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This web page's purpose is to compliment the printed and online installation manuals for the HSR series carburetors. Information that may be included in future versions of our manuals will be placed here so that our customers can have access to the latest and most complete information regarding the installation of the HSR carburetors.

As we become aware of any installation difficulties some of you may have, we will post answers or work-arounds on this page. We shall update this page as often as we discover and find solutions for any field installation difficulties that may arise.

Possible issues:

1. Choke Cable installation issues  (starts new page) (print version, pdf)
2. Carb hits cylinder fins on Twin Cam
3. Carb pushes seal into intake tract
4. carb hits tank or upper motor control arm
5. carb alignment (rotation in the manifold)

2.Carburetor hits cylinder fins on Twin Cam

About half of Mikuni/Twin Cam installations have an interference between the cylinder fins and the float bowl of the HSR carburetor. A small portion of the cylinder fin material must be removed from both front and rear cylinders to clear the float bowl. Failure to make this modification can result in alignment and wear problems.

A small file can be used to flatten the corner of the fins enough to clear the carburetor's float bowl. The modification is not visible with the carburetor mounted.

3.Carburetor pushes seal into intake tract

The carburetor spigot is normally a very tight fit in the intake manifold seal. The seal and carb spigot must be lubricated before installation. Soap, oil, grease or similar lubricants have all been used successfully.

Occasionally, the carburetor spigot may dig in and catch the rubber seal during installation. This, in turn, may cause a portion of the seal to be extruded into the throat of the manifold. That small flap of rubber can interfere with tuning and cause a loss of several horsepower.

Be sure to fully open the throttle after installation and check the bore of the carburetor-manifold to be sure it is clear of seal material. Should the carburetor spigot catch the seal upon insertion, check the corner of the spigot and round any sharp corners. Use a fine toothed file to break any sharp corners. The spigot mounts more easily if its leading edge is smooth and slightly rounded.

4.Carburetor hits tank or upper motor control arm

There are several possible reasons for this:

1: The manifold is out of position and is pointing "up" toward the tank. This is the most common cause of interference between the Mikuni HSR and the cycle's fuel tank. One clue that this is the case is that the air cleaner bolt holes do not line up with the bolt holes in the heads. Since many air cleaners have some slack in their installation, this is not always a definitive way to check manifold alignment. An angle gauge such as those made for home use can be used to check the alignment of the manifold's mouth.

Use the gauge to compare the angle across the mouth of the intake manifold with the front surface of the points cover or with the seating surfaces of the head breather bolts. The mouth of the carburetor should be parallel with the any of these surfaces.

If the mouth of the manifold is not parallel, loosen the manifold bolts and rotate it until it is.

NOTE: It is a good idea to lubricate the manifold seals as you do this. There is a tendency towards air leaks if the manifold is rotated in the manifold/head seals without some lubrication. WD-40 is convenient and effective. Always check for air leaks after moving the manifold.
2:  Some FXR/FLHT frames have the upper lateral control arm mount (this is the folded sheet metal arm that is welded to the frame's backbone just above the carburetor) located slightly low. This can result in interference between the lateral link's mounting bolt and the top of the carburetor. Occasionally, the arm can be so low that it directly hits the top of the HSR Mikuni.

A shorter bolt (Harley fits either of two bolt lengths) or a "button" head bolt mounted with the head down often cures the clearance problem. If it is the arm itself that strikes the top of the carburetor, you may have to consider modifying the arm.

5.Carburetor alignment (rotating in the manifold) 

Mikuni HSR carburetors are located by friction between the intake manifold seal and the body of the carburetor. This is a proven technique and is reliable due to the very tight fit between the carb spigot and the manifold seal. Some aftermarket manifold seals do not create as much friction and allow the carburetor to turn in the manifold.

The otherwise excellent James Gaskets seal, for instance, should not be used with the Mikuni HSR carburetors since it does not grip the carburetor tightly enough. Ironically, the advantage of the James seal, its ease of installation, works against it for use with our carburetors.

The very tight fit of the stock seal requires that some sort of lubricant be used when installing the Mikuni HSR. Many different lubes have been used, including: engine oil, bearing grease, WD-40, liquid soap. Any of these will do.

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