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S25 problems - sanity check please :)

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  • S25 problems - sanity check please :)

    I have been wrestling with trying to get my TES to run. Timing has been set statically and with a strobe. Starts within a few kicks, was able to get it to tick over fairly smoothly. However, trying to get to roadway speed (30 mph) and as engine warmed up, it would commence backfiring and spitting fuel out of the carburetor and 4 stroking. Leaned the main jet, backfiring ceased, but was still 4 stroking under load with throttle opened. The mixture was then enriched (approx. 2 &1/2 turns out on the needle). Working nominally better, however at one point engine decided to run away with throttle closed. Adjusted the needle out slightly and went for another mile run. Was able to get into a power band and get to approx. 40 mph . However, the plug does not look very happy afterwards. I plan on tuning the pilot again per Collins July Motorcycle article, would that alone cause the engine to run lean (pilot adjustment is out 2 & 1/8)? Is there some type of sealing required on the inlet manifold where the carburetor is slid on? Am I missing something really obvious? Picture is new spark plug after last nights testing - NGK B7HS.

  • #2
    Geoff. There seems something very wrong here, the S25 carb is a very simple device with only 2 jets (main and pilot) an air screw, and an adjustment to move needle up and down. The settings for these carbs are very much the same for all applications with mainly only changes in main jet size. So if they are set up as per spec for the particular engine the bike should run fairly well and only minor adjustment needed to get it running well. No sealant is needed on the carb mounting to manifold. The pilot jet only controls the fuel at idle speeds so has no effect at 1/2 to full throttle.
    So I think you need to look further, at the engine itself maybe.
    Has the engine been running OK previously? or is it a recently acquired or rebuilt engine? If the latter, maybe something not assembled correctly, if the parts have been acquired from eBay or other sources, the cylinder barrel may have been subjected to some amateur tuners files in the ports. Likewise someone may have filed the throttle slide of the carb.
    Best way to attack this problem is to try another carb from a bike that is known to run OK. Like all these sort of problems its difficult to diagnosed problems without actually seeing the machine.
    The B7HS plug may be a bit hard, have you tried a B6HS?


    • #3
      The engine was rebuilt by the PO with advice from Kenny Sykes. I figure it has to be an air leak at this point because enriching it no longer has any affect.


      • #4
        I was going to suggest that you may be able to get the bike to a member who could take a look at it but as i see you are situated an Westfield, Indiana, USA that may be difficult and involve a long journey. Best of luck in sorting the problem, as you say air leak looks to be the problem.