IACV explained

[Ulvetanna_imp]

Re post 8, from post 5:

vacuum loss (minor = high idle, major = close to stalling/stalling)
TPS (FSM symptom/fail safe: hard to start cold)
IACV (FSM symptom/fail safe: engine stalls, hard to start, rough idle
EOT (FSM symptom/fail safe: poor acceleration)

DTC 29-1, FSM page 5-16, lists IACV circuit malfunction as "loose or poor contact of IACV connector, IACV or its circuit malfunction".

Otherwise, per the FSM, other sensors failsafe is normal engine operation, this includes MAP, IAT and O2. There is no mass flow sensor or exhaust temp sensor on the CB1100.

The FSM incudes all sensors on pages 5-15 and 5-16.

Symptoms reported are classic for IACV.

The piston not moving as much as the stepper steps would not likely provide a DTC (opinion) and this may be more likely than any other aspect of the IACV operation.

This would likely be from carbon build-up, perhaps induced by short trips, low speeds, and maybe even by high octane fuel (slower burn, less energy), and/or non tier one (highest level of cleaning additives) fuel, etc.

Dave's issue is not yet resolved, but if the IACV is replaced, with proper cleaning and wiring inspected, it is a highly likely fix...if the IACV piston travels freely within the port. When the IACV was replaced, without success, the throttle body port may have been an issue.

BTW, the IACV is not a simple solution, access is a pain...which is why the team has gone through the process of elimination.

For a complete failure of the sensor, fail safe...yes. But a sensor operating but providing faulty corrupted information, not properly calibrated: no. Any one of those sensors could be providing data that is slightly out of calibration, just enough to cause this "hunting" by the step motor. Very common in these kinds of systems. Unplug each sensor one by one and check the result to find out what happens. (CAUTION when test riding).

Has anyone checked the crank sensor that provides RPM information? That would be a place to look right away. If that was dirty, damaged, or displaced for any reason that could sure be a potential cause.

Also, while access may be a pain, it's still simple compared to a complete tear-apart for access to the top of the engine to check the valves. That's an all-day job. Whatever it takes to get it running correctly, it has to be done.

But the thing of it is, it's a $1600 part! That's 20 percent of the cost of the bike new in most cases, and a third the value of a used bike that runs properly.

I can really see how this would chap the hide of some owners.

If I had this problem, here is what I would do:

1. Obtain an ECU from your buddy's CB1100 which runs perfectly, and replace yours with the good one. If your bike runs fine, maybe that's it, and you could have your ECU reflashed with the current program.

2. Methodically test every sensor involved in combustion; if they do all "fail safe" then just unplug every single one, one at a time, and see if the bike runs better.

3. Remove and test the IACV. If bad...fight with Honda to have it replaced or at least a discount, or perhaps find another from a salvage yard. Or despair in having to cough up the dough.

See where that takes you. Just remember to always use CAUTION when test riding.