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Surging, Hunting Idle

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  • Surging, Hunting Idle

    This might be a long shot but... Short story is I'm curious why my idle would be hunting. At idle the rpms go up and go down, up and down, up and down. It's not a massive range, but I expect and recall last year it was more steady. I'm wondering if I've got a bad sensor or the PCIII is on the way out?

    The long, long story...
    This is a modified SV650 with a sprint air filter, bored throttle bodies, cam swap, lightened/treated crank, wiseco pistons, high comp gaskets, PCIII. I can provide more details as needed.

    I had worn gears was popping out on hard third gear acceleration and had the motor apart over the winter. Put the motor back together with new transmission, gaskets, rings, repacked the can and put a baffle in the end of the pipe to stay under noise limits. Notice the surging right off the bat while heat cycling. Figured, meh, I'm over thinking it. Practice day, several sessions and laps in, the bike cut out. We thought it was the iap sensor (black sensor mounted to the air box with the vacuum line from the front intake) so without another available disconnected it. Next day, first race did 7 laps and it cut out again. Both times mid-corner. eek. No cough or sputter... just off. Re-started the engine fine both times. Tip over sensor is mounted up btw.

    My mechanic did some consultation and fiddling and figured the rear cylinder timing (?) was off, in that the cam chain was advanced one tooth to much (?). It was mentioned that he had some concern about this when he put it together the first time. We retarded it one tooth, removed the baffle from the end of the pipe and replaced the iap sensor with another used one. This all seemed to help, because prior to this the bike was now a complete mess and misfiring at idle. Did two dyno runs and three races the next day and the bike seemed to run well and didn't die which is awesome. The dyno runs didn't show any significant issues and the engine made just under 78HP. I believe it made maybe a couple more last year before the engine work but that could just be variance in dyno, weather?

    But still the idle still seems to surge and hunt.

    I've had the air box and throttle bodies off/on about 1/2 dozen times now and I'm pretty darn sure the throttle bodies are seated correctly. Ran the Suzuki diagnostic tool and no codes.

    Thoughts?

  • #2
    PC can do that. Unplug it from harness and run stock. See if that fixes problem.

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    • #3
      I'll give that a try. I did have the PCIII removed at one point, but not since changing the timing and removing the baffle.

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      • #4
        How much weight have you removed from the crank and how heavy is your flywheel?
        Do you have vibrations?Is crank balanced?



        Surging idle may indicate cam timing for top end power.
        Maybe too much overlap?
        Have you actually measured compression ratio?

        If you increase idle up to say 1500-1600 rpm does situation improve?

        Do you have adjustable cam sprockets?

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        • #5
          I haven't any idea how much weight was removed from the crank or the weight of the flywheel. It's a Falicon crank and the flywheel has been lightened but these were all done well previous to owning the bike and I didn't notice this idle issue last year and it never died on me. I do not have adjustable cams. I haven't measured the compression. The idle issue seems to get worse with a lower idle.

          The saga continues... I just did another track day and two race days. The bike still wasn't idling right and died on me once again while on track! Fires up fine each time. We now adjusted the front cylinder timing, tried replacing and calibrating the TPS (in PC3 as well) and the CDI but the idle issue continued until we replaced the throttle bodies.

          The throttle bodies which were removed were a set of Zoran bored units and the ones put on were stock. I'm now getting the injectors cleaned on the bored throttle bodies, but was wondering if Zoran or anyone else has any idea why these these particular throttle bodies would cause this hunting idle? No obvious damage and while the injectors didn't appear spotless I'm told when they were cleaned that they weren't plugged. With the stock throttle bodies the bike seemed to run fine for the following two races rounds in which I completed six races without issue (aside from crashing once [not related lol]).

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          • #6
            If throttles are causing it there are 3 things on them. Injectors, tps and servo. Try swapping one by one from your throttles and see if that solves it. Tps is just adjusting, no need to swap.

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            • #7
              I've gotten the throttle bodies back after having had the injectors cleaned. Fingers crossed. We did do the TPS adjustment and I don't believe these throttle bodies have the servo?
              Is the servo to do with the secondaries? All that extra junk has been disabled or removed.

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              • #8
                Yes, servo is motor driving secondary plates/shafts.

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