Engine is a ~300hp 300. 268H cam. 1.7 rockers.

I've been trying to tune out the infamous Edelbrock lean bog when the secondaries open. The secondaries haven't been responding to changes much at all.

First thing I did was put my wideband on it and confirm it was pig rich on only the primaries at 12:1 dipping into the low 11s, then when the secondaries open, it went way lean. It could go up to 20:1, but mostly 18:1 unless I rolled into it slowly. If I got into the secondaries at 1200rpm and let it run up to 5000rpm, it would start around 16:1 and end around 14:1.

The first thing I did was restrict the secondaries early feed air bleed with wire. No noticeable change.

I swapped primary rods and jets around(.083) to get good cruise AFR around stoich, but power mode without the secondaries is still a tad rich around 12:1. I upped the secondary jets to .098". From 1200 to 5000, it would start a in the 15's and just barley make it to 13:1 at 5000.

I then tried restrictions the secondary main air bleeds, and removing the crimped restriction at the bottom of the secondary early feed tube. No change.

I then drilled and added 5g lead weight to the secondary air door, soldered the early feed air bleed closed and restricted the secondary emilsion tube low speed hole. I removed the high speed air bleed restrictions I put in. No change noticed.

I then added another 5g of weight to the door and removed the secondary jets completely from the carb since I don't have any bigger than .098". FINALLY I got a SANE AFR when opening the secondaries. It's not stupid rich with though. It'll start out around 12.5:1 and creep down to 11:1 or maybe a high 10.8 at most right at 5000rpm. WHAT GIVES?

I can certainly drill out my jets or spend a bunch more money buying some, but it just doesn't seem right to me that it runs as good as it does without jets at all and doesn't respond hardly at all to any other mods that should make a difference. Any suggestions? I'd also like to be able to Taylor the AFR throughout the RPM range, so if there's any gurus out there let me know.

 

I am a Holley man myself so cant help too much with the Edel carb but it looks like you are making 2 changes at 1 time on the secondary side. Weight on the arm but then go bigger or no jets at all, why?


When doing anything to try and fix something only make 1 change at a time. This way you know if that worked or did not. Make 2 and you don't know if 1 canceled the other.


I would also fix the primary side to run right first. Wire the secondary side so it will not open. Once the primary side runs great then start working on the secondary side making 1 change at a time.


Now you say Edel 500? Is this a 500 cfm carb? If so I think it is too big for a 300 six. I know of Holley 390's that are sometimes too big. Then add the bad fuel distribution of the inline six and it just adds to the issues to get it tuned right.


BTW could the carb have a leak inside, bad gasket, on the secondary side causing it to run rich?


That is my .02
Dave ----

 

It may be a tad big, but not by much. 300ci @ 5000rpm needs close to 500cfm. 450cfm would probably be ideal.

Weighting the door pulls more fuel from the early feed circuit and less from the secondary main, as it blocks the airflow on the secondaries until the cfm gets high enough for the secondary main boosters to work and start pulling fuel. Once the door is open, fuel is all from the main boosters through the jet. Weighting the door attempts to richen the lower rpm range when the secondaries open. Going larger on the jets richens both lower rpm and higher rpm.

The primary side is fixed. It runs fine in cruise and power mode and afr is good.

To get the afr in a sane range, I had to drill out the secondary jets to 1/8" or the equivalent of a .125" jet. Max rich AFR now is 11.9:1 Edelbrock doesn't even offer a jet that big as far as I know. I might try a real .119" jet and see.

I'm still working on trying to flatten the afr curve some though. I have drilled and added bleeds in the emulsion tube and it has helped. I may add some more weight to the door though, as it's still trying to come in before 2500, if not below 2000rpm.

 

In my experience, 450cfm is just about right. The 390 was just too small.
I've also run several 600cfm Holleys with good results. They weren't my favorite because the throttle was a little less than crisp, but they worked fine.

I would think 500cfm is a very decent size for the 300.

Are you secondaries vacuum or mechanical?

 

Edelbrock "Performer"/Carter AFB and even Q-jet and Thermoquad secondary butterflies are all mechanical, with an air door or air flap above the secondary boosters that do not allow airflow fully through the secondary venturi until airflow demand is enough to pull it open. The Edelbrock "Thunder" AVS series has an adjustable spring tension that holds the secondary air flap closed. The Edelbrock "Performer" AFB series has the flap on a hinge with a weighted cam that holds it closed.

I was only impressed with my Holley 600 vacuum secondary when it was just out of the box and a very rich transition circuit. And that was only compared to my Holley 390 vacuum secondary that I had serious issues with. The 600 still took a lot of fiddling to the acceleration pump cams and squirters, biggest secondary spring, and restricting the idle circuit so I could get any fuel mileage because cruising was all on the rich transition circuit, and then part throttle drivability wasn't any good. This Edelbrock 500 drives GREAT compared to the Holleys, especially when the Edelbrock is rich on the primaries and even when tuned lean it never needs more than the smallest acceleration pump shot. The low RPM power is no comparison with the Edelbrock winning by a long shot and when the AFR is right on the secondaries, it seems the Edelbrock wins out too. I really want to do a dyno comparison to see what it really is, but money may not allow.

