Texas Two Step Taco

John Murray

Been Around the Block
Bowl vents clear and big enough? This sort of thing can happen even with a reed and it obviously reduces fuel flow, but it generally goes away at higher rpms. Check the vent and the overflow standpipe are open and don't have too much hose on them. Sufficient venting should make it go away.
 

Texasstar

Can't is a four letter dirty word
Bowl vents clear and big enough? This sort of thing can happen even with a reed and it obviously reduces fuel flow, but it generally goes away at higher rpms. Check the vent and the overflow standpipe are open and don't have too much hose on them. Sufficient venting should make it go away.
In the gas cap? Or on the carb bowl?


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Texasstar

Can't is a four letter dirty word
This article is very interesting. I don’t know if you can see it in the video but you can see the vapor being pushed out of the carburetor. https://www.hotrod.com/articles/ctrp-1008-intake-reversion/

“Airflow (pulses) delivered to or pressure differentials created across a carburetor will cause a release of fuel, whether the flow is toward or away from the combustion space, thereby upsetting intended carburetor calibrations.”


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John Murray

Been Around the Block
The carb. If it's a VM Mikuni there'll be one or possibly two vent passages that rise up through the carb body and emerge about halfway up the slide housing. There should also be a standpipe in the bowl itself that comes out the bottom of the bowl.
 

John Murray

Been Around the Block
This article is very interesting. I don’t know if you can see it in the video but you can see the vapor being pushed out of the carburetor. https://www.hotrod.com/articles/ctrp-1008-intake-reversion/

“Airflow (pulses) delivered to or pressure differentials created across a carburetor will cause a release of fuel, whether the flow is toward or away from the combustion space, thereby upsetting intended carburetor calibrations.”


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A little standoff is normal at certain speeds even with a reed. There are waves bouncing back and forth between the bellmouth and the closed reed and these can carry a little fog back out. Not normally a problem.
 

Texasstar

Can't is a four letter dirty word
A little standoff is normal at certain speeds even with a reed. There are waves bouncing back and forth between the bellmouth and the closed reed and these can carry a little fog back out. Not normally a problem.
Now that the airbox and wires are out of the way It is amazing to watch the pulse waves and see them at the carb, fuel filter, and tank. This is a TM38 Mikuni flatslide.


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teazer

Over 1,000 Posts
DTT BOTM WINNER
The fuel issue with frothing is most likely vibration causing the tank and fuel to vibrate and vaporize in the tank.

In the average intake system at any speed there are roughly 6 reversion waves per revolution, each being weaker that the the prior. pressure pulses travel at the speed of sound which is lower in a cold intake than a hot exhaust. The airflow is traveling a lot slower. We used to calculate mean air velocity to size carbs on 4 strokes. On any two stroke you can see a fuel stand off at lower revs. On something like a TZ250 it's not a fog, it's basically a standing wave.

The fix is a large volume air filter. It traps that fuel and more importantly, you can't see it so you stop worrying about it.

I wouldn't worry about what happens at low revs and low load, but rubber mounting the fuel tank should help a little. You could also try moving iy further forwards to try to minimize the effects of vibration. Try it in front of the forks to see if that's any better.

How rich is that jetting on the pilot jet?
 

Texasstar

Can't is a four letter dirty word
It is a Paris Hilton pilot jet. :) checked the flow and we are flowing 650ml per minute into the carb. Moved the tank to the outside of the mounts away from touching the frame.

This is fascinating to watch. Put the video on
IMG_0299.jpg

slow mo and you can almost see the pulse.


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Texasstar

Can't is a four letter dirty word
I can see why this becomes addictive. after and before on the race tech titanium sprocket bolts for an xr650l. You’ll notice the xr650l have 10mm bolts but we are converting to 8mm. Goin to sleeve the hub holes. 2.3 oz now 8.35 oz before.
Adjustments.jpg

IMG_0350.jpg



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Texasstar

Can't is a four letter dirty word
Going to try fuel cell foam it will leave us with 97% capacity



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Texasstar

Can't is a four letter dirty word
Removed the tank from the frame. Doug suggested hanging it from straps.


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teazer

Over 1,000 Posts
DTT BOTM WINNER
isolation from chassis resonance is the way to go.

Isn't it amazing to see how vibration is amplified? Suzuki triples are really bad for that at 1500 - 2000 rpm where everything shakes. Put a fairing on and it gets worse because it all resonates. BSA twins used to shake the front wheel back and forth at idle.
 
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Texasstar

Can't is a four letter dirty word
isolation from chassis resonance is the way to go.

Isn't it amazing to see how vibration is amplified? Suzuki triples are really dad for that at 1500 - 2000 rpm where everything shakes. Put a fairing on and it gets worse because it all resonates. BSA twins used to shake the front wheel back and forth at idle.
I wonder how many Bultaco riders have ever opened their gas caps and rev’d it and watched. I think they would be surprise to see how oxygenated their fuel was. I have an old pair of motorcycle boots I am going to use the leather straps off of to suspend the tank from.
Pretty sure I just broke some grammar rules.

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Texasstar

Can't is a four letter dirty word
Made aluminum spacers for the Xr650l hub conversion to 8mm racetech titanium sprocket bolts and nuts. Made the aluminum spacers .005” bigger than the hole and then froze them and pressed them in. Used an O drill bit which is a .001” over 8mm. I need to get a set of reamers. Aluminum JT sprocket came in yesterday. So Juan Verdes donated XR650L hub and wheel are a whole
IMG_0433.jpg

lot lighter. Just some trivia. My first motorcycle was an XR650L Honda. Also the new bearing retainer came in. Still need to peen it.

IMG_0431.jpg

IMG_0428.jpg

We are sitting at 9.7 lbs without the brake rotor and hardware. Our new rotor weighs 13.6 ounces and should be here by thanksgiving. How is that for Amazon service? Will use this one until it gets here.

The XR650L caliper and bracket will be here this weekend and then we can make a few modifications to the swing arm for fitment.


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pidjones

Over 1,000 Posts
If any more vibration abatement is needed (and this goes against all of the other work you are doing), try adding some mass to the tank. You might be able to tune out the resonant frequency.
 

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