Re: Importance of float level

Graham Motzing /
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EX:

just after posting i remembered a really good example of this. Due to manifold constraints and packaging i went from a VM22 on a moped build to a VM18. Despite being similar design and layout, these two carbs are (obviously) different castings and always end up running really different tuning parts.

I had a 165 jet in the VM22, when i put the VM18 i was running a 75 jet. since mikuni jets are sized based on fuel flow, its easy math to see that dropping the jet size resulted in a (1-(75/165))= 65% decrease in flow rate through the jet.

now, the speed of the moped didn't decrease much, and it was properly tuned in both scenarios, which means we can assume that mass air flow rate stayed relatively constant and fuel flow rate stayed relatively constant.

so, because mikuni did the hard work for us and linearized the mass fuel flow rate of the jets, the math is really easy to say that (give or take) there was a 65% increase in differential pressure across the main jet to accomplish the same mass fuel flow rate with a smaller jet.

so the differential pressure of having a different float level setting as a percentage change in mass fuel flow rate decreased by a factor of whatever that pressure drop is. if you wanted to just make up some numbers as an example you could say that the main jet delta p on the VM22 was, lets say, 10 cm. So then the 65% increase on the VM18 needs the pressure to to be (10/.45)= 22.2 cm differential to make the same mass flow rate. Now if you are .2 cm off on your float level, on the VM22 that will be a (.2/10) =2 % change in mass flow rate, whereas if you are .2 cm off on the VM18 it will be a (.2/22.2)= 0.9% change in mass flow rate.

I think that math is right but i'm not super sure that you can use the mikuni jet size flow rates in a linear fashion like that, either way it shows a pretty good example of how the original poster is way the fuck wrong.

This single post is part of a larger thread. Start from the top or view this post in context.
Subject Written By Posted
  Importance of float levelTopic by: Michael Forrest Michael Forrest 08/28/19 11:16AM
  Re: Importance of float levelRe: live ɘvil live ɘvil 08/28/19 11:44AM
  Re: Importance of float levelRe: Michael Forrest Michael Forrest 08/28/19 11:46AM
  Re: Importance of float levelRe: jayson sumpter jayson sumpter 08/28/19 12:00PM
  Re: Importance of float levelRe: live ɘvil live ɘvil 08/28/19 11:52AM
  Re: Importance of float levelRe: Michael Forrest Michael Forrest 08/28/19 11:58AM
  Re: Importance of float levelRe: Rocco Taco Rocco Taco 08/28/19 12:15PM
  Re: Importance of float levelRe: Michael Forrest Michael Forrest 08/28/19 12:26PM
  Re: Importance of float levelRe: live ɘvil live ɘvil 08/28/19 12:34PM
  Re: Importance of float levelRe: Michael Forrest Michael Forrest 08/28/19 12:42PM
  Re: Importance of float levelRe: live ɘvil live ɘvil 08/28/19 01:37PM
  Re: Importance of float levelRe: Michael Forrest Michael Forrest 08/28/19 03:05PM
  Re: Importance of float levelRe: live ɘvil live ɘvil 08/28/19 03:34PM
  Re: Importance of float levelRe: Michael Forrest Michael Forrest 08/28/19 04:06PM
  Re: Importance of float levelRe: live ɘvil live ɘvil 08/28/19 04:12PM
  Re: Importance of float levelRe: Rocco Taco Rocco Taco 08/28/19 01:59PM
  Re: Importance of float levelRe: Michael Forrest Michael Forrest 08/28/19 03:15PM
  Re: Importance of float levelRe: live ɘvil live ɘvil 08/28/19 04:07PM
  Re: Importance of float levelRe: Braedan Cvetkovich Braedan Cvetkovich 08/28/19 04:50PM
  Re: Importance of float levelRe: ken gilbert ken gilbert 08/28/19 04:55PM
  Re: Importance of float levelRe: Rocco Taco Rocco Taco 08/28/19 05:13PM
  Re: Importance of float levelRe: live ɘvil live ɘvil 08/28/19 07:02PM
  Re: Importance of float levelRe: Michael Forrest Michael Forrest 08/28/19 09:15PM
  Re: Importance of float levelRe: live ɘvil live ɘvil 08/28/19 09:36PM
  Re: Importance of float levelRe: Rocco Taco Rocco Taco 08/28/19 09:51PM
  Re: Importance of float levelRe: K Olena K Olena 08/30/19 11:25AM
  Re: Importance of float levelRe: Michael Forrest Michael Forrest 08/30/19 11:38AM
  Re: Importance of float levelRe: Rocco Taco Rocco Taco 08/30/19 11:56AM
  Re: Importance of float levelRe: Rocco Taco Rocco Taco 08/30/19 11:57AM
  Re: Importance of float levelRe: Graham Motzing Graham Motzing 08/30/19 11:58AM
  Re: Importance of float levelRe: Graham Motzing Graham Motzing 08/30/19 12:27PM
  Re: Importance of float levelRe: Rocco Taco Rocco Taco 08/30/19 12:59PM
  Re: Importance of float levelRe: live ɘvil live ɘvil 08/30/19 01:53PM
  Re: Importance of float levelRe: Rebel Moby Rebel Moby 08/30/19 05:33PM
  Re: Importance of float levelRe: Overpriced Parts Overpriced Parts 08/30/19 07:28PM
  Re: Importance of float levelRe: baird co baird co 09/01/19 01:17PM
  Re: Importance of float levelRe: Norris Hall Norris Hall 09/02/19 10:25AM
  Re: Importance of float levelRe: Rebel Moby Rebel Moby 09/02/19 05:38PM
  Re: Importance of float levelRe: Rocco Taco Rocco Taco 09/02/19 08:03PM
  Re: Importance of float levelRe: Rebel Moby Rebel Moby 09/03/19 06:04PM
  Re: Importance of float levelRe: Norris Hall Norris Hall 09/02/19 06:57PM
  Re: Importance of float levelRe: Rebel Moby Rebel Moby 09/03/19 06:03PM
  Re: Importance of float levelRe: Norris Hall Norris Hall 09/02/19 06:59PM
  Re: Importance of float levelRe: Norris Hall Norris Hall 09/04/19 12:42AM
  Re: Importance of float levelRe: Rebel Moby Rebel Moby 09/04/19 06:05PM

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