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Tiny carburetor cameras maybe?


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I have an issue (dual carb xr350) where going from idle to full throttle causes a partial stall and I want to know what's going on inside the carbs.

How much fuel is leaving each main jet when the stall happens and how much is leaving when the engine is running correctly? Is the fuel atomising properly etc..

I'm sure I could fix the issue by trial and error but I really want to know what the problem actually is..

So how can I find out what's going on? How do carburetor designers monitor what was coming out of their main jets? Tiny cameras? Glass carbs? hot-wire anemometers even? Anyone here ever tried to monitor how much fuel is shooting out of their main jets?

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Almost all dynos will have a wideband A/F monitor. This will provide a graph and you can see the deviation on the graph. This will give you an idea of what to adjust in the carb(s)

edit-when I have a driveability issue, my experience has led me to..

-reset carbs to factory specs including pilot size, clip position etc and only deviation is the mainjet size, which is dialed according to your altitude...and reassess after thats been done, assuming the motor isnt hammered. if the motor is tired, give it some rings/piston

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When you go from idle position to full throttle you are seriously upsetting the balance of air flow and fuel flow within the carburetor. Newer carburetors have accelerator pumps to add an extra shot of fuel during transient throttle conditions in order to keep the engine fed until the balance is re-established. Fattening up the pilot jets and needle positions helps but it hurts you in the steady state. Carburetors are not a science, they are a compromise. No there aren't mini cameras or flow meters on the jets. The best carb manufacturers can do is calibrate the carb on a flow bench prior to installing it on the engine. It would be too labor intensive to try to tune every carburetor on a mass produced engine.

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The cure for your problem is to replace your twin carbs with one tiny little one from something like a 125. That way, the bore of the carb will be small enough to provide a good vacuum signal at full throttle at a 1500 rpm idle.

Either that, or you will need to learn that you just can't expect the engine to be able to deal with the throttle being snapped open from idle to WOT.

At idle, the engine is running only on the pilot circuit, and no fuel flows through the main discharge nozzle at all. The carb is designed so that as the slide rises, allowing more air in, the pilot circuit can handle the demands of the accelerating engine for the first bit of throttle opening. This allows the engine to develop an air flow over the main nozzle to raise fuel through and get the main circuit started. As this process begins, the throttle can be opened farther and faster without the engine stumbling simply because the main circuit is operating and can respond to the demand.

If, however, you snap the throttle open at idle the suddenly opened throttle creates a drop in intake vacuum that not only prevents the main circuit start up, it can cause the pilot circuit to shut down. The engine then ceases to run altogether. Even with a properly tuned accelerator pump, a big single can't always deal with such a maneuver.

The engine does need to be able to respond reasonably well to the throttle being opened quickly, but not suddenly or completely from fully closed at such low speeds. You need to learn to "roll" the throttle open; opening the throttle just slightly, and increasing it as rapidly as the engine will respond to it. It's a technique as old as motorcycles themselves.

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Well I'm fairly old myself and I've owned several large singles (fantic 350, xt500, cr500, cagiva wmx500, Sherpa 350...) none of which behaved like this. I'm pretty sure it's not running like an xr350r should and it has new top end including valve train and clean carbs at factory settings.

So no ideas for monitoring what's coming out of the main jets? I'm tempted to try cameras like this looking in through the intake boots but the resolution looks kinda low..

I know it's not economical, I just think it would be an interesting way to approach the problem.

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Well I'm fairly old myself and I've owned several large singles (fantic 350, xt500, cr500, cagiva wmx500, Sherpa 350...) none of which behaved like this.
Two strokes don't count; the get an intake pulse twice as often and can tolerate the throttle snapping open much better.

I can tell you what you would see coming from the main nozzle if you could watch. Nothing. Or very little, at least. The way to improve it is to have the pilot circuit slightly rich (not too rich, or you encourage hard hot starting and stalling when closing the throttle), reducing the major diameter of the needle, and reducing the cutaway of the slide.

As I said, the carb can be and should be reasonably responsive to reasonably quick partial openings of the throttle, even at idle. It won't be capable of responding to instant full throttle snaps from idle unless you compromise other aspects of its performance to achieve that. You have to determine what "reasonable" means.

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A camera isn't going to tell you the fuel flow amount from the jet. You could probably double fuel flow and not be able to see it on camera. You can see fuel coming from the jets, I've watched this on flowbenches and dynos. You can go from dying rich to dying lean, and you aren't going to be able to judge which is which from the fuel mist leaving the nozzle.

Carb manufacturers do pretty much all calibration just running on an engine. After acceptible performance is found, then it will go on a flowbench. You need to spend some time doing some good ol tuning. As others have said, going from idle to WOT in a quick snap is a difficult task for any engine. Some engines / carbs deal with it better than others.

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A camera isn't going to tell you the fuel flow amount from the jet. You could probably double fuel flow and not be able to see it on camera. You can see fuel coming from the jets, I've watched this on flowbenches and dynos. You can go from dying rich to dying lean, and you aren't going to be able to judge which is which from the fuel mist leaving the nozzle.

But you can use an optical sensor to measure the opacity of the fuel/air charge and relate the opacity to a ppm content of fuel or a ratio. That's not quite a camera, though.

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There's an idea. When you say 'you can' do you mean this is actually a practiced method or are you theorising? I guess you'd just need a light source, light dependent resistor and a multimeter outputting to a laptop. I was thinking along similar lines using a hot-wire anemometer for the resistance but the LDR would be simpler assuming you could get the sensitivity.

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When I say you can I mean it hasn't been done yet and may not be possible because I'm not an expert in that field but...

I've drank enough beer tonight (been breaking down a seized Husky 510 motor) that I've had an out of the box idea.... So, it's important that we all understand that, on a fuel injected engine, as you roll on the throttle the O2 sensor has nothing to do with the fuel/air mixture. As you roll on the throttle, the computer has preset fuel injector pulse widths (fuel flow rates) for each throttle postion, engine rpm, intake air pressure, air temp. The O2 sensor only comes in to play during steady state conditions where it "trims" the fuel/air mixture to bring things closer to ideal. The problem is that the O2 sensor cannot respond fast enough to be used to affect the fuel/air mix during transient conditions. So, why can't we measure the fuel/air mixture before the cylinder or even in the cylinder to get a faster response? I think optical systems are capable of this. They could be reasonably reliable in the intake tract. In the cylinder they would require frequent cleaning.

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