Wb3a

George Clausen · Posted: Fri Oct 22, 2010 2:53 pm Post subject:

Mity-vacs or their generic equivalents are nice tools to have around. A carb that seeps pop off pressure but holds vacuum is almost certainly the condition of the needle tip or seat rather than an air leak in the stack assembly for instance. They are also one of the best tools for bleeding brake systems. +1 for Al noting the importance of pictures. The most carefully composed verbage is no where as good as one good picture. Maybe a Karting Komic book is in order. Remember you heard it here on 4 cycle first.
George

JIM SILVERHEELS · Posted: Fri Oct 22, 2010 4:06 pm Post subject: DARN

Had to reach past the fish hooks in my pockets and order some vac grease today.
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Pete Muller · Posted: Sat Oct 23, 2010 8:47 am Post subject:

I have a combo pressure/vacuum tester I picked up from Azusa Engineering around 20 years ago. Not sure if they still sell them...

PM

John Mulvihill · Posted: Sat Oct 23, 2010 1:00 pm Post subject:

Hi Pete,

Azusa has one in their product listing, but there is no link, pics, or prices.

I wonder if someone could make a dual purpose tester using a DA pneumatic cylinder?:

http://www.surpluscenter.com/item.asp?item=4-1486-A&catname=air

http://www.surpluscenter.com/item.asp?item=21-1654&catname=air

A smaller bore might be better(?) as it would be easier to control with more travel to get the same pressure.

John

John
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John Mulvihill · Posted: Wed Oct 27, 2010 6:25 pm Post subject:

Here are a couple photos of the internals of two Walbro WB3a's. Specifically the emulsion trough where gas and air mix before being drawn (or pushed) into the carb bore.

The trough is in the bottom center of the photo and there are two circular openings visible in the end of the trough. The top orifice is the air correction (air pre-mix) jet and the orifice directly below it is the opening of the transition jet.

Gasoline is introduced through a channel visible to the upper left of the trough. At speed, air comes through the air correction jet mixing with the gasoline and creating a 'froth' in the trough which then goes through the transition jet into the carb bore. This tasty, frothy mix of air and gas atomizes easier and more completely than a jet of straight gasoline would.

The interesting point to me is how badly (in the first photo) the holes are centered in the trough and in relationship to each other. In person they look alot worse than they do in the photo.
[img]

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In the second photo, the casting of this carb is much cleaner and the drilled (reamed?) holes seem to be more in alignment with each other.
[img]

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To me it would seem like the carb that has the air correction jet closely aligned with the transition jet would result in a greater and a more consistant atomization of the gasoline flowing through it. I'd also think that beveling the entrance to the transition jet and, if needed, shifting that bevel to align with the air correction jet would help.

Anyone have any thoughts or am I missing the boat?

Thanks,
John
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al nunley · Joined: 13 Nov 2006 Posts: 3037

Interesting. So which carb performs the best?
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John Mulvihill · Posted: Thu Oct 28, 2010 5:36 am Post subject:

Good point<LOL>.

The one where the holes line up works the best, but I'm not sure if that is why.

What I'd like to do is take a lousy carb and do a dyno comparison then start hacking away at it to what I can do, if anything to make it work better.

Anyone have a real lousy carb that they would like to contribute to the cause? The bad one in the photo has already been a test subject, not for improvements, but just for jig set-ups on my lathe so I can't use it for this.

Thanks,
John
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Colin Edwards · Posted: Thu Oct 28, 2010 2:19 pm Post subject:

John,

One rather inventive "tuner" who I know well was filling the emulsion delivery port with dental amalgam. He would then re-drill / ream the hole to then be more in line with the air corrector orifice. I'm certain he was not the only one to do this. Was more popular in the Yamaha "J" engine classes where more track time would be spent on the low speed fuel circuit. Scrutineers now look for this mod as it is illegal in Oz.
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John Mulvihill · Posted: Thu Oct 28, 2010 2:43 pm Post subject:

Hi Colin,

How did they catch on to you? (Just kidding!)

To me, that's the kind of modification that should be allowed. Its cheaper to drill one hole than to buy 10 carbs and stick 9 people with poor ones via E-Bay.

I'd lean more towards a good epoxy myself, but I bet the dental amalgam blended in better.

Any other areas that you know of that could benefit from re-working?

Thanks for the input,
John
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al nunley · Joined: 13 Nov 2006 Posts: 3037

Colin Edwards · Posted: Thu Oct 28, 2010 4:22 pm Post subject:

John,

One thing that may identify "better" carbys is the word "JAPAN" stamped into the body / manifold mating surface. The bloke that does my carbs reckons the bodies without "JAPAN" are not as good as those with "JAPAN".
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John Mulvihill · Posted: Thu Oct 28, 2010 5:00 pm Post subject:

Hi Colin.

He may have a good point. So far I've seen U.S. (old ones), Japan, and Singapore.

There may be more. Perhaps the real question is "How many molds are out there?"

Then there are probably multiple drill jigs.

We need a 'mole' in the Walbro factory to dig out the info. Anybody related to a Walbro engineer?

Hi Al,

Good point. 1 in 10? We need a Los Vegas bookie to establish the odds. Its probably a question of degree with a bunch of variables. One is a little better (or worse) the next one is a bunch better (or worse). Or, all the bad tuners get the good carbs and never know it? Vice versa. Then one day a good tuner gets a good carb.......... Probably have to buy a thousand carbs and dyno them on the same day<LOL> Probably more good carbs than we think.

John
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Walt Gifford · Posted: Thu Oct 28, 2010 8:11 pm Post subject:

John, My theory is that air doesn't flow through the air premix orifice at wide open throttle because there is a balancing vacuum at the choke.

When the throttle is closed there is a tremendous vacuum at the transition jet and pressure at the choke that causes air to mix with the fuel which leans the mixture so the engine doesn't flood.

Maybe if the air premix orifice comes out offset to the right of the transition jet then more fuel than air gets drawn in necessitating a leaner needle setting to get off the corner which in turn will cause a lean condition in the mid rpm range.

Or, maybe your good carb just flows more air and it's a coincidence that the holes line up.

Nice pics btw.

Gif Cool

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