Forget throttle tip-in, driveability, and vacuum at idle. Set the thing on Kill Mode via thimble-sized fuel jets, disregard the power valve, and let the carburetor dump as much fuel as possible down the throat of the manifold--then let it rip down the track. But what do you do when you need to drive it home? Here's where some of those basic carb-tuning skills come into play.
In most cases, dualities contradict. Not with a carburetor, though. The beauty of these old-fashioned gems is their ability to carry out different duties with the turn of a mixture screw and the swapping of fuel jets. The carburetor's ability to exercise dual workloads is the sole reason they are still around. Don't fix what ain't broke, right? Carburetors are not only simple to work on, but with a little basic knowledge your pig could become a sleek cheetah. It's easy to upgrade various components, such as manifold swaps, nitrous, or even gear swap changes on the car; driveability needs to parallel these situations accordingly. In the world of high performance, that means setting the tune for the track but then being able to set up for street duties. This may mean a slightly lower idle or possibly a bigger power valve.
To showcase the basics of carburetor tuning, we plopped on an out-of-the-box Mighty Demon 750 carburetor onto a stroker 383 small-block at Vaca Performance and Dyno in Downey, California. It was there we were schooled by John McKindley on basic Demon carburetor functions. McKindley explained what to look for and how to adjust the Demon for the perfect street tune. So read on as we illustrate our findings to teach you a few new tricks--or maybe remind you of old ones--for tuning your Demon for the street.
What We Did
Tuned a Mighty Demon 750 (PN 5402020GC) for the street on a low-compression 383 small-block
Get your carburetor to work more efficiently and make more power with less fuel
What does a power valve do? In a nutshell, it allows the carburetor to function much leaner for part-throttle fuel economy. However, once the throttle is in a wide-open position, vacuum in the manifold drops to almost zero and the power valve enriches the fuel mixture. The cool part is that when the engine is returned to basic part-throttle operation and vacuum rises, the power valve closes up, leaning out the mixture. This makes a power valve important because of its effect on everyday, around-town drivability. For instance, once the vehicle is put into Drive, the vacuum drops, sending a vacuum signal to the carburetor and telling it to open. This could make the engine go rich with fuel. The engine could start to blubber and run inefficiently. A rule of thumb: One power valve number is worth 6-8 fuel jet sizes.
38 Timing & 76/83 jets
The 383 was making nearly 380 lb-ft at a mere 3,000 rpm with a peak of 399.6 ending at 4,900 rpm. Horsepower came in a bit later as usual, with 398.4 hp at 5,500 rpm. But brake specific fuel consumption (BSFC) is what we were after. McKindley explained, "BFSCs are important because they can define how efficiently the carburetor is working." Our goal was to make the most useable power from the least amount of fuel. Ideally, we want BFSCs in the mid to low 0.400 range. BSFC are an indication of engine efficiency, not carburetor efficiency! A finely tuned engine with a higher compression ratio can get into the low 0.400 range. A 0.590-0.600 range is appropraite for 8:1 compression engines.
38 Timing & 73/80 Jets
For our second pull, we were curious to see what a leaner fuel environment would do. We swapped in smaller 73 primary and 80 secondary jets, but kept the timing at 38 degrees. To our surprise, the leaner fuel condition actually dropped power across the board. We even backed up this run with another one just to confirm the numbers.
38 Timing & 78/85 Jets
For our third pull, we opted to swing in the other direction by adding fuel. We popped open the bowls and installed an 78 primary and 85 secondary jets., while leaving the timing at 38 degrees. Our efforts didn't go unnoticed, as the increase surpassed our baseline numbers.
36 Timing & 78/85 jets
During our testing we were curious to find out what lower timing numbers would produce on the dyno while still utilizing the 78/85 primary/secondary jetting in the Mighty Demon. On average, the results illustrated that we were using more fuel (average and peak BFSC were 0.590 and 0.661) through the dyno pull compared with the previous average BFSC of 0.558. Horsepower and torque numbers were minimally affected. However, because we were using more fuel and made less power, we deemed the combo inefficient.
Air Bleeds And Idle Mixtures
Air bleeds work just like fuel jets, but instead of controlling fuel flow they allow more air in. Air bleeds can be adjusted three ways. The low-speed bleed is for idle; the mid bleed is for partial throttle; the high-speed bleed is for WOT. Mostly though, the mid can be adjusted for driveablitly since it pertains to partial throttle situations. Air bleeds change or reform the fuel curve. A large air bleed jet leans out the mixture. Example: If a good fuel curve is present but leans out slightly in the higher rpm, you can change the high-speed air bleed to clean up the fuel curve.Idle mixture adjustments change the air/fuel mixture. Turning them "out" or "in" will either fatten or lean out the fuel mixture.
40 Timing & 78/85 Jets
After reviewing the results, McKindley suggested we add a couple degrees of timing while using the larger 78/85 primary/secondary fuel jets. The added timing would burn the additional fuel being dumped into the engine., and since Goff's 383 only had 8.1:1 compression, there was little fear of detonation with the 91-octane we were feeding it. We added 4 degrees of timing to the engine, giving us 40 total, and made back-to-back pulls on the dyno. The results showed added power and used, on average, less fuel. When it came to torque, the 383 produced more at 300 rpms sooner and even carried it higher throughout the power curve.
40 Timing & 78/85 Jets & 8.5 Valve
For the final pull, we kept both the jetting and timing the same, but swapped out the stock 6.5 power valve for a larger 8.5 valve on the primary side of the Demon. The results didn't show any gains in the peak or average numbers, which made us revert back to the previous tune.
Mighty Demon 750
1. Annular boosters allow the fuel to be atomized more efficiently through the use of many small holes.
2. Billet aluminum base plates for strength.
3. Air bleeds located on the outside along the air cleaner ring.
4. Primary throttle linkage for adjustment.
5. Secondary throttle linkage for adjustment.
6. Curb idle mixture screws are located on each side of the metering block and control the amount of fuel mixture to be discharge at idle.