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The really accurate ones like some dyno shops use have a 'wideband' exhaust probe, but I've seen some advertised that use a regular emissions-control oxygen sensor for basic red-green readings. I think K&N makes one, for example. I've always wondered whether it would be worth getting one of these; what kind do you have?
Anyway, assuming that's what you've got, I can see that it might help jet your carb by taking some of the 'seat-of-the-pants' element out of the process. Dyno-shop guys can stick the broadband probe in the tailpipe and give you a graph of the exact air/fuel ratio throughout the entire rev range, but most of us doing it on our own have to rely on "feel" to decide whether the mix is too rich or too lean at any given point. Too lean give a very sharp stumble, as if you've suddenly run out of gas. Too rich gives a soft cutoff; as you rev, the engine just seems to go "soggy." So what you wind up doing is running through the gears under various conditions, noting where power delivery stops being smooth and trying to guess whether you've hit a "too rich" or "too lean" condition, then figuring out what carb adjustments are needed to fix that.
So, you're going to have to do the same thing, but if you've got a simple F/A ratio meter at least it should be a bit easier to figure out whether you're too rich or too lean at the "problem" areas!
Next question: What kind of carb do you have in your Sonett? I'm going to assume it's some kind of Weber, because if you're still running the original FooMooCow you can't even GET any alternate jets for it, so you're basically hosed!
If you've got a Weber, the exact steps for jetting will depend on which model you have. I won't try to get too detailed about this because (a) I'm not a carb guru and (b) there are so many variations from one carb model to the next. But here's a really basic outline of a procedure that has worked well for me when dinking with various models of Weber carbs on various V4s:
1) Make sure everything else on the engine is up to snuff: compression, electrical, tune-up specs etc. I know it's tempting to skip this, but don't! The reason it's important is that carburetion is based on the air being pumped through the engine; if your "pump" isn't pumping evenly, you'll never have a smooth enough flow to get the carb right!
2) Grab the data sheets for your particular carb off of one of the many Weber sites on the Internet, so you'll know where the different adjustments are located. Now, take notes on everything that is presently on your carb: all the jet sizes, all the settings. (If someone had done this before rejetting your carb for high altitude, you could now just go back to the original settings and save yourself a lot of headaches!)
3) Start by setting the idle speed and mixture according to the data sheets; you have to begin this way on a Weber because the "idle" circuit actually supplies everything up to about 3000 rpm, so if it's messed up you won't be able to set the main circuit either. If you can't get the engine to idle well at the settings on the data sheet (for example, if you have to turn up the idle speed to keep it running) you may need to change the idle jet. Your A/R meter should help.
4) Once you have the idle circuit set up correctly, you can move on to the main circuit. On most Webers there are at least three factors you can play with: main jet size, air corrector size, and emulsion tube.
-- Main jet controls mixture from the "transition zone" (where it starts taking over from the idle jet) up to wide-open throttle (WOT.) If low-speed running is good but you're too rich or too lean at normal highway cruise speeds, this is the most likely thing to change.
-- As you approach WOT, the air corrector kicks in to keep the mixture from going too rich. It bleeds additional air into the main circuit and leans it out. So, if the car runs well at highway cruise but goes too rich when you stand on the gas, you may need a larger air corrector; if it goes lean at full throttle you may need a smaller air corrector.
-- The emulsion tube helps mix the air into the fuel, but it also controls the point at which the air corrector kicks in (via the spacing of the holes up the tube.) Changing emulsion tubes is real guru stuff that you're probably best off leaving alone. With most Webers the float level also has some effect on where the air corrector kicks in, because it determines how far up the "main well" the fuel sits. So, if your jet sizes seem good but you need to change the transition point SLIGHTLY, you might want to try doing it by raising or lowering the float level 1 or 2 mm rather than by dinking with emulsion tubes.
As you can see, it's an intricate process but also can provide hours of fun. Just remember to change only one setting at a time, change it only in the smallest increment (since your car basically runs already; you're just trying to "dial in") and keep good notes on what you do!
posted by 204.76.11...
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