The KLR650 AirBox mod combined with jetting is the easiest and cheapest way to add power to the trusty old 650 engine. The factory air box does what it is designed to do but it does not do it efficiently and is therefore a major restriction to the performance of the engine. The design cause uneven and very turbulent airflow. Air, like water, does not like to change direction and prefer the shortest and most unobstructed path possible. A fully modified KLR650 air box cannot support air flow beyond 282 – 285 CFM thus preventing  100% volumetric efficiency.

A flow bench at 28 inches was used to check which steps in the KLR650 AirBox mod will yield the best results . It is true that a flow bench will not tell you what the actual performance results would be but it sure will give you an indication of what to expect.

A 650 sleeve was fitted on the flow bench a bone stock cylinder head was attached to it. The carburetor and an unmolested KLR650 airbox with a clean factory filter was attached to the cylinder head.

KLR650 AirBox Mod

Click on image to see larger version.

Maximum intake valve lift is 8.79mm or 0.346″. The flow bench was setup to test in increments of 0.050″ up to 0.400′. This was deliberately done to check what flow would look like if eventually a cam with more lift would be installed. For the KLR650 airbox Mod flow test, the industry standard of 28″ of water was used. Prevailing temperatures during the testing ranged from 76 – 79 degrees. The throttle butterfly was held fully open during the entire test. The vacuum slide will open with airflow generated by the flow bench. Full opening of the vacuum slide occurred right at 26 CFM of air flow. A Unifilter was also tested later in the setup.

The following abbreviation is used for the chart below:

A minus sign indicate something was remove and a plus sign means something was added. Therefore C-S+L means that the snorkel was removed and the L Shape mod was performed.

C = Cylinder Head + Carburetor  + Complete air box with clean factory air filter
CH =Cylinder Head Only
S = Snorkel
L = L Shape Mod
D = Filter Door
1/3D = Third bottom of door is closed up
Scrn = Anti Flash Back Screen
U = Unifilter
RT = Ram Tube
CB= Carburetor Boot

Lift C C-S C-S+L C-S+L-D C-D C-S-D+U C +1/3D-S-Scrn+U CH +Carb+RT CH+CB Only
.050 60.8  60.7 62.2 61.6 60.7 62.5 64.6 65.9 63.7
.100 116.3  122 124.9 125 123 125.8 126.1 124.0 129.5
.150 159.7 172.3 179.6 182.3 182 183.8 185.8 178.4 192.8
.200 188.4 206.1 219.3 222.4 219.8 226.1 229.1 222.2 248.3
.250 206.8 229.7 247.5 251.8 248.6 256.6 261.9 254.3 288.1
.300 216.9 242 262.7 267.4 264.0 272.7 278.6 279.5 313.4
.350 220.3 245.3 267.4 273.4 268.4 277.5 283 296.2 323.0
.400 220.8 245 266 271.4 268.3 277.5 282.1 299.9 312.5

Note: A airflow stayed fairly constant or started to dropped of after 0.350″ valve lift during the KLR650 airbox mod testing. The cause of the drop after 0.350 can be discussed later.

A pitot tube was used to test the airspeed at various locations inside the air box . The area between the air filter and the filter door had substantial  lower air speeds when the filter door was in place. This worsened towards the very bottom of the filter and the filter door. Removing the air filter door obviously solved this. (Click to see where dead spots are)

Airspeed and flow through the filter door started to diminish after the L-shape mod, the snorkel and one third of the bottom of the air box door was closed off.  (Click to see pic). The section of the uncovered or modified airbox door that is now exposed is covered by the side panel. This is the perfect workaround for possible crosswind interference when and if the filter door is removed. Close the bottom third up. If you don’t want to cut the air box door/lid, Duct tape does an excellent job.

Which KLR650 AirBox mod is the best?

The way you use your KLR will determine which modification you should perform.

If you ride the bike mostly on pavement and moderate dirt roads you might opt to  install a Unifilter, remove the air box door/lid and close 1/3 of the air box opening with duct tape or modify the air box door to look like this (click here). If you are going to traverse areas consisting of deep waters the preferred way to go would be to  remove the snorkel, performing the “L Shape Mod”, remove the Anti Flash Back Screen and install Unifilter.

Last but most important.  Always make sure that your bike is correctly jetted and use a well maintained air filter.

KLR650 CV carburetor vs Performance:

The KLR650’s 40mm KeiHin CV carburetor is very reliable and is more than sufficient to cater to most of the the KLR’s performance requirements. The carburetor by itself can support 327 CFM. It is the restrictiveness of air box that prevents full performance support.

The KLR in standard or in mild modified form does not benefit with performance carburetors such as the 40 mm Mikuni TM40-6 smooth bore or the KeiHin FCR. It is true that these carburetors can support a higher flow rate but since the the KLR cylinder head can only support 327 the higher flow performance carbs will have no additional advantage over the 40 mm CV carb.

Some feel that the accelerator pumps on theses carburetors will increase throttle response. We feel that the 40 mm CV carburetor with supporting modifications and correctly setup will be just as responsive.

Remove the KLR650 airbox from the equation and the it will benefit from these performance carburetors.