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
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.
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
4.Carburetor hits tank or upper motor control arm
There are several
possible reasons for this:
||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.
||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
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.