Re: Yamaha QT 50 oddity

Ron Brown /
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Bryan,

Some of this is conjecture and some is empirical from working on my own peds.

If your carb is like mine, it does not have a tapered needle extending from the bottom of the slide to the "needle jet", the opening in the venturi which allows gas/gas air mix into the air stream. In the absence of a needle, I do not know what to call this jet, so I will call it an atomizer which is what it would be called in a "butterfly" type carb such as in a car.

The purpose of this tapered needle is to regulate the available gas/gas air for mixing with the incoming air during intermediate throttle openings.

By the way, I use "gas/gas air" because many carbs have an emulsifying tube which mixes a small amount of air with the gas as it travels from the main jet to the atomizer. This assists in atomizing the fuel into the intake air stream.

If you are not thoroughly confused yet, consider the operation of the carb at full throttle. The venturi size is fixed by the diameter of the passage through the carb. The main jet and air jet to the emulsifier, if any, are also fixed. At some rpm, the velocity of the air through the venturi will create enough of a pressure drop to draw just enough fuel through the main jet/atomizer to give you a perfect mixture. If the rpm increases, the air will flow faster, creating a greater pressure drop in the venturi, drawing more fuel through the atomizer. This sounds perfect except that if diameter of the carb restricts air flow at all, because it was designed for optimum flow at lower rpm, then the pressure will drop even more at the atomizer. This causes too much gas to be mixed with the fuel, essentially a "choke" effect. The air cleaner/air box can also contribute to this.

The reason you never have the problem goin up hill is because you never attain this "excessive" rpm range.

If you are still with me, the contributing factors to this equation are:

Diameter of the carb venturi.

Main jet size.

Emulsifier air jet size.

Atomizer jet size.

Float bowl fuel level.

Any restrictions to air intake, air box, filter, air turbulence around the air box.

Any restrictions to gas flow, dirty jets, plugged filters, float bowl and gas tank vents.

Any of the components which are sized to regulate flow can be adversely affected by over agressive cleaning with hard wire, drill bits etc.

Any air leak in a position to allow more air into the mixture, carb to manifold, manifold to block, crankshaft seals..

Any fuel leak which allows more fuel into the mixture, leaky float needle, leaky, stuck or misadjusted enrichener (choke), loose main jet.

As you can see, the opportunities for error are many. Some cause the mixture to be more rich, some more lean.

As we know that you have a too rich problem, there are only so many things to look at which we can do something about. Main jet size, float level and free air flow are the simplest, although it is worth verifying that the enrichener is ok and the air passages to the emulsifier are clear. You have allready determined that the air filter/air box make no difference, although this surprises me. This leaves you with the float level and main jet size to check/play with.

It occurs to me that you should also check that the throttle slide is opening all the way when you twist the throttle.

If you are still awake, I hope this helps.

Ron

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Subject Written By Posted
  Yamaha QT 50 oddityTopic by: bryan bryan 07/10/01 12:52AM
  Re: Yamaha QT 50 oddityRe: MR MR 07/10/01 02:29AM
  Re: Yamaha QT 50 oddityRe: Carmen Carmen 07/11/01 12:14PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/10/01 04:01PM
  Re: Yamaha QT 50 oddityRe: don-ohio don-ohio 07/12/01 10:42PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/16/01 12:56AM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/16/01 01:04AM
  Re: Yamaha QT 50 oddityRe: don-ohio don-ohio 07/16/01 02:08AM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/16/01 03:01PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/16/01 03:20PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/18/01 01:41AM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/18/01 12:21PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/18/01 02:37PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/19/01 01:22AM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/19/01 11:57AM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/19/01 02:02PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/19/01 03:02PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/19/01 04:08PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/19/01 05:41PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/19/01 06:39PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/19/01 07:24PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/20/01 11:13PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/21/01 11:35AM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/21/01 03:43PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/22/01 06:30PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/22/01 08:58PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/23/01 01:06PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/23/01 10:47PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/24/01 10:59AM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/24/01 11:57AM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/24/01 01:22PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/24/01 09:06PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/25/01 10:18AM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/26/01 04:30PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/26/01 06:01PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/27/01 12:24AM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/27/01 11:38AM
  Re: Yamaha QT 50 oddityRe: Shawna Guy Shawna Guy 08/26/02 02:38AM
  Re: Yamaha QT 50 oddityRe: Bryan Bryan 08/26/02 03:26AM
  Re: Yamaha QT 50 oddityRe: Caton9 Caton9 10/21/02 06:02PM
  Re: Yamaha QT 50 oddity parts!Re: Bryan Bryan 10/21/02 06:17PM
  Re: Yamaha QT 50 oddityRe: Wayne Wayne 08/26/02 09:10AM
  Re: Yamaha QT 50 oddityRe: brynab brynab 07/21/01 07:26PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/30/01 12:24AM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/30/01 12:18PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/30/01 01:46PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/30/01 02:08PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 07/30/01 11:01PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 07/31/01 11:46AM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 08/10/01 11:47PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 08/10/01 11:51PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 08/11/01 12:34PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 08/13/01 08:54PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 08/14/01 12:19PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 08/14/01 03:55PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 08/15/01 01:03PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 08/16/01 01:17AM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 08/16/01 12:45PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 08/16/01 02:00PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 08/16/01 04:42PM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 08/16/01 08:38PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 08/17/01 02:02AM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 08/17/01 12:51PM
  Re: Yamaha QT 50 oddityRe: Brian Brian 08/17/01 03:27PM
  Re: Yamaha QT 50 oddityRe: bryanb bryanb 08/17/01 02:03AM
  Re: Yamaha QT 50 oddityRe: Roy Markee Roy Markee 11/12/01 02:03AM
  Re: Yamaha QT 50 oddityRe: Ron Brown Ron Brown 11/12/01 12:43PM
  Re: Yamaha QT 50 oddityRe: k rude k rude 10/19/02 06:02PM
  Re: Yamaha QT 50 oddityRe: Joe Hunt Joe Hunt 10/21/02 12:30AM

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