Have an Offy dual plane intake and Holley 390cfm carb on the intake, factory EFI exhaust manifolds into a single 2.5” pipe after a factory “Y”. Would have preferred to do headers, but no one makes I6 headers to fit a Ford van.

At light to half throttle the power and response are much better than in stock 1bbl trim, but as soon as the secondaries open up on the 4bbl the power goes flat. Installed a quick-change secondary spring kit, and even with the heaviest spring [max delay on secondary opening] the power still bogs or goes flat when the secondaries open. As this is a dual plane intake [mains on one circuit, secondaries on another], could it be that the near instant doubling of intake air volume is just making the manifold vacuum drop way too low? Is fuel demand too much for the stock pump? Any ideas??

 

Hmm...sounds like your going lean when the secondaries open. Give it a bigger pump shot and let the secondaries open a bit sooner.

 

If it goes flat, I mean really flat, then check the fuel level in the secondary bowl. Even if you checked it once, check it again. I've set it (or at least I thought it was set) only to go back and find it was too low. If this is the case, your starving for fuel and it will definitely go flat!

Ron

 

other than the bogging 83van, how do you like the 4bbl? And im curious what you get for gas milage. I would think you would get good gas mileage, because you are running on the small primaries most of the time

 

The van normally gets used for short delivery trips and I haven't checked the "light load" mileage since first installation of the parts. It was getting about 14 mpg city stock, but that actually dropped to about 12.5 after the new carb/intake/exhaust went on. Remember this is a big full size van [5200 lbs curb weight].

Gas mileage while towing, well, sucks. It was getting 12.5mpg in stock form loaded down and towing 2500 lbs worth of trailer, now that has dropped to 10.5 mpg. Big difference though is that on straight flat land I can cruise at 65-70mph, before it was topping out a good 10mph lower. This is a low mileage engine BTW, only 66K on the clock.

 

I like the set up but gas mileage was not an issue to begin with. A new car/truck payment was! I'm running 3.55 gears with 32 inch tires and an OD tranny in a Bronco. I usually have to keep it floored on the highway (75-85 mph) so gas goes quick!

 

I'm going to ask a stupid question because it needs to be asked. Are you sure it bogs when the secondaries open, or is it just at WOT? I ask because it is possible for the secondaries to open at part throttle and it is possible for them to be closed at WOT. Have you watched them while driving with the doghouse off to make sure the bog is when the secondaries open?

I have also seen people start with a float level too high and adjust it lower until it is at the correct height. This results in the level being way too low 99% of the time because the fuel level does not drop instantly when the float level is reduced.

Are you sure that carb is meant for your application? It's possible that your 390 is calibrated for use on a tunnel ram which would make it run like crap on anything else. Some 390's also come with a check valve in the secondary signal passage to change the opening characteristics. I never can remember if the valve speeds them up or slows them down. It seems like it should slow them down.

You said something about the near instant doubling of the airflow messing things up. The secondaries should not open instantly. If they do something is wrong. Secondary opening usually takes anywhere from 2-5 seconds depending on the engine and its rate of acceleration. If the engine bogs at some point during secondary opening the secondaries usually won't open any more until the bog has passed.

Is your ignition system up to the task of lighting the fire under the increased cylinder pressure?

If all that is in good shape you should spend some time on a chassis dyno while monitoring the air/fuel ratio. It should only cost you about $50 for 3 pulls or $100 for an hour of time. You can learn a lot by comparing a dyno sheet and an air/fuel graph on the same sheet of paper. I would link to a few dyno charts for cars I've dyno tuned, but I get in trouble for trying to help people with links to useful information.

 

The "flat spot" sems to hit right when the secondaries open. Can't see them without some remote camera setup, but I'm very familiar with the sound when they do open up. Full throtle response when the engine is relatively cold [secondaries can't open yet] is relatively good.

