Wb3a

[George Clausen]

This may be an area where us 2 cycle guys are behind the curve the 4 cycle guys are on. I know there are FH Briggs carbs that have never been reamed or honed winning national level races. Supposedlly because they have unique anomolies in the bores. Is this possible? How would this be recognizable without a very sophisticated wet flow bench? I have tested more than a hundred WB3's on my dry bench and recorded flows and fuel specifics on them. Does that help me predict preformance? not really. The best flowing tend to show the most HP at peak rpm but not average. Back in the day of .600 restricted junior classes, I could sort the ones out with high fuel specific for that class with good results.
At the end of the day, IMHO, a good driver, an open practice day, a lap timer, and a good procedure ( testing each carb on equally worn tires) will sort out the best and worse ones
The dyno is best used to find dips in the powerband that can be minimumised by altering the fuel curve of the carb
George

[Mark Horne]

After many years testing engines on the Dyno I have found this to be true:
If you have ten carbs, test them all regardless of what their appearance may be. Out of ten, 5-7 of them will be average (not too hard to tune but they will only work in a narrow range of jet settings. One may be a bit better than the other average samples). At least one will be junk (hard to get any kind of smooth power with a fairly large drop in HP at some point through the RPM Sweep). One or two will be easy to tune and make much better power than the rest. This of course is based upon a random assemblage of carbs.

I always test the customers carbs first and recommend they have at least 5 ready to test from the beginning. Will a "Blue Printed" carb make more power? Perhaps, if the work was performed correctly it has the best chance of being good. I have seen carbs that were basically stock with only the Venturi machined make excellent power. Not many though. As with anything, poor workmanship will deliver poor results. I agree that reaming the holes will produce repeatable results carb to carb. If the Venturi bore is symmetrical about the center line after machining, it has a better chance of making more power.

If you have a proven good carb, only sell it for the price of several new ones.

Have a great day,
Mark
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SLK PowerLab
2 Cycle Dyno Tuning

"Racing is life,.....everything else is just waiting"
Steve McQueen - LeMans

"There are only two things that make feel alive, racing is one of them."

[John Mulvihill]

Thanks Guys,

Hi George,

What was the "high fuel specific" you were looking at? Not sure that I follow that. Fuel flow?

So far in playing with our wideband O2 (Sportsman Muffler) the AFR was a very flat curve using our "Magic Carb". No dips or peaks. We haven't ried the O2 on a pipe yet

Hi Mark,

I agree with your findings. A nicely bleprinted carb works better but (so far) there's no making a "silk purse out of a sow's ear".

Passing thought: Ever try intentionally machining the bore off-center? I'd think that machining the bore to the max. but leaving as much material as possible in front of the orifices would maximize airflow and still leave a strong signal?

My ADHD brain is telling me that the inconsistency in Walbro's is due to the multiple maze-like passages the fuel travels through on the way to the exit orifices. A drilled (reamed?) hole runs into another drilled hole and they don't line up terribly well.

An orifice drilled/reamed in the bottom of a trough where the passage is off center and has a square shoulder on the entrance might not allow for consistant fuel flow.

Time for a sacrificial carb. Anybody got a really, really crappy Wabro that they've never been able to get to worK???? Frankly, I tossed the really bad one I talked about before. At least I didn't put it on E-Bay.....

John
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I bowl overhand..........

[Pete Muller]

John,

In my opinion, you'll be able to inspect all you want and you still won't find the difference.

I started doing the "batch of carbs" testing on Yamahas in the late 70's, and even cut a few "bad ones" apart to see what I could find. Of course I never had the courage to cut a good one apart to compare them too! I measured and inspected everything I could get to (and think of)... and never have been able to find a rock solid reason why carb A is better then carb B.

When I was road racing Yamahas, (and I think Steve will probably echo the same experience), I didn't even need to make a lap. I could tell from the sound of the engine leaving the pits whether the carb was a good one or a bad one.

