Home Forums Tech Talk Carbs and Engines – matching

This topic contains 6 replies, has 4 voices, and was last updated by  David Galownia 4 years ago.

  • Author
  • #48798

    Samir Shah

    I continue to have carb problems, but am figuring it out. However, I keep hearing some information that does not make sense to me. This also leads to some engine questions.

    I am referring to Leopard pre-X30 engines with the Tillotson HL334 carbs.

    Paraphrasing what I have heard: “engine builders will match the carb to the engine and rebuild both at the same time”

    These engines are single piston – if you have 2 engines that have the same compression, shouldn’t carbs be interchangable? I can see carbs with worn out needles, needing different settings between carbs, and weather and track design (ie fast vs twisty tracks) requiring different spring, fulcrum height and pop-off inside the carb, but I would think the individuality should be at the carb level, not the engine level.  Something I am missing here? Bored out piston diameter?

    EC distributing, for example, blue prints carbs – they mention nothing about the engine.

    Most information about carb settings seems to be from seniors. Would Jr engines with restricted headers need different carb settings? Lower popoff for example?

    People seem to send in Leopard engines at the 10 hour mark for a piston refreshing. Is this necessary? Doesn’t compression tell the whole story, and measuring and monitoring compression what one needs to do? Rather than time, shouldn’t piston wear be visually inspected for by opening the top instead? What should one look for?

    Or is the piston brittle, and running it lean (as people do on the high needle) cause it to explode with running time.

    Does a good engine builder check or do anything to the bottom end when a piston service is being done? What tell tales are there to indicate a bottom service is needed? (Besides the time rule-of-thumb).

    Thanks, Samir.


  • #48801

    David Galownia

    Hi Samir,

    Regarding carbs on leopard engines, we’ve never in our testing seen carbs matched better to specific engines. We test all our carbs and if we change the engine we keep our fastest carb on always.   The fastest carb from our experience is the fastest carb.

    We race tag senior so I can’t speak to the jr. stuff but I imagine the same principle would apply. However I would also imagine a junior carb is set up different than a senior carb. Couldn’t tell you for sure.

    On rebuilds, it also seems to matter how high you are revving your engine. If you are revving it consistently higher you might not get the 8-10 hour life but if lower you may actually get longer.  We’ve had a couple engines go on us around 7 hours and it has been the thrust washer not the piston.

    These are my non engine builder prospectives.  Hope this helps.





  • #48810

    Eric Stevens

    The motor builders will set the carbuerator for the fuel delivery curve that is right for the way the set up their motors. For driveability and maximum power the carb needs to be rich on the bottom and lean-out as the RPMs rise.

    So a motor builder will set the pop-off pressure, fulcrum height and suggest needle settings according to experience or on the dyno.

    Certain carbs will just flow more air and therfore they make more power but all can be made to run well. Some motor builders will set/adjust the throttle stop position for WOT at the point where the air flow is at its maximum.

    I find that regular maintenance like daily disassembly and cleaning keeps it running good and reliably. Also I recommend a good fuel filter as debris really screws with the operation of a pumper style carbuerator


  • #48813

    Samir Shah

    Thanks. Very helpful.

  • #48816

    Matt Dixon

    I will agree with above, your fastest carb will always be your fastest. Test different ones cherish the best.

    As for rebuilds, when the bottom lets go you loose the whole motor. So be careful only worrying about compression.


  • #48821

    Samir Shah

    Can you guys share your fulcrum arm position and popoff value? Thanks.

  • #49089

    David Galownia

    Hey Samir,

    I couldn’t tell you those off the top of my head but I believe the pop off on most of our carbs are around 10.  We don’t really mess with the settings at all, just test them one after the other and watch the stop watch.

    – Dave


You must be logged in to reply to this topic.