Walbro Venturi Size

Steve F · Joined: 05 Sep 2001 Posts: 134

I've measured a whole bunch of "blueprinted" Walbro carbs and found virtually no differences in the sizes of the venturi and the throat even though they are as much as .020 diameter smaller than the legal size prescribed by WKA and IKF.

I'm tempted to bore the carb throat and venturi out to the legal max diameters, but am concerned that I may disrupt the flow balance of the venturi effect in doing so. Anybody out there have any insight on modifying the carb as described?

Jimmy Moore · Joined: 26 Jul 2001 Posts: 664

Boring out the carb is not as easy as it looks. The casting plugs come together right there and it's rather uneven. You can get the carb running perfectly true then end up taking most of the material off from one side. I suggest you paint the area with a black magic marker then move the boring bar out untill you make first contact then re-center depending on what the cut looks like. You only have a very small amount of metal to work with so, cut a little and re-check for center several times. Don't cut too close to the legal limit, I would come up about .003" shy to compensate for heavy handed tech men and worn no-go gages. When you get done you will have a carb that will flow a little better but you'll need a stop watch to prove it. I don't really think it's worth the hassle.

Sean Cook · Joined: 28 Jul 2001 Posts: 82

Steve,

There are a few different ways to bore the venturi on a Yamaha. You can use a broach, that is made by Lad Specialties. It works very simple, you use an arber press to push the broach through the carb. I believe it punches it out to .947. Sometimes the broach will not be centered, but it only takes seconds to use.

If you take more time, indicate the carb in the lathe using a dial test indicator, the same kind of indicator used when aligning cranks. This way the machining will be done centered. It does take some time to do, and it is very easy to scrap a carb on the first try. You might consider sending the carb out, unless you are tooled up for the job.

Sean Cook
Cook Racing Engines

Steve F · Joined: 05 Sep 2001 Posts: 134

I've purchased all the no go gages needed for inspection of the enitre carb, so far the only difference I've found between the "blueprinted" carbs and one that is known to me to be totally stock ( I bought the motor new) is the fulcrum arm height/ the pop off pressure setting/ and replacing the throttle plate screw with a button head style screw.

I just can't believe that shops are getting $60 for that little amount of work. There must be something else I'm missing. Any ideas on where else I might look?

I've got to think that there's something going on in the high and low speed seats but I sure dont see it.

Jean Stafford · Posted: Fri Sep 07, 2001 8:30 am Post subject: Walbro Venturi Size

quote:
Originally posted by Steve F:
I've purchased all the no go gages needed for inspection of the enitre carb, so far the only difference I've found between the "blueprinted" carbs and one that is known to me to be totally stock ( I bought the motor new) is the fulcrum arm height/ the pop off pressure setting/ and replacing the throttle plate screw with a button head style screw.

I just can't believe that shops are getting $60 for that little amount of work. There must be something else I'm missing. Any ideas on where else I might look?

I've got to think that there's something going on in the high and low speed seats but I sure dont see it.

Reminds me of the story about the farmer who complained to the mechanic who charged him $20 to adjust one screw on his tractor to get it running. Mechanic said," It's $5.00 for the adjustment. It's $15 for knowing which screw to turn."

Sometimes it's not what you do, it's what you don't screw up that counts.

There are a lot of carbs that are scraped by testing the limits, and reputable builders don't sell them, they throw them away,

Jean Stafford · Posted: Fri Sep 07, 2001 8:38 am Post subject: Walbro Venturi Size

Steve:
I've found that all the "blueprinted" carbs I've come in contact with, have had the venturi bored to 0.948" +/- 0.001.

Also, If you can get enough fuel into the engine to make it four-cycle, aren't the holes big enough?

Most guys I know are almost closing the high end of the Walbro carb off, and running hte low end more than 2 turns open. Seems to me that the venturi is not being asked to work here.

I beleive the "secret" is getting the carb "balanced" correctly so the fuel metering that is supposed to go on is doing so at the proper time.

Demands on the carburetor by the engine vary as the RPM increases. When the engine is running at lower RPM's the pulses to the pump are greater than at high RPM's. I think this is why there is a venturi in the system.

I think I know what the secret is, but I'm not telling.

Jimmy Moore · Joined: 26 Jul 2001 Posts: 664

Steve, The amount of money people get to blueprint a Wb3a always amazes me too. It takes about a half hour and a $5 carb kit and believe me it's not rocket science. You get what you pay for ain't always true.