PM

[Steve O'Hara]

Pete,

So true... but we are dinosaurs.... now to be a real racer you need EGT, Lamba sensor, detonation sensor, GPS and a weather station to learn the same thing we learned in a hot run from the pits to turn one at Willow

Reaming, honing, boring, flow bench testing, dyno testing batches of carbs.... all very impressive efforts to solve a riddle that no one has an answer for.
The reason is simple.... they are all looking at the wrong part of the carbs for an explanation for why one works a little better than others.
Steve O'Hara

[al nunley]

[JIM SILVERHEELS]

Like my good buddy Al, I too would like Steve to cough the answer up PLEASE !
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Don't get stuck in someone else's discarded chewing gum with your thought process.

[Walt Gifford]

A couple of things I see that vary is the air premix orifice and the transition jet. Sometimes they line up sometimes they don't and the distance apart varies too. Easy to see with 5x.

Also, the split line for the casting comes together right at the choke and I think the molds must line up in various ways. Even if you line the carb up perfectly on a lathe it never seems to cut evenly. You have too take a little cut then adjust it take another cut ect. I haven't seen many that come out perfectly centered. It must somehow effect the flow over the jets.

Worst carb I ever had, you had to lift off the throttle X amount and the power would increase. Of course too much vibration to hold it perfect.

Who knows maybe all you have to do is wrap some teflon tape around around the low speed needle to keep the air from leaking in.



Gif
_________________
FAA certified jet engine and aircraft technician
Nicholson Speedway class champion 2001
Yamaha KT100 Service Center
40 years karting experience

[al nunley]

[Walt Gifford]

For an o ring to really seal you'd have to have an unthreaded section on the needle and a cup under the spring to keep the o ring from spreading.

I've tried teflon paste but you can't move the needle after it dries can't imagine loctite.

I was in the process of making a needle with slightly bigger threads then I found some blue RLV needles that were a little bigger than the black flex T ones I had. I put them in with grease to help seal the threads.

It's a problem, if someone came up with a product to solve it I'm sure it would sell.

Gif
_________________
FAA certified jet engine and aircraft technician
Nicholson Speedway class champion 2001
Yamaha KT100 Service Center
40 years karting experience

[George Clausen]

"fuel specific" is slang shorthand for the carburetors ability to draw and meter fuel into the airstream over the entire "fuel curve". Or to put it another way, how efficient the carb is at any given air flow rate to deliver the proper amount of fuel. For example, countless hours have been spent on restricted Nascar carburetors to smooth out the lean dips in the powerband that are caused by the restrictor plate and the reduced and erratic vacuum signal to the venturi's. Real proficiency at this is in the "black art" catagory, as most simply don't have the time or resource to spend the time to do the testing. Us common folk can indeed blueprint and fix what mass production didn't address. We can also test one against the other. We can tailor the fuel curve to suite a particular application. but its a rare day we can make a really good carb from a really bad one.
I have a couple of Cass City WB3 Walbros that are better than any others Iv'e tested. Neither has been blueprinted. Neither are for sale until my son or grandkids are done racing
George

[George Clausen]

oops, forgot to mention this, I have some mixture needles that are center drilled that Iv'e made up to attach to the manometer I use to measure fuel specific on my flow bench. When I first started doing this I noticed a wide variation in readings, more than what I would have expected from measuring Briggs carbs. This turned out to be air leaks around the needles as Al N. describes. May sound insignificant but it would equalize pressure from the wet side to the outside. An "O" ring. a tiny machine washer and 1 coil nipped off the spring solves it though
George

[JIM SILVERHEELS]

Teflon plumbers tape?
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Don't get stuck in someone else's discarded chewing gum with your thought process.

[Colin Edwards]

Its quite common in Australia for both high and low needles to be O ringed and the throttle shaft as well. Additionally the throttle shaft bores are teflon sleeved to very tight tolerances thus ensuring ALL air enters the inlet port via the venturi entry.
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Real Racing Cars don't have doors!

[George Clausen]

I would guess that would do it. I'd be very afraid that a small bit would find its way to the entrance of the idle or transition passage though. Perhaps if you started the "wrap" up 3 or 4 threads from the tip and always cleaned the carb after removing the needle it might be OK
George