Here it is - but check the threads....
I found this blog online which seems to detail some similar info for future reference for people doing searches:
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DEC 18, 2011
Rough Running Rotax 912/914 Engine
When we installed the new Rotax 912 in our plane 16 years and 1,400+ hours ago, it exhibited poor idle when started, uneven running up to 3,000 rpm, and worse yet, would really shake in flight when the throttle was pulled back to idle. The latter problem was really disconcerting when power was reduced for landing.
When flying on 100LL avgas on long-distance trips, the engine would quickly show the effect of lead-fouled spark plugs and run rough during start-up and taxiing.
We tried all the suggested cures, such as balancing carburetors, adjusting mixture, replacing spark plugs, cleaning carburetors. These remedies improved the problems somewhat, but performance never was really satisfactory.
Eventually, we assumed that this was just part of the Rotax performance. That is, until a recent annual inspection turned up the real culprit.
For the discussion, we need to look at the ignition system.
Each ignition cable of the Rotax engine is connected to the spark plug with NGK connector VD05FMH. The connector housing is made of rubber. Inside the connector is a threaded pin.
The ignition cable is pushed into the connector until it touches the pin, then the connector is twisted so that the connecting pin is threaded into the copper strands of the cable. The process stops when the cable runs against the bottom of the connector.
Once the cable has been installed into the connector, a tie wrap is pulled into the groove of the neck of the connector and tightened. The tie wrap seals the connector around the cable and thus prevents moisture from getting into the connector.
The rubber of a new connector is quite soft and pliable, and the opening where the ignition cable is to be inserted is undersized relative to the diameter of the cable.
When a new cable is pushed into a new connector and then the connector is twisted to thread the cable onto the pin, the dry surfaces of cable and connector induce quite some friction. Indeed, the friction may become so large that it may seem that the cable has bottomed out in the connector when it has barely touched the pin.
When this happens, the cable does not properly connect with the pin and fires with a weakened spark or not at all. In the latter case, a significant rpm drop during the ignition check points out the problem. But in the case of a weakened spark, there may be enough spark that the problem is not detected by the ignition check.
Exactly this installation mistake was made at the factory on my engine. The connector of the spark plug for the top #4 cylinder was never threaded onto the pin. After 16 years and 1,400+ hours, the cable suddenly pulled out of the connector during the recent annual inspection when we removed the connector from the spark plug for the compression test. That's how we became aware of the faulty installation. Throughout the years, the ignition check never showed a significant rpm drop.
It is easy to avoid this problem. When installing a connector, measure the depth of the hole of the connector into which the cable is installed. This is done best with a thin wire, such as safety wire, so that the depth to the bottom of the hole and not just up to the threaded pin is measured.
Mark that depth on the cable with masking tape. Now we know that the cable must be installed up to the masking tape mark.
For easier installation, apply a very, very thin coat of non-conducting silicon grease to the cable, and then push the cable into the connector and thread the pin onto the cable.
Due to the coat of silicon grease, this can be done with reasonable effort, and twisting of the connector will become impossible exactly when the cable hits the bottom of the hole and the masking tape mark is reached.
Finally, add the tie wrap to tighten the connector around the cable.
If you have a Rotax 912/914 engine where the above symptoms of roughness occur and all other remedies have been tried, you may want to test proper installation of each connector by placing masking tape on the cable next to the connector, cutting the tie wrap, unscrewing the connector from the cable, and comparing the masking tape mark with the depth of the hole.
If the installation had been done correctly, reinstall the connector using the above procedure.
If you find a faulty installation where a connector was not sufficiently twisted onto the cable, repair this as follows. Cut off about 1/2'' of the cable so that you see pristine copper strands, and install a new connector. Do NOT try to use the old connector since the pin likely is damaged due to arcing.
When we did this repair for our Rotax 912 engine, the engine had smooth idle immediately after cold start, had more power on takeoff, and ran smoothly when power was reduced to idle in the air.
It felt like a completely different engine. No wonder; for one of the cylinders, just one spark plug had been firing properly. Since that plug was at the bottom, it fouled easily with 100LL avgas use, producing a rough running engine with avgas use.
Of course, we also checked the remaining connectors for proper installation. They all turned out be correctly installed.
Posted 18th December 2011 by Klaus Truemper
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