So I've been battling this carb for three days now..... maybe more. It is a quadrajet, 17059288 to be exact. I traded Joe the one that was on the motor, an Edelbrock something or other i have no experience with for the quadrajet off one of his parts boats. The boat had a 350 in it so i thought a good cleaning would be all that it needed. After getting the carb back on the motor I did the usuall timing and idle adjustments. I have a few guages hooked up to keep an eye on oil pressure, temp, rpm and a vacuum guage for the idle tuning. I got the engine running pretty good but noticed the idle mixture screws had zero effect on the vacuum guage or rpm. The engine was getting the fuel from some place else. For nothing more then not understanding how the magic inside a quadrajet works i thought the transition slots above the idle mixture screws were feeding the engine the extra fuel and that those slots needed to be completely covered. Ok, tear it off and figure out what needs to be adjusted to achieve that. Before i go further you should know i have three other quadrajets i can rob parts off. Knowing what i just stated i tried bending the primary throttle blades down a bit to cover those slots. Back together it went and nothing changed. Ok, back off, back apart, what's next? Bent blades off and spare set on. I have a Cliff Ruggles book on the quadrajet so I started paging through the idle section. I didn't find anything that screamed this is your problem so i put it back together and just started going to the extremes with the adjustments. Lots of timing, idle mixture screws seated, 5 turns out, idle speed screw yada yada yada. At one point i knew the fuel was overloading and it died. I hit the key just to hear the bendix gear engage the flywheel and lock. Fuel hydro lock. Out come the plugs and of course it's #1. The last plug pulled. I had a spare set of plugs so in they went. At this time i also removed the 90's from the exhaust added some 3" pvc extensions to the exhaust. When i had the 90's on i was getting water all over the garage floor. Ok, back on and running.... my God a few minutes in and the inside off that perfectly white pvc is now black with soot. Ok that explains the crap floating on the exhaust water. A dark film almost like little pieces of torn up paper. They didn't have that oil or fuel on water look to them. Next i Google a bunch of stuff similar to rich idle.... nozzle drip seems to be the common issue in most the stuff i read. Nozzle drip was happening because the idle speed screw needed to be turned in to get enough idle air which allows that air to pull fuel out the primary nozzles. A lot of fuel in my case. Apparently this issue is common in modified engines. I know the po had the engine rebuilt and I was told it was 290hp. I figured they sold him what he wanted to hear and the engine wasn't really modified past rebuilt. I may have been wrong. Anyways, back to the troubleshooting. I bet I've had the carb off and a part no less then ten times. I'm pretty good at it now. I decided to call up one of my old coworkers and run some of this by him. The guy, Preston, is methodical and knows quadrajets. I wasn't quite done explaining my issue when he says i know what you need to do. "Idle bypass air." Somewhere I read most marine carbs do not have it factory, all can be added though. Mine was easy enough. A couple holes, one modded gasket and we were headed in the right direction. Back on and running i noticed more travel from the idle speed screw before the nozzle drip started. Ok, off it came, back apart it came. In Cliffs book he has a few different recipes for the hole diameters in the idle circuitry based on a few 350 cam and compression ratio setups. I picked one similar to a merc 260 that was slightly built. I took those numbers and upsized just to the lean side of his windows. Though i did up size the idle tubes to 0.040 from 0.030. I also upsized the idle bypass air a bit more to 0.088. Back together and on the engine ready to check the results and Preston calls back from a message I left earlier. We go over what I upsized and right away I can hear his eyes rolling. He tells me in all his years of building big cubic inch Pontiac motors he has never seen an idle tube upsized to 0.040 and it is going to be way too rich at idle.... Good thing I have spares. 0.040 was the smallest bit I had, guess I'll order smaller bits. The rest of the diameters sound good and could be upsized more depending on the needs of the engine. Start small... We settle on about 0.033-0.035 for the idle tubes. After our call I resumed testing the new adjustments. I now have mixture control with the idle mixture screws, good vacuum and zero nozzle drip. The exhaust is still floating the dark stuff on the water but im attributing that to my idle tube over sizing mistake. Il report back after i get the replacement tubes installed.
Here are some pictures of the idle bypass air install. The red X's show the nation body holes that needed to be drilled..
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