Replacing Intake Manifolds

Replacing Intake Manifolds

Given age and repeated heated cycles on top of the cylinder head, it is not uncommon for the rubber intake manifolds (a.k.a. boots) between the throttle bodies and cylinder head to develop cracks and leak air. This results into poor running and makes it impossible to properly tune a K bike.

A common symptom is that the bike runs roughly at idle but seems to run fine at higher RPMs. This is because at idle the air is leaking into the mixture but as the RPMs increase the vacuum below the throttle bodies increases and temporarily seals these air leaks.

Diagnosis:

The easiest way to diagnose leaking intake manifolds is by visual inspection using a flashlight. However, they may be leaking in the back or the cracks may be so small that they are not visible.

Method 1: Start the bike, let it idle and spray some starter fluid around the intake manifolds. If the RPMs increase then it is obvious there are leaks.

Method 2: Start the bike, let it idle and spray some water mist around the intake manifolds. If the RPMs decrease then it is obvious there are leaks.

(I've only used Method 1 but I've read of others using Method 2.)

Replacement:

If you have a leaking intake manifold or several then it makes sense to replace all of them with new ones. Also, if you've decided to renew an old bike that's got high miles then you might want to consider doing this as preventative maintenance.

Click here to jump to replacement instructions for a K100RS4V, K1 or K1100.

K75 or K100:

Intake manifold (1) - part 11611460408

Though it may or may not be necessary, you may want to also consider replacing the boots above the throttle bodies (4) as well - part 11611460755. From the factory these are attached with one-use 48.5 mm (lower) and 50 mm (upper) Oetiker clamps. I usually replace these with 5/16" stainless steel hose clamps to simplify future maintenance and eliminate the need for an Oetiker clamping tool.

1) Disconnect the front fuel line from the fuel tank and both fuel lines from the fuel rail.

2) Loosen the large hose clamp at the back (right front on the bike) of the air box above the throttle bodies. (a.k.a. plenum)

3) Remove the lower Oetiker clamps. (I cut them off with a Dremel cutting wheel.)

4) Disconnect the crank case breather hose at the rear of the plenum.

5) Pull the plenum towards you to remove it.

6) Pick the wire clip off of the throttle position sensor connector and disconnect it. (Light blue circle above.) Put the wire clip back on the connector so it doesn't get lost. When you reinstall this you don't need to take the wire clip off as it will slip over the tangs on the sides of the TPS's connector to lock the connector in place.

7) Disconnect choke cable. First undo its locknut and then unscrew it all of the way in order to move it to the side to get enough slack in it to disconnect it.

8) 1990 and earlier: follow the wires for the choke switch to its connector and disconnect it.

9) Remove the screw type hose clamps the hold the throttle bodies in the intake manifolds. Note their positions as you'll want to reinstall them in roughly the same place in order to keep them from interfering with throttle movement.

10) Remove the throttle body assembly.

11) Each intake manifold is held on by two machine screws. Remove those and then remove the intake manifolds.

12) Clean their mounting surfaces and install the new intake manifolds. (9 Nm - 80 in-lbs)

13) Installation of the remaining parts is the reverse of removal.

14) Synch the throttle bodies - click here

Click here for a one page printable copy of these instructions.

K100RS4V, K1 or K1100:

Intake manifold (1) - part 11611461621

O-ring (3) - part 11611465169

Manifold to TB boots (5) - part 11611461739

You should also replace the boots that go in between the intake manifolds and the throttle bodies too. From the factory these are attached to the bottoms of the throttle bodies with one-use 48.5 mm Oetiker clamps. I usually replace these with 5/16" stainless steel hose clamps to simplify future maintenance and eliminate the need for an Oetiker clamping tool.

1) Remove the two lines from the fuel rail.

2) Remove the two bolts holding the fuel rail to the cylinder head and pull the fuel rail and fuel injectors out of the intake manifolds.

3) Lift up the throttle bodies and remove the Oetiker clamps that hold them to the bottoms of the throttle bodies (I use a Dremel cutting wheel for this) and remove the boots.

4) Undo the two machine screws on each of the intake manifolds.

5) Clean the mounting surface for each intake manifold and install the new intake manifolds and O-rings. (9 Nm - 80 in-lbs)

6) Install the new boots to the bottoms of the throttle bodies. If using 5/16" hose camps instead of Oetiker clamps then be sure they are rotated so as not to interfere with throttle movement.

7) Install the throttle bodies on the intake manifolds.

8) Put a light coating of engine oil on the O-rings near the tips of the fuel injectors. Reinstall the fuel rail and fuel injectors into the intake manifolds.

9) Re-attach the fuel rail to the cylinder head. (7 Nm - 62 in-lbs)

10) Re-attach the fuel lines.

11) Synch the throttle bodies - click here