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....For fuel mileage I'd consider getting a wideband O2 sensor and dialing in that carb. What you will probably find is that the carb tuning is hard to get perfect everywhere and fuel injection is easier to get fine tuned...
I am no expert by any means but... If I can pull 15 mpg from my 454 which is untouched with dual air pumps you should be able to do at least that with your set up.
Alright guys, first post. ...I'm kind of a newbie at this: my experience so far only encompasses bolt-on mods. The Vehicle:1985 C10 shortbedlow-mileage GM Performance 350 long-block crate engineEddy Performer IntakeEddy Performer 600cfm CarbEddy TES HeadersMSD 6AL2.5" exhaustI carefully pulled off all of the emissions equipment, except for my fuel canister. I've also plate-blocked the EGR boss. Timed vac to dizzyPVC to PVCFull vac is tee'd to carb bowl vent and canister <<<==== IS THIS WRONG??TH350 through 3.08 gearsP235/75R15 all the way around @ 35psiThe Issue:After properly tuning my carb, new plugs, new air filter, I am only getting an avg 9mpg (12mpg freeway). My truck seems to run best at 8*btc, just the mileage sucks. The Question(s):How do I find the perfect timing for my specific combo?Is 9-12mpg the best I can hope for with factory low compression heads/pistons?What mods necessitate a change in timing?Could I be a tooth off?
To begin: Welcome to the site from California!Consider the following...Correct the hose routing to your carburetor and canister "vents." You should NEVER have vacuum plumbed to the float bowl of the carb! This alone could cause severe mileage and drivability issues. The carb bowl should be 'vented' to the charcoal canister. The canister, depending on application, may also have a 1/4" PCV connection that tees into the PCV valve/vacuum hose, plus a ported vacuum connection to the carburetor, in addition to the carb bowl vent connection. Check the vacuum routing diagram on the radiator support.The PCV valve should be properly plumbed between manifold vacuum and either valve cover, via a 3/8" PCV vacuum hose. The opposite valve cover should be vented (optimally to the air cleaner housing) through a fresh air filter. The PCV system is a passive device that uses no power to function and dramatically benefits engine health and longevity. It also has a metered effect on the air/fuel mixture ingested by the engine.Using manifold vacuum versus ported vacuum to operate the distributor vacuum advance is arguable. There are as many opinions about this as people posting. Neither method is perfect for every application. In the process of dialing in your vehicle, you may switch back-and-forth several times before settling."Perfect timing" is best determined using a chassis dyno. For your build, that's probably an unnecessary expense. Start by making sure the mechanical and vacuum advance mechanisms are functioning properly and not binding or worn out. Set your timing for a maximum of 31° to 34° total advance (initial timing + mechanical advance + vacuum advance, combined), all in by 3,000 to 3,200 engine RPM. This will be a good starting point, from which you can deviate, plus or minus, for best drivability and mileage. When setting your initial timing, be sure your vacuum advance hose is disconnected and plugged. All said and done, 8° BTC may be the optimum timing for your app. Unless you're having a problem with the distributor you currently have, replacing the distributor may be more of a "feel good" (aesthetic) upgrade than an actual benefit.Once you get the carburetor vacuum plumbing sorted out, recheck the carburetor jetting/fuel calibration.The worst mileage you'll ever get is with the engine running without moving. The next worst mileage is during acceleration. Minimize your idle time and accelerate modestly for a potentially surprising improvement. Or, you could do what a friend of mine does... never use the brakes!
I must tackle this fuel "vent" routing. I must've read my routing diagram wrong, for I thought the fuel bowl vent was tee'd into the PCV line somewhere, which was full manifold vacuum. It didn't seem right to me, either. I've noticed that some carbs seem to delete that vent altogether. Could I just cap it? Maybe delete the canister altogether? Sounds rather counterproductive to conserving fuel....
advance it til it pings and then retard it.