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How to tune a Weber DGV Carb!! by Ben Pender |
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Its actually really simple,
assuming that your carb is in good shape otherwise. This is the procedure to be used on ANY DGV carb once it's installed on the engine, correctly "plumbed" and the linkage is connected.. Essentially the final tuning after the carb has been rebuilt and you know that it has the correct, or "close enough" jetting for your application. 1. Gently bottom mixture adjusting screw (the one at the base) and back out 1.25-1.5 turns. 2. Turn idle speed adjusting screw so that it just hits the throttle bellcrank. Now turn another .5 turn. 3. Start engine using whatever means necessary-choke, ether, whatever. It shouldn't be very hard to start, but every once in a while... 4. Nurse the car with the throttle/choke/whatever until it warms up and you can completely open the choke. Note, you may need to bump the idle up to get it to idle without any throttle. Ok, now it's idling on its own without the throttle. 5. Now SLOWLY adjust the idle mixture screw (the one at the base) and find the spot where you get the FASTEST idle speed. On a properly rebuilt/jetted carb, you shouldn't have to go too far, probably less than a full turn in either direction. i.e., the setting should ultimately be somewhere between 0.5-2.5 turns from bottoming. 6. Now that you have the fastest idle speed attainable, back the idle speed adjustor down until you get the idle to 1000 rpm or less. If you can't do that, then you might want to make sure you don't have too much ignition advance. You basically want to get the idle to spec AND have the throttle as closed as possible. (The reason for this is if you have to have the throttle open any, you begin to bring the progression ports into play which suggests that you don't have the correct jetting for you application. In this case, your idle jet would be too lean and you'd need to go to a richer idle jet). 7. Now, you have got a relatively smooth idle with the idle screw turned out as far as possible, i.e., the throttle is as closed as possible. Now, very slowly, tweak the mixture screw to again find the fastest/smoothest idle. The changes made at this point will be subtle. When you find the fastest idle, then once again, adjust your idle speed screw so that the idle is where you want it (typically 900-1000). Again,, at this point you should be screwing the idle speed screw OUT to reduce idle, the idea being to have the throttle as closed as possible and still have a stable, smooth idle around 900 rpm (or whatever you're comfortable with). Congratulations! You're 9/10's finished fine-tuning your DGV! 8. The last thing I do after this is I make sure that there is little to no off-idle hesitation which would indicate a lean mixture. The accel pump in the Weber should prevent that, but sometimes, its possible that your final mixture setting is on the lean side and so you might get some hesitation. If you do, turn the mixture screw out just a hair, like 1/16th of a turn and then run the engine at 2000rpm for about 10 seconds and then let it idle. Now snap the throttle open quickly. Hesitation? No, great, you're done. Still some hesitation? Make another 1/16th turn out and repeat. 9. Now when you're doing this, its important that you are able to tell if you are hesitating due to a lean mixture, or stumbling due to an overly rich mixture. There are several clues that will help you out here. The first is if its leaning out, you may get some backfiring through the carb. The second is if its over-rich, you'll see lots of black smoke when you snap the throttle(s) open. The third is if turning the idle mixture screw out a sixteenth of a turn makes it worse, then its over-rich. Ultimately, what you >want to see when you snap the throttle open is a hint of black smoke out of the tailpipe. No smoke, you're too lean. Just a little smoke is what you want. Now, if you've got a nice, perhaps *slightly* burbly idle around 900rpm, and an engine that snaps enthusiastically off idle, you're done! Put the air cleaner on, and take a spirited drive around the block and enjoy your properly set-up and tuned Weber! And, like the DGV, this procedure works for any Weber application. The main key to remember is that you want your idle screw having to move the throttle as little as possible, since, again, if you have to open the throttle very far, it will uncover the transition ports, and that suggests that your idle jet is too lean. One other consideration is that if you have the throttle just about closed and the mixture adjusting screw pretty far in, then your idle jet may be too rich. Another note: I keep saying that you want the throttle as closed as possible, but in actuality, having the idle speed screw turned in about a half to one turn (after making contact with the bellcrank) is about right. Any more than that and your idle jet is too lean, any less than that and your idle jet maybe too rich. So, basically, set to initial settings (mixture screw 1.25-1.5 turns from bottom, idle speed screw 1 turn in once contacting bellcrank), install. Start, warm-up engine, make sure choke is fully open, then adjust mixture screw to get fastest idle, bring idle speed back to spec with idle speed screw, tweak mixture screw to get fastest/smoothest idle, adjust as necessary with idle speed screw. Now check for off-idle lean-ness/richness and adjust accordingly. Not too hard, and no special tools are necessary, just a tuned ear! Or a hand-held tach or dwell meter with tach function. And, I'd be ashamed if I didn't tell you to go through your ignition system once you get things running with the carb. Set ignition timing, etc. Once you do that, you may need to go back and make some final adjustments on the carb, but that should be minor, unless your ignition system is way out! Hope you enjoyed this! Now, go get your hands dirty! Regards, Ben Pender President, North Alabama British Motoring Society Owner, Ben Pender's Vintage Imported Auto Repair |