The Offy "dual port" intake is literally a 2-in-1 manifold, with primaries & secondaries having their own completely divided air paths. When the secondaries start to open the physical volume of the intake system doubles, which is going to reduce intake air velocity till the flow between the two halves balances out. Some dual plane intakes have a crossflow notch in the divider right under the carb which serves to balance the flow "left to right", and I'm thinking a similar notch may help here. Since you'd already have some flow in the secondary system, the transition when the back bores open is more smooth. I've considered pulling the intake and making such a crossover notch, but not enough time to tinker before the van leaves for its annual cross country treck.

The carb is a Holley 8004, exact one recommended for this intake. Ignition is the stock Duraspark II circa 1983. Stock coil and distributor, but new plugs/ wires/ rotor/ cap. Have considered swapping to a hotter coil, but I've never messed with changing a Ford 9 [?] volt coil to a full 12v model. Would love to drop in an MSD ignition but $$ is tight right now. Any suggestions for a low buck ignition upgrade?

 

Usually a flat spot when the secondaries open is a fuel starvation problem. I would check the float level first since that's pretty easy to do and has a good probability of being the problem.

Another thing I thought of is that the secondaries might be sticking slightly. Sometimes they are adjusted so that they close too much, which causes the blades to stick in the bores a little bit. When this happens they don't open when vacuum first acts on the diaphram, then when there is enough vacuum to unstick them open too quickly. The adjustment for this is on the bottom of the carb on the choke side. PITA to get to. If they aren't sticking I wouldn't mess with it. If there is any reistance other than what the diaphram provides there is something sticking.

I'm familiar with the dual port manifold. Even though the secondary side is much larger than the primary the volume is always the same. If the secondaries are slamming open as described above it might be an issue, but if they are opening in a controlled manner like they are designed to do it won't make any difference. The intake might make carb problems more pronounced, but if the carb is right the intake shouldn't be an issue.

 

Thanks streak, will check the fuel level again. It seemed fine right out of the box, but who knows. With mileage so poor, I figured it was running too rich if anything. Since this is a single feed carb, is there any type adjustment for the secondary bowls or just the one float on the primary side? I know Quadrajets inside and out but am new to Holley's.

 

There is a separate adjustment on the secondary bowl. Might as well check the primaries too. The primary side should be right at the level of the threads in the site hole. The secondary side should just barely trickle out the hole. You might also check the transfer tube between the bowls for blockage. This is very rare, but happens. If the tube is blocked no fuel will get to the secondaries.

 

Update: finally got a set of clear fuel bowl sight plugs and went to work. Float level on the primary side was actually a bit high, and with that corrected it now idles more evenly [used to surge a bit]. As for the "bog", raised the fuel level in the secondary bowl and it did get a touch better, but it's still there. Today I'm going to try swapping to a lighter secondary spring, and when Sat. rolls around will install a new fuel pump, Mallory Hyfire ignition box and MSD coil. Even though there's only 60k miles on the clock, the stock pump and coil are 21 years old, so it can't hurt. My wholeseller also had a great deal on the new Hyfire VI-A ignition box [$100], so what the heck. Assuming it gives me a mere 1mpg boost, it'll almost pay for itself on this upcoming trip. $$ is tight so I must rationalize the purchase :-)

 

I would suggest that you find that 1/8"checkball that goes in your vacuum secondary mechanism and install it. They leave it out on the 390 carb since they recommend it for light vehicles and yours and mine are very heavy. Also install a .035 discharge nozzle and make sure your accelerator pump is adjusted correctly. I would also make sure that your main jet size is correct. I had to go up two sizes to correct a lean condition. I'm running mine with the brown spring in the secondary housing and mine runs like a champ with no bogs or hesitation. Be sure your primary throttle blades are not open to far when adjusting your idle speed or you uncover to much of the transition slots which could also cause a bog.

 

Inliner, what type of vehicle do you have? I posted a similar problem in the "stumble, hesitation, miss" post. I have lots of questions. Ted

 

Believe there is a check ball in place, remember making sure it was there when I installed the quick change secondary spring kit as the tuning instructions mention it's necessity.

What size primary jets are you using? The model 8007 carb I have uses a metering plate for the secondaries instead of interchangable jets.

Installing the new ignition system made a definite improvement at all RPM levels.