Green 409 etc with the depth set to 1 4 of a pleat Remove immediately and let sit for approximately 10 minutes 3 Rinse the element with low pressure water Flush from the inside of the filter so the dirt is washed out of the filter and not into it IMPORTANT Do not use gasoline or cleaning solvent to wash the filter as this will damage the material 4 When the filter is dry sparingly apply K amp N filter oil with one pass per pleat Wait 10 minutes and re oil any white spots IMPORTANT Air dry only do not use compressed air HOW TO REMOVE amp REPLACE WARNING Please do not disassemble the throttle lever linkage It is not normally necessary to remove the throttle shaft bolt to change position of the jet needle However if you do remove the bolt be certain that you 1 Apply blue Loctite to the threads 2 Torque the bolt to 18 in lb 3 Bend the tab washer against a flat on the bolt head JET NEEDLE 1 Unscrew the Idle Adjuster to bottom the throttle valve slide 2 Remove the top cover 3 Loosen the Allen screw on the slide 2 5mm wrench 4 Swing the needle retainer aside and remove the jet needle Save the plastic washer under the E ring To reassemble Reverse Steps 1 through 4 Be sure to replace the plastic washer and snug the Allen screw Be certain that the needle retainer is on top of the E Ring www ClassicCycles
15. itting on side of carburetor Possible causes 1 Foreign matter around fuel needle valve amp seat including paint flakes rust or bits of fuel hose 2 Factory one way tank vents can cause pressure build up 3 Deteriorated gasoline may cause the fuel needle valve to stick POOR FUEL ECONOMY Mikuni HSR carbs normally deliver fuel economy very close to that of a stock Harley Possible causes 1 The choke cable must be fully bottomed in the metal elbow at car buretor end There must be some perceptible free play in the choke knob 2 Current stock Harley tank vents do not allow gasses to leave the tank and pressure may develop from engine heat Open the vent to allow two way air flow 3 An incorrect jet needle or pilot jet for a particular tuning set up or altitude can reduce fuel mileage The speed range in which mileage is usually recorded is controlled by the jet needle and pilot jet 4 Loose pilot or main jets adversely affect fuel mileage There must be some free play in the choke cable to ensure that the choke plunger is bottoming sealing Any amount of free play is okay but there must be some H D Check free play by slightly loosening the friction nut Then slide the knob in and out to check free play Mikuni If there is no free play use the adjuster under the rubber boot near the carburetor Mikuni s cable adjuster is cov ered by a rubber boot There must be some free play in the cable An
16. justing Screw A P 39 B30 205 ORing A P Screw 40 TM40 89 Bolt 40a TM42 17 Plate Lock Tab for Shaft 41 BN38 43 Pin Return Lever 42 TM42 46 Lever Return 43 B30 1069 Adjusting Screw Throttle 44 N3 04 Nut Throttle Stop 45 TM42 19 Spring Throttle Return 46 700 15012 Shaft Throttle 47 TM42 15 Plate Fuel Joint Retainer 48 C2 0410 B Screw Fuel Joint 49 604 26014 Screw Pilot Air 50 N133 206 Spring Pilot Air 51 VM12 205 Washer Pilot Air 52 N133 037 ORing Pilot Air 53 TM40 27 Fuel Joint 54 KV 10 O Ring Fuel Joint 55 B30 398 Packing Idle Adjuster 56 VM22 138 Washer Idle Adjuster 57 730 09018 Spring Idle Adjuster 58 925 15001 Ring Idle Adjuster 59 TM42 32 Idle Adjuster Long 59a 990 605 065 Idle Adjuster Short 60 BS30 97 00 Air Jet Blank 61 784 430000 Y 6 Needle Jet 723 62 TM42 11 70 Nozzle Accel Pump 63 N124 063 O Ring A P 64 VM28 486 25 Pilot Jet Sportster 20 65 TM42 12 Extender Main Jet 42 45 65a TM42 12 1A Extender Main Jet 48 66 N100 604 160 Main Jet 45 175 48 190 67 616 33003 O Ring N V 68 VM13 216 Screw N V Retainer 69 786 27001 4 2 Needle Valve Ass y 42 45 69a 786 27002 1A Needle Valve Ass y 48 70 859 32027 Float Ass y 71 BV26 22 Pin Float 72 C2 0410 Screw Float Pin 73 616 94028 Packing Float Bowl 74 TM42 05 Float Chamber Body 75 N122 02
17. mall and should be exchanged for a larger one 2 If the engine speed does not slow after two and a half turns out the pilot jet is too large and should be exchanged for a smaller one MAIN SYSTEM NOTE Cam design effects When testing consider the rpm effects of any accessory cam you may have installed Long duration cams with late closing intake timing tend to perform poorly below some critical minimum rpm If you attempt to test below this rpm the engine may seem soft flat and unresponsive No carburetor can compensate for the engine being off the cam All jet needle and main jet testing should be done with the engine near the middle of its rpm range but high enough to be on the cam All testing should be done with the engine at normal operating temperature Figure T2 JET NEEDLE off idle 1 4 throttle see Fig T2 The straight diameter portion of the jet needle controls the mixture from just above idle to approximately 1 4 throttle If the mixture is too rich or too lean in this throttle range the needle will need to be exchanged for one with a larger or smaller diameter HSR jet needles are available in four sizes Only the diameter of the straight part of the four jet needles differ The richest is the 95 and 98 is leanest LEAN CONDITION If the needle is too lean part throttle acceleration will be flat remember the possible effects of cams amp exhausts There may also be
