A Let's see if we can get you past that early lean backfire. First of all, that is quite a camshaft you're trying to run on the street! Also, your Weiand Tunnel Ram is a race-only drag piece. Years ago a few manu-facturers produced what they called street tunnel rams. They had reduced the runner sizes to better suit street rpm ranges. You're going to have a real tough time transitioning from idle to full power with your package. What the engine is looking for is a nice shot of fuel to wet the walls of that tunnel ram and get enough fuel to the cylinders before the air gets there. That is where your lean backfire is coming from. Also, your very low 5 inches of idle vacuum is getting a weak signal back to the carburetors. The idle circuit works OK because you have two carbs, and the idle feeds are below the throttle blades. When you open the throttle, the signal must activate the main circuit. This delay between the idle circuit, the accelerator pumps, and the main circuit is the lean bog. You may be able to tune around some of the lean transition by reducing the main circuit air bleeds. These air-bleed restrictors are located in the venturi area above the booster. The idle air bleed is located toward the outboard side of the carb, and the main air bleed is toward the inboard side of the carb. If you have some fine stainless steel wire, you can insert it into these air bleeds and see if it improves your drivability. With the reduced air bleed, the main circuit will come in sooner.
Also, you may want to look at the accelerator pump cams. You may be able to tune the squirters to get the fuel in on the early part of the throttle opening. Your 4160-model carbs are somewhat unique in that they are mechanical secondaries, but are only a single-pump accelerator system. Your carbs should have 0.031-inch accelerator squirters. That is quite a large squirter, but you can swap out the accelerator pump cam for a more aggressive one; this will help get you past the transition. Holley offers the Accelerator Pump Cam Assortment Kit (PN 10-12) with eight different ramp profiles to help tailor when you want to squirt the fuel. The only downside is that you'll need two kits. Next you need to swap out your power valves to a lower vacuum level. With only 5 inches of idle vacuum, you will want to go to a 3.5-inch valve. The rule of thumb is that you want your power valves tuned to 2 inches below the idle vacuum of the engine. The power valves are sold under PN 125-35.
To help your idle vacuum, you can increase the idle spark advance. We would recurve your ACCEL distributor to limit the mechanical advance. We like your 34 degrees total, but we'd like to see around 16 to 18 degrees at idle.
If all else fails you could swap out your very aggressive camshaft to a tamer one. But with the tuning advice listed above-and a little patience-you'll get there.
May Pops!QI recently installed a 410ci small-block in my '97 two-wheel-drive truck. It was built with decent parts and I ran it on a rear-wheel dyno, making 367 hp. What does that translate to on an engine dyno? One other thing, the rear-wheel diameter is 29 inches with a 3.70:1 gearset. The dyno techs ran the engine up to 6,800 rpm in High gear (it's built for that or more), which translates to over 150 mph wheel speed. The tires are rated for a max of 112 mph. I asked about this deal before they ran it, but they said they know about it and had never had a wheel blow on the dyno! Of course you know the weight of the truck is actually increased by the tie-downs. Why didn't they blow?Bowtie LaVigneVia e-mail