Tuning Two Wildly Different Holley Carburetors Engine Combinations

Words And Photos: Jeff Smith

The plan was simple – three of us were going to do a day-long cruise up the coast of Southern California to attend the annual Morro Bay Car Show in the spring. Doug Eisberg was going to take his 496c.i. big-block-powered ’69 Nova and Eric Rosendahl would be driving his 350c.i. small-block-powered ’66 El Camino. Both engines employed 4150 style dual inlet Holley carburetors, but that was where the similarities ended. The cars could not be more different as Eric’s small-block was a mild small-block cruiser while Doug’s 496c.i. Rat sported 10.5:1 compression, aluminum Dart heads, and a decent mechanical roller cam.

Before we took off for the cross-country cruise, the boys decided that a little carburetor tuning was in order to get the engines ready for the near 700 mile excursion. Both took their cars to the local muffler shop to have a wide band oxygen sensor bung welded in one side of the dual exhaust near the header collector so that we could employ an Innovate Motorsports wide-band oxygen sensor to help us dial in the carburetors. The Innovate would be the essential tool that would help us improve the drivability along with better highway and in-town fuel mileage while also dialing in the wide-open throttle (WOT) performance. Our goal was to improve power, fuel mileage, and drivability.

The best way to do this would have been to put the cars on a chassis dyno and spend the $500 or $600 to get everything dialed in. But since we already had an Innovate meter, we decided to use the local freeway as our chassis dyno. The only negative to this effort was that we would not be able to prove (or disprove) WOT power improvements. We left that up to the seat of the pants dyno.

This entire tuning effort would be based on using an Innovate Motorsports hand-held LM-2 air-fuel (A/F) ratio meter. We used the basic kit (PN 3837, $349 Summit Racing). There are many other wide band oxygen sensors to choose from including systems from Auto Meter, BigStuff3, FAST, Holley, and others. The Innovate has the ability to data log both rpm and A/F to plug into a laptop using Innovate’s free LogWorks software. Since we neglected to bring the laptop, we decided to keep it simple and just monitor the data on the LS-2 and take a few notes on a cheat sheet that we later used as a reference for the changes we wanted to make. This is a bit old school, but it allowed us to get where we needed to go.

Doug’s Nova

The big-block Nova was first up. This engine was using a 830 CFM Holley carburetor (PN 0-80509-1) with annual discharge boosters. Doug had also added a larger 1.750-inch throttle bore base plate so he could drill a hole in the back of the base plate for a PCV valve. The base plate was drilled with 1/8-inch holes in all four throttle plates for plenty of idle bypass air, which was overkill for this application. Once we calibrated the sensor and powered up the Innovate LS2 hand-held tuner, the LM-2 quickly informed us that Doug’s big-block was idling at a rather rich 11.5:1 A/F. A check of the idle mixture screws revealed they were turned 3 ½ turns out. By resetting all four to just a tad under 1 ½ turns, the idle A/F quickly leaned out to a surprisingly stable 14.5:1 A/F in gear with the engine at 180 degrees coolant temperature.

This pushed the idle speed up but we were just able to bring it back down to 950 rpm in gear by backing off both the primary and secondary idle speed adjustments. We didn’t do it at this time, but Doug will braze the secondary holes closed which will allow more idle speed adjustment with the idle speed screws.

A quick run up some city streets and a short freeway cruise revealed several problems. Both light acceleration and freeway cruise in overdrive produced very rich mid-11:1 A/F ratios although the deceleration A/F was surprisingly lean. Stock jetting for this carburetor is 86 for all four corners, but this carb was slightly leaner with 84 jets all the way around. It also had power valves in both the primary and secondary metering blocks which were actually good with lower numbers of 5.5 on both ends. This meant that the power valves would remain closed until the manifold vacuum fell below 5.5 inches Hg.

One thing we’ve noticed from tuning carburetors – especially those with big camshafts – is that they tend to run very rich during deceleration. What was surprising was that this carburetor tended to deliver a very lean 14:1 A/F even under very high engine speed deceleration. We called Holley to ask about this and because this carburetor is designed to be used in circle track racing, the metering blocks are drilled with anti-siphoning bleeds that prevent pulling a rich air-fuel ratio under high manifold vacuum deceleration. Holley’s Jon Sams told us that it’s also possible to add this to an existing metering block. We will look into this a little further and deliver updated information in a subsequent story.

Because this engine tended to run very rich at cruise yet the idle was very lean, we suspected that the throttle opening was enough to actually signal cruise speed fuel delivery from the main metering circuit. In most cases, engines with big carburetors tend to use idle circuit fuel for light throttle openings. But in this case, the annular discharge boosters are sensitive enough that they began to deliver fuel much earlier so that at a highway cruise rpm of around 2,300 rpm, this carb was operating on the main circuit.

Based on what we learned from the Innovate wide-band, we decided to lean the primary jets down four sizes to 80’s while richening the secondary to 86. This change would shift the WOT A/F ratio slightly leaner while hopefully leaning out the freeway cruise A/F ratio. The engine also had a mild stumble off idle that we decided was caused by our somewhat lean idle A/F ratio so we opened up all four idle mixture screws with a tiny adjustment that was enough to eliminate the hesitation.