18. s 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 choke cable is more flexible and less likely to jam Still it is best to check to be sure the cable is installed correctly IDLE SYSTEM Idle 1 8 throttle The HSR idle syst
19. ssible causes 1 Backfiring when the throttle is closed especially noticeable from high rpm is not necessarily caused by lean mixtures However lean mixtures can contribute to its inten sity 2 High performance mufflers with large exit area or low restriction baffles contribute to exhaust backfir ing 3 An exhaust system air leak can cause or intensify exhaust pop ping Air entering at the junction of the header pipes and mufflers can cause excessive popping upon deceleration 4 Out of time ignition together with misfiring may lead to loud backfir ing Such backfiring usually occurs irregularly and at large throttle openings NOTES _____________________________ _____________________________ _____________________________ _____________________________ _____________________________ _____________________________ _____________________________ HSR Accessories These parts may be ordered through your local Mikuni dealer Pilot Jet VM28 486 Size Throttle range 0 1 4 Std size 25 Normal range 17 5 to 30 Increments of 2 5 Main Jet N100 604 Size Throttle range 3 4 full Std size 160 Normal range 150 to 210 Increments of 2 5 Jet Needle Throttle range 1 8 3 4 HSR42 HSR45 48 Richer J8 8DDY01 95 J8 8CFY02 95 Richer J8 8DDY01 96 J8 8CFY02 96 Std J8 8DDY01 97 J8 8CFY02 97 Leaner J8 8DDY01 98 J8 8CFY02 98 Standard HSR Jet Needle straight diameter portion is 2 97 mm This
20. the needle one notch and repeat the test RICH CONDITION If acceleration is crisp but the engine hesitates or staggers as the throttle is quickly closed from 3 4 to 1 2 throttle the mixture is too rich Lower the needle one notch and repeat the test The needle will be correct when acceleration is crisp at mid rpm yet the engine does not load up during throttle shut down NOTE See the How To section at the back of this manual for jet needle adjustment and removal procedures Figure T3 MAIN JET SELECTION 3 4 full throttle DYNAMOMETER METHOD Dynamometer testing is a quick and safe way to determine the cor rect main jet size The main jet that delivers the most power is the best one If two jets produce the same power use the smaller jet One possible issue with dyno test ing is temperature Most dynamom eters do not have enough cooling capacity to allow endless engine runs without overheating An over heated engine leads to an incorrect main jet selection However skilled operators can get the main jet sized properly with only few runs and tem perature should not be a common problem www ClassicCycles org 6 7 WARNING The following tuning methods require riding the motorcycle on the open road and the use of full throttle This can be dangerous and if you choose to use either to these techniques be aware of your surroundings and personal safety Roll off Method This method is a good way
21. to get the main jet either correct or within one size of correct It is based on the fact that that as the throttle is closed the air fuel mixture richens momen tarily This normal enrichening can be used as a diagnostic tool The test is started with the engine running at an rpm high enough to ensure that it is on the cam Open the throttle fully and let the engine pull for several seconds Then quickly close the throttle to about the 7 8ths position If the engine seems to gain power the main jet is too small lean Fit a larger jet If the engine hesitates as the throttle is rolled off the main jet is too large Fit a smaller one When the main jet is correct the engine will continue to run smoothly and evenly as the throttle is closed Note that a main jet that is far too rich or lean may cause the engine to misfire at full throttle Roll on Method This is the method used by tuners at race tracks and is generally the most accurate way to set the main jet size as it reflects the needs of the engine under actual riding condi tions It is also the most dangerous and we do not recommend it for that reason The test consists of making full throttle runs between two points or markers on the track The jet that gives the highest speed at the second marker is the best main jet The gear selction and speed must always be the same at the first marker when full throttle is applied The distance between the markers must
