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mike martinez

4700 rpm misfire

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I have a turbo subaru motor in a sandcar with an older link unit. At 4700 rpm the motor begins to misfire. It sounds like it's hitting the rev limiter. The rev limiter is set at 6800 rpm. I set the programmer to the test rpm page then brought the motor up to 4700 rpm and noticed the readings started to jump around. With this info, the problem seems to be more ignition related rather than fuel. I have tried 3 different computers, a new coil, new spark plugs (gapped to .024"), new resitor spark plug wires, new timming belt, new cam sensor, checked all grounds, tried 3 different ignitors and I still get the same problem. It runs smooth up to this rpm level. It almost seems as if there is some sort of feed back or interfence signal that is corrupting the low voltage signals. Is it possibe the suppressor on the coil power supply might have gone bad causing voltage spikes? What else can I look at to determine were the problem might be? Thank, mike martinez

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If you have unstable RPM readings then you must have engine position sensor signal problems.  If interference is getting onto the trigger signals then the ECU will get funny RPM values and behave unpredictably.  It is important that these sensors are wired correctly, with the correct polartiy and wired using properly terminated screened cable.  Also, grounding problems can cause this kind of fault.  Interference problems are common in space-frame cehicles that ahve no steel firewall or full body.  Make sure you trigger sensors are wired correctly and that all sensitive wiring is routed away from noisy wiring such as ignition and injection wires.

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I have a turbo subaru motor in a sandcar with an older link unit. At 4700 rpm the motor begins to misfire. It sounds like it's hitting the rev limiter. The rev limiter is set at 6800 rpm. I set the programmer to the test rpm page then brought the motor up to 4700 rpm and noticed the readings started to jump around. With this info, the problem seems to be more ignition related rather than fuel. I have tried 3 different computers, a new coil, new spark plugs (gapped to .024"), new resitor spark plug wires, new timming belt, new cam sensor, checked all grounds, tried 3 different ignitors and I still get the same problem. It runs smooth up to this rpm level. It almost seems as if there is some sort of feed back or interfence signal that is corrupting the low voltage signals. Is it possibe the suppressor on the coil power supply might have gone bad causing voltage spikes? What else can I look at to determine were the problem might be?

> <br>

>Thank, mike martinez <blockquote>

I had similar problem this summer on my 6-cyl engine...but when i changed in "dwell" between risin and falling...it solved my problem, i dont know if u can adjust this...just an idea

:)

/Jonas

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Be super careful swapping between rising and falling dwell.  A wrong setting can easily burn out coils or igniters.  Use rising dwell if your igniter expects a high signal to start charging (dwell) and a change to a low (gnd) signal to fire the spark.  Use falling dwell if the igniter signal is low (gnd) to charge the coil. 

Nearly all igniters use rising dwell edge.  Only some ford, honda and CDI units use falling.

Sometimes changing the dwell edge can fix misfires as it modifies the timing offset in relation to the distributor rotor position.  That should actually be fixed by modifying the pickup location in the distributor.  Of course that only applies to distributed engines.

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