After a bunch of screwing around with the Edelbrock 500cfm AVS carb on my truck, I've pretty much concluded it is just too much carb. Even at the leanest possible tuned setting, I have a rich misfire condition/sputter under heavy load, such as a hill or passing at highway speeds, a headwind, or a large load in the bed. This condition doesn't clear up with further throttle, so I'm pretty sure it's not a step-up spring problem either.

Depending on the conditions, the engine is also really down on power, and my mileage is god-awful (11.6 is my best out of the last 3 or 4 tanks).

Manifold is heated with a Galaxie carb spacer, no vacuum leaks are present, DSII distributor was recurved professionally and tested on a distributor machine, to the new cam and induction specs of the motor. Base timing was set correctly during engine break-in, but I haven't double-checked it since. See signature for all specs.

Basically what it comes down to is this carb is too much for the elevation that I'm at. So that leaves me looking for a better setup. And that's what I'm not too sure about.

A 2bbl? Something stock for a different Ford application?

The Holley 390? Probably a better option for overall power, but can it also deliver decent mileage? More tuning involved with this setup as well?

*sigh* An EFI setup? If I did this, I would likely build my own MegaSquirt I ECU for fuel control only, and keep the DSII setup as a standalone. I've been toying with adapting a GM TBI to my Clifford intake (better flowing intake? and less modification from my current setup), or finding a later 4.9 EFI intake and components (advantages in fuel distribution, but will it even bolt up to my head?). I'm sure I could figure out the programming and tuning behind this project, it would just be the most time-intensive.


What input do you guys have to offer? I'm really burnt out with trying to get this carb setup running right, especially since this is my work truck this summer instead of just a 5-10 mile daily driver.

Thanks!

 

Did you change out metering rods and jets or just mess with the mixture screws?

 

Yes, I have the full tuning kit, and have tried rod settings all the way to 12% lean with improvement, but the symptoms are still there.

 

Was it a used or a new carb?

And you said it's an AVS? So it's a Thunder series and not the Performer. Have you adjusted the secondary air valve so it opens quicker/easier?

Usually when I run a carb that's too large it runs lean not rich. Since you can't lean it out enough any chance the carb isn't large enough? I'm not too familiar with high altitude tuning.

Maybe you should contact Edelbrock's tech support and see if they have any ideas to lean it out.

 

If you used a c series you can try a 2 barrel the motocraft 2100s/2150s are dime a dozen and pretty simple and they'd flow enough cfm to be an improvement over stock. Also I wound up going with a smaller air cleaner on mine to dial it in, my carb was a little to rich at the stock setting and would lean out to much and experience hesitation if I went one step leaner on the rods. Smaller air filters don't make as big a difference as metering but if you can get it pretty close they do lean it out a little bit. One thing I noted is even with mine running to lean my mileage wasn't as good and when I first put my carb on it was running so rich it would puff a cloud of fuel out the back however it didn't sputter like you stated it bogged and would just stall the engine so you may want to pull the plugs and make sure they're fouling out and not turning white.

 

I have been tunning mine for quite a while, I put a oxygen sensor on mine a while back with a gauge. what i thought was a rich bog was actually a lean bog. so everyday i drove the truck to work, watch gauge and get home change jets and rods until i got it right. engine 300, 50 over 390 pistons, 240 head, 260 comp cam, headman header, aftermarket ignition. I found that if i run 11 on edelbock graph, which is stock rod, 83 jet and and 86 secondary with silver spring 8hg, i can cruise down the freeway just on the lean side and with light throttle go just on the rich side, then at about 2000 rpms if i mash it, it will go from rich to optimal back to rich until i let off, and the bog, miss is gone, the stiffer spring helps alot with transition between primary and secondary.

 

Thats why I suggested he pull the plugs I am running the stock Edelbrock 1404 metering/jet setup on my 500 afb and have been to colorado. I'm not sure what part he is in but Ionly had to go one step leaner on the metering rods while leaving everything else alone. It seems like he leaned it out alot, also when your running lean opening up the carb allows more air and only makes it lean out further so don't just assume its running rich. Plus these things have a pretty broad variance so if moving it progressively leaner hasn't made it better you need to double check its not running to hot before you start damaging the motor.

 

I can say without a doubt that a 500cfm vacuum secondary carb is nowhere near too much carb a 300.

How did you determine you've got a rich condition? As AJ mentioned, you're far more likely to have a lean bog than rich. Being at elevation may make some difference, but if you've gone 12% lean and it didn't help you should try going 8% rich and see how that works. Or try the stock set-up with the stiffest step up springs and set up the accelerator pump for the max shot and see if that helps. A wideband O2 sensor would be your best friend at this point...