The second test drive delivered a leaner light acceleration numbers and the highway cruise improved slightly to 12.6:1 , but that is still way too rich if we expect to see any highway fuel mileage improvements. We had to move on to the second car but it appears that reducing the primary jetting to 78 jet will help with a change to a 88 secondary jet to make up the difference. But this still will probably not get us exactly where we would like to be. We could also improve this situation by slightly increasing the size of the power valve channel restrictor in the primary side to deliver more fuel on the primary side when heavier throttle reduces manifold vacuum. This will prevent the A/F ratio from spiking lean at 30 to 50 percent throttle opening.

Overall, Doug reported that the car smelled much leaner and seemed to respond to the throttle better and the light acceleration seemed a bit crisper. These were incremental changes, but still an improvement. A week later we quizzed him about how well the engine performed under cold start (60 degrees F) conditions. He said the engine did require manually maintaining the idle for the first minute or so to keep it running until sufficient heat built up in the manifold to vaporize the fuel. This is because the carb obviously has no choke. But this is a small price to pay for a three A/F ratio improvement at idle from 11.5:1 to 14.5:1. The engine and its spark plugs are much happier now.

496c.i. Nova A/F Ratio Test Results

Eric’s El Camino

The El Camino was nearly the complete opposite of his friend Doug’s big-block. The 350c.i. small-block Chevy has a mild hydraulic roller camshaft with a near-stock idle backed by a TH350 trans and a 3.31:1 rear gear. Eric calls his carburetor a Heinz 57 because it is a mix of all sorts of different parts. It started life as a normal Holley 3310-1 which received a Holley rear metering block conversion (Holley PN 34-13, $50.20 Summit Racing) that was followed by a Proform throttle body conversion (PN 67101C, $125.97 Summit Racing). So the only remaining original Holley parts are the baseplate, the float bowls, and the front metering block. A quick test drive monitored by the Innovate LM-2 delivered some interesting results. This engine was also suffering from a mild off-idle stumble. Cruise A/F was decent at 13.9:1 but light acceleration was not good and the WOT A/F ratio was terribly lean. You could feel the engine struggle at WOT on the freeway.

We returned to the garage and tore into the Holley. At first, we thought we’d fixed the off-idle stumble when we discovered the accelerator pump linkage was very loose. A proper adjustment helped, but didn’t eliminate the stumble. We tried three different accelerator pump nozzles (stock was a 31) both rich and lean with no success which seemed odd. Then we realized that because the LM-2 was telling us that both light and heavy acceleration was very lean, so richening the primary jets would probably solve most of these problems. The jetting was 72 in the primary and 75 in the secondary with a blocked secondary power valve. This engine needed more fuel so we increased the secondary jetting four sizes to a 79 and an equal number of jet sizes in the front to a 76.

After reassembling the carb, a test drive revealed light acceleration improved dramatically to 12.5:1 A/F while the idle came in at 14.7:1. Unfortunately, the WOT A/F had only improved to 13.8:1 – still too lean. We still had work to do. Back at the shop, we maintained the primary jet at 72 and bumped the secondary jetting from 79 to 85.

By now, the day was getting late and the planned dinner party was fast approaching so our last configuration would have to suffice – at least for the near future. A test drive revealed a WOT A/F of 13.2:1 so that improved. The highway cruise test showed an acceptable at 13.5:1 although we’d like to get that into the low 14’w. We richened the idle mixture just a touch, which did fatten up the idle to an acceptable 14.2:1 with light acceleration now also good at 14.1:1 with no noticeable stumble or hesitation.

El Camino A/F Ratio Test Results

Conclusions

Some of the things we’ve learned from tweaking carburetors for better drivability and mileage is that even big cam motors will run decent at lean A/F ratios at part throttle if the ignition system and timing are idealized. Some enthusiasts believe that running a lean part-throttle A/F mixture will make the engine run hot, which isn’t true. But ironically if the A/F ratio is excessively rich (richer than 12:1), that will make an engine run hot because the combustion process continues as the exhaust valve opens and combustion heat is exposed to the cylinder walls, transferring more heat into the cooling system.

The other big thing we’ve learned is that you can tune even a large CFM carburetor to run at a decent A/F ratio if you play with it enough. It wasn’t as obvious in these two examples, but in Doug’s big-block, the combination of the somewhat large 0.041-inch idle feed restrictor in the metering block with the built-in siphon break allowed the engine to run at a relatively lean A/F ratio. At very small throttle openings on highway cruise, most engines actually run on the idle circuit. There are exceptions to this of course, but most engines are actually fed by the transfer slot exposed by the primary throttle at roughly 10 percent throttle opening running down the freeway. The amount of fuel delivered by the transfer slot isn’t controlled by the idle mixture screws. It’s really determined by a tiny jet called the idle feed restrictor (IFR) in the main metering block. By adjusting the size of the IFR, this allows the tuner to idealize the A/F ratio at cruise. It’s a delicate balance – but it can be done.

In the case of Doug’s big engine, it doesn’t need much throttle opening at cruise yet it pulled fuel from the main metering circuit. This is due to the annular discharge boosters. Bigger cams with more overlap tend to need more throttle opening to run at freeway rpm so this pulls them into the main metering circuit. That’s the delicate balance you have to play with to determine which circuit the engine is using at cruise, but it can be done. What makes all this much easier to evaluate is the feedback from the wide-band oxygen sensor. Without this great little tool, tuning would be a far more difficult task. But don’t be afraid to make a few minor changes of your own on your street car. But be careful, you just might learn a few things!

Sources

Big Stuff 3
bigstuff3.com

Fuel Air Spark Technology (FAST)
fuelairspark.com

Holley Performance Products
holley.com

Innovate Motorsports
innovatemotorsports.com