Result of Bad TPS?

3 replies to this topic
  • angrybob

Posted April 27, 2008 - 09:28 AM


I'm curious of what that would be. I checked resistance and voltage with the following:

resistance (on the bench) was at WOT was 3300; spec says 4000-6000 ohms.

I couldn't get a voltage when installed with the bike running at 1900 rpm idle; spec says to set between .5X and .7x volts.

Anyway, the bike runs fine throughout the rpm range with nothing unusual. Of course it pops a little when I close the throttle, but notihing beyond normal.

Is there a default timing from the CDI if the TPS goes bad? If it is bad, and I'm running fine, am I just missing out on optimum perfomance through advanced timing at higher rpm?

  • grayracer513

Posted April 27, 2008 - 03:22 PM


The TPS provides information to the CDI as to the load state of the engine. The CDI maps the advance curve based first on RPM, which it determines by watching the signal from the trigger coil, and second on the load state.

The engine gets its maximum advance at high speeds and low loads, as when ripping along a fire road at 7000 at 1/3 throttle. The extra advance is necessary to get peak efficiency from the very thin (not dense) air fuel charge and low dynamic compression that are normal at high manifold vacuum levels. It is also made possible by this, since it's far less likely to detonate under such conditions.

At WOT, the advance is at its minimum for any given engine speed. If you disconnect the TPS, the CDI sees this as resistance in excess of the maximum range, and assumes it is full throttle. That's safe, since the timing is more retarded that way than if resistance were low.

In your case, that's where you could possibly run into problems. With the WOT resistance being lower than specified, your timing may be too far advanced at heavy throttle openings, and detonation could occur at low RPM under a big load.

Re-examine the test procedures and be certain of your results and your methods. Use only a digital VOM for the tests.

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  • angrybob

Posted April 27, 2008 - 04:12 PM


Thanks for the info. It was a PITA to pin out the connector with one set of hands and probes only. I just remembered that I have some loose female pins that will fit on the male pins in the connector. I'll make a little harness that I can hook alligator clips to. I am sure that was the problem.

The CDI input voltage was fine at 5.3 volts.

Will I be able to hear the detonation if it occurs?

  • grayracer513

Posted April 27, 2008 - 04:18 PM


When it gets bad enough to be an issue, you probably can, if you can pick it out from the chain slap, etc.

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