And for the record, my truck (300, Offy C, Edelbrock 500cfm) had a boggy spot that wouldn't go away no matter what the O2 sensor told me until I added a heating plate to the bottom of my Offy C. You've got heat under the carb but is the intake itself plumbed with heat?

My 300 runs best with the timing set to ~14-16 BTDC and the vacuum advance to a manifold source. Timing ended up there after I kept advancing a couple of degrees and driving it until it felt best then backing it down until it didn't push against the starter hot.

 

I'm running a 390 Holley and 12 mpg is about the best I get. It runs almost flawlessly, except for the finicky choke adjustment. The thing I don't like is whenever I'm towing the secondarys are open almost all the time, especially on hills. This makes for some real sad mileage figures. I am also running a Galaxy heated spacer but no heat on the bottom of the intake where you really need it. Plans are to experiment with a few different 2 bbls and get some manifold heat on it. I'll probably start with a 1.23 2100 2 bbl. I also have some Rochester 2GC both large and small to mess with if I get ambitious.
FWIW I am running a Comp 260 cam, EFI exhaust manifolds and a DSII dist with efi coil and GM module. The ignition is flawless. The cam I don't like and would prefer a stock cam with the lower torque curve.

 

I don't care what anyone says. The 390cfm 4V is too damn small for a 300!

 

"...Rochester 2GC both large and small to mess with..." Good carbs but mostly overlooked and underrated.

 

Sorry for not replying to this thread for a while. Plugs definitely read rich. I got it to run quite a bit better by switching to the second smallest step-up springs to keep it out of the power mode longer. So far I am getting the best results at the second leanest setting (8%?), I did try going richer which only worsened my issues.
I should clarify a little, it's not the type of bog that goes away with less throttle or more throttle, it just gets to a point under load where it doesn't want to accelerate or hold speed, then if you let off the throttle slightly you can hear and feel it misfiring and coughing.
With the spring change it does much better on highway hills, occasionally it does still fall on it's face when attempting to hold 55-60mph up an interstate incline or into a headwind, but it seems almost intermittent now.
No overheating issues, temp gauge never exceeds 190 either sitting in traffic or holding 70mph on the interstate. I feel like manifold heat is not an issue either, especially not now during the summer. The manifold gets too hot to touch just from radiant heat from the exhaust manifolds. I even had to build a heat shield between the two to get rid of a heat soak fuel boiling/dieseling issue I had.
A wideband and gauge setup would be pretty nice to have, but it's not in the cards right now.

Thanks for all the input guys, much appreciated.

 

http://www.edelbrock.com/automotive_...ers_manual.pdf

Read the bottom of page 5 I tried going to a smaller air filter assembly at the advice of one of edelbrocks tech support people and wound up putting my carb were it needed to be. While running a open element 14x3 the stock metering was to rich and going 1 stage 4% leaner on the metering resulted in white plugs and hesitation. In order to put it in the middle I did what they suggested and it seems to have created a happy medium. It also states it'll shift it leaner at WOT and at higher rpms. Running the 65x52 rods with stock metering jets and a bigger air cleaner my truck was sluggish and pulled 55 mph at 3000rpm with the same rods and same jets and a 9"x2" air filter it pulls smoothly and I can pull 65 mph at 3000 rpms with a light grey color to the plugs.

 

I'm not running an open-element filter, I have a stock style air cleaner from a 351HO equipped truck.
I'm a little bit confused about how a more restrictive air cleaner will lean out the fuel mixture. I'm used to small engines and cycle engines that will lean out and require a richer jet when switching the other direction, from a restrictive stock airbox to an open element pod filter.

 

I think it has to do with how quickly the air is pulled in that alters the metering of the air and fuel. I was led to believe the smaller filter pulls the same amount of air through a smaller area and therefore it increases the velocity of the air flow and alters the metering, leaning the mix. You'd have to honestly ask someone with more experience to get a definite proper answer. I wasn't to concerned with how it would go about obtaining a halfway point between the two settings on their chart. When I explained to the edelbrock tech my situation, the vehicle and how it was setup I was told to run the richer rods with a smaller air cleaner to lean it out by the desired 2%(roughly) as opposed to 4%. Not all setups are the same so sometimes you have to be creative when tuning.
Your engine having a cam and dual exhaust with a port & polished head will want to pull more air/fuel in and perhaps it may not be able to pull enough air to make for a decent a/f ratio because the intake is to restrictive and not allowing it to.. Its not a difficult adjustment to make and if you've gone as lean as possible on the metering and your close its about the last thing you could alter that I can think of provided nothing is malfunctioning with the carb.