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Modern engine race tuning question......

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Modern engine race tuning question......

Old 10-16-2020, 11:02 PM
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Default Modern engine race tuning question......

So, the old school way I was taught to set my carb slide EPA was to match it to the restrictor size I'm running. In other words, you don't need to have the slide open any further than the opening in the restrictor your running. Well, I'm not sure that still 100% applies because a lot of the old engines had a short spray bar needle, so past half throttle, it was all the way out of the spray bar hole. Most newer engines, Novarossi, O.S etc.... they use a long needle which barely comes all the way out of the spray bar hole at full opening of the slide.

So my question is..... Is it still recommended to match your slide opening to the size of the restrictor, or should you have your slide open past the restrictor to allow the needle to pull out of the spray bar further? Any advise would be greatly appreciated!!
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Old 10-17-2020, 05:55 AM
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Originally Posted by shotime View Post
So, the old school way I was taught to set my carb slide EPA was to match it to the restrictor size I'm running. In other words, you don't need to have the slide open any further than the opening in the restrictor your running. Well, I'm not sure that still 100% applies because a lot of the old engines had a short spray bar needle, so past half throttle, it was all the way out of the spray bar hole. Most newer engines, Novarossi, O.S etc.... they use a long needle which barely comes all the way out of the spray bar hole at full opening of the slide.

So my question is..... Is it still recommended to match your slide opening to the size of the restrictor, or should you have your slide open past the restrictor to allow the needle to pull out of the spray bar further? Any advise would be greatly appreciated!!
in the os carb (back with the 21j carb) you had the option of a long speed needle with a specific mid speed, and a short needle with its own mid speed needle. The general thought was the longer low speed needle was a bit easier to tune and was more consistent. Thatís about the farthest I got with it, I never tried to fully understand what was going on as I was just trying different stuff out and hated the short needle set up like most people. However I know some carbs flat out use short low speed needles that work great. I believe my older Werks or Orion carbs are short needles every time Iíve looked.

after re reading your post I see your question was different then i originally thought. So if you could measure the needle length, you can mathematically set up your epa without setting it by site, is this the question? Common sense here tells me you probably could if u understand how to do the math from an engineering standpoint. While in my mind I seem to think this is no different then popping your carburetors filter off and just setting epa by watching the slide open fully in the carb and setting it that way wouldnít it be much easier then doing a bunch of math? Might pull further then it looks till you do it by hand and use your eyes (and ears) to test where it opens to fullest.

Last edited by Matt Piva; 10-17-2020 at 06:05 AM.
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Old 10-17-2020, 10:42 AM
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I appreciate the response, but that's not exactly answering my question. Without over complicating my explanation, all I want to know is..... Do you limit your slide pull to the opening of the size restrictor your running regardless of what type of spray bar needle your carb has (short stubby needle or long pointy)? Is there ever an advanced to setting your slide opening past the size of the restrictor hole?
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Old 10-17-2020, 01:03 PM
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The venturi size isn't relevant. Just set the idle gap (with venturi removed) to .7mm or so and tune it from there. Try to avoid opening up the idle gap to get it to idle, covering up a rich low speed needle.
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Old 10-22-2020, 01:38 AM
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I think he's asking, do you set your throttle EPA to visibly fully open, with the restrictor in, or restrictor out?

I've always set mine to open barely more than the restrictor. Never fully open without the restrictor. Never gave a thought to carb type. And I've got as much rip on the straight as the pro's do. I haven't checked idle gap in 15 years. It's all tuned on the box by ear.
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Old 10-22-2020, 03:23 AM
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I made a thread like this and it seems most people will jsut set it to the end of the venturi. Ive found that should get an object the same length as the venturi you use and set your epa to match exactly. If using a 7mm venturi, get calipers and put them over your throttle slide and make it open by 7mm, take away 0.5mm to account for your idle gap which is already open. If your slide opens further than the venturi, theoretically you would have a dead spot on your transmitter trigger, because you are opening further than the venurti allows anyway
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Old 10-22-2020, 06:37 AM
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You do not want to have any influeance on the full throttle due flex and vibrations in the chassis so open it just a tiny bit beyond the carb body (not venturi)
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