Re: Yamaha QT 50 oddity

bryanb /
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Ron Brown wrote:

>

> 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 throttle cable comes into the carb on the top. 2 cables actually, one being the choke. THe throttle cable is connected to a slider, (that word seems right, but I actually have no idea what it is...) about the size of a quarter roll of dimes. its hollow, and inside, and sticking out the bottom is a long narrow needle valve. There is a compression 'spring' that holds the needle in the slider. This spring is compressed against the inside surface of the slider. I seem to remember this needle sitting at the bottom of the slider, but it seems possible for it to be set anywhere along the height of the slider, with only a small degree of an ability to stay there, so I set it at the bottom...

does this change your response?

>

> 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.

I think so... I remember palying with it when I took the carb apart the first time, and was trying to see how the damn thing worked...

>

> If you are still awake, I hope this helps.

Ahhhh Auto mechanics in high school comes rushing back.... I rebuilt a carb for my '74 Subaru wagon then... what a mess...this is far simpler!

This single post is part of a larger thread. Start from the top or view this post in context.
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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