22. to start too late backfiring may occur due to an overly lean mixture just off idle POOR IDLE Irregular idle too rich or too lean Possible causes 1 Choke cable routing see POOR FUEL ECONOMY section 2 Air leaks A Harley pattern engine may have air leaks around the intake manifold seals Such leaks result in an irregular or unreliable idle that does not respond properly to air screw adjustment 2 The pilot air screw is not adjusted correctly resulting in a too rich or too lean idle mixture setting 3 A pilot jet that is too large or too small can cause the engine to load up or lean out until it stalls 4 A loose pilot jet will adversely affect idle performance too rich POOR PART THROTTLE PERFOR MANCE 1 8 1 4 An overly rich or lean condition at normal cruise speeds Detonation at part throttle Possible causes 1 Choke cable routing see POOR FUEL ECONOMY section 2 A too rich or too lean jet needle for engine tuning set up or for operat ing altitude Highly tuned engines often require leaner jet needles as do engines operating at altitude 3 Factory one way tank vents can cause pressure in the tank 4 Harley pattern engines may have air leaks around the intake mani fold seals An air leak can lead to a lean running condition and possible detonation at part throttle 5 A loose main jet can result in an overly rich mixture www ClassicCycles org 14 15 BACKFIRING IN EXHAUST Po
23. y amount will do but there must be some to ensure that the choke plunger is fully bottomed www ClassicCycles org 12 13 POOR FULL THROTTLE PERFOR MANCE Detonation flat accelera tion misfiring Possible causes 1 One way tank vents may fail to allow enough air into the tank to replace fuel flow at high throttle openings The result is a partial vacuum and fuel starvation 2 A clogged fuel filter or petcock screen can result in fuel starvation 3 A too large or too small main jet can lead to poor power output 4 A partially clogged main jet may allow enough fuel flow for part throttle operation and yet starve the engine at full throttle 5 Air filters can become dirty enough to restrict air flow and power output This is more likely to be a problem with the stock filter than aftermarket filters BACKFIRE OUT CARBURETOR Usually when the engine is cold Possible causes 1 When an engine is cold this can be considered normal A cold engine does not vaporize fuel com pletely which can result in an overly lean mixture and backfiring through the carburetor 2 Cams with early compared to stock intake valve opening angles encourage backfiring at low rpm 3 Some very open or incorrectly designed exhaust systems encour age backfiring Stock length and diameter header pipes together with slip ons like those from Harley Davidson typically do not have this problem 4 If the accelerator pump adjustment is set
24. zzle The 1 screw is normally adjusted to establish a gap of about 2mm 0 080 inches between the white plastic lever and pump rod end If there is a hesitation just off idle reduce the gap The 2 screw is normally adjusted to stop the pump action at about 3 4 throttle This setting should meet most requirements including acceleration from low rpm If you downshift before accelerating and or have a lighter bike then you might consider shortening the pump stroke NOTE For best results the accelerator pump nozzle should be pointed directly at the jet needle The nozzle is held in place by the friction of an O ring and can be turned easily with a of long nose plier MAINTENANCE There are few moving parts in the HSR series carburetors and they do not require frequent servicing However here are a few suggestions that if followed will assure good performance season after season 1 If the motorcycle is to be stored for more than a couple of weeks drain the float bowl 2 Periodically remove and clean the float bowl drain plug once each season should be adequate 3 If a jet or passage does become plugged use only carburetor cleaner and compressed air Drill bits will alter the size and tuning A small wire such as a single strand from a bicycle cable or motorcycle throttle cable can be used to clean pilot or main jets as it does not remove material from the jet and alter its size 4 Occasionally inspect and lubricate
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