I have a 1978 Bayliner w/ a AQ170 Volvo engine. It has three single barrel carbs. Can anyone please tell me the most accurate way to adjust the air/fuel mix on the three carburators. The engine appears to be running rich. (Black exhaust and carbon build-up on the plugs)
08-31-2001, 03:41 PM
" I use a combination of methods to adjust mine.
First, make sure you have good water flow to the engine and it is fully warmed up. Then get the idle speed stable where it won't stall. Hook up a tach, preferably an old one with a meter rather than digital for ease of reading. Start at the middle carb: screw the mixture plug in slowly until you get at least a 50rpm drop on the tach, then back out the screw about 1/8 turn. If you don't get the idle drop, lightly bottom the screw and back it out 1 1/2 turns then try again. Then do the front one, then rear one. Actually, I guess the sequence doesn't really matter. A word of caution: the mixture screw is a long pointed thing that is easily damaged, so be carefull you don't jam it into the bottom of the threads. If you have a flooding carb, you'll likely bottom the screw and not get the idle drop.
Once you have the idle mixtures set, it's time to synchronize them. You'll have borrow or buy what is a called a "Unisyn". This is a device for measuring air flow. A good quality mercury manometer can also be used, or anything that can show relative air flow. With my unisyn, I took a 3 pound coffee can and cut a hole in the bottom large enough for the Unisyn to fit over. The open end of the coffee can fits quite nicely on the top of the carb. Then you go from carb to carb matching the relative air flow at idle with the idle speed screw, making sure the throttle is returned to full closed after each adjustment. You alternate from the air flow check to the mixture check until they are balanced.
A down and dirty way to deal with it would be to do the mixture (first) adjustment. If there is one that won't give a lean drop, they need overhauling and then you just hire someone to do it.
Hope this helps. "
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