WB3 carb rebuild on KT100

R

racingman

Member
I'm starting to rebuild my own Walbro carbs on my kid's kt100. It's a Jr. Supercan series with a 4 hole can. Not sure if you need anything other info. Asphalt track with many turns. More turns than straights, but there is one long straight.

I have a fulcrum gauge and a pop off gauge.

I'm wondering what to set the fork height at (I call it the fulcrum). Also, what is the pop off pressure I should be using?.

Thanks
 
Steve o'hara has pop off at 15. From what I read that is off the chart.

I'm wondering now what is the importance of pop off pressure. How does it effect the carb performance. When is lower better or higher pop off better. Does it even matter?
 
racingman said:
Steve o'hara has pop off at 15. From what I read that is off the chart.

I'm wondering now what is the importance of pop off pressure. How does it effect the carb performance. When is lower better or higher pop off better. Does it even matter?
Click to expand...
Pop-off, you need some, 10 psi is some!

Fulcrum arm. I always set mine as high as possible. As long as it didn't pop the needle off the seat. You may get confused listening to all the recommendations for pop-off pressure and fulcrum arm height. My recommendations, along with numerous others, all work. Whether one or the other works better, I don't know for certain, but I've never seen a difference.
 
One more for 10 lbs . If you have multiple carbs set one up at 10 , one at 15 .
Testing each . If your driver's good with the needles , then I think less is better .
Your engine guy or setup /dyno sheet have any info ?
 
Agree 15 sounds a little excessive imo,I would tend to think too high of pop off could potentially cause a lean condition with piston stick to soon follow...again, unless your an ace with the needles and have a keen sense of performance and mixture under power...I'd sure think 8-10 range...but I'm no where near the experienced tuner that others that have commented so far are...
 
Steve O'hara set up is used when you want the engine to idle and still run ok on the track. It's not ideal for every track.

The standard set up is 10 psi pop off, fulcrum arm .050" below the factory gasket (which is .032") High jet 1/16th of a turn open and low jet 2 turns open. You may find the optimum low jet setting anywhere between 1 1/2 turns to 2 1/2 turns open depending on how the jets are drilled from the factory. Also set the idle screw 1 turn open. Don't forget to use the after market fuel screen with the course mesh, not the one in the rebuild kit.

Sundog
 
sundog said:
Steve O'hara set up is used when you want the engine to idle and still run ok on the track. It's not ideal for every track.

The standard set up is 10 psi pop off, fulcrum arm .050" below the factory gasket (which is .032") High jet 1/16th of a turn open and low jet 2 turns open. You may find the optimum low jet setting anywhere between 1 1/2 turns to 2 1/2 turns open depending on how the jets are drilled from the factory. Also set the idle screw 1 turn open. Don't forget to use the after market fuel screen with the course mesh, not the one in the rebuild kit.

Sundog
Click to expand...
This is more along with what I have been reading. thanks I called Comet and they said 6 or 7 pop off. Talked with engine builder. Not sure what the real purpose of pop off pressure is. Why is less or more better on a wb3a walbro carb. I'm still trying to figure that out.
 
Is 6 or 7 pop-off for asphalt or dirt or both? I only run dirt and 10 is all I`ve ever run.
Is elevation an issue? I race in Florida, not much elevation. Know about barometric and humidity pressures.
 
sundog said:
Steve O'hara set up is used when you want the engine to idle and still run ok on the track. It's not ideal for every track.

The standard set up is 10 psi pop off, fulcrum arm .050" below the factory gasket (which is .032") High jet 1/16th of a turn open and low jet 2 turns open. You may find the optimum low jet setting anywhere between 1 1/2 turns to 2 1/2 turns open depending on how the jets are drilled from the factory. Also set the idle screw 1 turn open. Don't forget to use the after market fuel screen with the course mesh, not the one in the rebuild kit.

Sundog
Click to expand...
Pretty close to the same settings I've been using, and preaching, as a guide since 1985. The needle settings are exactly the same. The pop-off is the same. I always ran the fulcrum arm up higher. On a warm (80° or higher) high air density day, these settings are just barely sufficient. With a compression pressure in the neighborhood of 160 psi and 15,000+ RPM, be very cautious. It helps, when setting your cc's, to cut the barrel and not the head. Leave the head "STOCK"!!
 
The low pop off is safe because it ensures fuel will always flow. A big long track sure 6-8psi is a good recommendation especially when you don't know someones skill level with the carb.

When you open the throttle the valve opens to full travel and it should open enough so it never regulates fuel. Pop off comes into play in the braking zone. If you have a full speed straight into a very tight corner the crankcase will have more time to load up as the engine winds down. This is why some will blip the throttle as they brake. If you like your low needle setting for power vs cooling but find you're sluggish getting back on the throttle after a long braking zone you can go up to 12 psi to shut the valve sooner.
 
sundog said:
When you open the throttle the valve opens to full travel and it should open enough so it never regulates fuel. Pop off comes into play in the braking zone. If you have a full speed straight into a very tight corner the crankcase will have more time to load up as the engine winds down. This is why some will blip the throttle as they brake. If you like your low needle setting for power vs cooling but find you're sluggish getting back on the throttle after a long braking zone you can go up to 12 psi to shut the valve sooner.
Click to expand...

