Look at the throttle stop before you pull the carbs. A 1976 Honda CB500T that fires and runs straight to 8,000 rpm has its throttle plates open. On this bike the stop screw was sitting off the fixed stop, held there by the cable. The intake O-rings were the first guess, and they were the wrong one.
The bike had been idling quietly for several minutes, shut off, started again for people standing around it, and then did this on the next start. Kill it. Do not keep cranking it to 8,000 while you look.
The cables and the idle circuit are separate problems. Once this arm was back on the stop, a different symptom showed up, and that one is the enrichener guide.
What 8,000 rpm was, and what it was not
| Check | On this bike | What it means |
|---|---|---|
| Grip released, stop screw versus the fixed stop | A gap. The screw was off the stop | The plates are open before the grip moves |
| Cable adjusters backed off | The arm snapped onto the stop by itself | The cable had been holding it open |
| Lever marked CLOSE | Left alone | That is the enrichener. It does not hold this arm up |
| Carb O-rings, the first plan | Not the fault | A small leak cannot make 8,000 rpm |
The arm and the stop
These are Keihin CV carbs, 32 mm, one per cylinder, linked so one arm opens both butterflies. The stop is a fixed pin. The screw you can see passes through the moving arm. With the grip released, that screw should be touching the pin. A gap means the plates are already open, and the starter is lighting an engine at full song.
The cable's only job is to pull the arm off that pin when the grip turns. The spring under the arm is what closes it. If the spring is not winning, the screw never gets back to the pin.
The usual reason, after the bars have been turned and the bike has been sat on, is the cable housing. If the housing has popped out of its socket, the inner wire gets shorter and holds the arm open. The barrel in the arm can do the same thing if it is out of its pocket.
Slack it, then put a little play back
Engine off.
- Back the adjusters off at the twist grip until the inner cable is slack. If there is a splitter under the tank, slack that one too. The cable should be doing nothing.
- Push the arm toward the stop and let go. It has to snap against the pin on its own. If it sits wherever you leave it, the return spring is off the arm or hooked on the wrong side. One end belongs on the arm, the other on the post on the carb, pulling the screw onto the pin.
- Seat the cable barrel in the pocket on the arm, and seat the housing in its socket.
- Tighten the adjusters only until the grip has a few millimeters of free play. Twist and release. The screw has to hit the pin every time, before the grip reaches its own stop. If the arm lifts while the grip is still sitting still, the cable is too tight. Back it off.
The other carb is linked to this one. One arm off its stop opens both.
The lever marked CLOSE, on the right in this linkage, is the enrichener. Leave it. It adds fuel for a cold start. It was not the piece holding the throttle open.
If the screw is already on the stop
Then the plates are shut and 8,000 rpm is air getting in somewhere else. That is a carb that is not fully in the manifold, a clamp that is snug on the rubber and loose on the joint, or a manifold-to-head leak. A vacuum hose pulled off a petcock does it too. A small leak is a few hundred rpm, not a runaway. Do not spray for leaks while the engine is screaming. Hold the throttle shut by hand only long enough to confirm it will idle, then shut it down.
Putting the carbs back is a torque job on the clamps and the manifold nuts, with the wrench that starts at 20 ft-lb only where the manual spec is inside that range. Cover screws under 20 ft-lb are outside it. A stuck cross-point screw on the way in is the JIS impact driver, not a Phillips bit.
On this bike none of that was required. The cable was slacked, the spring put the screw back on the stop, and the 8,000 rpm start was gone.