Anyway, the point of getting the Edelbrock right wasn't ever for drivability, it is actually so I can add a blow through turbo. The better drivability and power was just a side effect benefit. Edelbrock mods for blow through are simple compared to Holley and that's even if I had a Holley double pumper small enough to work and have any drivability with mechanical secondaries. When I get the money, it'll all be moot anyway, as I'll be going EFI.

 

Your secondaries open before the primaries are fully open. The primaries may still be contributing to your lean condition. Disconnect and wire the secondaries closed to confirm that the primaries stay in the ideal range. As large as the secondary jets are now, I am suspicious of this possibility. Remember, we are typically stuck with two stage rod profiles.

The auto manufacturers sometimes had three step or even four step metering rod profiles to solve these problems. Finding those rods is mighty tough now. Some of those rods also had a different stroke/ travel range so they may not be useful without the original carb either! Aggravating.

One tough thing about tuning the secondaries is that there is no metering rod system and as the air door opens and as the airflow vs. vacuum relationship changes so does af ratio. That's a lot to ask the air door to take care of.

Considering the airflow rate when the secondaries open up, I have doubts that any changes in the secondary transition circuits/ air bleeds will make a noticeable difference. In an application where rpm and load levels held steady it could, but under acceleration at wide throttle openings they just don't contribute more than a minute percentage to the mix.

Ever hear of a Mikuni Powerjet? The add on kits may still be available. Check Dennis Kirk/ Parts Unlimited. Basically it is a tiny brass straw with interchangeable jets that was fed by a small fuel line tapped into the fuel bowl of a Mikuni sidedraft carb used on motorcycles, snowmobiles, etc. They were very popular with snow machine guys out here in the mountains since they allowed quick re-jetting for high altitude use and changing conditions. Jet several steps lean then add fuel/ fine tune with the power jets.

Proper placement of the discharge tube meant that as engine demand increased so did fuel flow. Modification will be required for a down draft carb, but I will say that the discharge should be located high and centered in the venturi. Darn near the existing booster discharge I would say. Possibly right across or slightly above would be a good start. An extended discharge tube will likely be needed (nothing is ever easy!). Small changes in placement height will affect the amount of vacuum pulling fuel from the power jet, so make sure both sides are equal. I would try a couple if a blow through turbo is your goal. It could save may head aches and a burn down.

Personally, I wouldn't touch a turbo without EFI. Too many headaches and potential for engine damage from a plugged jet. I can't afford to start over on my budget.

 

I'm not too concerned with trying a turbo carb combo. It's not my first time adding a turbo to an engine and tuning, just the first with a carb. And I've already been eyeing cheap EFI longblocks to swap the bottom end out with when this one goes.

I guess I really need to disconnect the secondaries and make sure the primaries are good, BUT the primaries are good up to 65% throttle when the secondaries start to open. I can hold it there and watch the AFR and it's in the 12's. I also figured the primaries would never see any more flow than that, since the secondaries steal from the primaries when they open. But, it'll be good to know for sure anyway.

Yeah, messing with the secondary air bleeds haven't done a whole bunch very easily and I attitude it to low airflow rate. But it has worked to smooth the steep lean to rich AFR ramp I've had from about 3k to 5k.

Maybe once I get my RPM pickup to work on my wideband, it'll be much easier to see what needs work where.

 

65%. Think about that. Things are good at 65% throttle. Now you are going to add 35% more air. If the rods are already in the full rich position, How much additional fuel will they add as the vacuum on the fuel circuit drops due to wider throttle opening. Ain't carbs fun?

I wish these carbs had a metering needle and venturi slide like motorcycle carbs. With those you just match the taper of the needle ( a science of its own) to the jet combo until things are right from 1/8 throttle to wide open.

 

So, I was able to disconnect the secondaries and run it just on the primaries to see how it fared, and it didn't change any. The primaries are fine, though maybe a tad richer than I'd like still. It stays in the mid to low 12s all the way to 5k.

I also noticed that I've got some fuel seepage and/or boil over on the secondaries after it sits for a few minutes. I'm not too happy about that, as it was not hot at all. I could hold my arm on the radiator or engine without any trouble. I'll change the gasket between the carb halves, as it's worn I'm sure from being apart so many times, but the boil over I don't really know what to do about. I may have to drill and tap the opposite bowl for a feed to keep the fuel line from being over the manifolds for any extended run, but I'm not sure what to do about the carb boiling. I already run an open spacer on top of the Offy-C.

 

If "65%" refers to the amount of throttle plate opening then that statement is not necessarily true. At closed throttle the plates are usually between 9 and 14 degrees. And airflow transitioning from 65% to 100% is not necessarily linear, especially at lower speeds.

 

The more you describe things the more I think warpage or gasket failure. Lean conditions that are unruly, leaking fuel.
The secondary side may also have a dirty float valve which could result in overfill when the engine is shut off and then restrict fuel flow too much during high demand (thus a low fuel level when fuel is being demanded the most). This may very well cause the symptoms. Truth be known, the secondary side should require a smaller jet than the primary since there is no metering rod plugging most of it off. Of course I did have a 390 that liked things one step rich on the secondary side.