Sundog,

I don't "think" a diaphragm carb works entirely like that. The inlet needle *must* regulate the fuel flow into the space under the diaphragm, because if it didn't... the mixture would be at the mercy of the volume and pressure the pump provides... which is not consistent.

There is of course no air under the diaphragm, only fuel. The engine is drawing a certain amount of fuel through the 2 fuel circuits... based on airflow through the carb and jet position. Assuming the kart is in a corner where the driver was off the throttle, the volume/space under the diaphragm fills with fuel and expands, and at some point, the inlet needle is fully closed. I believe, a combination of: no airflow (pressure drop) past the high speed orifice, the fuel filling the space under the diaphragm and flexing the diaphragm upward, and the spring pressure under the fulcrum arm... are what determine how much fuel is in the chamber and the position of the diaphragm when this happens.

So let's skip right to going down a long straight at fairly stable rpm. Fuel flow into the venturi is "steady state", which I believe will cause the diaphragm to also be in a fixed/static position. That position will be determined by some combination of spring pressure and fuel draw. If the jet gets opened further, the diaphragm flexes downward a bit more, and opens the inlet needle just that slight bit more to match the incoming flow (past the inlet needle) to the outgoing flow (into the bore of the carb).

The transients (constantly changing fuel demand due to rpm and load) are what is super interesting. The carb is "dumb" though, and only knows how much air is going through the bore of the carb and consequently what the pressure drop is past the low and high speed orifices. Coming off a corner full throttle, is where (I believe) spring pressure makes a difference in "feel" on the track. Assuming the above is correct (yea... could be a big assumption, I admit, LOL)... it seems logical to assume that if the spring pressure is high(er), more fuel needs to be drawn from the cavity before the inlet needle gets lifted off the seat (and I'm sure this all happens very quickly). Depending on a LOT of other things (like how "draggy" the clutch is, what type of exhaust pipe, blah blah), the spring pressure acts (to me) like it influences the rapid "transients" (transition from full throttle to off, or off to full throttle).

fwiw: I built a KT100 for a friend for the last VKA race we had out here on the west coast, and I set it up with 16 psi pop-off, and arm about .050" below the top of the gasket. Idled in the pits, was responsive to jet settings, and was very fast.

I've never been a big believer in the 2 turn low speed with the high speed just barely open. As you know, the Walbro has an angled hole that *should* lean the high speed fuel circuit at high air flow (dilutes it with air). Pretty much any 2-cycle engine will over-rev much better if it's on the lean side up at really high revs, and that "feature" gets negated to some degree if more of the required fuel is run though the low speed circuit.

Anyways.... great topic, and always fun to discuss. :)

PM
 
The reason they're recommending the lower Pop off is because it's a 4 hole can engine, for whatever reason the can motor will perform better with the lower Pop. We figured that out long ago building carbs for can racers. Comet has too . I think it's because that can disrupts the engines ability to breathe. As far as what lower Pop will do , it starts the fuel flow with less of a low pressure area, under the diaphragm. So the needle ends up staying open longer overall. The Walbro WB3A is not nearly as sensitive to pop off changes as some. A built WB32 for JICA was not tuneable at 10 lbs ,you could barely crack the high side. Took it to 15, now the high side came out to 1/2 or so and was bloody fast. If I'd have left it at 10, which is "some" , they would have went to 105th , in Jacksonville an been uncompetitive instead of destroying the class like they did. So , you can't tell me it makes no difference, seen it too many times. Tilly's need their Pop off within a couple pounds or so off where they should be or it's not gonna work like it should. Al, we do this every five years or so. And I hope we can continue. "Some" Pop off is not good enough for all situations.
 
We used this routine for 3-hole can Senior Sportsman, 4-hole can and piped classes.

A lot of guys like to set the pop-off as low as 7-8 pounds. I found 11 worked best for me. 8 was just to finicky to find the sweet spot. Here is the routine we use.

High needle 1/8 to 1/4. You can find the best setting by revving on the stand. Tiny changes until the high end gets crisp and quick.

Low needle. Start motor at 3/4, hit the track at 1 1/4, pace laps at 1 1/4, start race at 1 1/2, open 1/4 after each two or three laps till 2 1/4 out. Back down to 1 1/4 on yellows and start all over. This was for dirt speedway. The 2 1/4 is judgemental or best EGT. I really never could see the display so mine was always by the seat of the pants. Typically after the race it was around 2 or a little less for the can classes.

DK
 
You must log in or register to reply here.
Back
Top