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Tech: Mikuni Carburetor - Secondary Throttle Valve Fix

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Author: Richard Solomon, May 9, 2000. Editor: Philip Hansford

 

Background

 

2-6engine.jpg
The actuator is a UFO-shaped device on the back of the carb

The 4 cylinder 2.6L Mitsubishi engine (as seen on the Montero/Raider) is fed by a Mikuni carburetor. The Mikuni carburetor is a two-barrel design, which has a mechanically operated primary throttle and a vacuum actuated secondary throttle. When you press on the gas pedal, you are only opening the primary throttle valve. Vacuum formed in the primary throttle body by air passing through operates on a diaphragm actuator (called the "depression chamber" by Mitsubishi) which opens the secondary throttle in proportion to how much vacuum is formed, thus how widely the primary is opened. In many older 2.6L engines, the actuator for the vacuum secondary loses integrity and will no longer hold a vacuum. This limits or prevents the opening of the secondary throttle, and your truck ends up with a one-barrel carburetor!

The primary throttle is fairly easily found and accessed on the side of the carburetor. The vacuum actuator is a little harder. (The following is from my ’87 and I understand that ‘87-’89 are interchangeable, while ’86 and earlier are slightly different in size and appearance.) The actuator is a rounded, sort of UFO-shaped device with a coiled spring protruding from the bottom, and a vacuum hose/fitting on the top. It’s located around the back of the carburetor, and the spring should be connected close to the primary throttle.



 

Tools

Vacuum Pump & Gauge - Optional, but highly recommended.

Phillips screwdriver.

Clean container to hold ~1gal of antifreeze.

Friend/assistant - Optional, but recommended.

 

Do I Have a Problem?

The primary symptom of a failed secondary throttle actuator is loss of power, especially at wide open throttle (WOT). If your Montero/Raider has trouble with hills, passing Yugos or other strenuous full throttle activities, check your secondary!

 

Okay, I Have a Problem...Is It My Secondary Throttle Actuator?

With the engine OFF you should be able to easily open the primary throttle by hand. With the primary throttle held open, you should be able to easily open the secondary as well. Note that there is a mechanical interlock which should prevent the secondary from opening when the primary is not open. If you cannot manually operate the secondary then you may have a problem beyond the scope of this article. If your secondary opens easily, remove the vacuum hose and attach your vacuum pump/gauge to the actuator fitting. A "good" actuator will hold vacuum, while a "bad" actuator will not. My actuator was so far gone that I couldn’t even draw a vacuum, let alone hold it!

 

Okay, my secondary throttle actuator is bad, now what? (The Procedure)
actuator.jpg
This is the elusive Secondary

First, acquire a replacement. The Mitsubishi dealer CAN order this part separate from the carburetor, although they probably have no idea what the heck you're talking about at first. The key words are "depression chamber". This is what the factory shop manual calls the device that I call the secondary throttle vacuum actuator. Once I spoke those magic words, my dealer said "Son of a #$@!&, here it is..." while looking at his parts diagram for the carburetor area. FWIW, the Mitsubishi part # for my '87 is MD612713, which was *ONLY* $96.29 special order from Japan, then tossed in a bottle into the ocean with my name on it… I politely declined! I believe that this part is interchangeable between 1987-1989 models. It definitely appears that my 1987 is compatible with a 1988. I called around various junkers and found a fella with an '88 2.6L who'd look for the part for me. We found it right on the back of the carburetor (duh!) :) and a quick check with my vacuum pump showed it holding! (Also, movement of the spring was clearly visible - maybe .5" to 1" of travel or so?) Despite a long lecture from the junker on how you "can't buy this separate from the carburetor, which is $600" I escaped for only $25. The junker had to use an impact driver to loosen the two screws holding the actuator to the carburetor…remember this later!

Second, remove your old actuator. When I tried to pull my actuator, I too was unable to loosen the screws with the carburetor attached to the engine. Since the Mikuni is warmed by engine coolant, attempting to remove it will normally result in antifreeze going EVERYWHERE. I drained about a gallon of antifreeze out of the radiator, and was able to remove the carburetor without pouring antifreeze into the intake manifold. (Note: my truck was on a reasonably steep driveway facing nose-down, so I can't swear 1gal is enough on level.) For peace of mind, I also stuffed a clean rag into the opening left by the carburetor, so I was 100% sure no loose pieces/gunk ended up in the engine! With one person holding the carburetor, and one wielding a plain Phillips screwdriver, a friend and I were able to remove the actuator without incident. The end of the actuator is a coiled spring which attaches to the secondary throttle arm with a TINY C-clip - DON'T LOSE THIS CLIP!!!!

Finally, attach the new actuator. Simply hook the coiled spring back over the secondary throttle arm and reattach the C-clip that you hopefully didn’t lose in step 2. If you had to pull the carburetor off in step 2, reattach same and dump your antifreeze back into the radiator! Fire up your truck and enjoy 2-barrel power once again.

 

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